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Pages 401-729 and Plates III-VIII are in Volume II.




STUDIES IN THE THEORY OF DESCENT.




  LONDON:
  PRINTED BY GILBERT AND RIVINGTON, LIMITED,
  ST. JOHN’S SQUARE,




  STUDIES IN THE THEORY
  OF DESCENT

  BY

  DR. AUGUST WEISMANN
  PROFESSOR IN THE UNIVERSITY OF FREIBURG

  _WITH NOTES AND ADDITIONS BY THE AUTHOR_

  TRANSLATED AND EDITED, WITH NOTES, BY

  RAPHAEL MELDOLA, F.C.S.
  LATE VICE-PRESIDENT OF THE ENTOMOLOGICAL SOCIETY OF LONDON

  WITH A PREFATORY NOTICE BY

  CHARLES DARWIN, LL.D., F.R.S.
  _Author of “The Origin of Species,” &c._

  IN TWO VOLUMES
  VOL. I.

  WITH EIGHT COLOURED PLATES

  London:

  SAMPSON LOW, MARSTON, SEARLE, & RIVINGTON
  CROWN BUILDINGS, 188, FLEET STREET

  1882

  [_All rights reserved._]




PREFATORY NOTICE.


The present work by Professor Weismann, well known for his profound
embryological investigations on the Diptera, will appear, I believe, to
every naturalist extremely interesting and well deserving of careful
study. Any one looking at the longitudinal and oblique stripes, often
of various and bright colours, on the caterpillars of Sphinx-moths,
would naturally be inclined to doubt whether these could be of the
least use to the insect; in the olden time they would have been called
freaks of Nature. But the present book shows that in most cases the
colouring can hardly fail to be of high importance as a protection.
This indeed was proved experimentally in one of the most curious
instances described, in which the thickened anterior end of the
caterpillar bears two large ocelli or eye-like spots, which give to
the creature so formidable an appearance that birds were frightened
away. But the mere explanation of the colouring of these caterpillars
is but a very small part of the merit of the work. This mainly consists
in the light thrown on the laws of variation and of inheritance by
the facts given and discussed. There is also a valuable discussion
on classification, as founded on characters displayed at different
ages by animals belonging to the same group. Several distinguished
naturalists maintain with much confidence that organic beings tend to
vary and to rise in the scale, independently of the conditions to which
they and their progenitors have been exposed; whilst others maintain
that all variation is due to such exposure, though the manner in which
the environment acts is as yet quite unknown. At the present time
there is hardly any question in biology of more importance than this
of the nature and causes of variability, and the reader will find in
the present work an able discussion on the whole subject, which will
probably lead him to pause before he admits the existence of an innate
tendency to perfectibility. Finally, whoever compares the discussions
in this volume with those published twenty years ago on any branch of
Natural History, will see how wide and rich a field for study has been
opened up through the principle of Evolution; and such fields, without
the light shed on them by this principle, would for long or for ever
have remained barren.

            CHARLES DARWIN.




TRANSLATOR’S PREFACE.


In offering to English readers this translation of Professor Weismann’s
well-known “Studies in the Theory of Descent,” the main part of which
is devoted to entomological subjects, I have been actuated by the
desire of placing in the hands of English naturalists one of the most
complete of recent contributions to the theory of Evolution as applied
to the elucidation of certain interesting groups of facts offered by
the insect world. Although many, if not most, working naturalists
are already familiar with the results of Dr. Weismann’s researches,
of which abstracts have from time to time appeared in English and
American scientific journals, I nevertheless believe that a study
of the complete work, by enabling the reader to follow closely the
detailed lines of reasoning and methods of experiment employed by the
author, will be found to be of considerable value to those biologists
who have not been able to follow the somewhat difficult phraseology of
the original. It is not my intention, nor would it be becoming in me to
discuss here the merits of the results arrived at by the minute and
laborious investigations with which Dr. Weismann has for many years
occupied himself. I may however point out that before the appearance of
the present work the author, in addition to his well-known papers on
the embryology and development of insects, had published two valuable
contributions to the theory of descent, viz. one entitled “Über die
Berechtigung der Darwin’schen Theorie” (1868), and another “Über den
Einfluss der Isolirung auf die Artbildung” (1872). These works, which
are perhaps not so well known in this country as could be desired,
might be advantageously studied in connection with the present volume
wherein they are frequently referred to.

Since every new contribution to science is a fresh starting-point for
future work, I may venture without any great breach of propriety to
dwell briefly upon one or two of the main points which appear to me to
be suggested by Prof. Weismann’s investigations.

Although the causes of Glacial Epochs is a subject which has much
occupied the attention of geologists and physiographers, the question
is one of such great complexity that it cannot yet be regarded as
finally settled. But apart from the question of causes--a most able
discussion of which is given by the author of “Island Life”--there
is not the least doubt that at no very distant geological period
there occurred such an epoch, which, although intermittent, was of
considerable duration. The last great geological event which our globe
experienced was in fact this Ice Age, and the pure naturalist has not
hitherto attributed in my opinion sufficient importance to the _direct_
modifying effects of this prolonged period of cold. It is scarcely
possible that such a vast climatic change as that which came on at the
close of the Pliocene Period should have left no permanent effect upon
our present fauna and flora, all the species of which have survived
from the glacial age. The great principle of Natural Selection leads
us to see how pre-glacial forms may have become adapted to the new
climatic conditions (which came on gradually) by the “survival of the
fittest” or “indirect equilibration.” The influence of the last Glacial
Epoch as a factor in determining the present geographical distribution
of animals and plants has already been amply treated of by many writers
since the broad paths were traced out by Darwin, Lyell, and Wallace.
The last-named author has indeed quite recently discussed this branch
of the subject most exhaustively in his work on “Island Life” above
mentioned. The reference of a particular group of phenomena--the
seasonal dimorphism of butterflies--to the direct action of the Glacial
Period and the subsequent influence of the ameliorating climate, was
however the first step taken in this neglected field by the author
of the present work in 1875. It is possible, and indeed probable,
that future researches will show that other characters among existing
species can be traced to the same causes.

The great generalizations of embryology, which science owes so largely
to the researches of Karl Ernst von Baer, bear to the theory of
descent the same relations that Kepler’s laws bear to the theory of
gravitation. These last-named laws are nothing more than generalized
statements of the motions of the planets, which were devoid of meaning
till the enunciation of the theory of gravitation. Similarly the
generalized facts of embryology are meaningless except in the light
of the theory of descent. It has now become a recognized principle in
biology that animals in the course of their development from the ovum
recapitulate more or less completely the phases through which their
ancestors have passed. The practical application of this principle to
the determination of the line of descent of any species or group of
species is surrounded by difficulties, but attempts have been made of
late years--as by Haeckel in his _Gastrula_ theory--to push the law
to its legitimate consequences. In this country Sir John Lubbock, in
1874, appealed to the embryonic characters of larvæ in support of his
views on the origin of insects. To the author of this work (1876) is
due the first application of the principle of Ontogeny as revealing
the origin of the markings of caterpillars. A most valuable method
of research is thus opened up, and entomologists should not be long
in availing themselves of it. Our knowledge of the subject of larval
development in Lepidoptera is still most imperfect, and it cannot as
yet be foreseen to what extent the existing notions of classification
in this much-studied order may have to be modified when a minute
study of the Comparative Ontogeny of larval characters, worked out as
completely as possible for each family, has enabled a true genealogical
system to be drawn up. The extent to which such a larval genealogy
would coincide with our present classification cannot now be decided,
but he who approaches this fruitful line of inquiry in the true spirit
of an investigator, will derive much instruction from Prof. Weismann’s
remarks on “Phyletic Parallelism in Metamorphic Species.” The
affinities of the larger groups among Lepidoptera would most probably
be made out once and for ever if systematists would devote more time to
observation in this field, and to the co-ordination and working up of
the numerous data scattered throughout the vast number of entomological
publications.

The doctrine of development by no means implies, as has sometimes been
maintained, a continuous _advancement_ in organization. Although the
scale of organic nature has continued to rise as a whole, cases may
occasionally occur where a _lower grade_ of organization is better
adapted to certain conditions of life. This principle of “degeneration”
was recognized by Darwin as early as in the first edition of the
“Origin of Species;” it was soon perceived to be applicable to the
phenomenon of parasitism, and was first definitely formulated by
Dr. Anton Dohrn in 1875. In a lecture delivered before the British
Association at Sheffield in 1879, Prof. E. Ray Lankester ascribed to
“degeneration” a distinct and well-defined function in the theory of
descent. Dr. Weismann’s explanation of the transformation of Axolotl
given in the fourth essay of this work, may be regarded as a special
contribution to this phase of Darwinism. Whilst refuting the idea
held by certain naturalists, that such cases are arguments against
the origin of species by the accumulation of minute variations, and
prove the possibility of development _per saltum_, the theory here
advanced (that _Siredon_ at a former period existed at a higher
stage of development as _Amblystoma_, and that the observed cases of
metamorphosis are but reversions to this lost higher stage) suggests
the question whether there may not still be in existence many other
degenerated forms quite unsuspected by naturalists.

Many of the opponents of Evolution have from time to time denounced
this doctrine as leading to “pure materialism,” a denunciation which
may appear somewhat alarming to the uninitiated, but which may not
seem fraught with any serious consequences to those who have followed
the course of philosophical speculation during the last few years.
Those who attack the doctrine on this ground will however do well to
consider Prof. Weismann’s views set forth in the last essay in this
volume, before hastily assuming that the much dreaded “materialism” is
incompatible with any other conception of Nature.

The small amount of leisure time which I have been able to devote to
the translation of this volume has delayed its completion considerably
beyond the anticipated time, and it was with a view to meeting this
difficulty that I departed from the original form of the German edition
and issued it in parts. Owing to the extremely idiomatic character
of the German text, I have throughout endeavoured to preserve only
the author’s meaning, regardless of literal translation or of the
construction of the original. In some few cases, however, I have
intentionally adopted literal translations of certain technical
expressions which might, I think, be advantageously introduced into
our biological vocabularies. Some alterations have been made in the
original text by the author for the present edition, and many new notes
have been added. For those bearing my initials I am alone responsible.

It gives me much pleasure in conclusion to express my thanks to Dr.
Weismann, not only for the readily given permission to publish an
English translation of his work, but also for much valuable assistance
during the execution of the task. The author has been good enough to
superintend the drawing of the plates for this edition, and he has also
read through the greater part of the manuscript. From Mr. Darwin also
I have received much kindly encouragement, and among entomologists I
am especially indebted to Mr. W. H. Edwards of West Virginia, for his
valuable additions to the first part. To my friends Mr. A. G. Butler,
Mr. Roland Trimen, and Mr. F. Moore, I owe acknowledgments for much
useful information concerning the caterpillars of exotic _Sphingidæ_,
which I have incorporated in the notes and appendices, and Mr. W. S.
Simpson has given me occasional advice in the translation of some of
the more difficult passages.

            R. M.
  _London, November, 1881._




PREFACE TO THE ENGLISH EDITION.


With the appearance of Charles Darwin’s work “On the Origin of
Species,” in the year 1858, there commenced a new era in biology.
Weary of the philosophical speculations which, at the beginning of
this century, had at first been started with moderation but had
afterwards been pushed to excess, biologists had entirely let drop all
general questions and confined themselves to special investigations.
The consideration even of general questions had quite fallen into
disuse, and the investigation of mere details had led to a state of
intellectual shortsightedness, interest being shown only for that which
was immediately in view. Immense numbers of detailed facts were thus
accumulated, but they could not possibly be mastered; the intellectual
bond which should have bound them together was wanting.

But all this was changed in a short time. At first only single
and mostly the younger naturalists fell in with the new theory of
development proclaimed by Darwin, but the conviction soon became
general that this was the only scientifically justifiable hypothesis
of the origin of the organic world.

The materials accumulated in all the provinces of biology now for the
first time acquired a deeper meaning and significance; unexpected
inter-relations revealed themselves as though spontaneously, and
what formerly appeared as unanswerable enigmas now became clear and
comprehensible. Since that time what a vast modification has the
subject of animal embryology undergone; how full of meaning appear the
youngest developmental stages, how important the larvæ; how significant
are rudimentary organs; what department of biology has not in some
measure become affected by the modifying influence of the new ideas!

But the doctrine of development not only enabled us to understand the
facts already existing; it gave at the same time an impetus to the
acquisition of unforeseen new ones. If at the present day we glance
back at the development of the biological sciences within the last
twenty years, we must be astonished both at the enormous array of new
facts which have been evoked by the theory of development, and by the
immense series of special investigations which have been called forth
by this doctrine.

But while the development theory for by far the greater majority of
these investigations served as a light which more and more illuminated
the darkness of ignorance, there appeared at the same time some
other researches in which this doctrine itself became the object of
investigation, and which were undertaken with a view to establish it
more securely.

To this latter class of work belong the “Studies” in the present volume.

It will perhaps be objected that the theory of descent has already
been sufficiently established by Darwin and Wallace. It is true that
their newly-discovered principle of selection is of the very greatest
importance, since it solves the riddle as to how that which is useful
can arise in a purely mechanical way. Nor can the transforming
influence of direct action, as upheld by Lamarck, be called in
question, although its extent cannot as yet be estimated with any
certainty. The _secondary_ modifications which Darwin regards as the
consequence of a change in some other organ must also be conceded. But
are these three factors actually competent to explain the complete
transformation of one species into another? Can they transform more
than mere single characters or groups of characters? Can we consider
them as the sole causes of the regular phenomena of the development
of the races of animals and plants? Is there not perhaps an unknown
force underlying these numberless developmental series as the true
motor power--a “developmental force” urging species to vary in certain
directions and thus calling into existence the chief types and
sub-types of the animal and vegetable kingdoms?

At the time these “Studies” first appeared (1875) they had been
preceded by a whole series of attempts to introduce into science such
an unknown power. The botanists, Nägeli and Askenasy, had designated
it the “perfecting principle” or the “fixed direction of variation;”
Kolliker as the “law of creation;” the philosophers, Von Hartmann and
Huber, as the “law of organic development,” and also “the universal
principle of organic nature.”

It was thus not entirely superfluous to test the capabilities of the
known factors of transformation. We had here before us a question of
the highest importance--a question which entered deeply into all our
general notions, not only of the organic world, but of the universe as
a whole.

This question--does there exist a special “developmental
force”?--obviously cannot be decided by mere speculation; it must also
be attempted to approach it by the inductive method.

The five essays in this volume are attempts to arrive, from various
sides, somewhat nearer at a solution of the problem indicated.

The first essay on the “Seasonal Dimorphism of Butterflies” is
certainly but indirectly connected with the question; it is therein
attempted to discover the causes of this remarkable dimorphism, and
by this means to indicate at the same time the extent of one of the
transforming factors with reference to a definite case. The experiments
upon which I base my views are not as numerous as I could desire,
and if I were now able to repeat them they would be carried out more
exactly than was possible at that time, when an experimental basis had
first to be established. In spite of this, the conclusions to which
I was led appear to be on the whole correct. That admirable and most
conscientious observer of the North American butterflies, Mr. W. H.
Edwards, has for many years experimented with American species in a
manner similar to that which I employed for European species, and his
results, which are published here in Appendix II. to the first essay,
contain nothing as far as I can see which is not in harmony with my
views. Many new questions suggest themselves, however, and it would
be a grateful task if some entomologist would go further into these
investigations.

The second essay directly attacks the main problem above indicated.
It treats of the “Origin of the Markings of Caterpillars,” and is to
some extent a test of the correctness and capabilities of the Darwinian
principles; it attempts to trace the differences in form in a definite
although small group entirely to known factors.

Why the markings of caterpillars have particularly been chosen for this
purpose will appear for two reasons.

The action of Natural Selection, on account of the nature of this
agency, can only be exerted on those characters which are of
biological importance. As it was to be tested whether, besides Natural
Selection and the direct action of external conditions, together
with the correlative results of these two factors, there might not
lie concealed in the organism some other unknown transforming power,
it was desirable to select for the investigation a group of forms
which, if not absolutely excluding, nevertheless appeared possibly to
restrict, the action of one of the two known factors of transformation,
that of Natural Selection; a group of forms consisting essentially
of so-called “purely morphological” characters, and not of those the
utility of which was obvious, and of which the origin by means of
Natural Selection was both possible and probable _ab initio_. Now,
although the _colouring_ can readily be seen to be of value to the life
of its possessors, this is not the case with the quite independent
_markings_ of caterpillars; excepting perhaps those occasional forms
of marking which have been regarded as special cases of protective
resemblance. The markings of caterpillars must in general be considered
as “purely morphological” characters, _i.e._ as characters which we
do not know to be of any importance to the life of the species, and
which cannot therefore be referred to Natural Selection. The most
plausible explanation of these markings might have been that they were
to be regarded as ornaments, but this view precludes the possibility
of referring them either to Natural Selection or to the influence of
direct changes in the environment.

The markings of caterpillars offered also another advantage which
cannot be lightly estimated; they precluded from the first any attempt
at an explanation by means of Sexual Selection. Although I am strongly
convinced of the activity and great importance of this last process
of selection, its effects cannot be estimated in any particular case,
and the origin of a cycle of forms could never be clearly traced to
its various factors, if Sexual Selection had also to be taken into
consideration. Thus, we may fairly suppose that many features in the
markings of butterflies owe their origin to Sexual Selection, but we
are, at least at present, quite in the dark as to how many and which of
these characters can be traced to this factor.

An investigation such as that which has been kept in view in this
second essay would have been impracticable in the case of butterflies,
as well as in the analogous case of the colouring and marking of birds,
because it would have always been doubtful whether a character which
did not appear to be attributable to any of the other transforming
factors, should not be referred to Sexual Selection. It would have been
impossible either to exclude or to infer an unknown developmental
force, since we should have had to deal with two unknowns which could
in no way be kept separate.

We escape this dilemma in the markings of caterpillars, because the
latter do not propagate in this state. If the phenomena are not here
entirely referable to Natural Selection and the direct action of the
environment--if there remains an inexplicable residue, this cannot be
referred to Sexual Selection, but to some as yet unknown power.

But it is not only in this respect that caterpillars offer especial
advantages. If it is to be attempted to trace transformations in
form to the action of the environment, an exact knowledge of this
environment is in the first place necessary, _i.e._ a precise
acquaintance with the conditions of life under the influence of
which the species concerned exist. With respect to caterpillars, our
knowledge of the life conditions is certainly by no means as complete
as might be supposed, when we consider that hundreds of Lepidopterists
have constantly bred and observed them during a most extended period.
Much may have been observed, but it has not been thought worthy of
publication; much has also been published, but so scattered and
disconnected and at the same time of such unequal credibility, that
a lifetime would be required to sift and collect it. A comprehensive
biology of caterpillars, based on a broad ground, is as yet wanting,
although such a labour would be both most interesting and valuable.
Nevertheless, we know considerably more of the life of caterpillars
than of any other larvæ, and as we are also acquainted with an immense
number of species and are able to compare their life and the phenomena
of their development, the subject of the markings of caterpillars must
from this side also appear as the most favourable for the problem set
before us.

To this must be added as a last, though not as the least, valuable
circumstance, that we have here preserved to us in the development of
the individual a fragment of the history of the species, so that we
thus have at hand a means of following the course which the characters
to be traced to their causes--the forms of marking--have taken during
the lapse of thousands of years.

If with reference to the question as to the precise conditions of life
in caterpillars I was frequently driven to my own observations, it was
because I found as good as no previous work bearing upon this subject.
It was well known generally that many caterpillars were differently
marked and coloured when young to what they were when old; in some
very striking cases brief notices of this fact are to be found in the
works,[1] more especially, of the older writers, and principally
in that of the excellent observer Rösel von Rosenhof, the Nuremberg
naturalist and miniature painter. In no single case, however, do the
available materials suffice when we have to draw conclusions respecting
the phyletic development. We distinctly see here how doubtful is the
value of those observations which are made, so to speak, at random,
_i.e._ without some definite object in view. Many of these observations
may be both good and correct, but they are frequently wanting precisely
in that which would make them available for scientific purposes. Thus
everything had to be established _de novo_, and for this reason the
investigations were extended over a considerable number of years,
and had to be restricted to a small and as sharply defined a group
as possible--a group which was easily surveyed, viz. that of the
Hawk-moths or Sphinges.

Since the appearance of the German edition of this work many new
observations respecting the markings of caterpillars have been
published, such, for example, as those of W. H. Edwards and Fritz
Müller. I have, however, made but little use of them here, as I had no
intention of giving anything like a _complete_ ontogeny of the markings
in all caterpillars: larval markings were with me but means to an end,
and I wished only to bring together such a number of facts as were
necessary for drawing certain general conclusions. It would indeed
be most interesting to extend such observations to other groups of
Lepidoptera.

The third essay also, for similar reasons, is based essentially upon
the same materials, viz. the Lepidoptera. It is therein attempted to
approach the general problem--does there or does there not exist an
internal transforming force?--from a quite different and, I may say,
opposite point of view. The form-relationships of Lepidoptera in their
two chief stages of development, imago and larva, are therein analysed,
and by an examination of the respective forms it has been attempted to
discover the nature of the causes which have led thereto.

I may be permitted to say that the fact here disclosed of a _different
morphological_, with the _same genealogical_ relationship, appears
to me to be of decided importance. The agreement of the conclusions
following therefrom with the results of the former investigation has,
at least in my own mind, removed the last doubts as to the correctness
of the latter.

The fourth and shortest essay on the “Transformation of the Axolotl
into Amblystoma,” starts primarily with the intention of showing
that cases of sudden transformation are no proof of _per saltum_
development. When this essay first appeared the view was still widely
entertained that we had here a case proving _per saltum_ development.
That this explanation was erroneous is now generally admitted, but I
believe that those who suppose that we have here to deal with some
quite ordinary phenomenon which requires no explanation, now go too
far towards the other extreme. The term “larval reproduction” is an
_expression_, but no _explanation_; we have therefore to attempt to
find out the true interpretation, but whether the one which I have
given is correct must be judged of by others.

These four essays lead up to a fifth and concluding one “On the
Mechanical Conception of Nature.” Whilst the results obtained are here
summed up, it is attempted to form them into a philosophical conception
of Nature and of the Universe. It will be thought by many that this
should have been left to professed philosophers, and I readily admit
that I made this attempt with some misgiving. Two considerations,
however, induced me to express here my own views. The first was that
the facts of science are frequently misunderstood, or at any rate
not estimated at their true value, by philosophers;[2] the second
consideration was, that even certain naturalists and certainly very
many non-naturalists, turn distrustfully from the results of science,
because they fear that these would infallibly lead to a view of
the Universe which is to them unacceptable, viz. the materialistic
view. With regard to the former I wished to show that the views of
the development of organic Nature inaugurated by Darwin and defended
in this work are certainly correctly designated _mechanical_; with
reference to the latter I wished to prove that such a mechanical
conception of the organic world and of Nature in general, by no means
leads merely to one single philosophical conception of Nature, viz. to
Materialism, but that on the contrary it rather admits of legitimate
development in a quite different manner.

Thus in these last four essays much that appears heterogeneous will
be found in close association, viz. scientific details and general
philosophical ideas. In truth, however, these are most intimately
connected, and the one cannot dispense with the other. As the detailed
investigations of the three essays find their highest value in the
general considerations of the fourth, and were indeed only possible
by constantly keeping this end in view, so the general conclusions
could only grow out of the results of the special investigations as
out of a solid foundation. Had the new materials here brought together
been already known, the reader would certainly have been spared the
trouble of going into the details of special scientific research.
But as matters stood it was indispensable that the facts should be
examined into and established even down to the most trifling details.
The essay “On the Origin of the Markings of Caterpillars” especially,
had obviously to commence with the sifting and compilation of extensive
morphological materials.

            AUGUST WEISMANN.

  _Freiburg in Baden,
      November, 1881._




CONTENTS.


=Part I.=

ON THE SEASONAL DIMORPHISM OF BUTTERFLIES.


I.

_The Origin and Significance of Seasonal Dimorphism_, p. 1.

Historical preliminaries, 1. Does not occur in other orders of insects,
4. Beginning of experimental investigation, 5. Lepidopterous foes, 7.
First experiments with _Araschnia Levana_, 10. Experiments with _Pieris
Napi_, 13. Discussion of results, 17. Origination of _Prorsa_ from
_Levana_, 19. Theoretical considerations, 23. The case of _Papilio
Ajax_, 30. Experiments with _Pieris Napi var. Bryoniæ_, 39. The summer
generations of seasonally dimorphic butterflies the more variable, 42.


II.

_Seasonal Dimorphism and Climatic Variation_, p. 45.

Distinction between climatic and local varieties, 45. The case of
_Euchloe Belia_ and its varieties, 47. The case of _Polyommatus
Phlæas_, 49. The case of _Plebeius Agestis_, 50.


III.

_Nature of the Causes producing Climatic Varieties_, p. 52.

Seasonal dimorphism of the same nature as climatic variation, 52.
How does climatic change influence the markings of a butterfly? 52.
The cause of this to be found in temperature, 54. Part played by the
organism itself, 58. Analogous seasonal dimorphism in _Pierinæ_, 60.
The part played by sexual selection, 62.


IV.

_Why all Polygoneutic Species are not Seasonally Dimorphic_, p. 63.

Homochronic heredity, 63. Caterpillars, pupæ and eggs of summer and
winter generations of seasonally dimorphic butterflies alike, 64. The
law of cyclical heredity, 65. Climatic variation of _Pararga Ægeria_,
68. Continuous as distinguished from alternating heredity, 68. Return
from dimorphism to monomorphism, 70. Seasonally dimorphic species
hibernate as pupæ, 71. Retrogressive disturbance of winter generations,
72. The case of _Plebeius Amyntas_, 75.


V.

_On Alternation of Generations_, p. 80.

Haeckel’s classification of the phenomena, 80. Proposed modification,
81. Derivation of metagenesis from metamorphosis, 82. Primary
and secondary metagenesis, 84. Seasonal dimorphism related to
heterogenesis, 86. Heterogenesis and adaptation, 89. Differences
between seasonal dimorphism and other cases of heterogenesis, 89. The
case of _Leptodora Hyalina_, 93.


VI.

_General Conclusions_, p. 100.

Species produced by direct action of environment, 100. The transforming
influences of climate, 103. The origin of variability, 107. The
influence of isolation, 109. Cyclically acting causes of change produce
cyclically recurring changes, 111. Specific constitution an important
factor, 112. A “fixed direction of variation,” 114.


_Appendix I._, p. 117.

Experiments with _Araschnia Levana_, 117. Experiments with _Pierinæ_,
122.


_Appendix II._, p. 126.

Experiments with _Papilio Ajax_, 126. Additional experiments with _Pap.
Ajax_, 131. Experiments with _Phyciodes Tharos_, 140: with _Grapta
Interrogationis_, 149. Remarks on the latter, 152.


_Explanation of the Plates_, p. 159.


=Part II.=

ON THE FINAL CAUSES OF TRANSFORMATION.


=I.=

THE ORIGIN OF THE MARKINGS OF CATERPILLARS.


_Introduction_, p. 161.


I.

_Ontogeny and Morphology of Sphinx-Markings_, p. 177.

The genus _Chærocampa_, 177; _C. Elpenor_, 177; _C. Porcellus_, 184.
Results of the development of these species and comparison with
other species of the genus, 188. The genus _Deilephila_, 199; _D.
Euphorbiæ_, 201; _D. Nicæa_, 207; _D. Dahlii_, 208; _D. Vespertilio_,
209; _D. Galii_, 211; _D. Livornica_, 215; _D. Zygophylli_, 217; _D.
Hippophaës_, 218. Summary of facts and conclusions from this genus,
223. The genus _Smerinthus_, 232; _S. Tiliæ_, 233; _S. Populi_, 236;
_S. Ocellatus_, 240. Results of the development of these species, 242.
The genus _Macroglossa_, 245; _M. Stellatarum_, 245; comparison of this
with other species, 253. The genus _Pterogon_, 255; _P. Œnotheræ_,
256; comparison with other species, 256. The genus _Sphinx_, 259;
_S. Ligustri_, 259; comparison with other species, 261. The genus
_Anceryx_, 264; _A. Pinastri_, 265; comparison with other species, 268.


II.

_Conclusions from Phylogeny_, p. 270.

The Ontogeny of Caterpillars is a much abbreviated but slightly
falsified repetition of the Phylogeny, 270. Three laws of development,
274. The backward transference of new characters to younger stages is
the result of an innate law of growth, 278. Proof that new characters
always originate at the end of the development; the red spots of _S.
Tiliæ_, 282.


III.

_Biological Value of Marking in general_, p. 285.

Markings of Caterpillars most favourable to inquiry, 285. Are the
Sphinx-markings purely morphological, or have they a biological value?
287.


IV.

_Biological Value of Colour_, p. 289.

General prevalence of protective colouring among caterpillars, 289.
Polymorphic adaptive colouring in _C. Elpenor_, _C. Porcellus_,
_P. Œnotheræ_, _D. Vespertilio_, _D. Galii_, _D. Livornica_, _D.
Hippophaës_, 295. Habit of concealment primary; its causes, 298.
Polymorphism does not here depend upon contemporaneous but upon
successive double adaptation; displacement of the old by a new
adaptation; proof in the cases of _D. Hippophaës_, _D. Galii_, _D.
Vespertilio_, _M. Stellatarum_, _C. Elpenor_, and _S. Convolvuli_, 300.


V.

_Biological Value of special Markings_, p. 308.

Four chief forms of marking among _Sphingidæ_, 309. Complete absence of
marking among small caterpillars and among those living in obscurity,
310. Longitudinal stripes among grass caterpillars, 312. Oblique
striping. Coloured edges are the shadows of leaf ribs, 317. Eye-spots
and ring-spots. Definition, 326: Eye-spots not originally signs of
distastefulness, 328; they are means of alarm, 329; experiments with
birds, 330; possibility of a later change of function in eye-spots,
334. Ring-spots. Are they signs of distastefulness? Are there
caterpillars which are edible and which possess bright colours? 335;
experiments with lizards, 336. In _D. Galii_, _D. Euphorbiæ_, _D.
Dahlii_ and _D. Mauritanica_ the ring-spots are probably signs of
distastefulness, 341. In _D. Nicæa_ they are perhaps also means of
exciting terror, 342. The primary ring-spot in _D. Hippophaës_ is a
means of protection, 344. Subordinate markings. Reticulation, 347. The
dorsal spots of _C. Elpenor_ and _C. Porcellus_, 348. The lateral dots
of _S. Convolvuli_, 348. Origination of subordinate markings by the
blending of inherited but useless markings with new ones, 349.


VI.

_Objections to a Phyletic Vital Force_, p. 352.

Independent origination of ring-spots in species of the genus
_Deilephila_, 352. Possible genealogy of this genus, 358. Independent
origination of red spots in several species of _Smerinthus_, 360.
Functional change in the elements of marking, 365. Colour change in the
course of the ontogeny, 367.


VII.

_Phyletic Development of the Markings of the Sphingidæ. Summary and
Conclusion_, p. 370.

The oldest _Sphingidæ_ were devoid of marking, 370. Longitudinal
stripes the oldest form of marking, 371. Oblique striping, 373. Spot
markings, 375. The first and second elements of marking are mutually
exclusive, but not the first and third, or the second and third, 377.
Results with reference to the origin of markings; picture of their
origin and gradual complication, 380. General results; rejection of a
phyletic vital force, 389.


=II.=

ON PHYLETIC PARALLELISM IN METAMORPHIC SPECIES.

_Introduction_, p. 390.


I.

_Larva and Imago vary in Structure independently of each other_, p. 401.

Dimorphism of one stage only, 402. Independent variability of the
stages (heterochronic variability), 403. Constancy and variability are
not inherent properties of certain forms of marking, 407. Heterochronic
variability is not explained by assuming a phyletic vital force,
410. Rarity of greater variability in pupæ. Greater variability more
common among caterpillars than among the imagines. Causes of this
phenomenon, 412. Apparent independent variability of the single larval
stages. Waves of variability, 416. _Saturnia Carpini_ an instance of
_secondary_ variability, 419. Causes of the exact correlation between
the larval stages and its absence between the larva and imago, 429.


II.

_Does the Form-relationship of the Larva coincide with that of the
Imago?_ p. 432.

Family groups, 432. Families frequently completely congruent, 435.
Exception offered by the _Nymphalidæ_, 435. In transitional families
the larvæ also show intermediate forms, 441. Genera; almost completely
congruent; the Nymphalideous genera can be based on the structure of
the larvæ, 444. So also can certain sub-genera, as _Vanessa_, 445.
Incongruence in _Pterogon_, 450. Species; incongruence very common; _S.
Ocellatus_ and _Populi_, 451. Species of _Deilephila_ show a nearer
form-relationship as imagines than as larvæ, 454. Systemy not only the
expression of morphological relationship, 455. Varieties; incongruence
the rule; seasonal dimorphism; climatic varieties; dimorphism of
caterpillars; local varieties of caterpillars, 456. Result of the
investigation, 458. Causes of incongruence, 460. A phyletic vital force
does not explain the phenomena, 461. This force is superfluous, 464.


III.

_Incongruences in other Orders of Insects_, p. 481.

Hymenoptera. The imagines only possess ordinal characters, 481. Double
incongruence: different distance and different group-formation,
483. Diptera, 488. The larvæ form two types depending on different
modes of life, 489. The similarity of the grub-like larvæ of Diptera
and Hymenoptera depends upon convergence, 494. These data again
furnish strong arguments against a phyletic vital force, 496. The
tribe _Aphaniptera_, 498. Results furnished by the form-relationship
of Diptera and Hymenoptera, 499. Difference between typical and
non-typical parts transient, 501.


IV.

_Summary and Conclusion,_ p. 502.

First form of incongruence, 503. Second form of incongruence, 506.
General conclusion as to the elimination of a phyletic vital force,
511. Parallelism with the transformation of systems of organs, 513.


_Appendix I._, p. 520.

Additional notes on the Ontogeny, Phylogeny, &c., of Caterpillars.
Ontogeny of _Noctua_ larvæ, 520. Additional descriptions of
Sphinx-larvæ, 521. Retention of the subdorsal line by ocellated larvæ,
529. Phytophagic variability, 531. Sexual variation in larvæ, 534.


_Appendix II._, p. 536.

_Acræa_ and the _Maracujà_ butterflies as larvæ, pupæ, and imagines,
536.


_Explanation of the Plates_, p. 546.


=Part III.=

ON THE FINAL CAUSES OF TRANSFORMATION (_continued_).


=III.=

THE TRANSFORMATION OF THE MEXICAN AXOLOTL INTO AMBLYSTOMA.

_Introduction_, p. 555.

_Experiments_, 558. Significance of the facts, 563. The Axolotl rarely
or never undergoes metamorphosis in its native country, 565. North
American Amblystomas, 570. Does the exceptional transformation depend
upon a phyletic advancement of the species? 571. Theoretical bearing
of the case, 574. Differences between Axolotl and Amblystoma, 575.
These are not correlative results of the suppression of the gills,
578. Explanation by reversion, 581. Cases of degeneration to a lower
phyletic stage: Filippi’s sexually mature “_Triton_ larvæ,” 583.
Analogous observations on _Triton_ by Jullien and Schreibers, 591. The
sterility of the artificially produced Amblystomas tells against the
former importance of the transformation, 594. It is not opposed to the
hypothesis of reversion, 596. Attempted explanation of the sterility
from this point of view, 597. Causes which may have induced reversion
in the hypothetical Mexican Amblystomas, 600. Saltness of the water
combined with the drying up of the shores by winds, 604. Consequences
of the reversion hypothesis, 609; Systematic, 609; an addendum to the
“fundamental biogenetic law,” 611; General importance of reversion,
612. _Postscript_; dryness of the air the probable cause of the assumed
reversion of the Amblystoma to the Axolotl, 613. _Addendum_, 622.


=IV.=

ON THE MECHANICAL CONCEPTION OF NATURE.

_Introduction_, p. 634.

Results of the three foregoing essays: denial of a phyletic vital
force, 634. Application of these results to inductive conclusions with
reference to the organic world in general, 636. The assumption of such
a force is opposed to the fundamental laws of natural science, 637. The
“vital force” of the older natural philosopher, 640. Why was the latter
abandoned? Commencement of a mechanical theory of life, 642.


I.

_Are the Principles of the Selection Theory Mechanical?_ p. 645.

Refutation of Von Hartmann’s views, 645. Variability, 646. The
assumption of unlimited variability no postulate of the selection
theory, 647. The acknowledgment of a fixed and directed variability
does not necessitate the assumption of a phyletic vital force, 647.
Heredity, 657. Useful modifications do not occur only singly, 657.
New characters appearing singly may also acquire predominance, 659.
A mechanical theory of heredity is as yet wanting, 665. Haeckel’s
“Perigenesis of the Plastidule,” 667. Correlation, 670. The “specific
type” depends upon the physiological equilibrium of the parts of the
organism, 671. The theoretical principles of the doctrine of selection
are thus mechanical, 675. Importance of the physical constitution
of the organism in determining the quality of variations, 676. All
individual variability depends upon unequal external influences, 677.
Deduction of the limitability of variation, 682. Deduction of local
forms, 686. Parallelism between the ontogenetic and the phyletic vital
force, 687. The two are inseparable, 690.


II.

_Mechanism and Teleology_, p. 694.

Von Baer’s exaction from the theory of selection, 694. Justification of
his claim, but the impossibility of the co-operation of a metaphysical
principle with the mechanism of Nature, 695. _Per saltum_ development
(heterogeneous generation), 698. Weakness of the positive basis of
this hypothesis, 699. The latter refuted by the impossibility of the
co-operation of “heterogeneous generation” with natural selection, 702.
The interruption by a metaphysical principle cannot be reconciled with
gradual transformation, 705. The metaphysical (teleological) principle
can only be conceived of as the ultimate ground of the mechanism of
Nature, 709. Value of this knowledge for the harmonious conception of
the Universe, 711. Explanation of the spiritual by the assumption of
conscious matter, 714. The theory of selection does not necessarily
lead to Materialism, 716.

  INDEX                                                   p. 719.




STUDIES IN THE THEORY OF DESCENT.




Part I.

ON THE SEASONAL DIMORPHISM OF BUTTERFLIES.




I.

THE ORIGIN AND SIGNIFICANCE OF SEASONAL DIMORPHISM.


The phenomena here about to be subjected to a closer investigation have
been known for a long period of time. About the year 1830 it was shown
that the two forms of a butterfly (_Araschnia_) which had till that
time been regarded as distinct, in spite of their different colouring
and marking really belonged to the same species, the two forms of
this dimorphic species not appearing simultaneously but at different
seasons of the year, the one in early spring, the other in summer.
To this phenomenon the term “seasonal dimorphism” was subsequently
applied by Mr. A. R. Wallace, an expression of which the heterogeneous
composition may arouse the horror of the philologist, but, as it is
as concise and intelligible as possible, I propose to retain it in the
present work.

The species of _Araschnia_ through which the discovery of seasonal
dimorphism was made, formerly bore the two specific names _A. Levana_
and _A. Prorsa_. The latter is the summer and the former the winter
form, the difference between the two being, to the uninitiated, so
great that it is difficult to believe in their relationship. _A.
Levana_ (Figs. 1 and 2, Plate I.) is of a golden brown colour with
black spots and dashes, while _A. Prorsa_ (Figs. 5 and 6, Plate I.)
is deep black with a broad white interrupted band across both wings.
Notwithstanding this difference, it is an undoubted fact that both
forms are merely the winter and summer generations of the same species.
I have myself frequently bred the variety _Prorsa_ from the eggs of
_Levana_, and _vice versâ_.

Since the discovery of this last fact a considerable number of
similar cases have been established. Thus P. C. Zeller[3] showed, by
experiments made under confinement, that two butterflies belonging to
the family of the ‘Blues,’ differing greatly in colour and marking,
and especially in size, which had formerly been distinguished as
_Plebeius_ (_Lycæna_) _Polysperchon_ and _P. Amyntas_, were merely
winter and summer generations of the same species; and that excellent
Lepidopterist, Dr. Staudinger, proved the same[4] with species
belonging to the family of the ‘Whites,’ _Euchloe Belia_ Esp. and _E.
Ausonia_ Hüb., which are found in the Mediterranean countries.

The instances are not numerous, however, in which the difference
between the winter and summer forms of a species is so great as to
cause them to be treated of in systematic work as distinct species.
I know of only five of these cases. Lesser differences, having the
systematic value of varieties, occur much more frequently. Thus, for
instance, seasonal dimorphism has been proved to exist among many of
our commonest butterflies belonging to the family of the ‘Whites,’ but
the difference in their colour and marking can only be detected after
some attention; while with other species, as for instance with the
commonest of our small ‘Blues,’ _Plebeius Alexis_ (= _Icarus_, Rott.),
the difference is so slight that even the initiated must examine
closely in order to recognize it. Indeed whole series of species might
easily be grouped so as to show the transition from complete similarity
of both generations, through scarcely perceptible differences, to
divergence to the extent of varieties, and finally to that of species.

Nor are the instances of lesser differences between the two generations
very numerous. Among the European diurnal Lepidoptera I know of about
twelve cases, although closer observation in this direction may
possibly lead to further discoveries.[5] Seasonal dimorphism occurs
also in moths, although I am not in a position to make a more precise
statement on this subject,[6] as my own observations refer only to
butterflies.

That other orders of insects do not present the same phenomenon depends
essentially upon the fact that most of them produce only one generation
in the year; but amongst the remaining orders there occur indeed
changes of form which, although not capable of being regarded as pure
seasonal dimorphism, may well have been produced in part by the same
causes, as the subsequent investigation on the relation of seasonal
dimorphism to alternation of generation and heterogenesis will more
fully prove.

Now what are these causes?

Some years ago, when I imparted to a lepidopterist my intention of
investigating the origin of this enigmatical dimorphism, in the hope
of profiting for my inquiry from his large experience, I received
the half-provoking reply: “But there is nothing to investigate:
it is simply the specific character of this insect to appear in
two forms; these two forms alternate with each other in regular
succession according to a fixed law of Nature, and with this we
must be satisfied.” From his point of view the position was right;
according to the old doctrine of species no question ought to be
asked as to the causes of such phenomena in particular. I would not,
however, allow myself to be thus discouraged, but undertook a series of
investigations, the results of which I here submit to the reader.

The first conjecture was, that the differences in the imago might
perhaps be of a secondary nature, and have their origin in the
differences of the caterpillar, especially with those species which
grow up during the spring or autumn and feed on different plants,
thus assimilating different chemical substances, which might induce
different deposits of colour in the wings of the perfect insect. This
latter hypothesis was readily confuted by the fact, that the most
strongly marked of the dimorphic species, _A. Levana_, fed exclusively
on _Urtica major_. The caterpillar of this species certainly exhibits
a well-defined dimorphism, but it is not seasonal dimorphism: the two
forms do not alternate with each other, but appear mixed in every brood.

I have repeatedly reared the rarer golden-brown variety of the
caterpillar separately, but precisely the same forms of butterfly
were developed as from black caterpillars bred at the same time under
similar external conditions. The same experiment was performed, with a
similar result, in the last century by Rösel, the celebrated miniature
painter and observer of nature, and author of the well-known “Insect
Diversions”--a work in use up to the present day.

The question next arises, as to whether the causes originating the
phenomena are not the same as those to which we ascribe the change of
winter and summer covering in so many mammalia and birds--whether the
change of colour and marking does not depend, in this as in the other
cases, upon the _indirect action_ of external conditions of life, i.e.,
on adaptation through natural selection. We are certainly correct in
ascribing white coloration to adaptation[7]--as with the ptarmigan,
which is white in winter and of a grey-brown in summer, both colours
of the species being evidently of important use.

It might be imagined that analogous phenomena occur in butterflies,
with the difference that the change of colour, instead of taking place
in the same brood, alternates in different broods.[8] The nature of
the difference which occurs in seasonal dimorphism, however, decidedly
excludes this view; and moreover, the environment of butterflies
presents such similar features, whether they emerge in spring or in
summer, that all notions that we may be dealing with adaptational
colours must be entirely abandoned.

I have elsewhere[9] endeavoured to show that butterflies in general are
not coloured protectively during flight, for the double reason that
the colour of the background to which they are exposed continually
changes, and because, even with the best adaptation to the background,
the fluttering motion of the wings would betray them to the eyes of
their enemies.[10] I attempted also to prove at the same time that the
diurnal Lepidoptera of our temperate zone have few enemies which pursue
them when on the wing, but that they are subject to many attacks during
their period of repose.

In support of this last statement I may here adduce an instance. In the
summer of 1869 I placed about seventy specimens of _Araschnia Prorsa_
in a spacious case, plentifully supplied with flowers. Although the
insects found themselves quite at home, and settled about the flowers
in very fine weather (one pair copulated, and the female laid eggs),
yet I found some dead and mangled every morning. This decimation
continued--many disappearing entirely without my being able to find
their remains--until after the ninth day, when they had all, with one
exception, been slain by their nocturnal foes--probably spiders and
_Opilionidæ_.

Diurnal Lepidoptera in a position of rest are especially exposed to
hostile attacks. In this position, as is well known, their wings are
closed upright, and it is evident that the adaptational colours on
the under side are displayed, as is most clearly shown by many of our
native species.[11]

Now, the differences in the most pronounced cases of seasonal
dimorphism--for example, in _Araschnia Levana_--are much less manifest
on the _under_ than on the _upper_ side of the wing. The explanation by
adaptation is therefore untenable; but I will not here pause to confute
this view more completely, as I believe I shall be able to show the
true cause of the phenomenon.

If seasonal dimorphism does not arise from the _indirect_ influence of
varying seasons of the year, it may result from the _direct_ influence
of the varying external conditions of life, which are, without doubt,
different in the winter from those of the summer brood.

There are two prominent factors from which such an influence may be
expected--temperature and duration of development, i.e., duration
of the chrysalis period. The duration of the larval period need not
engage our attention, as it is only very little shorter in the winter
brood--at least, it was so with the species employed in the experiments.

Starting from these two points of view, I carried on experiments for
a number of years, in order to find out whether the dual form of the
species in question could be traced back to the direct action of the
influences mentioned.

The first experiments were made with _Araschnia Levana_. From the eggs
of the winter generation, which had emerged as butterflies in April,
I bred caterpillars, and immediately after pupation placed them in a
refrigerator, the temperature of the air of which was 8°-10° R. It
appeared, however, that the development could not thus be retarded to
any desired period by such a small diminution of temperature, for,
when the box was taken out of the refrigerator after thirty-four days,
all the butterflies, about forty in number, had emerged, many being
dead, and others still living. The experiment was so far successful
that, instead of the _Prorsa_ form which might have been expected
under ordinary circumstances, most of the butterflies emerged as the
so-called _Porima_ (Figs. 3, 4, 7, 8, and 9, Plate I.); that is to say,
in a form intermediate between _Prorsa_ and _Levana_ sometimes found
in nature, and possessing more or less the marking of the former, but
mixed with much of the yellow of _Levana_.

It should be here mentioned, that similar experiments were made
in 1864 by George Dorfmeister, but unfortunately I did not get
this information[12] until my own were nearly completed. In these
well-conceived, but rather too complicated experiments, the author
arrives at the conclusion “that temperature certainly affects the
colouring, and through it the marking, of the future butterfly, and
chiefly so during pupation.” By lowering the temperature of the air
during a portion of the pupal period, the author was enabled to produce
single specimens of _Porima_, but most of the butterflies retained
the _Prorsa_ form. Dorfmeister employed a temperature a little higher
than I did in my first experiments, viz. 10°-11° R., and did not leave
the pupæ long exposed, but after 5½-8 days removed them to a higher
temperature. It was therefore evident that he produced transition forms
in a few instances only, and that he never succeeded in bringing about
a complete transformation of the summer into the winter form.

In my subsequent experiments I always exposed the pupæ to a temperature
of 0°-1° R.; they were placed directly in the refrigerator, and
taken out at the end of four weeks. I started with the idea that it
was perhaps not so much the reduced temperature as the retardation of
development which led to the transformation. But the first experiment
had shown that the butterflies emerged between 8° and 10° R., and
consequently that the development could not be retarded at this
temperature.

A very different result was obtained from the experiment made at a
lower temperature.[13] Of twenty butterflies, fifteen had become
transformed into _Porima_, and of these three appeared very similar to
the winter form (_Levana_), differing only in the absence of the narrow
blue marginal line, which is seldom absent in the true _Levana_. Five
butterflies were uninfluenced by the cold, and remained unchanged,
emerging as the ordinary summer form (_Prorsa_). It thus appeared from
this experiment, that a large proportion of the butterflies inclined
to the _Levana_ form by exposure to a temperature of 0°-1° R. for four
weeks, while in a few specimens the transformation into this form was
nearly perfect.

Should it not be possible to perfect the transformation, so that
each individual should take the _Levana_ form? If the assumption of
the _Prorsa_ or _Levana_ form depends only on the direct influence
of temperature, or on the duration of the period of development, it
should be possible to compel the pupæ to take one or the other form
at pleasure, by the application of the necessary external conditions.
This has never been accomplished with _Araschnia Prorsa_. As in the
experiment already described, and in all subsequent ones, single
specimens appeared as the unchanged summer form, others showed an
appearance of transition, and but very few had changed so completely
as to be possibly taken for the pure _Levana_. In some species of the
sub-family _Pierinæ_, however, at least in the case of the summer
brood, there was, on the contrary, a complete transformation.

Most of the species of our ‘Whites’ (_Pierinæ_) exhibit the phenomenon
of seasonal dimorphism, the winter and summer forms being remarkably
distinct. In _Pieris Napi_ (with which species I chiefly experimented)
the winter form (Figs. 10 and 11, Plate I.) has a sprinkling of deep
black scales at the base of the wings on the upper side, while the tips
are more grey, and have in all cases much less black than in the summer
form; on the underside the difference lies mainly in the frequent
breadth, and dark greenish-black dusting, of the veins of the hind
wings in the winter form, while in the summer form these greenish-black
veins are but faintly present.

I placed numerous specimens of the summer brood, immediately after
their transformation into chrysalides, in the refrigerator (0°-1° R.),
where I left them for three months, transferring them to a hothouse
on September 11th, and there (from September 26th to October 3rd)
sixty butterflies emerged, the whole of which, without exception--and
most of them in an unusually strong degree--bore the characters of the
winter form. I, at least, have never observed in the natural state
such a strong yellow on the underside of the hind wings, and such a
deep blackish-green veining, as prevailed in these specimens (see, for
instance, Figs. 10 and 11). The temperature of the hothouse (12°-24°
R.) did not, however, cause the emergence of the whole of the pupæ; a
portion hibernated, and produced in the following spring butterflies of
the winter form only. I thus succeeded, with this species of _Pieris_,
in completely changing every individual of the summer generation into
the winter form.

It might be expected that the same result could be more readily
obtained with _A. Levana_, and fresh experiments were undertaken, in
order that the pupæ might remain in the refrigerator fully two months
from the period of their transformation (9-10th July). But the result
obtained was the same as before--fifty-seven butterflies emerged in the
hothouse[14] from September 19th to October 4th, nearly all of these
approaching very near to the winter form, without a single specimen
presenting the appearance of a perfect _Levana_, while three were of
the pure summer form (_Prorsa_).

Thus with _Levana_ it was not possible, by refrigeration and
retardation of development, to change the summer completely into the
winter form in all specimens. It may, of course, be objected that
the period of refrigeration had been too short, and that, instead
of leaving the pupæ in the refrigerator for two months, they should
have remained there six months, that is, about as long as the winter
brood remains under natural conditions in the chrysalis state. The
force of this last objection must be recognized, notwithstanding the
improbability that the desired effect would be produced by a longer
period of cold, since the doubling of this period from four to eight
weeks did not produce[15] any decided increase in the strength of the
transformation. I should not have omitted to repeat the experiment in
this modified form, but unfortunately, in spite of all trouble, I was
unable to collect during the summer of 1873 a sufficient number of
caterpillars. But the omission thus caused is of quite minor importance
from a theoretical point of view.

For let us assume that the omitted experiment had been performed--that
pupæ of the summer brood were retarded in their development by cold
until the following spring, and that every specimen then emerged in
the perfect winter form, Levana. Such a result, taken in connexion with
the corresponding experiment upon _Pieris Napi_, would warrant the
conclusion that the direct action of a certain amount of cold (or of
retardation of development) is able to compel all pupæ, from whichever
generation derived, to assume the winter form of the species. From this
the converse would necessarily follow, viz. that a certain amount of
warmth would lead to the production of the summer form, _Prorsa_, it
being immaterial from which brood the pupæ thus exposed to warmth might
be derived. But the latter conclusion was proved experimentally to be
incorrect, and thus the former falls with it, whether the imagined
experiment with _Prorsa_ had succeeded or not.

I have repeatedly attempted by the application of warmth to change the
winter into the summer form, but always with the same negative result.
_It is not possible to compel the winter brood to assume the form of
the summer generation._

_A. Levana_ may produce not only two but three broods in the year, and
may, therefore, be said to be _polygoneutic_.[16] One winter brood
alternates with two summer broods, the first of which appears in July,
and the second in August. The latter furnishes a fourth generation of
pupæ, which, after hibernation, emerge in April, as the first brood of
butterflies in the form _Levana_.

I frequently placed pupæ of this fourth brood in the hothouse
immediately after their transformation, and in some cases even during
the caterpillar stage, the temperature never falling, even at night,
below 12° R., and often rising during the day to 24° R. The result was
always the same: all, or nearly all, the pupæ hibernated, and emerged
the following year in the winter form as perfectly pure _Levana_,
without any trace of transition to the _Prorsa_ form. On one occasion
only was there a _Porima_ among them, a case for which an explanation
will, I believe, be found later on. It often happened, on the other
hand, that some few of the butterflies emerged in the autumn, about
fourteen days after pupation; and these were always _Prorsa_ (the
summer form), excepting once a _Porima_.

From these experiments it appeared that similar causes (heat) affect
different generations of _A. Levana_ in different manners. With
both summer broods a high temperature always caused the appearance
of _Prorsa_, this form arising but seldom from the third brood (and
then only in a few individuals), while the greater number retained
the _Levana_ form unchanged. We may assign as the reason for this
behaviour, that the third brood has no further tendency to be
accelerated in its development by the action of heat, but that by a
longer duration of the pupal stage the _Levana_ form must result. On
one occasion the chrysalis stage was considerably shortened in this
brood by the continued action of a high temperature, many specimens
thus having their period of development reduced from six to three
months. The supposed explanation above given is, however, in reality no
explanation at all, but simply a restatement of the facts. The question
still remains, why the third brood in particular has no tendency to be
accelerated in its development by the action of heat, as is the case
with both the previous broods?

The first answer that can be given to this question is, that the cause
of the different action produced by a similar agency can only lie in
the _constitution_, i.e., in the _physical nature_ of the broods in
question, and not in the external influences by which they are acted
upon. Now, what is the difference in the physical nature of these
respective broods? It is quite evident, as shown by the experiments
already described, that cold and warmth cannot be the _immediate_
causes of a pupa emerging in the _Prorsa_ or _Levana_ form, since the
last brood always gives rise to the _Levana_ form, whether acted on by
cold or warmth. The first and second broods only can be made to partly
assume, more or less completely, the _Levana_ form by the application
of cold. In these broods then, a low temperature is the _mediate_ cause
of the transformation into the _Levana_ form.

The following is my explanation of the facts. The form _Levana_ is
the original type of the species, and _Prorsa_ the secondary form
arising from the gradual operation of summer climate. When we are able
to change many specimens of the summer brood into the winter form by
means of cold, this can only depend upon reversion to the original, or
ancestral, form, which reversion appears to be most readily produced
by cold, that is, by the same external influences as those to which
the original form was exposed during a long period of time, and the
continuance of which has preserved, in the winter generations, the
colour and marking of the original form down to the present time.

I consider the origination of the _Prorsa_ from the _Levana_ form
to have been somewhat as follows:--It is certain that during the
diluvial period in Europe there was a so-called ‘glacial epoch,’
which may have spread a truly polar climate over our temperate zone;
or perhaps a lesser degree of cold may have prevailed with increased
atmospheric precipitation. At all events, the summer was then short
and comparatively cold, and the existing butterflies could have
only produced one generation in the year; in other words, they were
_monogoneutic_. At that time _A. Levana_ existed only in the _Levana_
form.[17] As the climate gradually became warmer, a period must have
arrived when the summer lasted long enough for the interpolation of
a second brood. The pupæ of _Levana_, which had hitherto hibernated
through the long winter to appear as butterflies in the following
summer, were now able to appear on the wing as butterflies during the
same summer as that in which they left their eggs as larvæ, and eggs
deposited by the last brood produced larvæ which fed up and hibernated
as pupæ. A state of things was thus established in which the first
brood was developed under very different climatic conditions from the
second. So considerable a difference in colour and marking between the
two forms as we now witness could not have arisen suddenly, but must
have done so gradually. It is evident from the foregoing experiments
that the _Prorsa_ form did not originate suddenly. Had this been the
case it would simply signify that every individual of this species
possessed the faculty of assuming two different forms according as it
was acted on by warmth or cold, just in the same manner as litmus-paper
becomes red in acids and blue in alkalies. The experiments have shown,
however, that this is not the case, but rather that the last generation
bears an ineradicable tendency to take the _Levana_ form, and is not
susceptible to the influence of warmth, however long continued; while
both summer generations, on the contrary, show a decided tendency to
assume the _Prorsa_ form, although they certainly can be made to assume
the _Levana_ form in different degrees by the prolonged action of cold.

The conclusion seems to me inevitable, that the origination of the
_Prorsa_ form was gradual--that those changes which originated in
the chemistry of the pupal stage, and led finally to the _Prorsa_
type, occurred very gradually, at first perhaps remaining completely
latent throughout a series of generations, then very slight changes
of marking appearing, and finally, after a long period of time, the
complete _Prorsa_ type was produced. It appears to me that the quoted
results of the experiments are not only easily explained on the view
of the _gradual_ action of climate, but that this view is the only one
admissible. The action of climate is best comparable with the so-called
cumulative effect of certain drugs on the human body; the first small
dose produces scarcely any perceptible change, but if often repeated
the effect becomes cumulative, and poisoning occurs.

This view of the action of climate is not at all new, most zoologists
having thus represented it; only the formal proof of this action is
new, and the facts investigated appear to me of special importance as
furnishing this proof. I shall again return to this view in considering
climatic varieties, and it will then appear that also the nature of the
transformation itself confirms the slow operation of climate.

During the transition from the glacial period to the present climate
_A. Levana_ thus gradually changed from a monogoneutic to a digoneutic
species, and at the same time became gradually more distinctly
dimorphic, this character originating only through the alteration of
the summer brood, the primary colouring and marking of the species
being retained unchanged by the winter brood. As the summer became
longer a third generation could be interpolated--the species became
polygoneutic; and in this manner two summer generations alternated with
one winter generation.

We have now to inquire whether facts are in complete accordance with
this theory--whether they are never at variance with it--and whether
they can all be explained by it. I will at once state in anticipation,
that this is the case to the fullest extent.

In the first place, the theory readily explains why the summer but not
the winter generations are capable of being transformed; the latter
cannot possibly revert to the _Prorsa_ form, because this is much the
younger. When, however, it happens that out of a hundred cases there
occurs one in which a chrysalis of the winter generation, having been
forced by warmth, undergoes transformation before the commencement of
winter, and emerges in the summer form,[18] this is not in the least
inexplicable. It cannot be atavism which determines the direction of
the development; but we see from such a case that the changes in the
first two generations have already produced a certain alteration in
the third, which manifests itself in single cases under favourable
conditions (the influence of warmth) by the assumption of the _Prorsa_
form; or, as it might be otherwise expressed, the _alternating_
heredity (of which we shall speak further), which implies the power
of assuming the _Prorsa_ form, remains latent as a rule in the winter
generation, but becomes _continuous_ in single individuals.

It is true that we have as yet no kind of insight into the nature of
heredity, and this at once shows the defectiveness of the foregoing
explanation; but we nevertheless know many of its external phenomena.
We know for certain that one of these consists in the fact that
peculiarities of the father do not appear in the son, but in the
grandson, or still further on, and that they may be thus transmitted
in a latent form. Let us imagine a character so transmitted that it
appears in the first, third, and fifth generations, remaining latent
in the intermediate ones; it would not be improbable, according to
previous experiences, that the peculiarity should exceptionally, i.e.,
from a cause unknown to us, appear in single individuals of the second
or fourth generation. But this completely agrees with those cases in
which “exceptional” individuals of the winter brood took the _Prorsa_
form, with the difference only that a cause (warmth) was here apparent
which occasioned the development of the latent characters, although we
are not in a position to say in what manner heat produces this action.
These exceptions to the rule are therefore no objection to the theory.
On the contrary, they give us a hint that after one _Prorsa_ generation
had been produced, the gradual interpolation of a second _Prorsa_
generation may have been facilitated by the existence of the first.
I do not doubt that even in the natural state single individuals of
_Prorsa_ sometimes emerge in September or October; and if our summer
were lengthened by only one or two months this might give rise to a
third summer brood (just as a second is now an accomplished fact),
under which circumstances they would not only emerge, but would also
have time for copulation and for depositing eggs, the larvæ from which
would have time to grow up.

A sharp distinction must be made between the first establishment
of a new climatic form and the transference of the latter to newly
interpolated generations. The former always takes place very slowly;
the latter may occur in a shorter time.

With regard to the duration of time which is necessary to produce a
new form by the influence of climate, or to transmit to a succeeding
generation a new form already established, great differences occur,
according to the physical nature of the species and of the individual.
The experiments with _Prorsa_ already described show how diverse are
individual proclivities in this respect. In Experiment No. 12 it was
not possible out of seventy individuals to substitute _Prorsa_ for the
_Levana_ form, even in one solitary case, or, in other words, to change
alternating into continuous inheritance; whilst in the corresponding
experiments of former years (Experiment 10, for example), out of an
equal number of pupæ three emerged as _Prorsa_, and one as _Porima_. We
might be inclined to seek for the cause of this different behaviour in
external influences, but we should not thus arrive at an explanation of
the facts. We might suppose, for instance, that a great deal depended
upon the particular period of the pupal stage at which the action of
the elevated temperature began--whether on the first, the thirtieth,
or the hundredth day after pupation--and this conjecture is correct
in so far that in the two last cases warmth can have no further
influence than that of somewhat accelerating the emergence of the
butterflies, but cannot change the _Levana_ into the _Prorsa_ form. I
have repeatedly exposed a large number of _Levana_ pupæ of the third
generation to the temperature of an apartment, or even still higher
(26° R.), during winter, but no _Prorsa_ were obtained.[19]

But it would be erroneous to assume a difference in the action of heat
according as it began on the first or third day after transformation;
whether during or before pupation. This is best proved by Experiment
No. 12, in which caterpillars of the fourth generation were placed in
the hothouse several days before they underwent pupation; still, not
a single butterfly assumed the _Prorsa_ form. I have also frequently
made the reverse experiment, and exposed caterpillars of the first
summer brood to cold during the act of pupation. A regular consequence
was the dying off of the caterpillars, which is little to be wondered
at, as the sensitiveness of insects during ecdysis is well known, and
transformation into the pupal state is attended by much deeper changes.

Dorfmeister thought that he might conclude from his experiments that
temperature exerts the greatest influence in the first place during
the act of pupation, and in the next place immediately after that
period. His experiments were made, however, with such a small number
of specimens that scarcely any safe conclusion can be founded on them;
still, this conclusion may be correct, in so far as everything depends
on whether, from the beginning, the formative processes in the pupa
tended to this or that direction, the final result of which is the
_Prorsa_ or _Levana_ form. If once there is a tendency to one or the
other direction, then temperature might exert an accelerating or a
retarding influence, but the tendency cannot be further changed.

It is also possible--indeed, probable--that a period may be fixed in
which warmth or cold might be able to divert the original direction of
development most easily; and this is the next problem to be attacked,
the answer to which, now that the main points have been determined,
should not be very difficult. I have often contemplated taking the
experiments in hand myself, but have abandoned them, because my
materials did not appear to me sufficiently extensive, and in all such
experiments nothing is to be more avoided than a frittering away of
experimental materials by a too complicated form of problem.

There may indeed be a period most favourable for the action of
temperature during the first days of the pupal stage; it appears from
Experiment No. 12 that individuals tend in different degrees to
respond to such influences, and that the disposition to abandon the
ordinary course of development is different in different individuals.
In no other way can it be explained that, in all the experiments made
with the first and second generations of _Prorsa_, only _a portion_ of
the pupæ were compelled by cold to take the direction of development
of _Levana_, and that even from the former only a few individuals
completely reverted, the majority remaining intermediate.

If it be asked why in the corresponding experiments with _Pieris
Napi_ complete reversion always occurred without exception, it may
be supposed that in this species the summer form has not been so
long in existence, and that it would thus be more easily abandoned;
or, that the difference between the two generations has not become
so distinct, which further signifies that here again the summer form
is of later origin. It might also be finally answered, that the
tendency to reversion in different species may vary just as much as
in different individuals of the same species. But, in any case, the
fact is established that all individuals are impelled by cold to
complete reversion, and that in these experiments it does not depend so
particularly upon the moment of development when cold is applied, but
that differences of individual constitution are much more the cause why
cold brings some pupæ to complete, and others to partial, reversion,
while yet others are quite uninfluenced. In reference to this, the
American _Papilio Ajax_ is particularly interesting.

This butterfly, which is somewhat similar to the European _P.
Podalirius_, appears, wherever it occurs, in three varieties,
designated as var. _Telamonides_, var. _Walshii_, and var. _Marcellus_.
The distinguished American entomologist, W. H. Edwards, has proved
by breeding experiments, that all three forms belong to the same
cycle of development, and in such a manner that the first two appear
only in spring, and always come only from hibernating pupæ, while
the last form, var. _Marcellus_, appears only in summer, and then
in three successive generations. A seasonal dimorphism thus appears
which is combined with ordinary dimorphism, winter and summer forms
alternating with each other; but the first appears itself in two forms
or varieties, vars. _Telamonides_ and _Walshii_. If for the present we
disregard this complication, and consider these two winter forms as
one, we should thus have four generations, of which the first possesses
the winter form, and the three succeeding ones have, on the other hand,
the summer form, var. _Marcellus_.

The peculiarity of this species consists in the fact that in all
three summer generations only a portion of the pupæ emerge after a
short period (fourteen days), whilst another and much smaller portion
remains in the pupal state during the whole summer and succeeding
winter, first emerging in the following spring, and then always in the
winter form. Thus, Edwards states that out of fifty chrysalides of the
second generation, which had pupated at the end of June, forty-five
_Marcellus_ butterflies appeared after fourteen days, whilst five pupæ
emerged in April of the following year, and then as _Telamonides_.

The explanation of these facts is easily afforded by the foregoing
theory. According to this, both the winter forms must be regarded as
primary, and the _Marcellus_ form as secondary. But this last is not
yet so firmly established as _Prorsa_, in which reversion of the summer
generations to the _Levana_ form only occurs through special external
influences; whilst in the case of _Ajax_ some individuals are to be
found in every generation, the tendency of which to revert is still so
strong that even the greatest summer heat is unable to cause them to
diverge from their original inherited direction of development, or to
accelerate their emergence and compel them to assume the _Marcellus_
form. It is here beyond a doubt that it is not different external
influences, but internal causes only, which maintain the old hereditary
tendency, for all the larvæ and pupæ of many different broods were
simultaneously exposed to the same external influences. But, at the
same time, it is evident that these facts are not opposed to the
present theory; on the contrary, they confirm it, inasmuch as they
are readily explained on the basis of the theory, but can scarcely
otherwise be understood.

If it be asked what significance attaches to the duplication of the
winter form, it may be answered that the species was already dimorphic
at the time when it appeared in only one annual generation. Still,
this explanation may be objected to, since a dimorphism of this kind
is not at present known, though indeed some species exhibit a sexual
dimorphism,[20] in which one sex (as, for instance, the case of the
female _Papilio Turnus_) appears in two forms of colouring, but not a
dimorphism, as is here the case, displayed by both sexes.[21] Another
suggestion, therefore, may perhaps be offered.

In _A. Levana_ we saw that reversion occurred in very different degrees
with different individuals, seldom attaining to the true _Levana_
form, and generally only reaching the intermediate form known as
_Porima_. Now it would, at all events, be astonishing if with _P. Ajax_
the reversion were always complete, as it is precisely in this case
that the tendency to individual reversion is so variable. I might,
for this reason, suppose that one of the two winter forms, viz. the
var. _Walshii_, is nothing else than an incomplete reversion-form,
corresponding to _Porima_ in the case of _A. Levana_. Then
_Telamonides_ only would be the original form of the butterfly, and
this would agree with the fact that this variety appears later in the
spring than _Walshii_. Experiments ought to be able to decide this.[22]
The pupæ of the first three generations placed upon ice should give,
for the greater part, the form _Telamonides_, for the lesser portion
_Walshii_, and for only a few, or perhaps no individuals, the form
_Marcellus_. This prediction is based on the view that the tendency
to revert is on the whole great; that even with the first summer
generation, which was the longest exposed to the summer climate, a
portion of the pupæ, without artificial means, always emerged as
_Telamonides_, and another portion as _Marcellus_. The latter will
perhaps now become _Walshii_ by the application of cold.

One would expect that the second and third generations would revert
more easily, and in a larger percentage, than the first, because
this latter first acquired the new _Marcellus_ form; but the present
experiments furnish no safe conclusion on this point. Thus, of the
first summer generation only seven out of sixty-seven pupæ hibernated,
and these gave _Telamonides_; while of the second generation forty
out of seventy-six, and of the third generation twenty-nine out of
forty-two pupæ hibernated. But to establish safer conclusions, a still
larger number of experiments is necessary. According to the experience
thus far gained, one might perhaps still be inclined to imagine
that, with seasonal dimorphism, external influences operating on the
individual might directly compel it to assume one or the other form.
I long held this view myself, but it is, nevertheless, untenable.
That cold does not produce the one kind of marking, and warmth the
other, follows from the before-mentioned facts, viz. that in _Papilio
Ajax_ every generation produces both forms; and, further, in the case
of _A. Levana_ I have frequently reared the fourth (hibernating)
generation entirely in a warm room, and yet I have always obtained the
winter form. Still, one might be inclined not to make the temperature
_directly_ responsible, but rather the retardation or acceleration of
development produced through the action of temperature. I confess that
I for a long time believed that in this action I had found the true
cause of seasonal dimorphism. Both with _A. Levana_ and _P. Napi_ the
difference between the duration of the pupal period in the winter and
summer forms is very great, lasting as a rule, in the summer generation
of _A. Levana_, from seven to twelve days, and in the winter generation
about two hundred days. In this last species the pupal state can
certainly be shortened by keeping them at an elevated temperature;
but I have, nevertheless, only in one case obtained two or three
butterflies at the end of December from caterpillars that had pupated
in September, these generally emerging in the course of February and
March, and are to be seen on the wing in warm weather during the latter
month. The greatest reduction of the pupal period still leaves for this
stage more than 100 days.

From this last observation it follows that it is not the duration of
development which, in individual cases, determines the form of the
butterfly, and which consequently decides whether the winter or summer
form shall emerge, but that, on the contrary, the duration of the pupal
stage is dependent on the tendency which the forthcoming butterfly
had taken in the chrysalis state. This can be well understood when we
consider that the winter form must have had a long, and the summer form
a short pupal period, during innumerable generations. In the former
the habit of slow development must have been just as well established
as that of rapid development in the latter; and we cannot be at all
surprised if we do not see this habit abandoned by the winter form
when the opportunity presents itself. But that it may be occasionally
abandoned the more proves that the duration of the pupal development
less determines the butterfly form than does the temperature directly,
in individual cases.

Thus, for instance, Edwards explicitly states that, whereas the two
winter forms of _P. Ajax_, viz. the vars. _Walshii_ and _Telamonides_,
generally appear only after a pupal period of 150 to 270 days, yet
individual cases occur in which the pupal stage is no longer than in
the summer form, viz. fourteen days.[23] A similar thing occurs with
_A. Levana_, for, as already explained, not only may the development
of the winter form be forced to a certain degree by artificial warmth,
but the summer generation frequently produces reversion-forms without
protraction of development. The intermediate reversion-form _Porima_
was known long before it was thought possible that it could be produced
artificially by the action of cold; it appears occasionally, although
very rarely, at midsummer in the natural state.

If, then, my explanation of the phenomena is correct, the winter form
is primary and the summer the secondary form, and those individuals
which, naturally or artificially, assume the winter form must be
considered as cases of atavism. The suggestion thus arises whether
low temperature alone is competent to bring about this reversion, or
whether other external influences are not also effective. Indeed,
the latter appears to be the case. Besides purely internal causes,
as previously pointed out in _P. Ajax_, warmth and mechanical motion
appear to be able to bring about reversion.

That an unusually high temperature may cause reversion, I conclude
from the following observation. In the summer of 1869 I bred the first
summer brood of _A. Levana_; the caterpillars pupated during the second
half of June, and from that time to their emergence, on 28th June-3rd
July, great heat prevailed. Now, while the intermediate form _Porima_
had hitherto been a great rarity, both in the free state and when bred,
having never obtained it myself, for example, out of many hundreds
of specimens, there were among the sixty or seventy butterflies that
emerged from the above brood, some eight to ten examples of _Porima_.
This is certainly not an exact experiment, but there seems to me a
certain amount of probability that the high summer temperature in this
case brought about reversion.

Neither for the second cause to which I have ascribed the power of
producing reversion can I produce any absolute evidence, since the
experimental solution of all these collateral questions would demand an
endless amount of time. I am in possession of an observation, however,
which makes it appear probable to me that continuous mechanical
movement acts on the development of the pupæ in a similar manner
to cold, that is, retarding them, and at the same time producing
reversion. I had, in Freiburg, a large number of pupæ of the first
summer brood of _Pieris Napi_, bred from eggs. I changed residence
while many caterpillars were in course of transformation and travelled
with the pupæ in this state seven hours by rail. Although this brood of
_P. Napi_, under ordinary circumstances, always emerges in the summer,
generally in July of the same year, as the summer form (var. _Napeæ_),
yet out of these numerous pupæ I did not get a single butterfly
during the year 1872. In winter I kept them in a warm room, and the
first butterflies emerged in January, 1873, the remainder following
in February, March, and April, and two females not until June. All
appeared, however, as exquisite winter forms. The whole course of
development was precisely as though cold had acted on the pupæ; and in
fact, I could find no other cause for this quite exceptional deportment
than the seven hours’ shaking to which the pupæ were exposed by the
railway journey, immediately after or during their transformation.

It is obviously a fact of fundamental importance to the theory of
seasonal dimorphism, that the summer form can be readily changed into
the winter form, whilst the latter cannot be changed into the summer
form. I have thus far only made experiments on this subject with _A.
Levana_, but the same fact appears to me to obtain for _P. Napi_. I
did not, however, operate upon the ordinary winter form of _P. Napi_,
but chose for this experiment the variety _Bryoniæ_, well known to
all entomologists. This is, to a certain extent, the potential winter
form of _P. Napi_; the male (Fig. 14, Plate I.) exactly resembles the
ordinary winter form in the most minute detail, but the female is
distinguished from _Napi_ by a sprinkling of greyish brown scales over
the whole of the upper side of the wings (Fig. 15, Plate I.). This
type, _Bryoniæ_, occurs in Polar regions as the only form of _Napi_,
and is also found in the higher Alps, where it flies in secluded
meadows as the only form, but in other localities, less isolated,
mixed with the ordinary form of the species. In both regions _Bryoniæ_
produces but one generation in the year, and must thus, according to my
theory, be regarded as the parent-form of _Pieris Napi_.

If this hypothesis is correct--if the variety _Bryoniæ_ is really the
original form preserved from the glacial period in certain regions of
the earth, whilst _Napi_ in its winter form is the first secondary form
gradually produced through a warm climate, then it would be impossible
ever to breed the ordinary form _Napi_ from pupæ of _Bryoniæ_ by
the action of warmth, since the form of the species now predominant
must have come into existence only by a cumulative action exerted on
numerous generations, and not _per saltum_.

The experiment was made in the following manner: In the first part
of June I caught a female of _Bryoniæ_ in a secluded Alpine valley,
and placed her in a capacious breeding-cage, where she flew about
among the flowers, and laid more than a hundred eggs on the ordinary
cabbage. Although the caterpillars in the free state feed upon another
plant unknown to me, they readily ate the cabbage, grew rapidly, and
pupated at the end of July. I then brought the pupæ into a hothouse
in which the temperature fluctuated between 12° and 24° R.; but, in
spite of this high temperature, and--what is certainly of more special
importance--notwithstanding the want of cooling at night, only one
butterfly emerged the same summer, and that a male, which, from certain
minute characteristic markings, could be safely identified as var.
_Bryoniæ_. The other pupæ hibernated in the heated room, and produced,
from the end of January to the beginning of June, 28 butterflies, all
of which were exquisite _Bryoniæ_.

Experiment thus confirmed the view that _Bryoniæ_ is the parent-form
of _Napi_, and the description hitherto given by systematists ought
therefore properly to be reversed. _Pieris Bryoniæ_ should be elevated
to the rank of a species, and the ordinary winter and summer forms
should be designated as vars. _Napi_ and _Napeæ_. Still I should not
like to take it upon myself to increase the endless confusion in the
synonomy of butterflies. In a certain sense, it is also quite correct
to describe the form _Bryoniæ_ as a climatic variety, for it is, in
fact, established, if not produced, by climate, by which agency it
is likewise preserved; only it is not a secondary, but the primary,
climatic variety of _Napi_. In this sense most species might probably
be described as climatic varieties, inasmuch as under the influence of
another climate they would gradually acquire new characters, whilst,
under the influence of the climate now prevailing in their habitats,
they have, to a certain extent, acquired and preserved their present
form.

The var. _Bryoniæ_ is, however, of quite special interest, since it
makes clear the relation which exists between climatic variation
and seasonal dimorphism, as will be proved in the next section. The
correctness of the present theory must first here be submitted to
further proof.

It has been shown that the secondary forms of seasonally dimorphic
butterflies do not all possess the tendency to revert in the same
degree, but that this tendency rather varies with each individual. As
the return to the primary form is synonymous with the relinquishing of
the secondary, the greater tendency to revert is thus synonymous with
the greater tendency to relinquish the secondary form, but this again
is equivalent to a lesser stability of the latter; it must consequently
be concluded that the individuals of a species are very differently
influenced by climatic change, so that with some the new form must
become sooner established than with others. From this a variability of
the generation concerned must necessarily ensue, i.e., the individuals
of the summer generation must differ more in colour and marking than
is the case with those of the winter generation. If the theory is
correct, the summer generations should be more variable than the winter
generations--at least, so long as the greatest possible equalization of
individual variations has not occurred through the continued action of
warmth, combined with the constant crossing of individuals which have
become changed in different degrees. Here also the theory is fully in
accord with facts.

In _A. Levana_ the _Levana_ form is decidedly more constant than the
_Prorsa_ form. The first is, to a slight extent, sexually dimorphic,
the female being light and the male dark-coloured. If we take into
consideration this difference between the sexes, which also occurs to
a still smaller extent in the _Prorsa_ form, the foregoing statement
will be found correct, viz. that the _Levana_ form varies but little,
and in all cases considerably less than the _Prorsa_ form, in which
the greatest differences occur in the yellow stripes and in the
disappearance of the black spots on the white band of the hind wing,
these black spots being persistent _Levana_ markings. It is, in fact,
difficult to find two perfectly similar individuals of the _Prorsa_
form. It must, moreover, be considered that the _Levana_ marking,
being the more complicated, would the more readily show variation.
Precisely the same thing occurs in _Pieris Napi_, in which also the
var. _Æstiva_ is considerably more variable than the var. _Vernalis_.
From the behaviour of the var. _Bryoniæ_, on the other hand, which I
regard as the parent-form, one might be tempted to raise an objection
to the theory; for this form is well known to be extraordinarily
variable in colour and marking, both in the Alps and Jura, where it is
met with at the greatest altitudes. According to the theory, _Bryoniæ_
should be less variable than the winter form of the lowlands, because
it is the older, and should therefore be the more constant in its
characters. It must not be forgotten, however, that the variability of
a species may not only originate in the one familiar manner of unequal
response of the individual to the action of varying exciting causes,
but also by the crossing of two varieties separately established in
adjacent districts and subsequently brought into contact. In the
Alps and Jura the ordinary form of _Napi_ swarms everywhere from the
plains towards the habitats of _Bryoniæ_, so that a crossing of the
two forms may occasionally, or even frequently, take place; and it is
not astonishing if in some places (Meiringen, for example) a perfect
series of intermediate forms between _Napi_ and _Bryoniæ_ is met with.
That crossing is the cause of the great variability of _Bryoniæ_ in
the Alpine districts, is proved by the fact that in the Polar regions
this form “is by no means so variable as in the Alps, but, judging
from about forty to fifty Norwegian specimens, is rather constant.”
My friend, Dr. Staudinger, who has twice spent the summer in Lapland,
thus writes in reply to my question. A crossing with _Napi_ cannot
there take place, as this form is never met with, so that the ancient
parent-form _Bryoniæ_ has been able to preserve its original constancy.
In this case also the facts thus accord with the requirements of the
theory.




II.

SEASONAL DIMORPHISM AND CLIMATIC VARIATION.


If, as I have attempted to show, seasonal dimorphism originates
through the slow operation of a changed summer climate, then is this
phenomenon nothing else than the splitting up of a species into two
climatic varieties in the same district, and we may expect to find
various connexions between ordinary simple climatic variation and
seasonal dimorphism. Cases indeed occur in which seasonal dimorphism
and climatic variation pass into each other, and are interwoven in
such a manner that the insight into the origin and nature of seasonal
dimorphism gained experimentally finds confirmation. Before I go more
closely into this subject, however, it is necessary to come to an
understanding as to the conception “climatic variation,” for this term
is often very arbitrarily applied to quite dissimilar phenomena.

According to my view there should be a sharp distinction made between
climatic and local varieties. The former should comprehend only such
cases as originate through the direct action of climatic influences;
while under the general designation of “local forms,” should be
comprised all variations which have their origin in other causes--such,
for example, as in the indirect action of the external conditions of
life, or in circumstances which do not owe their present existence to
climate and external conditions, but rather to those geological changes
which produce isolation. Thus, for instance, ancient species elsewhere
long extinct might be preserved in certain parts of the earth by the
protecting influence of isolation, whilst others which immigrated in
a state of variability might become transformed into local varieties
in such regions through the action of ‘amixia,’[24] i.e. by not being
allowed to cross with their companion forms existing in the other
portions of their habitat. In single cases it may be difficult, or for
the present impossible, to decide whether we have before us a climatic
form, or a local form arising from other causes; but for this very
reason we should be cautious in defining climatic variation.

The statement that climatic forms, in the true sense of the word, do
exist is well known to me, and has been made unhesitatingly by all
zoologists; indeed, a number of authentically observed facts might
be produced, which prove that quite constant changes in a species may
be brought about by the direct action of changed climatic conditions.
With butterflies it is in many cases possible to separate pure climatic
varieties from other local forms, inasmuch as we are dealing with
only unimportant changes and not with those of biological value, so
that natural selection may at the outset be excluded as the cause of
the changes in question. Then again the sharply defined geographical
distribution climatically governed, often furnishes evidence of
transition forms in districts lying between two climatic extremes.

In the following attempt to make clear the relationship between simple
climatic variation and seasonal dimorphism, I shall concern myself only
with such undoubted climatic varieties. A case of this kind, in which
the winter form of a seasonally dimorphic butterfly occurs in other
habitats as the only form, i.e., as a climatic variety, has already
been adduced in a former paragraph. I allude to the case of _Pieris
Napi_, the winter form of which seasonally dimorphic species occurs
in the temperate plains of Europe, whilst in Lapland and the Alps it
is commonly found as a monomorphic climatic variety which is a higher
development of the winter type, viz., the var. _Bryoniæ_.

Very analogous is the case of _Euchloe Belia_, a butterfly likewise
belonging to the _Pierinæ_, which extends from the Mediterranean
countries to the middle of France, and everywhere manifests a very
sharply pronounced seasonal dimorphism. Its summer form was, until
quite recently, described as a distinct species, _E. Ausonia_.
Staudinger was the first to prove by breeding that the supposed two
species were genetically related.[25] This species, in addition to
being found in the countries named, occurs also at a little spot
in the Alps in the neighbourhood of the Simplon Pass. Owing to the
short summer of the Alpine climate the species has in this locality
but one annual brood, which bears the characters of the winter form,
modified in all cases by the coarser thickly scattered hairs of the
body (peculiar to many Alpine butterflies,) and some other slight
differences. The var. _Simplonia_ is thus in the Alps a simple climatic
variety, whilst in the plains of Spain and the South of France it
appears as the winter form of a seasonally dimorphic species.

This _Euchloe_ var. _Simplonia_ obviously corresponds to the var.
_Bryoniæ_ of _Pieris Napi_, and it is highly probable that this form of
_E. Belia_ must likewise be regarded as the parent-form of the species
surviving from the glacial epoch, although it cannot be asserted, as
can be done in the case of _Bryoniæ_, that the type has undergone no
change since that epoch, for _Bryoniæ_ from Lapland is identical with
the Alpine form,[26] whilst _E. Simplonia_ does not appear to occur in
Polar countries.

Very interesting also is the case of _Polyommatus Phlæas_, Linn.,
one of our commonest _Lycænidæ_, which has a very wide distribution,
extending from Lapland to Spain and Sicily.[27] If we compare specimens
of this beautiful copper-coloured butterfly from Lapland with those
from Germany, no constant difference can be detected; the insect has,
however, but one annual generation in Lapland, whilst in Germany it is
double-brooded; but the winter and summer generations resemble each
other completely, and specimens which had been caught in spring on the
Ligurian coast were likewise similarly coloured to those from Sardinia.
(Fig. 21, Plate II.). According to these facts we might believe this
species to be extraordinarily indifferent to climatic influence; but
the South European summer generation differs to a not inconsiderable
extent from the winter generation just mentioned, the brilliant coppery
lustre being nearly covered with a thick sprinkling of black scales.
(Plate II., Fig. 22.) The species has thus become seasonally dimorphic
under the influence of the warm southern climate, although this is
not the case in Germany where it also has two generations in the
year.[28] No one who is acquainted only with the Sardinian summer form,
and not with the winter form of that place, would hesitate to regard
the former as a climatic variety of our _P. Phlæas_; or, conversely,
the north German form as a climatic variety of the southern summer
form--according as he accepts the one or the other as the primary form
of the species.

Still more complex are the conditions in another species of _Lycænidæ_,
_Plebeius Agestis_ (= _Alexis_ Scop.), which presents a double seasonal
dimorphism. This butterfly appears in three forms; in Germany A and B
alternate with each other as winter and summer forms, whilst in Italy
B and C succeed each other as winter and summer forms. The form B
thus occurs in both climates, appearing as the summer form in Germany
and as the winter form in Italy. The German winter variety A, is
entirely absent in Italy (as I know from numerous specimens which I
have caught), whilst the Italian summer form, on the other hand, (var.
_Allous_, Gerh.), does not occur in Germany. The distinctions between
the three forms are sufficiently striking. The form A (Fig. 18, Plate
II.) is blackish-brown on the upper side, and has in the most strongly
marked specimens only a trace of narrow red spots round the borders;
whilst the form B (Fig. 19, Plate II.) is ornamented with vivid red
border spots; and C (Fig. 20, Plate II.) is distinguished from B by the
strong yellowish-brown of the under side. If we had before us only the
German winter and the Italian summer forms, we should, without doubt,
regard them as climatic varieties; but they are connected by the form
B, interpolated in the course of the development of both, and the two
extremes thus maintain the character of mere seasonal forms.




III.

NATURE OF THE CAUSES PRODUCING CLIMATIC VARIETIES.


It has been shown that the phenomenon of seasonal dimorphism has the
same proximate cause as climatic variation, viz. change of climate,
and that it must be regarded as identical in nature with climatic
variation, being distinguished from ordinary, or, as I have designated
it, simple (monomorphic) climatic variation by the fact that, besides
the new form produced by change of climate, the old form continues to
exist in genetic connexion with it, so that old and new forms alternate
with each other according to the season.

Two further questions now present themselves for investigation, viz.
(1) by what means does change of climate induce a change in the marking
and colouring of a butterfly? and (2) to what extent does the climatic
action determine the nature of the change?

With regard to the former question, it must, in the first place, be
decided whether the true effect of climatic change lies in the action
of a high or low temperature on the organism, or whether it may not
perhaps be produced by the accelerated development caused by a high
temperature, and the retarded development caused by a low temperature.
Other factors belonging to the category of external conditions of life
which are included in the term “climate” may be disregarded, as they
are of no importance in these cases. The question under consideration
is difficult to decide, since, on the one hand, warmth and a short
pupal period, and, on the other hand, cold and a long pupal period,
are generally inseparably connected with each other; and without great
caution one may easily be led into fallacies, by attributing to the
influence of causes now acting that which is but the consequence of
long inheritance.

When, in the case of _Araschnia Levana_, even in very cold summers,
_Prorsa_, but never the _Levana_ form, emerges, it would still be
erroneous to conclude that it is only the shorter period of development
of the winter generation, and not the summer warmth, which occasioned
the formation of the _Prorsa_ type. This new form of the species
did not come suddenly into existence, but (as appears sufficiently
from the foregoing experiments) originated in the course of many
generations, during which summer warmth and a short development period
were generally associated together. From the fact that the winter
generation always produces _Levana_, even when the pupæ have not been
exposed to cold but kept in a room, it would be equally erroneous to
infer that the cold of winter had no influence in determining the type.
In this case also the determining causes must have been in operation
during innumerable generations. After the winter form of the species
has become established throughout such a long period, it remains
constant, even when the external influence which produced it (cold) is
occasionally withdrawn.

Experiments cannot further assist us here, since we cannot observe
throughout long periods of time; but there are certain observations,
which to me appear decisive. When, both in Germany and Italy, we see
_Polyommatus Phlæas_ appearing in two generations, of which both the
German ones are alike, whilst in Italy the summer brood is black,
we cannot ascribe this fact to the influence of a shorter period of
development, because this period is the same both in Germany and Italy
(two annual generations), so that it can only be attributed to the
higher temperature of summer.

Many similar cases might be adduced, but the one given suffices for
proof. I am therefore of opinion that it is not the duration of the
period of development which is the cause of change in the formation
of climatic varieties of butterflies, but only the temperature to
which the species is exposed during its pupal existence. In what
manner, then, are we to conceive that warmth acts on the marking
and colouring of a butterfly? This is a question which could only
be completely answered by gaining an insight into the mysterious
chemico-physiological processes by which the butterfly is formed in the
chrysalis; and indeed only by such a complete insight into the most
minute details, which are far beyond our scrutiny, could we arrive
at, or even approximate to, an explanation of the development of any
living organism. Nevertheless an important step can be taken towards
the solution of this problem, by establishing that the change does not
depend essentially upon the action of warmth, but upon the organism
itself, as appears from the nature of the change in one and the same
species.

If we compare the Italian summer form of _Polyommatus Phlæas_ with its
winter form, we shall find that the difference between them consists
only in the brilliant coppery red colour of the latter being largely
suffused in the summer form with black scales. When entomologists speak
of a “black dusting” of the upper side of the wings, this statement
must not of course be understood literally; the number of scales is the
same in both forms, but in the summer variety they are mostly black,
a comparatively small number being red. We might thus be inclined
to infer that, owing to the high temperature, the chemistry of the
material undergoing transformation in _Phlæas_ is changed in such a
manner that less red and more black pigment is produced. But the case
is not so simple, as will appear evident when we consider the fact
that the summer forms have not originated suddenly, but only in the
course of numerous generations; and when we further compare the two
seasonal forms in other species. Thus in _Pieris Napi_ the winter is
distinguished from the summer form, among other characters, by the
strong black dusting of the base of the wings. But we cannot conclude
from this that in the present case more black pigment is produced in
the winter than in the summer form, for in the latter, although the
base of the wings is white, their tips and the black spots on the
fore-wings are larger and of a deeper black than in the winter form.
The quantity of black pigment produced does not distinguish between the
two forms, but the mode of its distribution upon the wings.

Even in the case of species the summer form of which really possesses
far more black than the winter form, as, for instance, _Araschnia
Levana_, one type cannot be derived from the other simply by the
expansion of the black spots present, since on the same place where
in _Levana_ a black band crosses the wings, _Prorsa_, which otherwise
possesses much more black, has a white line. (See Figs. 1-9, Plate
I.) The intermediate forms which have been artificially produced
by the action of cold on the summer generation present a graduated
series, according as reversion is more or less complete; a black spot
first appearing in the middle of the white band of _Prorsa_, and then
becoming enlarged until, finally, in the perfect _Levana_ it unites
with another black triangle proceeding from the front of the band,
and thus becomes fused into a black bar. The white band of _Prorsa_
and the black band of _Levana_ by no means correspond in position; in
_Prorsa_ quite a new pattern appears, which does not originate by a
simple colour replacement of the _Levana_ marking. In the present case,
therefore, there is no doubt that the new form is not produced simply
because a certain pigment (black) is formed in larger quantities, but
because its mode of distribution is at the same time different, white
appearing in some instances where black formerly existed, whilst in
other cases the black remains. Whoever compares _Prorsa_ with _Levana_
will not fail to be struck with the remarkable change of marking
produced by the direct action of external conditions.

The numerous intermediate forms which can be produced artificially
appear to me to furnish a further proof of the gradual character
of the transformation. Ancestral intermediate forms can only occur
where they have once had a former existence in the phyletic series.
Reversion may only take place completely in some particular characters,
whilst in others the new form remains constant--this is in fact the
ordinary form of reversion, and in this manner a mixture of characters
might appear which never existed as a phyletic stage; but particular
characters could certainly never appear unless they were normal to the
species at some stage of phyletic development. Were this possible it
would directly contradict the idea of reversion, according to which
new characters never make their appearance, but only such as have
already existed. If, therefore, the ancestral forms of _A. Levana_
(which we designate as _Porima_) present a great number of transitional
varieties, this leads to the conclusion that the species must have gone
through a long series of stages of phyletic development before the
summer generation had completely changed into _Prorsa_. The view of the
slow cumulative action of climatic influences already submitted, is
thus confirmed.

If warmth is thus without doubt the agency which has gradually changed
the colour and marking of many of our butterflies, it sufficiently
appears from what has just been said concerning the nature of the
change that the chief part in the transmutation is not to be attributed
to the agency in question, but to the organism which is affected by it.
Induced by warmth, there begins a change in the ultimate processes of
the matter undergoing transformation, which increases from generation
to generation, and which not only consists in the appearance of the
colouring matter in one place instead of another, but also in the
replacement of yellow, in one place by white and in another by black,
or in the transformation of black into white on some portions of the
wings, whilst in others black remains. When we consider with what
extreme fidelity the most insignificant details of marking are, in
constant species of butterflies, transmitted from generation to
generation, a total change of the kind under consideration cannot but
appear surprising, and we should not explain it by the nature of the
agency (warmth), but only by the nature of the species affected. The
latter cannot react upon the warmth in the same manner that a solution
of an iron salt reacts upon potassium ferrocyanide or upon sulphuretted
hydrogen; the colouring matter of the butterfly’s wing which was
previously black does not become blue or yellow, nor does that which
was white become changed into black, but a new marking is developed
from the existing one--or, as I may express it in more general terms,
the species takes another course of development; the complicated
chemico-physical processes in the matter composing the pupa become
gradually modified in such a manner that, as the final result, a new
marking and colouring of the butterfly is produced.

Further facts can be adduced in support of the view that in these
processes it is the constitution of the species, and not the external
agency (warmth), which plays the chief part. The latter, as Darwin
has strikingly expressed it, rather performs the function of the
spark which ignites a combustible substance, whilst the character of
the combustion depends upon the nature of the explosive material.
Were this not the case, increased warmth would always change a given
colour[29] in the same manner in all butterflies, and would therefore
always give rise to the production of the same colour. But this does
not occur; _Polyommatus Phlæas_, for example, becoming black in the
south, whilst the red-brown _Vanessa Urticæ_ becomes black in high
northern latitudes, and many other cases well known to entomologists
might be adduced.[30] It indeed appears that species of similar
physical constitution, i.e., nearly allied species, under similar
climatic influences, change in an analogous manner. A beautiful example
of this is furnished by our _Pierinæ_. Most of the species display
seasonal dimorphism; as, for instance, _Pieris Brassicæ_, _Rapæ_,
_Napi_, _Krueperi_, and _Daplidice_, _Euchloe Belia_ and _Belemia_,
and _Leucophasia Sinapis_, in all of which the difference between the
winter and the summer forms is of a precisely similar nature. The
former are characterized by a strong black dusting of the base of the
wings, and by a blackish or green sprinkling of scales on the underside
of the hind wings, while the latter have intensely black tips to the
wings, and frequently also spots on the fore-wings.

Nothing can prove more strikingly, however, that in such cases
everything depends upon the physical constitution, than the fact that
in the same species the males become changed in a different manner to
the females. The parent-form of _Pieris Napi_ (var. _Bryoniæ_) offers
an example. In all the _Pierinæ_ secondary sexual differences are
found, the males being differently marked to the females; the species
are thus sexually dimorphic. Now the male of the Alpine and Polar var.
_Bryoniæ_, which I conceive to be the ancestral form, is scarcely to
be distinguished, as has already been mentioned, from the male of our
German winter form (_P. Napi_, var. _Vernalis_), whilst the female
differs considerably.[31] The gradual climatic change which transformed
the parent-form _Bryoniæ_ into _Napi_ has therefore exerted a much
greater effect on the female than on the male. The external action on
the two sexes was exactly the same, but the response of the organism
was different, and the cause of the difference can only be sought for
in the fine differences of physical constitution which distinguish the
male from the female. If we are unable to define these differences
precisely, we may nevertheless safely conclude from such observations
that they exist.

I have given special prominence to this subject because, in my idea,
Darwin ascribes too much power to sexual selection when he attributes
the formation of secondary sexual characters to the sole action of
this agency. The case of _Bryoniæ_ teaches us that such characters may
arise from purely innate causes; and until experiments have decided
how far the influence of sexual selection extends, we are justified in
believing that the sexual dimorphism of butterflies is due in great
part to the differences of physical constitution between the sexes.
It is quite different with such sexual characters as the stridulating
organs of male Orthoptera which are of undoubted importance to that
sex. These can certainly be attributed with great probability to sexual
selection.

It may perhaps not be superfluous to adduce one more similar case, in
which, however, the male and not the female is the most affected by
climate. In our latitudes, as also in the extreme north, _Polyommatus
Phlæas_, already so often mentioned, is perfectly similar in both
sexes in colour and marking; and the same holds good for the winter
generation of the south. The summer generation of the latter, however,
exhibits a slight sexual dimorphism, the red of the fore wings of the
female being less completely covered with black than in the male.




IV.

WHY ALL POLYGONEUTIC SPECIES ARE NOT SEASONALLY DIMORPHIC.


If we may consider it to be established that seasonal dimorphism is
nothing else than the splitting up of a species into two climatic
varieties in one and the same locality, the further question at once
arises why all polygoneutic species (those which produce more than
_one_ annual generation) are not seasonally dimorphic.

To answer this, it will be necessary to go more deeply into the
development of seasonal dimorphism. This evidently depends upon a
peculiar kind of periodic, alternating heredity, which we might be
tempted to identify with Darwin’s “inheritance at corresponding
periods of life.” It does not, however, in any way completely agree
with this principle, although it presents a great analogy to it and
must depend ultimately upon the same cause. The Darwinian “inheritance
at corresponding periods of life”--or, as it is termed by Haeckel,
“homochronic heredity”--is characterized by the fact that new
characters always appear in the individuals at the same stage of life
as that in which they appeared in their progenitors. The truth of this
principle has been firmly established, instances being known in which
both the first appearance of a new (especially pathological) character
and its transmission through several generations has been observed.
Seasonally dimorphic butterflies also furnish a further valuable proof
of this principle, since they show that not only variations which
arise suddenly (and which are therefore probably due to purely innate
causes) follow this mode of inheritance, but also that characters
gradually called forth by the influence of external conditions and
accumulating from generation to generation, are only inherited at that
period of life in which these conditions were or are effective. In all
seasonally dimorphic butterflies which I have been able to examine
closely, I found the caterpillars of the summer and winter broods to
be perfectly identical. The influences which, by acting on the pupæ,
split up the imagines into two climatic forms, were thus without effect
on the earlier stages of development. I may specially mention that
the caterpillars, as well as the pupæ and eggs of _A. Levana_, are
perfectly alike both in the summer and winter forms; and the same is
the case in the corresponding stages of _P. Napi_ and _P. Bryoniæ_.

I shall not here attempt to enter more deeply into the nature of the
phenomena of inheritance. It is sufficient to have confirmed the law
that influences which act only on certain stages in the development of
the individual, even when the action is cumulative and not sudden, only
affect those particular stages without having any effect on the earlier
or later stages. This law is obviously of the greatest importance to
the comprehension of metamorphosis. Lubbock[32] has briefly shown in a
very clear manner how the existence of metamorphosis in insects can be
explained by the indirect action of varying conditions on the different
life-stages of a species. Thus the mandibles of a caterpillar are,
by adaptation to another mode of nourishment, exchanged at a later
period of life for a suctorial organ. Such adaptation of the various
development-stages of a species to the different conditions of life
would never give rise to metamorphosis, if the law of homochronic, or
periodic, heredity did not cause the characters gradually acquired at
a given stage to be transferred to the same stage of the following
generation.

The origin of seasonal dimorphism depends upon a very similar law, or
rather form, of inheritance, which differs from that above considered
only in the fact that, instead of the ontogenetic stages, a whole
series of generations is influenced. This form of inheritance may
be formulated somewhat as follows:--When dissimilar conditions
alternatingly influence a series of generations, a cycle is produced
in which the changes are transmitted only to those generations
which are acted upon by corresponding conditions, and not to the
intermediate ones. Characters which have arisen by the action of a
summer climate are inherited by the summer generation only, whilst
they remain latent in the winter generation. It is the same as with
the mandibles of a caterpillar which are latent in the butterfly, and
again make their appearance in the corresponding (larval) stage of the
succeeding generation. This is not mere hypothesis, but the legitimate
inference from the facts. If it be admitted that my conception of
seasonal dimorphism as a double climatic variation is correct, the law
of “cyclical heredity,”[33] as I may term it--in contradistinction
to “homochronic heredity,” which relates only to the ontogenetic
stages--immediately follows. All those cases which come under the
designation of ‘alternation of generation,’ can obviously be referred
to cyclical heredity, as will be explained further on. In the one case
the successive generations deport themselves exactly in the same
manner as do the successive stages of development of the individual in
the other; and we may conclude therefrom (as has long been admitted on
other grounds) that a generation is, in fact, nothing else than a stage
of development in the life of a species. This appears to me to furnish
a beautiful confirmation of the theory of descent.

Now if, returning to questions previously solved, the alternating
action of cold in winter and warmth in summer leads to the production
of a winter and summer form, according to the law of cyclical heredity,
the question still remains: why do we not find seasonal dimorphism in
all polygoneutic butterflies?

We might at first suppose that all species are not equally sensitive
to the influence of temperature: indeed, the various amounts
of difference between the winter and summer forms in different
species would certainly show the existence of different degrees of
sensitiveness to the modifying action of temperature. But even this
does not furnish an explanation, since there are butterflies which
produce two perfectly similar[34] generations wherever they occur,
and which, nevertheless, appear in different climates as climatic
varieties. This is the case with _Pararga Ægeria_ (Fig. 23, Plate
II.), the southern variety of which, _Meione_ (Fig. 24, Plate II.),
is connected with it by an intermediate form from the Ligurian coast.
This species possesses, therefore, a decided power of responding to
the influence of temperature, and yet no distinction has taken place
between the summer and the winter form. We can thus only attribute
this different deportment to a different kind of heredity; and we
may therefore plainly state, that changes produced by alternation
of climate are not always inherited _alternatingly_, i.e. by the
corresponding generations, but sometimes _continuously_, appearing
in every generation, and never remaining latent. The causes which
determine why, in a particular case, the one or the other form of
inheritance prevails, can be only innate, i.e. they lie in the organism
itself, and there is as little to be said upon their precise nature as
upon that of any other process of heredity. In a similar manner Darwin
admits a kind of double inheritance with respect to characters produced
by sexual selection; in one form these characters remain limited to the
sex which first acquired them, in the other form they are inherited by
both sexes, without it being apparent why, in any particular case, the
one or the other form of heredity should take place.

The foregoing explanation may obtain in the case of sexual selection,
in which it is not inconceivable that certain characters may not be
so easily produced, or even not produced at all, in one sex, owing
to its differing from the other in physical constitution. In the
class of cases under consideration, however, it is not possible that
the inherited characters can be prevented from being acquired by one
generation owing to its physical constitution, since this constitution
was similar in all the successive generations before the appearance of
dimorphism. The constitution in question first became dissimilar in
the two generations to the extent of producing a change of specific
character, through the action of temperature on the alternating broods
of each year, combined with cyclical heredity. If the law of cyclical
heredity be a general one, it must hold good for all cases, and
characters acquired by the summer generation could never have been also
transmitted to the winter generation from the very first.

I will not deny the possibility that if alternating heredity
should become subsequently entirely suppressed throughout numerous
generations, a period may arrive when the preponderating influence of a
long series of summer generations may ultimately take effect upon the
winter generation. In such a case the summer characters would appear,
instead of remaining latent as formerly. In this manner it may be
imagined that at first but few, and later more numerous individuals,
approximate to the summer form, until finally the dimorphism entirely
disappears, the new form thus gaining ascendency and the species
becoming once more monomorphic. Such a supposition is indeed capable
of being supported by some facts, an observation on _A. Levana_
apparently contradicting the theory having been already interpreted in
this sense. I refer to the fact that whilst some butterflies of the
winter generation emerge in October as _Prorsa_, others hibernate,
and appear the following spring in the _Levana_ form. The winter form
of _Pieris Napi_ also no longer preserves, in the female sex, the
striking coloration of the ancestral form _Bryoniæ_, a fact which may
indicate the influencing of the winter generation by numerous summer
generations. The double form of the spring generation of _Papilio Ajax_
can be similarly explained by the gradual change of alternating into
continuous heredity, as has already been mentioned. All these cases,
however, are perhaps capable of another interpretation; at any rate,
the correctness of this supposition can only be decided by further
facts.

Meanwhile, even if we suppose the above explanation to be correct,
it will not apply to the absence of seasonal dimorphism in cases like
that of _Pararga Ægeria_ and _Meione_, in which only _one_ summer
generation appears, so that a preponderating inheritance of summer
characters cannot be admitted. Another explanation must thus be
sought, and I believe that I have found it in the circumstance that
the butterflies named do not hibernate as pupæ but as caterpillars, so
that the cold of winter does not directly influence those processes of
development by which the perfect insect is formed in the chrysalis.
It is precisely on this point that the origin of those differences of
colour which we designate as the seasonal dimorphism of butterflies
appears to depend. Previous experiments give great probability to this
statement. From these we know that the eggs, caterpillars, and pupæ of
all the seasonally dimorphic species experimented with are perfectly
similar in the summer and winter generations, the imago stage only
showing any difference. We know further from these experiments, that
temperature-influences which affect the caterpillars never entail a
change in the butterflies; and finally, that the artificial production
of the reversion of the summer to the winter form can only be brought
about by operating on the pupæ.

Since many monogoneutic species now hibernate in the caterpillar
stage (e.g. _Satyrus Proserpina_, and _Hermione_, _Epinephele
Eudora_, _Furtina, Ithonus_, _Hyperanthus_, _Ida_, _&c._), we may
admit that during the glacial period such species did not pass the
winter as pupæ. As the climate grew warmer, and in consequence
thereof a second generation became gradually interpolated in many of
these monogoneutic species, there would ensue (though by no means
necessarily) a disturbance of the winter generation, of such a kind
that the pupæ, instead of the caterpillars as formerly, would then
hibernate. It may, indeed, be easily proved _à priori_ that whenever
a disturbance of the winter generation takes place it only does so
retrogressively, that is to say--species which at one time pass the
winter as caterpillars subsequently hibernate in the egg, while those
which formerly hibernate as pupæ afterwards do so as caterpillars.
The interpolation of a summer generation must necessarily delay till
further towards the end of summer, the brood about to hibernate; the
remainder of the summer, which serves for the development of the
eggs and young caterpillars, may possibly under these conditions be
insufficient for pupation, and the species which hibernated in the
pupal state when it was monogoneutic, may perhaps pass the winter in
the larval condition after the introduction of the second brood. A
disturbance of this kind is conceivable; but it is certain that many
species suffer no further alteration in their development than that of
becoming digoneutic from monogoneutic. This follows from the fact that
hibernation takes place in the caterpillar stage in many species of
the sub-family _Satyridæ_ which are now digoneutic, as well as in the
remaining monogoneutic species of the same sub-family. But we cannot
expect seasonal dimorphism to appear in all digoneutic butterflies the
winter generation of which hibernates in the caterpillar form, since
the pupal stage in these species experiences nearly the same influences
of temperature in both generations. We are hence led to the conclusion
that seasonal dimorphism must arise in butterflies whenever the pupæ of
the alternating annual generations are exposed throughout long periods
of time to widely different regularly recurring changes of temperature.

The facts agree with this conclusion, inasmuch as most butterflies
which exhibit seasonal dimorphism hibernate in the pupa stage. Thus,
this is the case with all the _Pierinæ_, with _Papilio Machaon_,
_P. Podalirius_, and _P. Ajax_, as well as with _Araschnia Levana_.
Nevertheless, it cannot be denied that seasonal dimorphism occurs also
in some species which do not hibernate as pupæ but as caterpillars; as,
for instance, in the strongly dimorphic _Plebeius Amyntas_. But such
cases can be explained in a different manner.

Again, the formation of a climatic variety--and as such must we
regard seasonally dimorphic forms--by no means entirely depends
on the magnitude of the difference between the temperature which
acts on the pupæ of the primary and that which acts on those of the
secondary form; it rather depends on the absolute temperature which
the pupæ experience. This follows without doubt from the fact that
many species, such as our common Swallow-tail (_Papilio Machaon_), and
also _P. Podalirius_, in Germany and the rest of temperate Europe,
show no perceptible difference of colour between the first generation,
the pupæ of which hibernate, and the second generation, the pupal
period of which falls in July, whereas the same butterflies in South
Spain and Italy are to a small extent seasonally dimorphic. Those
butterflies which are developed under the influence of a Sicilian
summer heat likewise show climatic variation to a small extent. The
following consideration throws further light on these conditions. The
mean summer and winter temperatures in Germany differ by about 14.9°
R.; this difference being therefore much more pronounced than that
between the German and Sicilian summer, which is only about 3.6° R.
Nevertheless, the winter and summer generations of _P. Podalirius_ are
alike in Germany, whilst the Sicilian summer generation has become a
climatic variety. The cause of this change must therefore lie in the
small difference between the mean summer temperatures of 15.0° R.
(Berlin) and 19.4° R. (Palermo). According to this, a given absolute
temperature appears to give a tendency to variation in a certain
direction, the necessary temperature being different for different
species. The latter statement is supported by the facts that, in the
first place, in different species there are very different degrees of
difference between the summer and winter forms; and secondly, many
digoneutic species are still monomorphic in Germany, first becoming
seasonally dimorphic in Southern Europe. This is the case with _P.
Machaon_ and _P. Podalirius_, as already mentioned, and likewise with
_Polyommatus Phlæas_. Zeller in 1846-47, during his journey in Italy,
recognized as seasonally dimorphic in a small degree a large number of
diurnal Lepidoptera which are not so in our climate.[35]

In a similar manner the appearance of seasonal dimorphism in species
which, like _Plebeius Amyntas_, do not hibernate as pupæ, but as
caterpillars, can be simply explained by supposing that the winter
generation was the primary form, and that the increase in the summer
temperature since the glacial period was sufficient to cause this
particular species to become changed by the gradual interpolation
of a second generation. The dimorphism of _P. Amyntas_ can,
nevertheless, be explained in another manner. Thus, there may have
been a disturbance of the period of development in the manner already
indicated, the species which formerly hibernated in the pupal stage
becoming subsequently disturbed in its course of development by the
interpolation of a summer generation, and hibernating in consequence
in the caterpillar state. Under these circumstances we must regard the
present winter form (var. _Polysperchon_) as having been established
under the influence of a winter climate, this form, since the supposed
disturbance in its development, having had no reason to become changed,
the spring temperature under which its pupation now takes place not
being sufficiently high. The interpolated second generation on the
other hand, the pupal period of which falls in the height of summer,
may easily have become formed into a summer variety.

This latter explanation agrees precisely with the former, both starting
with the assumption that in the present case, as in that of _A. Levana_
and the _Pierinæ_, the winter form is the primary one, so that the
dimorphism proceeds from the said winter form and does not originate
the winter but the summer form, as will be explained. Whether the
winter form has been produced by the action of the winter or spring
temperature is immaterial in judging single cases, inasmuch as we are
not in a position to state what temperature is necessary to cause any
particular species to become transformed.

The reverse case is also theoretically conceivable, viz., that in
certain species the summer form was the primary one, and by spreading
northwards a climate was reached which still permitted the production
of two generations, the pupal stage of one generation being exposed
to the cold of winter, and thus giving rise to the production of a
secondary winter form. In such a case hibernation in the pupal state
would certainly give rise to seasonal dimorphism. Whether these
conditions actually occur, appears to me extremely doubtful; but it may
at least be confidently asserted that the first case is of far more
frequent occurrence. The beautiful researches of Ernst Hoffmann[36]
furnish strong evidence for believing that the great majority of the
European butterflies have immigrated, not from the south, but from
Siberia. Of 281 species, 173 have, according to Hoffmann, come from
Siberia, 39 from southern Asia, and only 8 from Africa, whilst during
the greatest cold of the glacial period, but very few or possibly no
species existed north of the Alps. Most of the butterflies now found
in Europe have thus, since their immigration, experienced a gradually
increasing warmth. Since seasonal dimorphism has been developed in
some of these species, the summer form must in all cases have been the
secondary one, as the experiments upon the reversion of _Pieris Napi_
and _Araschnia Levana_ have also shown.

All the seasonally dimorphic butterflies known to me are found in
Hoffmann’s list of Siberian immigrants, with the exception of two
species, viz., _Euchloe Belemia_, which is cited as an African
immigrant, and _Pieris Krueperi_, which may have come through Asia
Minor, since at the present time it has not advanced farther west
than Greece. No considerable change of climate can be experienced by
migrating from east to west, so that the seasonal dimorphism of _Pieris
Krueperi_ can only depend on a cause similar to that which affected the
Siberian immigrants, that is, the gradual increase of temperature in
the northern hemisphere since the glacial period. In this species also,
the winter form must be the primary one. In the case of _E. Belemia_,
on the other hand, the migration northwards from Africa certainly
indicates removal to a cooler climate, which may have originated a
secondary winter form, even if nothing more certain can be stated. We
know nothing of the period of migration into southern Europe; and even
migration without climatic change is conceivable, if it kept pace with
the gradual increase of warmth in the northern hemisphere since the
glacial epoch. Experiments only would in this case be decisive. If the
summer generation, var. _Glauce_, were the primary form, it would not
be possible by the action of cold on the pupæ of this brood to produce
the winter variety _Belemia_, whilst, on the other hand, the pupæ of
the winter generation by the influence of warmth would be made to
revert more or less completely to the form _Glauce_. It is by no means
to be understood that the species would actually comport itself in this
manner. On the contrary, I am of opinion that in this case also, the
winter form is primary. The northward migration (from Africa to south
Spain) would be quite insufficient, and the winter form is now found in
Africa as well as in Spain.




V.

ON ALTERNATION OF GENERATIONS.


Seasonal dimorphism has already been designated by Wallace as
alternation of generation,[37] a term which cannot be disputed so long
as it is confined to a regular alternation of dissimilar generations.
But little is gained by this definition, however, unless it can be
proved that both phenomena are due to similar causes, and that they
are consequently brought about by analogous processes. The causes of
alternation of generation have, until the present time, been scarcely
investigated, owing to the want of material. Haeckel alone has quite
recently subjected these complicated phenomena generally to a searching
investigation, and has arrived at the conclusion that the various
forms of metagenesis can be arranged in two series. He distinguishes
a progressive and a retrogressive series, comprising under the former
those species “which, to a certain extent, are still in a transition
stage from monogenesis to amphigenesis (asexual to sexual propagation),
and the early progenitors of which, therefore, never exclusively
propagated themselves sexually” (_Trematoda_, _Hydromedusæ_). Under the
other, or retrogressive form of metagenesis, Haeckel includes a “return
from amphigenesis to monogenesis,” this being the case with all those
species which now manifest a regular alternation from amphigenesis to
parthenogenesis (_Aphides_, _Rotatoria_, _Daphniidæ_, _Phyllopoda_,
&c.). Essentially I can but agree entirely with Haeckel. Simply
regarding the phenomena of alternation of generation as at present
known, it appears to me to be readily admissible that these multiform
modes of propagation must have originated in at least two different
ways, which can be aptly formulated in the manner suggested by Haeckel.

I will, however, venture to adopt a somewhat different mode of
conception, and regard the manner of propagation (whether sexual or
asexual) not as the determining, but only as the secondary cause. I
will further hazard the separation of the phenomena of alternating
generations (in their widest sense) into two main groups according to
their origin, designating the cases of one group as true metagenesis
and those of the other as heterogenesis.[38] Metagenesis takes
its origin from a phyletic series of dissimilar forms, whilst
heterogenesis originates from a phyletic series of similar forms--this
series, so far as we can at present judge, always consisting of
similar sexual generations. The former would thus nearly coincide
with Haeckel’s progressive, and the latter with his retrogressive
metagenesis. Metagenesis may further originate in various ways. In the
first place, from metamorphosis, as for example, in the propagation
of the celebrated _Cecidomyia_ with nursing larvæ. The power which
these larvæ possess of propagating themselves asexually has evidently
been acquired as a secondary character, as appears from the fact that
there are many species of the same genus the larvæ of which do not
nurse, these larvæ being themselves undoubted secondary forms produced
by the adaptation of this stage of phyletic development to a mode of
life widely different from that of the later stages. In the form now
possessed by these larvæ they could never have represented the final
stage of their ontogeny, neither could they have formerly possessed
the power of sexual propagation. The conclusion seems inevitable that
metagenesis has here proceeded from metamorphosis; that is to say, one
stage of the ontogeny, by acquiring asexual propagation, has changed
the originally existing metamorphosis into metagenesis.

Lubbock[39] is undoubtedly correct when, for cases like that just
mentioned, he attempts to derive alternation of generations from
metamorphosis. But if we exclude heterogenesis there still remain a
large number of cases of true metagenesis which cannot be explained
from this point of view.

It must be admitted, with Haeckel, that the alternation of generations
in the Hydromedusæ and Trematoda does not depend, as in the case of
_Cecidomyia_, upon the larvæ having acquired the power of nursing,
but that the inferior stages of these species always possessed this
power which they now only preserve. The nursing Trematode larvæ now
existing may possibly have been formerly able to propagate themselves
also sexually, this mode of propagation having at the present time
been transferred to a later phyletic stage. In this case, therefore,
metagenesis was not properly produced by metamorphosis, but arose
therefrom in the course of the phyletic development, the earlier
phyletic stages abandoning the power of sexual reproduction, and
preserving the asexual mode of propagation. A third way in which
metagenesis might originate is through polymorphosis. When the
latter is combined with asexual reproduction, as is especially the
case with the Hydrozoa, metagenesis may be derived therefrom. The
successive stages of transformation of one and the same physiological
individual do not in these cases serve as the point of departure
for alternation of generation, but the different contemporary forms
living gregariously into which the species has become divided through
functional differentiation of the various individuals of the same
stock. Individuals are here produced which alone acquire the power
of sexual reproduction, and metagenesis is thus brought about,
these individuals detaching themselves from the stock on which they
originated, while the rest of the individuals remain in combination,
and retain the asexual mode of propagation. No sharp distinction can be
otherwise drawn between this and the cases previously considered.[40]
The difference consists only in the whole cycle of reproduction being
performed by one stock; both classes have the common character that
the different phyletic stages never appear in the same individual
(metamorphosis), but in the course of further phyletic development
metagenesis at the same time arises, i.e. the division of these stages
among a succession of individuals. We are therefore able to distinguish
this _primary metagenesis_ from the _secondary metagenesis_ arising
from metamorphosis.

It is not here my intention to enter into the ultimate causes of
metagenesis; in this subject we should only be able to advance by
making vague hypotheses. The phenomenon of seasonal dimorphism,
with which this work has mainly to deal, is evidently far removed
from metagenesis, and it was to make this clear that the foregoing
observations were brought forward. The characters common in the origin
of metagenesis are to be found, according to the views previously
set forth, in the facts that here the faculty of asexual and of
sexual reproduction is always distributed among _several_ phyletic
stages of development which succeed each other in an ascending series
(progressive metagenesis of Haeckel), whereas I find differences only
in the fact that the power of asexual propagation may (in metagenesis)
be either newly acquired (larva of _Cecidomyia_) or preserved from
previous ages (_Hydroida_). It seems that in this process sexual
reproduction is without exception lost by the earlier, and remains
confined solely to the most recent stages.

From the investigations on seasonal dimorphism it appears that a
cycle of generations can arise in an entirely different way. In this
case a series of generations originally alike are made dissimilar by
external influences. This appears to me of the greatest importance,
since seasonal dimorphism is without doubt closely related to that
mode of reproduction which has hitherto been exclusively designated
as heterogenesis, and a knowledge of its mode of origination must
therefore throw light on the nature and origin of heterogenesis in
general.

In seasonal dimorphism, as I have attempted to show, it is the
direct action of climate, and indeed chiefly that of temperature,
which brings about the change in some of the generations. Since
these generations have been exposed to the alternating influence
of the summer and winter temperature a periodical dimorphism has
been developed--a regular cycle of dissimilar generations. It has
already been asserted that the consecutive generations of a species
comport themselves with respect to heredity in a manner precisely
similar to that of the ontogenetic stages, and at the same time such
succeeding generations point out the parallelism between metamorphosis
and heterogenesis. If influences capable of directly or indirectly
producing changes operate on any particular stage of development,
these changes are always transmitted to the same stage. Upon this
metamorphosis depends. In a precisely similar manner changes which
operated periodically on certain generations (1, 3, 5, for instance)
are transmitted to these generations only, and not to the intermediate
ones. Upon this depends heterogenesis. We have just been led to the
comprehension of heterogenesis by cyclical heredity, by the fact
that a cycle is produced whenever a series of generations exists
under regularly alternating influences. In this cycle newly acquired
changes, however minute in character at first, are only transmitted to
a later, and not to the succeeding generation, appearing only in the
one corresponding, i.e. in that generation which exists under similar
transforming influences. Nothing can more clearly show the extreme
importance which the conditions of life must have upon the formation
and further development of species than this fact. At the same time
nothing shows better that the action of these conditions is not
suddenly and violently exerted, but that it rather takes place by small
and slow operations. In these cases the long-continued accumulation
of imperceptibly small variations proves to be the magic means by
which the forms of the organic world are so powerfully moulded. By the
application of even the greatest warmth nobody would be able to change
the winter form of _A. Levana_ into the summer form; nevertheless, the
summer warmth, acting regularly on the second and third generations
of the year, has, in the course of a lengthened period, stamped these
two generations with a new form without the first generation being
thereby changed. In the same region two different climatic varieties
have been produced (just as in the majority of cases climatic varieties
occur only in separate regions) which alternate with each other, and
thus give rise to a cycle of which each generation propagates itself
sexually.

But even if seasonal dimorphism is to be ascribed to heterogenesis, it
must by no means be asserted that those cases of cyclical propagation
hitherto designated as heterogenesis are completely identical with
seasonal dimorphism. Their identity extends only to their origin
and manner of development, but not to the mode of operation of the
causes which bring about their transformation. Both phenomena have a
common mode of origination, arising from similar (monomorphic) sexual
generations and course of development, a cycle of generations with
gradually diverging characters coming into existence by the action of
alternating influences. On the other hand, the nature of the changes
by which the secondary differs from the primary generation may be
referred to another mode of action of the exciting causes. In seasonal
dimorphism the differences between the two generations are much less
than in other cases of heterogenesis. These differences are both
quantitatively less, and are likewise qualitative, affecting only
characters of biological insignificance.[41] The variations in question
are mostly restricted to the marking and colouring of the wings and
body, occasionally affecting also the form of the wing, and in a few
cases the size of the body (_Plebeius Amyntas_), whilst the bodily
structure--so far at least as my investigations extend--appears to be
the same in both generations.[42]

The state of affairs is quite different in the remaining cases of
heterogenesis; here the entire structure of the body appears to be
more or less changed, and its size is often very different, nearly all
the internal organs differing in the two generations. According to
Claus,[43] “we can scarcely find any other explanation of the mode of
origination of heterogenesis than the gradual and slow advantageous
adaptation of the organization to important varying conditions of
life”--a judgment in which this author is certainly correct. In all
such cases the change does not affect unimportant characters, as it
does in butterflies, but parts of biological or physiological value;
and we cannot, therefore, consider such changes to have originated
through the _direct_ action of altered conditions of life, but
_indirectly_ through natural selection or adaptation.

Thus, the difference between seasonal dimorphism and the other known
cases of heterogenesis consists in the secondary form in which the
species appears in the former originating through the direct action
of external conditions, whilst in the latter this form most probably
originates through the indirect action of such influences. The first
half of the foregoing proposition is alone capable of provisional
proof, but it is in the highest degree probable that the latter half
is also correct. Naturally we cannot say to what extent the direct
action of external conditions plays also a part in true heterogenesis,
as there have been as yet no experiments made on its origin. That
direct action, working to a certain extent co-operatively, plays only
a secondary part, while the chief cause of the change is to be found
in adaptation, no one can doubt who keeps in view, for instance, the
mode of propagation discovered by Leuckart in _Ascaris nigrovenosa_. In
this worm, the one generation lives free in the water, and the other
generation inhabits the lungs of frogs, the two generations differing
from one another in size of body and structure of internal organs to an
extent only possible with the true Nematoda.

To prevent possible misunderstanding, let it be finally noted--even
if superfluous--that the changes causing the diversity of the two
generations in seasonal dimorphism and heterogenesis are not of such
a nature that the value of different “specific characters” can be
attached to them. Distinctly defined specific characters are well
known not to occur generally, and it would therefore be erroneous to
attach but little value to the differences in seasonal dimorphism
because these chiefly consist in the colouring and marking of the
wings. The question here under consideration is not whether two animal
forms have the value of species or of mere varieties--a question which
can never be decided, since the reply always depends upon individual
opinion of the value of the distinctions in question, and the idea
of both species and varieties is moreover purely conventional. The
question is, rather, whether the distinguishing characters possess
an equal constancy--that is, whether they are transmitted with the
same force and accuracy to all individuals; and whether they occur,
therefore, in such a manner that they can be practically employed
as specific characters. With respect to this, it cannot be doubtful
for a moment that the colouring and marking of a butterfly possess
exactly the same value as the constant characters in any other group
of animals, such as the palate-folds in mice, the structure of the
teeth in mammals, the number and form of the wing and tail feathers in
birds, &c. We have but to remember with what wonderful constancy often
the most minute details of marking are transmitted in butterflies. The
systematist frequently distinguishes between two nearly allied species,
as for instance in the _Lycænidæ_, chiefly by the position of certain
insignificant black spots on the under side of the wing (_P. Alexis_
female, and _P. Agestis_); and this diagnosis proves sufficient, since
_P. Alexis_, which has the spots in a straight row, has a different
caterpillar to _P. Agestis_, in which the central spot is nearer the
base of the hind wing!

For the reasons just given, I maintain that it is neither justifiable
nor useful to designate the di- and polymorphism of butterflies as
di- and polychroism, and thereby to attribute but little importance to
these phenomena.[44] This designation would be only justifiable if the
differences of colour were due to other causes than the differences of
form, using this last word in a narrow sense. But it has been shown
that the same direct action of climate which originates new colours,
produces also in some species differences of form (contour of wing,
size, &c.); whilst, on the other hand, it has long been known that many
protective colours can only be explained by the indirect action of
external conditions.

When I raise a distinction in the nature of the changes between
seasonal dimorphism and the remaining known cases of heterogenesis,
this must be taken as referring only to the biological or physiological
result of the change in the transformed organism itself. In seasonal
dimorphism only insignificant characters become prominently changed,
characters which are without importance for the welfare of the species;
while in true heterogenesis we are compelled to admit that useful
changes, or adaptations, have occurred.

Heterogenesis may thus be defined either in accordance with my proposal
or in the manner hitherto adopted, since it may be regarded as more
morphological than the cyclical succession of differently formed sexual
generations; or, with Claus, as the succession of different sexual
generations, “living under different conditions of existence”--a
definition which applies in all cases to seasonal dimorphism. Varying
conditions of existence, in their widest sense, are the result of the
action of different climates; and a case has been made known recently
in which it is extremely probable that the climatic differences of
the seasons have produced a cycle of generations by influencing the
processes of nutrition. This case is quite analogous to that which
we have observed in the seasonal dimorphism of butterflies, but with
the distinction that the difference between the winter and summer
generations does not, at least entirely, consist in the form of the
reproductive adult, but almost entirely in its ontogeny--in the mode
of its development. A comparison of this case with the analogous
phenomenon in butterflies, may be of interest. In the remarkable
fresh-water Daphnid, _Leptodora hyalina_ Lillejeborg, it was proved
some years ago by P. E. Müller,[45] who studied the ontogeny, that this
last was direct, since the embryo, before leaving the egg, already
possesses the form, members, and internal organs of the adult. This
was, at least, the case with the summer eggs. It was subsequently
shown by Sars[46] that this mode of development only holds good for
the summer brood, the winter eggs producing an embryo in the spring
which possesses only the three first pairs of limbs, and, instead of
compound eyes, only a single frontal eye, thus exhibiting briefly, at
first, the structure of a _Nauplius_, and gradually acquiring that
of _Leptodora_. The mature form derived from the winter eggs is not
distinguishable from the later generations, except by the presence
of the simple larval eye, which appears as a small black spot. The
generations when fully developed are thus distinguished only by this
minute marking, but the summer generation undergoes direct development,
whilst the winter generation, on the contrary, is only developed
by metamorphosis, beginning with the simplest Crustacean type, and
thus fairly representing the phyletic development of the species. We
therefore see, in this case, the combination of a metamorphic and a
direct development taking place to a certain extent under our eyes. It
cannot be proved with certainty what the cause of this phenomenon may
be, but the conjecture is almost unavoidable that it is closely related
to the origin of the seasonal dimorphism of butterflies, since both
depend on the alternating climatic influences of summer and winter:
it is most probable that these influences have directly[47] brought
about a shortening of the period of development in summer. Thus we have
here a case of heterogenesis nearly related to the seasonal dimorphism
of butterflies in a twofold manner--first, because the cycle of
generations is also in this case brought about by the direct action of
the external conditions of life; and secondly, the winter form is here
also the primary, and the summer form the secondary one.

In accordance with the idea first introduced into science by Rudolph
Leuckart, we have hitherto understood heterogenesis to be only the
alternation of dissimilar sexual generations. From this point of
view the reproduction of _Leptodora_ can be as little ascribed to
heterogenesis as can that of _Aphis_ or _Daphnia_, although the
apparent agamic reproduction of the winter and a portion of the summer
generation is undoubtedly parthenogenesis and not propagation by
nursing.[48] As has already been said, however, I would attribute no
fundamental importance to the criterion of agamic reproduction--the
more especially because we are ignorant of the physiological
significance of the two modes of propagation; and further, because this
principle of classification is entirely external, and only valuable
in so far as no better one can be substituted for it. A separation of
the modes of cyclical propagation according to their genesis appears
to me--especially if practicable--not alone to be of greater value,
but the only correct one, and for this the knowledge of the origin of
seasonal dimorphism seems to me to furnish a possible method.

If, as was indicated above, we designate as metagenesis (in the narrow
sense) all those cases in which it must be admitted that a series
of differently aged phyletic stages have furnished the points of
departure, and as heterogenesis those cases in which similar phyletic
stages have been compelled to produce a cycle of generations by the
periodic action of external influences, it is clear that the scope of
heterogenesis is by this means considerably extended, and at the same
time sharply and precisely defined.

Under heterogenesis then is comprised, not only as heretofore the
reproduction of _Ascaris nigrovenosa_, of _Leptodora appendiculata_,
and of the cattle-lice, but also that of the _Aphides_, _Coccidæ_,
_Daphniidæ_, _Rotatoria_, and _Phyllopoda_, and, in short, all those
cases in which we can determine the former identity of the two kinds
of generations from their form, anatomical structure, and mode of
reproduction. This conclusion is essentially supported by a comparison
of the most closely allied species. Thus, for instance, when we see
the genus _Aphis_ and its allies related on all sides to insects which
propagate sexually in all generations, and when we further observe
the great similarity of the whole external and internal structure
in the two kinds of generations of _Aphis_, we are forced to the
conjecture that the apparent asexual reproduction of the _Aphidæ_ is in
reality parthenogenesis, i.e., that it has been developed from sexual
reproduction. Neither can it be any longer disputed that in this case,
as well as in that of _Leptodora_ and other _Daphniidæ_, the same female
alternately propagates parthenogenetically, and produces eggs requiring
fertilization. This was established by Von Heyden[49] some years ago,
in the case of _Lachnus Querci_, and has been since confirmed by
Balbiani.[50]

There can be no doubt that in all these cases the cycle of generations
has been developed from phyletically similar generations. But
instances are certainly conceivable which present themselves with less
clearness and simplicity. In the first place, we do not know whether
parthenogenesis may not finally settle down into complete asexual
reproduction. Should this be the case, it might be possible that from
heterogenesis a mode of propagation would ultimately arise, which
was apparently indistinguishable from pure metagenesis. Such a state
of affairs might result, if the generations settling into asexual
reproduction (as, for instance, the plant-lice), at the same time
by adaptation to varying conditions of life, underwent considerable
change of structure, and entered upon a metamorphosis to some extent
retrogressive. We should then be inclined to regard these generations
as an earlier phyletic stage, whilst, in fact, they would be a later
one, and the idea of metagenesis would thus have been formed after the
manner of heterogenesis.

On the other hand, it is equally conceivable that heterogenesis may
have been developed from true metagenesis in the case of larvæ which,
having acquired the faculty of asexual propagation, are similar in
function to sexually mature insects. This possibility is not at first
sight apparent. If the nursing-larvæ of the _Cecidomyiæ_ were as much
like the sexual insects as are the young Orthoptera to the sexually
mature forms, we should not know whether to regard them as degraded
sexual insects, or as true larvæ which had attained the power of
asexual propagation. Their propagation would be considered to be
parthenogenesis; and as it could not be denied that heterogenesis was
here manifest, the mode of development of their particular kind of
propagation might be proved, i.e., it might be demonstrated, that the
generations now parthenogenetic were formerly mere reproductive larval
stages.

I have only offered these last observations in order to show on what
uncertain ground we are still standing with regard to this subject
whenever we deal with the meaning of any particular case, and how
much still remains to be done. It appears certain that the two forms
of cyclical propagation, heterogenesis and metagenesis, originate in
entirely distinct ways, so that it must be admitted that, under these
circumstances, the idea of the existing conditions respecting the true
genesis may possibly be erroneous. To indicate the manner in which the
cyclical mode of propagation has arisen in any single case, would only
be possible by a searching proof and complete knowledge of existing
facts in addition to experiments.




VI.

GENERAL CONCLUSIONS.


I shall not here give a repetition and summary of the results arrived
at with respect to seasonal dimorphism, but rather the general
conclusions derived from these results; and, at the same time, I may
take the opportunity of raising certain questions which have not
hitherto found expression, or have been but briefly and casually stated.

It must, in the first place, be admitted that differences of specific
value can originate through the direct action of external conditions
of life only. Of the truth of this proposition there can be no doubt,
after what has been above stated concerning the difference between
the two forms of any seasonally dimorphic species. The best proof is
furnished by the older systematists, to whom the genetic relationship
of the two forms was unknown, and who, with unprejudiced taxonomy, in
many cases indicated their distinctness by separate specific names.
This was the case with _Araschnia Levana_ and _Prorsa_, _Euchloe Belia_
and _Ausonia_, _E. Belemia_ and _Glauce_, _Plebeius Polysperchon_ and
_Amyntas_. In the presence of these facts it can scarcely be doubted
that new species can be formed in the manner indicated; and I believe
that this was and is still the case, with butterflies at least, to
a considerable extent; the more so with these insects, because the
striking colours and markings of the wings and body, being in most
cases without biological significance, are useless for the preservation
of the individual or the species, and cannot, therefore, be objects of
natural selection.

Darwin must have obtained a clear insight into this, when he attempted
to attribute the markings of butterflies to sexual and not to natural
selection. According to this view, every new colour or marking first
appears in one sex accidentally,[51] and is there fixed by being
preferred by the other sex to the older coloration. When the new
ornamentation becomes constant (in the male for example), Darwin
supposes that it becomes transferred to the female by inheritance,
either partially or completely, or not at all; so that the species,
therefore, remains more or less sexually dimorphic, or (by complete
transference) becomes again sexually monomorphic.

The admissibility of such different, and, to a certain extent,
arbitrarily limited inheritance, has already been acknowledged.
The question here concerned is, whether Darwin is correct when he
in this manner attributes the entire coloration of butterflies to
sexual selection. The origin of seasonal dimorphism appears to me to
be against this view, howsoever seductive and grand the latter may
seem. If differences as important as those which exist between the
summer and winter forms of many butterflies can be called forth by the
direct action of a changed climate, it would be extremely hazardous to
attribute great importance to sexual selection in this particular case.

The principle of sexual selection appears to me to be incontestible,
and I will not deny that it is also effective in the case of
butterflies; but I believe that as a final explanation of colour this
agency can be dispensed with, inasmuch as we see that considerable
changes of colour can occur without the influence of sexual
selection.[52]

The question now arises, how far does the transforming influence of
climate extend? When a species has become transformed by climatic
change to such an extent that its new form possesses the systematic
value of a new species, does it return to its older form by removal
to the old climatic conditions? or would it under these circumstances
become again transformed in a new manner? This question is not without
importance, inasmuch as in the first case climatic influences would be
of little value in the formation of species, and there would result
at most only a fluctuation between two extremes. In the same manner
as in seasonally dimorphic species the summer and winter forms now
alternate with each other every year, so would the forms produced by
warmth and cold then alternate in the greater periods of the earth’s
history. Other groups of animals are certainly changed by the action of
different climatic influences; but in butterflies, as I believe I have
proved, temperature plays the chief part, and as this only oscillates
between rather narrow limits, it admits of no great differences of
coloration.

The question thus suggests itself, whether species of butterflies
only oscillate between two forms, or whether climatic change, when
sufficiently great to produce variation, does not again originate a new
form. Inasmuch as the reversion experiments with seasonally dimorphic
butterflies appear to correspond with the latter view, I believe that
this must be admitted. I am of opinion that an old form never again
arises through change of climate, but always a new one; so that a
periodically recurring change of climate is alone sufficient, in the
course of a long period of time, to admit of new species arising from
one another. This, at least, may be the case with butterflies.

My views rest essentially upon theoretical considerations. It
has already been insisted upon, as results immediately from the
experiments, that temperature does not act on the physical constitution
of the individual in the same manner as acid or alkali upon litmus
paper, i.e., that one and the same individual does not produce this or
that coloration and marking according as it is exposed to warmth or
cold; but rather that climate, when it influences in a similar manner
many succeeding generations, gradually produces such a change in
the physical constitution of the species that this manifests itself
by other colours and markings. Now when this newly acquired physical
constitution, established, as we may admit, throughout a long series
of generations, is again submitted to a constant change of climate,
this influence, even if precisely similar to that which obtained during
the period of the first form of the species, cannot possibly reproduce
this first form. The nature of the external conditions may be the same,
but not so the physical constitution of the species. Just in the same
manner as a _Pieris_ (as has been already shown), a _Lycæna_, or a
_Satyrus_, produces quite different varieties under the transforming
influence of the same climate, so must the variation originating
from the transformed species of our present case after the beginning
of the primary climate be different from that primary form of the
species, although perhaps in a less degree. In other words, if only
two different climates alternated with each other during the earth’s
geological periods, every species of butterfly submitted to these
changes of climate would give rise to an endless series of different
specific forms. The difference of climate would in reality be greater
than supposed, and for any given species the climatic variation would
not only occur through the periodic shifting of the ecliptic, but also
through geological changes and the migrations of the species itself, so
that a continuous change of species must have gone on from this sole
cause of alternation of climate. When we consider that many species
elsewhere extinct have become locally preserved, and when, further, to
these we add those local forms which have arisen by the prevention of
crossing (amixia), and finally take into consideration the important
effects of sexual selection, we can no longer be astonished at the vast
numbers of species of butterflies which we now meet with on the earth.

Should any one be inclined to conclude, from my reversion experiments
with seasonally dimorphic butterflies, that the secondary species
when exposed to the same climate as that which produced it must
revert to the primary, he forgets that this reversion to the winter
form is nothing but a reversion--i.e., a sudden return to a primary
form through peculiar laws of inheritance--and by no means a gradual
re-acquisition of this primary form under the gradual influence of
the primary climate. Reversion to the winter form occurs also through
other influences, as, for instance, by high temperature. Reversions
of this kind, depending on laws of heredity, certainly happen with
those cases of transmutation which do not alternate with the primary
form, as in seasonal dimorphism, but which occur continuously. They
would, however probably be more quickly suppressed in such cases than
in seasonal dimorphism, where the constant alternation of the primary
and secondary forms must always maintain the tendency of the latter to
produce the former.

That the above conclusion is correct--that a secondary species, when
exposed to the external conditions under the influence of which the
primary form originated, does not again revert to the latter--is proved
by experience with plants. Botanists[53] assure us “that cultivated
races which become wild, and are thus brought back to their former
conditions of life, do not become changed into the original wild form,
but into some new one.”

A second point which appears to me to be elucidated by seasonal
dimorphism, is the origin of variability. It has already been
prominently shown that secondary forms are for the most part
considerably more variable than primary forms. From this it follows
that similar external influences either induce different changes in
the different individuals of a species, or else change all individuals
in the same manner, variability arising only from the unequal time
in which the individuals are exposed to the external influence. The
latter is undoubtedly the case, as appears from the differences which
are shown by the various individuals of a secondary form. These are
always only differences of degree and not of kind, as is perhaps most
distinctly shown by the very variable _A. Prorsa_ (summer form), in
which all the occurring variations differ only by the _Levana_ marking
being more or less absent, and, at the same time, by approximating
more or less to the pure _Prorsa_ marking; but changes in a totally
different direction never occur. It is likewise further evident, as has
been mentioned above, that allied species and genera, and even entire
families (_Pieridæ_), are changed by similar external inducing causes
in the same manner--or, better, in the same direction.

In accordance with these facts the law may be stated, that, in
butterflies at least, all the individuals of a species respond to the
same external influences by similar changes, and that, consequently,
the changes brought about by climatic influences take a fixed
direction, determined by the physical constitution of the species.
When, however, new climatic forms of butterflies, in which natural
selection is completely excluded, and the nature of the species itself
definitely determines the direction of the changes, nevertheless show
variability from the very beginning, we may venture to conclude that
every transformation of a species generally begins with a fluctuation
of its characters. But when we find the primary forms of butterflies
always far more constant, this shows that the continued crossing of the
individuals of a species to a certain extent balances the fluctuations
of form. Both facts taken together confirm the law formerly enunciated
by me,[54] that in every species a period of variability alternates
with one of (relative) constancy--the latter indicating the
culmination, and the former the beginning or end, of its development.
I here call to mind this law, because the facts which I advanced at
that time, viz., Hilgendorf’s history of the phyletic development of
the Steinheim fossil shells, having since become somewhat doubtful, one
might easily be inclined to go too far in mistrusting them and refuse
to give them any weight at all.[55]

In the essay just indicated I traced the origin of a certain class
of local forms to local isolation. I attempted to show that when a
species finds itself in an isolated district in a condition (period)
of variability, it must there necessarily acquire somewhat deviating
characters by being prevented from crossing with the individuals
of other regions, or, what comes to the same thing, a local form
must originate. This production of local forms results because
the different variations which, for the time being, constitute the
variability of the species, would always be in a different numerical
proportion in the isolated district as compared with other regions;
and further, because constancy is produced by the crossing of these
(isolated) varieties among themselves; so that the resultant of
the various components is (local) variation. If the components are
dissimilar the resultant would also be different, and thus, from a
theoretical point of view, there seems to me no obstacle in the way
of the production of such local forms by the process of ‘amixia.’ I
believe that I have further shown that numerous local forms can be
conceived to have arisen through this process of preventive crossing,
whilst they cannot be explained by the action of climatic influences.

That I do not deny the existence of true climatic forms in admitting
this principle of ‘amixia,’ as has been frequently imagined, appears
sufficiently from the treatise in question. The question arises,
however, whether climatic influences may not also originate forms
by ‘amixia’ by making a species variable. It would be difficult at
present to decide finally upon this subject. If, however, in all cases
a variation in a certain fixed direction occurred through climatic
influences, a form could not arise by ‘amixia’ from such a variability,
since the components could then produce resultants different only
in degree and not in kind. But we are not yet able to extend our
researches to such fine distinctions.

As a final, and not unimportant result of these investigations, I may
once more insist that dissimilar influences, when they alternatingly
affect a long series of originally similar generations in regularly
recurring change, only modify the generations concerned, and not
intermediate ones. Or, more briefly, cyclically acting causes of
change produce cyclically recurring changes: under their influence
series of monomorphic generations become formed into a cycle of di- or
polymorphic generations.

There is no occasion to return here to the immediate evidence and
proof of the foregoing law. In the latter, however, is comprised the
question--is not the cycle of generations produced by cyclical heredity
ultimately equivalent to Darwin and Haeckel’s homochronic heredity
which forms the ontogenetic stages into a cycle? It is possible
that from this point, in the future, the nature of the processes of
heredity, which are still so obscure, may be penetrated into, and both
phenomena traced to the same cause, as can now be only surmised but not
clearly perceived.

Finally, the most general, and in so far chief result of these
investigations, appears to me to lie in the conclusion, which may
be thus formulated:--A species is only caused to change through the
influence of changing external conditions of life, this change being
in a fixed direction which entirely depends on the physical nature of
the varying organism, and is different in different species, or even in
the two sexes of the same species.

I am so little disposed to speak in favour of an unknown transforming
power that I may here again insist that the transformation of a
species only partly depends upon external influences, and partly on
the specific constitution of the particular form. I designate this
constitution ‘specific,’ inasmuch as it responds to the same inciting
cause in a manner different to the constitution of another species.
We can generally form a clear conception why this should be the case;
for not only is there in another species a different kind of latent
vital activity, but each species has also a different developmental
history. It must be admitted that, from the earliest period of the
formation of an organism, and throughout all its intermediate stages,
properties which have become established, such as growth, nutrition,
or tendency to development, have been transferred to the species now
existing, each of which bears these tendencies in itself to a certain
extent. It is these innate tendencies which determine the external
and internal appearance of the species at every period of its life,
and which, by their reaction to external factors, represent the life
of the individual as well as that of the species. Since the sum of
these inherited tendencies must vary more or less in every species,
not only is the different external appearance of species as well as
their physiological and biological diversity thus explained, but it
necessarily follows therefrom, that different species must respond
differently to those external causes which tend to produce a change in
their form.

Now, this last conclusion is equivalent to the statement that every
species, through its physical constitution, (in the sense defined) is
impressed with certain fixed powers of variation, which are evidently
extraordinarily numerous in the case of each species, but are not
unlimited; they permit of a wide range for the action of natural
selection, but they also limit its functions, since they certainly
restrain the course of development, however wide the latter may be. I
have elsewhere previously insisted[56] that too little is ascribed to
the part played by the physical constitution of species in the history
of their transformation, when the course of this transformation is
attributed entirely to external conditions. Darwin certainly admits
the importance of this factor, but only so far as it concerns the
individual variation, the nature of which appears to him to depend on
the physical constitution of the species. I believe, however, that in
this directive influence lies the precise reason why, under the most
favourable external circumstances, a bird can never become transformed
into a mammal--or, to express myself generally, why, from a given
starting-point, the development of a particular species cannot now
attain, even under the most favourable external conditions, any desired
goal; and why, from this starting-point, given courses of development,
even when of considerable latitude, must be restricted, just as a ball
rolling down a hill is diverted by a fixed obstacle in a direction
determined by the position of the latter, and depending on the
direction of motion and the velocity at the moment of being diverted.

In this sense I agree with Askenasy’s “fixed” direction of variation;
but not if another new physical force directing variation itself
is thereby intended.[57] The explanation of the phenomena does not
appear to me to require such an admission, and, if unnecessary, it
is certainly not legitimate. According to my view, transmutation by
purely internal causes is not to be entertained. If we could absolutely
suspend the changes of the external conditions of life, existing
species would remain stationary. The action of external inciting
causes, in the widest sense of the word, is alone able to produce
modifications; and even the never-failing “individual variations,”
together with the inherited dissimilarity of constitution, appear
to me to depend upon unlike external influences, the inherited
constitution itself being dissimilar because the individuals have been
at all times exposed to somewhat varying external influences.

A change arising from purely internal causes seems to me above all
quite untenable, because I cannot imagine how the same material
substratum of physical constitution of a species can be transferred to
the succeeding generation as two opposing tendencies. Yet this must
be the case if the direction of development transferred by heredity
is to be regarded as the ultimate ground both of the similarity
and dissimilarity to the ancestors. All changes, from the least to
the greatest, appear to me to depend ultimately only on external
influences; they are the response of the organism to external inciting
causes. It is evident that this response must be different when a
physical constitution of a different nature is affected by the same
inciting cause, and upon this, according to my view, depends the great
importance of these constitutional differences.

If, under “heredity,” we comprise the totality of inheritance--that
is to say, the physical constitution of a species at any time, and
therefore the restricted and, in the foregoing sense, pre-determined
power of variation, whilst under “adaptation” we comprehend the direct
and indirect response of this physical constitution to the changes in
the conditions of life, I can agree with Haeckel’s mode of expression,
and with him trace the transformation of species to the two factors of
heredity and adaptation.




APPENDIX I.

EXPERIMENTS.


EXPERIMENTS WITH ARASCHNIA LEVANA.

1. Bred from eggs laid by a female of the winter form on 12th-15th
May, 1868, in a breeding-cage. The caterpillars emerged on 20th-22nd
May, and pupated on 7th-9th June. The pupæ, kept at the ordinary
temperature, produced:--

  On the 19th of June  4 butterflies.
    ”    20th    ”     5     ”
    ”    21st    ”    10     ”
    ”    22nd    ”     9     ”
    ”    23rd    ”     7     ”
    ”    25th    ”    13     ”
                      --
         Total        48     ”

All these butterflies were of the _Prorsa_ type, 3 females having a
considerable amount of yellow, but none with so much as figs. 3, 4, 7,
8, or 9. Pl. I.

2. August 12th, 1868, found larvæ of the third generation, which
pupated at the beginning of September, and were kept in a room not
warmed. In September three butterflies emerged in the _Prorsa_ form,
the remainder hibernating and producing, after being placed in a heated
room at the end of February, from the 1st to the 17th of March, 1869,
more butterflies, all of the _Levana_ form.

3. Larvæ found on the 17th June, 1869, were sorted according to colour;
the yellow ones, with light brown spines, produced, at the ordinary
temperature, on 8th-12th July, 13 butterflies, 12 of which showed the
ordinary _Prorsa_ type, and one, a male, possessing more yellow than
fig. 3, Pl. I., must be considered as a _Porima_ type.

4. From caterpillars of the second generation, found at the same
time as those of Exp. 3, 30 pupæ were placed in the refrigerator
(temperature 8°-10° R.) on June 25th. When the box was opened on
August 3rd, almost all had emerged, many being dead, and all, without
exception, were of the intermediate form (_Porima_), although nearer
the _Prorsa_ than the _Levana_ type.

5. A large number of caterpillars of the second generation, found at
the same time, pupated, and were kept at a high summer temperature.
After a pupal period of about 19 days, some 70 butterflies emerged from
28th June to 5th July, all of the _Prorsa_ form, with the exception of
5, which were strongly marked with yellow (_Porima_).

6. The 70 butterflies of the foregoing experiment were placed in an
enclosure 6 feet high, and 8 feet long, in which, during warm weather,
they freely swarmed on flowers. Copulation was only once observed, and
but one female laid eggs on nettle on July 4th. At the high summer
temperature prevailing at the time, these eggs produced butterflies
after 30-31 days (third generation). All were _Prorsa_, with more or
less yellow; among 18 none were completely _Porima_.

7. Young larvæ of the fourth generation, found on the 8th of August,
were reared in a hothouse (17°-20° R.). They pupated on 21st-23rd
August. Of these:--

A. 56 pupæ were placed on ice (0°-1° R.) for five weeks, and then
allowed to hibernate in a room not warmed. In April, 1870, they all
gave the _Levana_ form, with the exception of a single _Porima_.

B. About an equal number of pupæ were placed in the hothouse, but
without any result; for, notwithstanding a temperature of 12°-24° R.,
not a single butterfly emerged in the course of October and November.
The pupæ were then allowed to hibernate in an unheated room, and in
April and May gave nothing but _Levana_.

8. Caterpillars of the second generation, found at the beginning of
June, 1870, pupated on 13th-15th June, and gave, at the ordinary
temperature, on June 29th-30th, 7 butterflies of the _Prorsa_ form.

9. Pupæ of the same (second) generation were placed immediately after
pupation on June 18th, 1870, in a refrigerator (0°-1° R.), and after
remaining there four weeks (till July 18th) gave, at the ordinary
summer temperature:--

  On the 22nd of July, 2 _Prorsa_.
      ”  23rd     ”    3    ”
      ”  24th     ”    6 _Porima_, 4 of which were
                           very similar to _Levana_.
      ”  25th     ”    1 _Levana_, without the blue
                           marginal line.
      ”  26th     ”    2 _Levana_, also without the
                           blue marginal line.
      ”  2nd August,   6 _Porima_.
                      --
         Total        20

Of these 20 butterflies only 5 were of the pure _Prorsa_ form.

10. Full grown larvæ of the fourth generation, found on August 20th,
1870, pupated on August 26th to September 5th. The pupæ were divided
into three portions:--

A. Placed in the hothouse (12°-25° R.), immediately after pupation and
left there till October 20th. Of about 40 pupæ only 4 emerged, 3 of
which were _Prorsa_ and 1 _Porima_. The remaining pupæ hibernated and
all changed into _Levana_ the following spring.

B. Kept in a room heated to 6°-15° R. from November. Not a single
specimen emerged the same year. This lot of pupæ were added to C from
November.

C. Placed on ice for a month immediately after pupation; then,
from September 28th to October 19th in the hothouse, where no more
butterflies emerged. The pupæ hibernated, together with those from lot
B, in a room heated by water to 6°-15° R., and gave:--

  On the 6th of February,  1 female _Levana_.
    ”   22nd      ”        1 male _Levana_.
    ”   23rd      ”        1 male _Levana_.
    ”   24th      ”        1 female _Levana_.
    ”   25th      ”        1 male and 1 female _Levana_.
    ”   28th      ”        1 male and 1 female _Levana_.
    ”    1st of March,     1 male _Levana_.
    ”   13th      ”        1 female _Levana_.
    ”   15th      ”        1 female _Levana_.
    ”   19th      ”        1 male _Levana_.
    ”    2nd of April,     2 male and 1 female _Levana_.
    ”    7th      ”        1 female _Levana_.
    ”   21st      ”        1 female _Levana_.
    ”    2nd of May,       1 female _Levana_.
                          --
        Total             18 _Levana_, 10 of which were females.

The exact record of the time of emergence is interesting, because it is
thereby rendered apparent that different individuals respond more in
different degrees to a higher than to the ordinary temperature. Whilst
with many an acceleration of development of 1-2 months occurred, others
emerged in April and May, i.e. at the time of their appearance in the
natural state.

11. Reared the second generation from eggs of the first generation.
Emerged from the eggs on June 6th, 1872, pupated on July 9th. The pupæ
were placed on ice (0°-1° R.) from July 11th till September 11th, and
then transferred to a hothouse, where all emerged:--

  On the 19th of September,   3  male _Prorsa_, 1 male _Porima_.
     ”   21st       ”        13  _Porima_ (12 males, 1 female),
                                   2 female _Levana_.
     ”   22nd       ”        14  _Porima_ (12 males, 2 females)
                                   and 1 female _Levana_.
     ”   23rd       ”        10  female _Levana_, 3 male _Porima_.
     ”   24th       ”         5  female _Levana_.
     ”   25th       ”         1  female _Levana_.
     ”   27th       ”         3  female _Levana_.
     ”    4th of October,     1  male _Porima_.
                             --
             Total           57  butterflies (32 males and 25 females),
   only 3 of which were _Prorsa_, 32 _Porima_, and 22 _Levana_.

It must be pointed out, however, that among those specimens marked
as “_Levana_” there were none which entirely corresponded with the
natural _Levana_, or which indeed approximated so nearly to this form
as did some of the specimens in Exp. 9. All were larger than the
natural _Levana_, and possessed, notwithstanding the large amount of
yellow, more black than any true _Levana_. In all artificially bred
_Levana_ the black band of the basal half of the hind wings is always
interrupted with yellow, which is seldom the case with true _Levana_.
The whole appearance of the artificial _Levana_ is also coarser, and
the contour of the wings somewhat different, the fore-wings being
broader and less pointed. (See figs. 7 to 9, Pl. I.).

12. Larvæ of the fourth generation, found on September 22nd, 1872, were
divided into two portions:--

A. Placed for pupation in an orchid-house at 12°-25° R., and allowed
to remain there till December. In spite of the high temperature not
a single butterfly emerged during this time, whilst pupæ of _Vanessa
C-album_ and _Pyrameis Atalanta_, found at the same time, and placed in
the same hothouse, emerged in the middle of October. From the middle of
December the pupæ were kept in an unheated room, and they emerged very
late in the spring of 1873, all as _Levana_:--

  On the 6th of June, 7 _Levana_.
    ”    8th      ”   2    ”
    ”   11th      ”   2    ”
    ”   12th      ”   1    ”
    ”   15th      ”   6    ”
    ”   16th      ”   1    ”
    ”   19th      ”   2    ”
                     --
       Total         21    ”

B. Kept in an unheated room during the winter. The butterflies emerged
from the 28th of May, all as _Levana_.


EXPERIMENTS WITH PIERINÆ.

13. Females of _Pieris Rapæ_, captured in April, laid eggs on
_Sisymbrium Alliaria_. From these caterpillars were obtained, which
pupated on 1st-3rd June. The pupæ were placed on ice from June 3rd till
September 11th (0°-1° R.), and from September 11th till October 3rd in
the hothouse (12°-24° R.), where there emerged:--

  On the 23rd of October, 1 female.
     ”   24th      ”      1 female.
     ”   25th      ”      2 males, 1 female.
     ”   26th      ”      1 female.
     ”   28th      ”      1 male, 1 female.
                          -------------------
         Total            3 males, 5 females.

All these were sharply impressed with the characters of the winter
form, the females all strongly yellow on the upper side, the males pure
white; on the under side a strong black dusting on the hind wings,
particularly on the discoidal cell. One pupa did not emerge in the
hothouse, but hibernated, and gave in a heated room on January 20th,
1873, a female, also of the winter form.

14. Females of _Pieris Napi_, captured on 27th-28th April, 1872, laid
eggs on _Sisymbrium Alliaria_. The larvæ bred from these pupated on May
28th to June 7th. The pupæ, shortly after transformation, were placed
on ice, where they remained till Sept. 11th (three months). Transferred
to the hothouse on October 3rd, they produced, up to October 20th, 60
butterflies, all with the sharply-defined characters of the winter
form. The remaining pupæ hibernated in a room, and produced:--

  On the 28th of April, 3 males, 6 females.
     ”    4th of May,   1 female.
     ”   12th     ”     4 males.
     ”   15th     ”     1 male, 1 female.
     ”   16th     ”     1 male.
     ”   18th     ”     1 male, 1 female.
     ”   19th     ”     1 female.
     ”   20th     ”     2 males, 1 female.
     ”   23rd     ”     2 males.
     ”   26th     ”     1 male.
     ”   29th     ”     1 female.
     ”    3rd of June,  3 females.
     ”    6th     ”     1 female.
     ”    9th     ”     1 female.
     ”   21st     ”     1 female.
     ”    2nd of July,  1 female.
                       ---------------------
              Total    15 males, 19 females.

15. Several butterflies from Exp. 14, which emerged in May, 1873,
were placed in a capacious breeding-house, where they copulated and
laid eggs on rape. The caterpillars fed on the living plants in the
breeding-house, and after pupation were divided into two portions:--

A. Several pupæ, kept at the ordinary summer temperature, gave
butterflies on July 2nd, having the characters of the summer form.

B. The remainder of the pupæ were placed on ice immediately after
transformation, and remained over three months in the refrigerator
(from July 1st till October 10th). Unfortunately most of them perished
through the penetration of moisture into the box. Only 8 survived, 3
of which emerged on the 20th of October as the winter form; the others
hibernated in an unheated room, and emerged at the beginning of June,
1874. All 5 were females, and all exhibited the characters of the
winter form. Notwithstanding a pupal period of eleven months, they did
not possess these characters to a greater extent than usual, and did
not, therefore, approximate to the parent-form _Bryoniæ_.

16. On June 12th, 1871, specimens of _Pieris Napi_, var. _Bryoniæ_,
were captured on a mountain in the neighbourhood of Oberstorf
(Allgäuer Alpen), and placed in a breeding-house, where they flew
freely about the flowers; but although copulation did not take place,
several females laid eggs on the ordinary garden cabbage. From these
caterpillars were hatched, which at all stages of growth were exactly
like those of the ordinary form of _Napi_. They throve well until
shortly before pupation, when a fungoid epidemic decimated them, so
that from 300 caterpillars only about 40 living pupæ were obtained.
These also completely resembled the ordinary form of _Napi_, and showed
the same polymorphism, some being beautifully green, others (the
majority) straw yellow, and others yellowish grey. Only one butterfly
emerged the same summer, a male, which, by the black dusting of the
veins on the margin of the wings (upper side), could be with certainty
recognized as var. _Bryoniæ_. The remaining pupæ hibernated in a heated
room, and gave, from the end of January to the beginning of June, 10
males and 5 females, all with the characters of the var. _Bryoniæ_.
They emerged:--

  On the 22nd of January,  1 male.
    ”    26th      ”       1 male.
    ”     3rd of February, 1 male.
    ”     4th      ”       1 male.
    ”     5th      ”       1 male.
    ”     7th      ”       1 female.
    ”     9th      ”       1 male.
    ”    24th      ”       1 male.
    ”     4th of March,    1 female.
    ”    11th      ”       1 male, 1 female.
    ”     6th of April,    1 female.
    ”    17th      ”       1 male.
    ”    11th of May,      1 female.
    ”     3rd of June,     1 male.

We here perceive that the tendency to accelerate development through
the action of warmth is, in this case, also very different in different
individuals. Of the 16 butterflies only 1 kept to the normal period
of development from July 27th to June 3rd, fully ten months; all the
others had this period abbreviated, 1 male to eleven days, 8 specimens
to six months, 4 to seven months, 2 to eight months, and 1 to nine
months.




APPENDIX II.


EXPERIMENTS WITH PAPILIO AJAX.[58]

From eggs of var. _Telamonides_ laid on the last of May larvæ were
obtained, which gave on June 22nd-26th, 122 pupæ. These, as fast as
formed, were placed on ice in the refrigerator in small tin boxes, and
when all the larvæ had become transformed the pupæ were transferred
to a cylindrical tin box (4 in. diam. and 6 in. high), and packed in
layers between fine shavings. The tin box was set in a small wooden
one, which was put directly on the ice and kept there till July 20th.
From that date, by an unfortunate accident, the box, instead of being
kept on the surface of the ice in an ice-house, as was intended, was
placed on straw near the ice, so that the action of the cold was
modified, the outside pupæ certainly experiencing its full effects,
but the inside ones were probably at a somewhat higher temperature.
The ice failed on August 20th, so that the pupæ had been subjected
to an equable low temperature in the refrigerator for three to four
weeks, and to a lesser degree of cold in the ice-house for five weeks,
the temperature of the last place rising daily, as the ice had all
thawed by August 20th. On opening the box it was found (probably owing
to the cold not having been sufficiently severe) that the butterflies
had commenced to emerge. Twenty-seven dead and crippled specimens were
removed, together with several dead pupæ. One butterfly that had just
emerged was taken out and placed in a box, and when its wings had fully
expanded it was found to be a “_Telamonides_ of the most pronounced
type.” The experimenter then states:--“Early in the morning I made
search for the dead and rejected butterflies, and recovered a few. It
was not possible to examine them very closely from the wet and decayed
condition they were in, but I was able to discover the broad crimson
band which lies above the inner angle of the hind wings, and which is
usually lined on its anterior side with white, and is characteristic of
either _Walshii_ or _Telamonides_, but is not found in _Marcellus_. And
the tip only of the tail being white in _Walshii_, while both tip and
sides are white in _Telamonides_, enabled me to identify the form as
between these two. There certainly were no _Walshii_, but there seemed
to be a single _Marcellus_, and excepting that all were _Telamonides_.”

The remaining pupæ were kept in a light room where 3 _Telamonides_
emerged the following day, and by September 4th 14 specimens of the
same variety had emerged, but no _Marcellus_ or intermediate forms.
From the 4th to the 20th of September a few more _Telamonides_
appeared, but between the 4th and 15th of the month 12 out of 26
butterflies that had emerged were intermediate between _Telamonides_
and _Marcellus_, some approximating to one form and some to the other
form. The first pure _Marcellus_ appeared on September 4th, and was
followed by one specimen on the 6th, 8th, 13th and 15th respectively.
From this last date to October 3rd, 6 out of 10 were _Marcellus_ and
3 intermediate. On September 3rd, a specimen intermediate between
_Telamonides_ and _Walshii_ emerged, “in which the tails were white
tipped as in _Walshii_, but in size and other characters it was
_Telamonides_, though the crimson band might have belonged to either
form.” Butterflies continued to emerge daily up to September 20th,
after which date single specimens appeared at intervals of from four
to six days, the last emergence being on October 16th. Thus, from
the time the box was removed from the ice-house, the total period of
emerging was fifty-seven days, some specimens having emerged before the
removal of the box. With specimens of _P. Ajax_ which appear on the
wing the first season the natural pupal period is about fourteen days,
individuals rarely emerging after a period of four to six weeks.

Between August 20th and October 16th, the 50 following butterflies
emerged:--

  On the 20th of August,     1 male _Telamonides_.
    ”    21st       ”        1 male and 2 female _Telamonides_.
    ”    22nd       ”        1 female _Telamonides_.
    ”    24th       ”        1 female _Telamonides_.
    ”    29th       ”        1 male _Telamonides_.
    ”    31st       ”        1 female _Telamonides_.
    ”     1st of September,  1 female _Telamonides_.
    ”     2nd       ”        1 female _Telamonides_.
    ”     3rd       ”        1 female intermediate between
                               _Telamonides_ and _Walshii_.
    ”      ”        ”        1 male _Telamonides_.
    ”     4th       ”        4 males and 1 female _Telamonides_.
    ”      ”        ”        2 males, medium, nearest _Telamonides_.
    ”      ”        ”        2 males, medium, nearest _Marcellus_.
    ”      ”        ”        2 males, _Marcellus_.
    ”     5th       ”        1 male and 1 female _Telamonides_.
    ”      ”        ”        1 male medium, nearest _Telamonides_.
    ”     6th       ”        1 male _Marcellus_.
    ”     7th       ”        1 male _Telamonides_.
    ”     8th       ”        1 male _Marcellus_ and 1 female
                               _Telamonides_.
    ”     9th       ”        1 male _Marcellus_ and 1 female medium,
                               nearest _Marcellus_.
    ”    13th       ”        1 male medium, nearest _Marcellus_.
    ”     ”         ”        1 male medium, nearest _Telamonides_.
    ”     ”         ”        1 male _Marcellus_.
    ”    14th       ”        1 male _Marcellus_ and 1 female medium,
                               nearest _Marcellus_.
    ”     ”         ”        1 male medium, nearest _Telamonides_.
    ”    15th       ”        1 male _Marcellus_.
    ”    16th       ”        1 female _Marcellus_ and 1 male
                               _Telamonides_.
    ”    18th       ”        1 male medium, nearest _Marcellus_.
    ”    19th       ”        1 female _Marcellus_.
    ”    20th       ”        1 male _Telamonides_.
    ”    24th       ”        1 male _Marcellus_.
    ”    30th       ”        1 female _Marcellus_.
    ”     2nd of October,    1 female _Marcellus_.
    ”     3rd       ”        1 female medium, nearest _Telamonides_.
    ”     8th       ”        1 female medium, nearest _Telamonides_.
    ”    16th       ”        1 female medium, nearest _Telamonides_.

                        Total.

  _Telamonides_                   22  12 males, 10 females.
  _Telamonides_ partly _Walshii_   1             1 female.
  Medium, nearest _Telamonides_    8   5 males,  3 females.
  Medium, nearest _Marcellus_      6   4 males,  2 females.
  _Marcellus_                     13   9 males,  4 females.

                                  50  30 males, 20 females.

All these butterflies were very uniform in size, being about that of
the ordinary _Telamonides_. The specimens of _Telamonides_ especially
were “strongly marked, the crimson band in a large proportion of them
being as conspicuous as is usual in _Walshii_, and the blue lunules
near the tail were remarkably large and bright coloured. Of the
_Marcellus_, in addition to the somewhat reduced size, the tails were
almost invariably shorter than usual and narrower, and instead of the
characteristic single crimson spot, nearly all had two spots, often
large. In all these particulars they approach _Telamonides_.”

Adding to the _Telamonides_ which emerged after August 20th most of
those specimens which were found dead in the box at that date, the
total number of this form is thus brought up to nearly 50. Of the 122
pupæ with which Mr. Edwards started, 28 remained in a state fit for
hibernation, several having died without emerging. Previous experiments
had shown that 28 out of 122 pupæ is not an unreasonable number to
hibernate, so that the author concludes that the butterflies which
emerged the same season would have done so naturally, and the effect
of the artificial cold was not “to precipitate the emerging of any
which would have slept” till the following spring. Now under ordinary
circumstances all the butterflies which emerged the same season would
have been of the _Marcellus_ form, so that the cold changed a large
part of these into the form _Telamonides_, some (probably from those
pupæ which experienced the lowest temperature) being completely
changed, and others (from those pupæ which were only imperfectly
subjected to the cold) being intermediate, _i.e._, only partly changed.
It appears also that several pupæ experienced sufficient cold to retard
their emergence and stunt their growth, but not enough to change their
form, these being the 13 recorded specimens of _Marcellus_. Had the
degree of cold been equal and constant, the reversion would probably
have been more complete. The application of cold produced great
confusion in the duration of the pupal period, the emergence, instead
of taking place fourteen days after the withdrawal of the cold, as
might have been expected from Dr. Weismann’s corresponding experiment
with _Pieris Napi_ (Appendix I. Exps. 13 and 14), having been extended
over more than two months.

From the results of this experiment it must be concluded that
_Telamonides_ is the primary form of the species.


ADDITIONAL EXPERIMENTS WITH PAPILIO AJAX.

[_Communicated by_ Mr. W. H. EDWARDS, _November 18th, 1879_.]

EXP. 1.--In 1877 chrysalides of _P. Ajax_ and _Grapta Interrogationis_
(the eggs laid by females of the form _Fabricii_) were experimented
upon; but the results were not satisfactory, for the reason that the
author having been absent from home most of the time while the pupæ
were in the ice-box, on his return found the temperature above 5°-6°
R. And so far as could be told, the ice had been put in irregularly,
and there might have been intervals during which no ice at all was in
the box. Six chrysalides of the _Grapta_ so exposed produced unchanged
_Umbrosa_, the co-form with _Fabricii_. But all chrysalides from the
same lot of eggs, and not exposed to cold, also produced _Umbrosa_.
Nothing was learnt, therefore, respecting this species.

But chrysalides of _Ajax_, exposed at same time, did give changed
butterflies to some extent. From a lot of 8, placed in the box when
under twelve hours from pupation, and left for twenty-four days,
there came 5 males and 3 females. Of these was 1 _Telamonides_ in
markings and coloration, and all the rest were between _Marcellus_ and
_Telamonides_. Two other chrysalides on ice for twenty-three days gave
_Telamonides_, but 3 more exposed twenty-six days, and all one hour old
when put on ice, were unchanged, producing _Marcellus_.

During the same season 6 other _Ajax_ chrysalides were placed in the
box, and kept at about 0°-1° R. One was one hour old, and remained
for five days; 1 was one hour old, and remained for two days and
three-quarters; 3 at three hours old for eight days; and 1 (age
omitted), six days. All these gave unchanged butterflies of the form
_Marcellus_.

EXP. 2.--In May, 1878, many chrysalides were placed in the ice-box,
being from eggs laid by _Ajax_, var. _Walshii_. The youngest were
but ten to fifteen minutes from pupation, and were soft; others at
intervals up to twenty-four hours (the chrysalis is hard at about
twelve hours); after that, each day up to eight days after pupation.
All were removed from the box on the same day, 28th May. The exposure
was from nineteen to five days, those chrysalides which were put on ice
latest having the shortest exposure. The author wished to determine if
possible whether, in order to effect any change, it was necessary that
cold should be applied immediately after pupation or if one or several
days might intervene between pupation and refrigeration. Inasmuch as no
colour begins to show itself in the pupæ till a few hours, or at most
a day or two, before the butterfly emerges, it was thought possible
that cold applied shortly before that time would be quite as effective
as if applied earlier and especially very soon after pupation. The
result was, that more than half of the chrysalides exposed before they
had hardened died: 1 exposed at ten minutes, 2 at one hour, 1 at two
hours, 2 at three hours after pupation. On the other hand 1 at fifteen
minutes produced a butterfly, 1 at two hours, another at twelve hours.
The temperature was from 0°-1° R. most of the time, but varied somewhat
each day as the ice melted. The normal chrysalis period is from eleven
to fourteen days, in case the butterfly emerges the same season, but
very rarely an individual will emerge several weeks after pupation.

  On the  14th day after taking the pupæ from the ice, 1 _Telamonides_
          emerged from a chrysalis which had been placed in the ice-box
          three days after pupation, and was on ice sixteen days.

  On 19th day, 1 _Telamonides_ emerged from a pupa put on the ice
          twelve hours after pupation, and kept there eleven days.

  On 19th day, 1 _Walshii_ emerged from a pupa two hours old, and on
          ice eleven days.

All the rest emerged _Marcellus_, unchanged, but at periods prolonged
in a surprising way.

  1 on 43rd day exposed 15 minutes after pupation.
    ”  46th       ”      2 hours           ”
    ”  53rd       ”     24 hours           ”
    ”  62nd       ”      6 days            ”
    ”  63rd       ”      4 days            ”
    ”  66th       ”      7 days            ”
    ”  77th       ”      4 days            ”
    ”  81st       ”     12 hours           ”
    ”  91st       ”      5 days            ”
    ”  96th       ”     19 hours           ”

Five chrysalides lived through the winter, and all gave _Telamonides_
in the spring of 1879.

It appeared, therefore, that the only effect produced by cold in all
chrysalides exposed more than three days after pupation was to retard
the emergence of the butterfly. But even in some of these earliest
exposed, and kept on the ice for full nineteen days, the only effect
seemed to be to retard the butterfly.

EXP. 3.--In June, 1879, eggs of the form _Marcellus_ were obtained, and
in due time gave 104 chrysalides. Of these one-third were placed in the
ice-box at from twelve to twenty-four hours after pupation, and were
divided into 3 lots.

  1st,  9 pupæ, kept on ice 14 days.
  2nd, 12   ”        ”      20 days.
  3rd, 11   ”        ”      25 days.

Temperature 0°-1° R. most of the time, but varying somewhat as the ice
melted. (Both in 1878 and 1879 Mr. Edwards watched the box himself, and
endeavoured to keep a low temperature.)

Of the 69 chrysalides not exposed to cold, 34 gave butterflies at from
eleven to fourteen days after pupation, and 1 additional male emerged
11th August, or twenty-two days at least past the regular period of the
species.

Of the iced chrysalides, from lot No. 1 emerged 4 females at eight days
and a half to nine days and a half after removal from the ice, and 5
are now alive (Nov. 18) and will go over the winter.

From lot No. 2 emerged 1 male and 5 females at eight to nine days;
another male came out at forty days; and 5 will hibernate.

From lot No. 3 emerged 4 females at nine to twelve days; another male
came out at fifty-four days; and 6 were found to be dead.

In this experiment the author wished to see as exactly as
possible--First, in what points changes would occur. Second, if there
would be any change in the shape of the wings, as well as in markings
or coloration--that is, whether the shape might remain as that of
_Marcellus_, while the markings might be of _Telamonides_ or _Walshii_;
a summer form with winter markings. Third, to ascertain more closely
than had yet been done what length of exposure was required to bring
about a decided change, and what would be the effect of prolonging
this period. After the experiments with _Phyciodes Tharos_, which had
resulted in a suffusion of colour, the author hoped that some similar
cases might be seen in _Ajax_. The decided changes in 1878 had been
produced by eleven and sixteen days’ cold. In 1877, an exposure of two
days and three-quarters to eight days had failed to produce an effect.

From these chrysalides 11 perfect butterflies were obtained, 1 male and
10 females. Some emerged crippled, and these were rejected, as it was
not possible to make out the markings satisfactorily.

From lot No. 1, fourteen days, came:--

  1 female between _Marcellus_ and _Telamonides_.
  2 females, _Marcellus_.

These 2 _Marcellus_ were pale coloured, the light parts a dirty white;
the submarginal lunules on hind wings were only two in number and
small; at the anal angle was one large and one small red spot; the
frontal hairs were very short. The first, or intermediate female, was
also pale black, but the light parts were more green and less sordid;
there were 3 large lunules; the anal red spot was double and connected,
as in _Telamonides_; the frontal hairs short, as in _Marcellus_. These
are the most salient points for comparing the several forms of _Ajax_.
In nature, there is much difference in shape between _Marcellus_ and
_Telamonides_, still more between _Marcellus_ and _Walshii_; and the
latter may be distinguished from the other winter forms by the white
tips of the tails. It is also smaller, and the anal spot is larger,
with a broad white edging.

From lot No. 2, twenty days, came:--

  1 female _Marcellus_, with single red spot.

  1 female between _Marcellus_ and _Telamonides_; general coloration
    pale; the lunules all obsolescent; 2 large red anal spots not
    connected; frontal hairs medium length, as in _Telamonides_.

  1 female between _Marcellus_ and _Telamonides_; colour bright and
    clear; 3 lunules; 2 large red spots; frontal hairs short.

  1 female _Telamonides_; colours black and green; 4 lunules; a large
    double and connected red spot; frontal hairs medium.

  2 female _Telamonides_; colours like last; 3 and 4 lunules; 2 large
    red spots; frontal hairs medium.

From lot No. 3, twenty-five days, came:--

  1 male _Telamonides_; clear colours; 4 large lunules; 1 large, 1
    small red spot; frontal hairs long.

  1 female _Telamonides_; medium colours; 4 lunules; large double
    connected red spot; frontal hairs long.

In general shape all were _Marcellus_, the wings produced, the tails
long.

From this it appeared that those exposed twenty-five days were fully
changed; of those exposed twenty days, 3 were fully, 2 partly, 1 not at
all; and of those exposed fourteen days, 1 partly, 2 not at all.

The butterflies from this lot of 104 chrysalides, but which had not
been iced, were put in papers. Taking 6 males and 6 females from the
papers just as they came to hand, Mr. Edwards set them, and compared
them with the iced examples.

Of the 6 males, 4 had 1 red anal spot only, 2 had 1 large 1 small; 4
had 2 green lunules on the hind wings, 2 had 3, and in these last
there was a 4th obsolescent, at outer angle; all had short frontal
hairs.

Of the 6 females, 5 had but 1 red spot, 1 had 1 large 1 small spot; 5
had 2 lunules only, 1 had 3; all had short frontal hairs.

Comparing 6 of the females from the iced chrysalides, being those in
which a change had more or less occurred, with the 6 females not iced:

  1. All the former had the colours more intense, the black deeper,
      the light, green.

  2. In 5 of the former the green lunules on hind wings were decidedly
      larger; 3 of the 6 had 4 distinct lunules, 1 had 3, 1 had 3, and
      a 4th obsolescent. Of the 6 females not iced none had 4, 2 had
      2, and a 3rd, the lowest of the row, obsolescent; 3 had 3, the
      lowest being very small; one had 3, and a 4th, at outer angle,
      obsolescent.

  3. In all the former the subapical spot on fore wing and the stripe
      on same wing which crosses the cell inside the common black band,
      were distinct and green; in all the latter these marks were
      either obscure or obsolescent.

  4. In 4 of the former there was a large double connected red spot,
      and in one of the 4 it was edged with white on its upper side;
      2 had 1 large and 1 small red spot. Of the latter 5 had 1 spot
      only, and the 6th had 1 spot and a red dot.

  5. The former had all the black portions of the wing of deeper
      colour but less diffused, the bands being narrower; on the other
      hand, the green bands were wider as well as deeper coloured.
      Measuring the width of the outermost common green band along the
      middle of the upper medium interspace on fore wing in tenths of a
      millimetre, it was found to be as follows:

  On the iced pupæ    81, 66, 76, 76, 66, 66.
  On the not iced     56, 56, 51, 51, 46, 51.

Measuring the common black discal band across the middle of the lower
medium interspace on fore wing:

  On the iced pupæ    51, 66, 51, 51, 56, 61.
  On the not iced     76, 71, 66, 63, 71, 76.

In other words the natural examples were more melanic than the others.

No difference was found in the length of the tails or in the length and
breadth of wings. In other words, the cold had not altered the shape of
the wings.

Comparing 1 male iced with 6 males not iced:

  1. The former had a large double connected red anal spot, edged with
      white scales at top. Of the 6 not iced, 3 had but 1 red spot, 2
      had 1 large 1 small, 1 had 1 large and a red dot.

  2. The former had 4 green lunules; of the latter 3 had 3, 3 had
      only 2.

  3. The former had the subapical spot and stripe in the cells clear
      green; of the latter 1 had the same, 5 had these obscure or
      obsolescent.

  4. The colours of the iced male were bright; of the others, 2 were
      the same, 4 had the black pale, the light sordid white or
      greenish-white.

Looking over all, male and female, of both lots, the large size of the
green submarginal lunules on the fore wings in the iced examples was
found to be conspicuous as compared with all those not iced, though
this feature is included in the general widening of the green bands
spoken of.

In all the experiments with _Ajax_, if any change at all has been
produced by cold, it is seen in the enlarging or doubling of the red
anal spot, and in the increased number of clear green lunules on the
hind wings. Almost always the frontal hairs are lengthened and the
colour of the wings deepened, and the extent of the black area is also
diminished. All these changes are in the direction of _Telamonides_, or
the winter form.

That the effect of cold is not simply to precipitate the appearance of
the winter form, causing the butterfly to emerge from the chrysalis
in the summer in which it began its larval existence instead of the
succeeding year, is evident from the fact that the butterflies come
forth with the shape of _Marcellus_, although the markings may be of
_Telamonides_ or _Walshii_. And almost always some of the chrysalides,
after having been iced, go over the winter, and then produce
_Telamonides_, as do the hibernating pupæ in their natural state. The
cold appears to have no effect on these individual chrysalides.[59]

With every experiment, however similar the conditions seem to be, and
are intended to be, there is a difference in results; and further
experiments--perhaps many--will be required before the cause of this
is understood. For example, in 1878, the first butterfly emerged on
the fourteenth day after removal from ice, the period being exactly
what it is (at its longest) in the species in nature. Others emerged
at 19-96 days. In 1879, the emergence began on the ninth day, and by
the twelfth day all had come out, except three belated individuals,
which came out at twenty, forty, and fifty-four days. In the last
experiment, either the cold had not fully suspended the changes which
the insect undergoes in the chrysalis, or its action was to hasten them
after the chrysalides were taken from the ice. In the first experiment,
apparently the changes were absolutely suspended as long as the cold
remained.

It might be expected that the application of heat to the hibernating
chrysalides would precipitate the appearance of the summer form, or
change the markings of the butterfly into the summer form, even if the
shape of the wings was not altered; that is, to produce individuals
having the winter shape but the summer markings. But this was not
found to occur. Mr. Edwards has been in the habit for several years of
placing the chrysalides in a warm room, or in the greenhouse, early in
the winter, thus causing the butterflies to emerge in February, instead
of in March and April, as otherwise they would do. The heat in the
house is 19° R. by day, and not less than 3.5° R. by night. But the
winter form of the butterfly invariably emerged, usually _Telamonides_,
occasionally _Walshii_.


EXPERIMENTS WITH PHYCIODES THAROS.

EXP. 1.--In July, 1875, eggs of _P. Tharos_ were obtained on _Aster
Nova-Angliæ_ in the Catskill Mountains, and the young larvæ, when
hatched, taken to Coalburgh, West Virginia. On the journey the larvæ
were fed on various species of _Aster_, which they ate readily. By the
4th of September they had ceased feeding (after having twice moulted),
and slept. Two weeks later part of them were again active, and fed for
a day or two, when they gathered in clusters and moulted for the third
time, then becoming lethargic, each one where it moulted with the cast
skin by its side. The larvæ were then placed in a cellar, where they
remained till February 7th, when those that were alive were transferred
to the leaves of an _Aster_ which had been forced in a greenhouse, and
some commenced to feed the same day. In due time they moulted twice
more, making, in some cases, a total of five moults. On May 5th the
first larva pupated, and its butterfly emerged after thirteen days.
Another emerged on the 30th, after eight days pupal period, this stage
being shortened as the weather became warmer. There emerged altogether
8 butterflies, 5 males and 3 females, all of the form _Marcia_, and
all of the variety designated C, except 1 female, which was var. B.[60]

EXP. 2.--On May 18th the first specimens (3 male _Marcia_) were seen on
the wing at Coalburgh; 1 female was taken on the 19th, 2 on the 23rd,
and 2 on the 24th, these being all that were seen up to that date, but
shortly after both sexes became common. On the 26th, 7 females were
captured and tied up in separate bags on branches of _Aster_. The next
day 6 out of the 7 had laid eggs in clusters containing from 50 to
225 eggs in each. Hundreds of caterpillars were obtained, each brood
being kept separate, and the butterflies began to emerge on June 29th,
the several stages being:--egg six days, larva twenty-two, chrysalis
five. Some of the butterflies did not emerge till the 15th of July.
Just after this date one brood was taken to the Catskills, where they
pupated, and in this state were sent back to Coalburgh. There was no
difference in the length of the different stages of this brood and the
others which had been left at Coalburgh, and none of either lot became
lethargic. The butterflies from these eggs of May were all _Tharos_,
with the exception of 1 female _Marcia_, var. C. Thus the first
generation of _Marcia_ from the hibernating larvæ furnishes a second
generation of _Tharos_.

EXP. 3.--On July 16th, at Coalburgh, eggs were obtained from several
females, all _Tharos_, as no other form was flying. In four days the
eggs hatched; the larval stage was twenty-two, and the pupal stage
seven days; but, as before, many larvæ lingered. The first butterfly
emerged on August 18th. All were _Tharos_, and none of the larvæ had
been lethargic. This was the third generation from the second laying of
eggs.

EXP. 4.--On August 15th, at Coalburgh, eggs were obtained from a female
_Tharos_, and then taken directly to the Catskill Mountains, where they
hatched on the 20th. This was the fourth generation from the third
laying of eggs. In Virginia, and during the journey, the weather had
been exceedingly warm, but on reaching the mountains it was cool, and
at night decidedly cold. September was wet and cold, and the larvæ
were protected in a warm room at night and much of the time by day, as
they will not feed when the temperature is less than about 8° R. The
first pupa was formed September 15th, twenty-six days from the hatching
of the larvæ, and others at different dates up to September 26th, or
thirty-seven days from the egg. Fifty-two larvæ out of 127 became
lethargic after the second moult on September 16th, and on September
26th fully one half of these lethargic larvæ commenced to feed again,
and moulted for the third time, after which they became again lethargic
and remained in this state. The pupæ from this batch were divided into
three portions:--

A. This lot was brought back to Coalburgh on October 15th, the weather
during the journey having been cold with several frosty nights, so
that for a period of thirty days the pupæ had at no time been exposed
to warmth. The butterflies began to emerge on the day of arrival, and
before the end of a week all that were living had come forth, viz., 9
males and 10 females. “Of these 9 males 4 were changed to _Marcia_ var.
C, 3 were var. D, and 2 were not changed at all. Of the 10 females 8
were changed, 5 of them to var. B, 3 to var. C. The other 2 females
were not different from many _Tharos_ of the summer brood, having large
discal patches on under side of hind wing, besides the markings common
to the summer brood.”

B. This lot, consisting of 10 pupæ, was sent from the Catskills to
Albany, New York, where they were kept in a cool place. Between October
21st and Nov. 2nd, 6 butterflies emerged, all females, and all of the
var. B. Of the remaining pupæ 1 died, and 3 were alive on December
27th. According to Mr. Edwards this species never hibernates in the
pupal state in nature. The butterflies of this lot were more completely
changed than were those from the pupæ of lot A.

C. On September 20th 18 of the pupæ were placed in a tin box directly
on the surface of the ice, the temperature of the house being 3°-4° R.
Some were placed in the box within three hours after transformation and
before they had hardened; others within six hours, and others within
nine hours. They were all allowed to remain on the ice for seven days,
that being the longest summer period of the chrysalis. On being removed
they all appeared dead, being still soft, and when they had become
hard they had a shrivelled surface. On being brought to Coalburgh they
showed no signs of life till October 21st, when the weather became
hot (24°-25° R.), and in two days 15 butterflies emerged, “every one
_Marcia_, not a doubtful form among them in either sex.” Of these
butterflies 10 were males and 5 females; of the former 5 were var. C, 4
var. D, and 1 var. B, and of the latter 1 was var. C, and 4 var. D. The
other 3 pupæ died. All the butterflies of this brood were diminutive,
starved by the cold, but those from the ice were sensibly smaller
than the others. All the examples of var. B were more intense in the
colouring of the under surface than any ever seen by Mr. Edwards in
nature, and the single male was as deeply coloured as the females, this
also never occurring in nature.

Mr. Edwards next proceeds to compare the behaviour of the Coalburgh
broods with those of the same species in the Catskills:--

EXP. 5.--On arriving at the Catskills, on June 18th, a few male
_Marcia_, var. D, were seen flying, but no females. This was exactly
one month later than the first males had been seen at Coalburgh. The
first female was taken on June 26th, another on June 27th, and a
third on the 28th, all _Marcia_, var. C. Thus the first female was
thirty-eight days later than the first at Coalburgh. No more females
were seen, and no _Tharos_. The three specimens captured were placed
on _Aster_, where two immediately deposited eggs[61] which were
forwarded to Coalburgh, where they hatched on July 3rd. The first
chrysalis was formed on the 20th, its butterfly emerging on the 29th,
so that the periods were: egg six, larva seventeen, pupa nine days.
Five per cent. of the larvæ became lethargic after the second moult.
This was, therefore, the second generation of the butterfly from the
first laying of eggs. All the butterflies which emerged were _Tharos_,
the dark portions of the wings being intensely black as compared with
the Coalburgh examples, and other differences of colour existed, but
the general peculiarities of the _Tharos_ form were retained. This
second generation was just one month behind the second at Coalburgh,
and since, in 1875, eggs were obtained by Mr. Mead on July 27th and
following days, the larvæ from which all hibernated, this would be
the second laying of eggs, and the resulting butterflies the first
generation of the following season.

Thus in the Catskills the species is digoneutic, the first generation
being _Marcia_ (the winter form), and the second the summer form. A
certain proportion of the larvæ from the first generation hibernate,
and apparently all those from the second.

_Discussion of Results._--There are four generations of this butterfly
at Coalburgh, the first being _Marcia_ and the second and third
_Tharos_. None of the larvæ from these were found to hibernate. The
fourth generation under the exceptional conditions above recorded (Exp.
4) produced some _Tharos_ and more _Marcia_ the same season, a large
proportion of the larvæ also hibernating. Had the larvæ of this brood
been kept at Coalburgh, where the temperature remained high for several
weeks after they had left the egg, the resulting butterflies would have
been all _Tharos_, and the larvæ from their eggs would have hibernated.

The altitude of the Catskills, where Mr. Edwards was, is from 1650
to 2000 feet above high water, and the altitude of Coalburgh is 600
feet. The transference of the larvæ from the Catskills to Virginia
(about 48° lat.) and _vice-versa_, besides the difference of altitude,
had no perceptible influence on the butterflies of the several broods
except the last one, in which the climatic change exerted a direct
influence on part of them both as to form and size. The stages of
the June Catskill brood may have been accelerated to a small extent
by transference to Virginia, but the butterflies reserved their
peculiarities of colour. (See Exp. 5.) So also was the habit of
lethargy retained.[62] The May brood, on the other hand, taken from
Virginia to the Catskills, suffered no retardation of development.
(See Exp. 2.) It might have been expected that all the larvæ of this
last brood taken to the mountains would have become lethargic, but
the majority resisted this change of habit. From all these facts it
may be concluded “that it takes time to naturalize a stranger, and
that habits and tendencies, even in a butterfly, are not to be changed
suddenly.”[63]

It has been shown that _Tharos_ is digoneutic in the Catskills and
polygoneutic in West Virginia, so that at a great altitude, or in a
high latitude, we might expect to find the species monogoneutic and
probably restricted to the winter form _Marcia_. These conditions are
fulfilled in the Island of Anticosti, and on the opposite coast of
Labrador (about lat. 50°), the summer temperature of these districts
being about the same. Mr. Edwards states, on the authority of Mr.
Cooper, who collected in the Island, that _Tharos_ is a rare species
there, but has a wide distribution. No specimens were seen later
than July 29, after which date the weather became cold, and very few
butterflies of any sort were to be seen. It seems probable that none of
the butterflies of Anticosti or Labrador produce a second brood. All
the specimens examined from these districts were of the winter form.

In explanation of the present case Dr. Weismann wrote to Mr.
Edwards:--“_Marcia_ is the old primary form of the species, in the
glacial period the only one. _Tharos_ is the secondary form, having
arisen in the course of many generations through the gradually working
influence of summer heat. In your experiments cold has caused the
summer generation to revert to the primary form. The reversion which
occurred was complete in the females, but not in all the males. This
proves, as it appears to me, that the males are changed or affected
more strongly by the heat of summer than the females. The secondary
form has a stronger constitution in the males than in the females. As
I read your letter, it at once occurred to me whether in the spring
there would not appear some males which were not pure _Marcia_, but
were of the summer form, or nearly resembling it; and when I reached
the conclusion of the letter I found that you especially mentioned that
this was so! And I was reminded that the same thing is observable in
_A. Levana_, though in a less striking degree. If we treated the summer
brood of _Levana_ with ice, many more females than males would revert
to the winter form. This sex is more conservative than the male--slower
to change.”

The extreme variability of _P. Tharos_ was alluded to more than a
century ago by Drury, who stated:--“In short, Nature forms such a
variety of this species that it is difficult to set bounds, or to know
all that belongs to it.” Of the different named varieties, according
to Mr. Edwards, “A appears to be an offset of B, in the direction
most remote from the summer form, just as in _Papilio Ajax_, the var.
_Walshii_ is on the further side of _Telamonides_, remote from the
summer form _Marcellus_.” Var. C leads from B through D directly to the
summer form, whilst A is more unlike this last variety than are several
distinct species of the genus, and would not be suspected to possess
any close relationship were it not for the intermediate forms. The
var. B is regarded as nearest to the primitive type for the following
reasons:--In the first place it is the commonest form, predominating
over all the other varieties in W. Virginia, and moreover appears
constantly in the butterflies from pupæ submitted to refrigeration.
Its distinctive peculiarity of colour occurs in the allied species
_P. Phaon_ (Gulf States) and _P. Vesta_ (Texas), both of which are
seasonally dimorphic, and both apparently restricted in their winter
broods to the form corresponding to B of _Tharos_. In their summer
generation both these species closely resemble the summer form of
_Tharos_, and it is remarkable that these two species, which are the
only ones (with the exception of the doubtful _Batesii_) closely allied
to _Tharos_, should alone be seasonally dimorphic out of the large
number of species in the genus.

Mr. Edwards thus explains the case under consideration:--“When _Phaon_,
_Vesta_, and _Tharos_ were as yet only varieties of one species, the
sole coloration was that now common to the three. As they gradually
became permanent, or in other words, as these varieties became species,
_Tharos_ was giving rise to several sub-varieties, some of them in time
to become distinct and well marked, while the other two, _Phaon_ and
_Vesta_, remained constant. As the climate moderated and the summer
became longer, each species came to have a summer generation; and in
these the resemblance of blood-relationship is still manifest. As the
winter generations of each species had been much alike, so the summer
generations which sprung from them were much alike. And if we consider
the metropolis of the species _Tharos_, or perhaps of its parent
species, at the time when it had but one annual generation, to have
been somewhere between 37° and 40° on the Atlantic slope, and within
which limits all the varieties and sub-varieties of both winter and
summer forms of _Tharos_ are now found in amazing luxuriance, we can
see how it is possible, as the glacial cold receded, that only part of
the varieties of the winter form might spread to the northward, and but
one of them at last reach the sub-boreal regions and hold possession to
this day as the sole representative of the species. And at a very early
period the primary form, together with _Phaon_ and _Vesta_, had made
its way southward, where all three are found now.”


EXPERIMENTS WITH GRAPTA INTERROGATIONIS.[64]

[_Communicated by_ Mr. W. H. EDWARDS, _November 15th, 1879_.]

The experiments with this species were made in June, 1879, on pupæ
from eggs laid by the summer form _Umbrosa_ of the second brood of the
year, and obtained by confining a female in a bag on a stem of hop.
As the pupæ formed, and at intervals of from six to twenty-four hours
after pupation (by which time all the older ones had fully hardened),
they were placed in the ice-box. In making this experiment Mr. Edwards
had three objects in view. 1st. To see whether it was essential that
the exposure should take place immediately after pupation, in order to
effect any change. 2ndly. To see how short a period would suffice to
bring about any change. 3rdly. Whether exposing the summer pupæ would
bring about a change in the form of the resulting butterfly. Inasmuch
as breeding from the egg of _Umbrosa_, in June, in a former year,[64]
gave both _Umbrosa_ (11) and _Fabricii_ (6), the butterflies from the
eggs obtained, if left to nature, might be expected to be of both
forms. The last or fourth brood of the year having been found up to the
present time to be _Fabricii_, and the 1st brood of the spring, raised
from eggs of _Fabricii_ (laid in confinement), having been found to be
wholly _Umbrosa_, the latter is probably the summer and _Fabricii_ the
winter form. The two intervening broods, _i.e._ the 2nd and 3rd, have
yielded both forms. This species hibernates in the imago state.

After the pupæ had been in the ice-box fourteen days they were all
removed but 5, which were left in six days longer. Several were dead at
the end of fourteen days. The temperature most of the time was 1°-2°
R.; but for a few hours each day rose as the ice melted, and was found
to be 3°-6° R.

From the fourteen-day lot 7 butterflies were obtained, 3 males and
4 females. From the twenty-day lot 4 males and 1 female; every one
_Umbrosa_. All had changed in one striking particular. In the normal
_Umbrosa_ of both sexes,[65] the fore wings have on the upper side on
the costal margin next inside the hind marginal border, and separated
from it by a considerable fulvous space, a dark patch which ends a
little below the discoidal nervule; inside the same border at the inner
angle is another dark patch lying on the submedian interspace. Between
these two patches, across all the median interspaces, the ground-colour
is fulvous, very slightly clouded with dark.

In all the 4 females exposed to cold for fourteen days a broad black
band appeared crossing the whole wing, continuous, of uniform shade,
covering the two patches, and almost confluent from end to end with the
marginal border, only a streak of obscure fulvous anywhere separating
the two. In the case of the females from pupæ exposed for twenty days,
the band was present, but while broad, and covering the space between
the patches, it was not so dark as in the other females, and included
against the border a series of obscure fulvous lunules. This is just
like many normal females, and this butterfly was essentially unchanged.

In all the males the patches were diffuse, that at the apex almost
coalescing with the border. In the 3 from chrysalides exposed fourteen
days these patches were connected by a narrow dark band (very different
from the broad band of the females), occupying the same position as
the clouding of the normal male, but blackened and somewhat diffused.
In the 4 examples from the twenty-day pupæ, this connecting band was
scarcely as deeply coloured and continuous as in the other 3. Beyond
this change on the submarginal area, whereby a band is created where
naturally would be only the two patches, and a slight clouding of the
intervening fulvous surfaces, there was no difference of the upper
surface apparent between these examples of both sexes, and a long
series of natural ones placed beside them.

On the under side all the males were of one type, the colours being
very intense. There was considerably more red, both dark and pale, over
the whole surface, than in a series of natural examples in which shades
of brown and a bluish hue predominate. No change was observed in the
females on the under side.

It appears that fourteen days were as effective in producing changes
as a longer period. In fact, the most decided changes were found in
the females exposed the shorter period. It also appears that with
this species cold will produce change if applied after the chrysalis
has hardened. The same experiments were attempted in 1878 with pupæ
of _Grapta Comma_. They were put on ice at from ten minutes to six
hours after forming, and subjected to a temperature of about 0°-1° R.
for eighteen to twenty days, but every pupa was killed. Chrysalides
of _Papilio Ajax_ in the same box, and partly exposed very soon after
pupation, were not injured. It was for this reason that none of the
_Interrogationis_ pupæ were placed in the box till six hours had passed.

It appears further that cold may change the markings on one part of the
wing only, and in cases where it does change dark or dusky markings
melanises them; or it may deepen the colours of the under surface (as
in the females of the present experiment). The females in the above
experiment were apparently most susceptible to the cold, the most
decided changes having been effected in them.

The resulting butterflies were all of one form, although both might
have been expected to appear under natural circumstances.

_Dr. Weismann’s remarks on the foregoing experiments._--The author
of the present work has, at my request, been good enough to furnish
the following remarks upon Mr. Edward’s experiments with _G.
Interrogationis_:--

The interesting experiments of Mr. Edwards are here principally
introduced because they show how many weighty questions in connexion
with seasonal dimorphism still remain to be solved. The present
experiments do not offer a _direct_ but, at most, only an _indirect_
proof of the truth of my theory, since they show that the explanation
opposed to mine is also in this case inadmissible. Thus we have here,
as with _Papilio Ajax_, two out of the four annual generations mixed,
_i.e._, consisting of summer and winter forms, and the conclusion is
inevitable that these forms were not produced by the _gradual_ action
of heat or cold. When, from pupæ of the same generation which are
developed under precisely the same external conditions, both forms of
the butterfly are produced, the cause of their diversity cannot lie
in these conditions. It must rather depend on causes innate in the
organism itself, _i.e._, on inherited duplicating tendencies which
meet in the same generation, and to a certain extent contend with
each other for precedence. The two forms must have had their origin
in earlier generations, and there is nothing against the view that
they have arisen through the gradual augmentation of the influences of
temperature.

In another sense, however, one might perceive, in the facts discovered
by Edwards, an objection to my theory.

By the action of cold the form _Umbrosa_, which flies in June, was
produced. Now we should be inclined to regard the var. _Umbrosa_ as
the summer form, and the var. _Fabricii_, which emerges in the autumn,
hibernates in the imago state, and lays eggs in the spring, as the
winter form. It would then be incomprehensible why the var. _Umbrosa_
(_i.e._, the summer form) should be produced by cold.

But it is quite as possible that the var. _Umbrosa_ as that the var.
_Fabricii_ is the winter form. We must not forget that, in this
species, _not one of the four annual generations is exposed to the cold
of winter in the pupal state_. When, therefore, we have in such cases
seasonal dimorphism, to which complete certainty can only be given by
continued observations of this butterfly, which does not occur very
commonly in Virginia, this must depend on the fact that the species
formerly hibernated in the pupal stage. This question now arises, which
of the existing generations was formerly the hibernating one--the first
or the last?

Either may have done so _à priori_, according as the summer was
formerly shorter or longer than now for this species. If the former
were the case, the var. _Fabricii_ is the older winter form; were the
latter the case, the var. _Umbrosa_ is the original winter form, as
shall now be more closely established.

Should the experiments which Mr. Edwards has performed in the course of
his interesting investigations be repeated in future with always the
same results, I should be inclined to explain the case as follows:--

It is not the var. _Fabricii_, but _Umbrosa_, which is the winter
generation. By the northward migration of the species and the relative
shortening of the summer, this winter generation would be pushed
forward into the summer, and would thereby lose only a portion of the
winter characters which it had till that time possessed. The last of
the four generations which occurs in Virginia is extremely rare, so
that it must be regarded either as one of the generations now supposed
to be originating, or as one now supposed to be disappearing. The
latter may be admitted. Somewhat further north this generation would
be entirely suppressed, and the third brood would hibernate, either
in the imago state or as pupæ or caterpillars. In Virginia this third
generation consists of both forms. We may expect that further north, at
least, where it hibernates as pupæ, it will consist entirely, or almost
entirely, of the var. _Umbrosa_. Still further north in the Catskill
Mountains, as Edwards states from his own observations, the species
has only two generations, and one might expect that the var. _Umbrosa_
would there occur as the winter generation.

Should the foregoing be correct, then the fact that the second
generation assumes the _Umbrosa_ form by the action of cold, as was
the case in Edward’s experiments, becomes explicable. The new marking
peculiar to this form produced by this means must be regarded as a
complete reversion to the true winter form, the characters of which are
becoming partly lost as this generation is no longer exposed to the
influence of winter, but has become advanced to the beginning of summer.

_The foregoing explanation is, of course, purely hypothetical_;
it cannot at present be asserted that it is the correct one. Many
investigations based on a sufficiently large number of facts are still
necessary to be able to attempt to explain this complicated case with
any certainty. Neither should I have ventured to offer any opinion on
the present case, did I not believe that even such a premature and
entirely uncertain explanation may always be of use in serving the
inventive principle, _i.e._, in pointing out the way in which the truth
must be sought.

As far as I know, no attempt has yet been made to prove from a general
point of view the interpolation of new generations, or the omission
of single generations from the annual cycle, with respect to causes
and effects. An investigation of this kind would be of the greatest
importance, not only for seasonal dimorphism, but also for the
elucidation of questions of a much more general nature, and would be a
most satisfactory problem for the scientific entomologist. I may here
be permitted to develope in a purely theoretical manner the principles
in accordance with which such an investigation should be made:--

_On the change in the number of generations of the annual cycle._--A
change in the number of generations which a species produces annually
must be sought chiefly in changes of climate, and therefore in a
lengthening or shortening of the period of warmth, or in an increase or
diminution of warmth within this period; or, finally, in both changes
conjointly. The last case would be of the most frequent occurrence,
since a lengthening of the period of warmth is, as a rule, correlated
with an elevation of the mean temperature of this period, and _vice
versâ_. Of other complications I can here perceive the following:--

Climatic changes may be _active_ or _passive_, _i.e._, they occur by a
change of climate or by a migration and extension of the species over
new districts having another climate.

By a lengthening of the summer, as I shall designate the shorter
portion of the whole annual period of warmth, the last generation of
the year would be advanced further in its development than before;
if, for instance, it formerly hibernated in the pupal state, it would
now pass the winter in the imago stage. Should a further lengthening
of the summer occur, the butterflies might emerge soon enough to lay
eggs in the autumn, and by a still greater lengthening the eggs also
might hatch, the larvæ grow up and hibernate as pupæ. In this manner
we should have a new generation interpolated, owing to the generation
which formerly hibernated being made to recede step by step towards the
autumn and the summer. _By a lengthening of the summer there occurs
therefore a retrogressive interruption of generations._

The exact opposite occurs if the summer should become shortened. In
this case the last generation would no longer be so far developed
as formerly; for instance, it might not reach the pupal stage, as
formerly, at the beginning of winter, and would thus hibernate in
a younger stage, either as egg or larvæ. Finally, by a continual
shortening of the summer it would no longer appear at the end of
this period but in the following spring; in other words, it would be
eliminated. _By a shortening of the summer accordingly the interruption
of generations occurs by advancement._

The following considerations, which submit themselves with reference to
seasonal dimorphism, are readily conceivable, at least, in so far as
they can be arrived at by purely theoretical methods. Were the summer
to become shorter the generation which formerly hibernated in the pupal
stage would be advanced further into the spring. It would not thereby
necessarily immediately lose the winter characters which it formerly
possessed. Whether this would happen, and to what extent, would rather
depend upon the intensity of the action of the summer climate on the
generation in question, and on the number of generations which have
been submitted to this action. Hitherto no attempts have been made to
expose a monomorphic species to an elevated temperature throughout
several generations, so as to obtain an approximate measure of the
rapidity with which such climatic influences can bring about changes.
For this reason we must for the present refrain from all hypothesis
relating to this subject.

The disturbance of generations by the shortening of summer might also
occur to a species in such a manner that the generation which formerly
hibernated advances into the spring, the last of the summer generations
at the same time reaching the beginning of winter. The latter would
then hibernate in the pupal state, and would sooner or later also
assume the winter form through the action of the cold of winter. We
should, in this case, have two generations possessing more or less
completely the winter form, the ancient winter generation now gradually
losing the winter characters, and the new winter generation gradually
acquiring these characters.

In the reverse case, _i.e._, by a lengthening of the summer, we
should have the same possibilities only with the difference that the
disturbance of generations would occur in a reverse direction. In this
case it might happen that the former winter generation would become the
autumnal brood, and more or less preserve its characters for a long
period. Here also a new winter generation would be produced as soon as
the former spring brood had so far retrograded that its pupæ hibernated.

I am only too conscious how entirely theoretical are these conjectures.
It is very possible that observation of nature will render numerous
corrections necessary. For instance, I have assumed that every species
is able, when necessary, to adapt any one of its developmental stages
to hibernation. Whether this is actually the case must be learnt from
further researches; at present we only know that many species hibernate
in the egg stage, others in the larval state, others as pupæ, and yet
others in the perfect state. We know also that many species hibernate
in several stages at the same time, but we do not know whether each
stage of every species has an equal power of accommodation to cold.
Should this not be the case the above conjectures would have to be
considerably modified. To take up this subject, so as to completely
master all the facts connected therewith, naturalists would have to
devote their whole time and energy to the order Lepidoptera, which I
have been unable to do.

From the considerations offered, it thus appears that the phenomena
of seasonal dimorphism may depend on extremely complex processes, so
that one need not be surprised if only a few cases now admit of certain
analysis. We must also admit, however, that it is more advantageous to
science to be able in the first place to analyze the simplest cases by
means of breeding experiments, than to concern oneself in guessing at
cases which are so complicated as to make it impossible at present to
procure all the materials necessary for their solution.

[Illustration:

  Plate I.

  Aug. Weismann pinx.      Lith. J.A. Hofmann, Würzburg.
]

[Illustration:

  Plate II.

  Aug. Weismann pinx.      Lith. J.A. Hofmann, Würzburg.
]




EXPLANATION OF THE PLATES.


PLATE I.

Fig. 1. Male _Araschnia Levana_, winter form.

Fig. 2. Female _A. Levana_, winter form.

Fig. 3. Male _A. Levana_, artificially bred intermediate form
(so-called _Porima_).

Fig. 4. Female _A. Levana_, intermediate form (_Porima_), artificially
bred from the summer generation, agreeing perfectly in marking with
the winter form, and only to be distinguished from it by the somewhat
darker ground colour.

Fig. 5. Male _A. Levana_, summer form (_Prorsa_).

Fig. 6. Female _A. Levana_, summer form (_Prorsa_).

Figs. 7 to 9. Intermediate forms (_Porima_), artificially bred from the
first summer generation.

Figs. 10 and 11. Male and female _Pieris Napi_, winter form,
artificially bred from the summer generation; the yellow ground-colour
of the underside of the hind wings brighter than in the natural winter
form.

Figs. 12 and 13. Male and female _Pieris Napi_, summer form.

Figs. 14 and 15. _Pieris Napi_, var. _Bryoniæ_, male and female reared
from eggs.


PLATE II.

Fig. 16. _Papilio Ajax_, var. _Telamonides_, winter form.

Fig. 17. _P. Ajax_, var. _Marcellus_, summer form.

Fig. 18. _Plebeius Agestis_ (_Alexis_, Scop.), German winter form.

Fig. 19. _P. Agestis_ (_Alexis_, Scop.), German summer form.

Fig. 20. _P. Agestis_ (_Alexis_, Scop.), Italian summer form. (The
chief difference between figs. 19 and 20 lies on the under-side, which
could not be here represented.)

Fig. 21. _Polyommatus Phlæas_, winter form, from Sardinia; the German
winter and summer generations are perfectly similar.

Fig. 22. _P. Phlæas_, summer form, from Genoa.

Fig. 23. _Pararga Ægeria_, from Freiburg, Baden.

Fig. 24. _P. Meione_, southern climatic form of _Ægeria_ from Sardinia.


END OF PART I.




STUDIES IN THE THEORY OF DESCENT.




Part II.

ON THE FINAL CAUSES OF TRANSFORMATION.




=I.=

THE ORIGIN OF THE MARKINGS OF CATERPILLARS.




INTRODUCTION.


The general idea which has instigated the researches described in the
present essay has already been expressed in the Preface, where it has
also been explained why the markings of caterpillars, and especially
those of the Sphinx-larvæ, were chosen for testing this idea.

The task presented itself in the following form:--In order to test
the idea referred to, it must be investigated whether all the forms
of marking which occur in the Sphinx-larvæ can or cannot be traced to
known transforming factors.

That natural selection produces a large number of characters can
be as little doubted as that many varying external influences can
bring about changes in an organism by direct action. That these two
transforming factors, together with their correlatively induced
changes, are competent to produce _all_ characters, howsoever
insignificant, has indeed been truly asserted, but has never yet
been proved. The solution of the problem, however, appeared to me to
depend particularly on this point. We are now no longer concerned in
proving that a changing environment reacts upon the organism--this
has already been shown--but we have to deal with the question whether
_every change_ is the result of the action of the environment upon the
organism. Were it possible to trace all the forms of markings which
occur, to one of the known factors of species transformation, it could
be thus shown that here at least an “innate power of development” was
of no effect; were this not possible, _i.e._ did there remain residual
markings which could not be explained, then the notion of an “innate
principle of development” could not be at once entirely discountenanced.

The attempt to solve this problem should commence by the acquisition
of a morphological groundwork, so that the phyletic development of
the markings might by this means be represented as far as possible.
It cannot be stated with certainty, _primâ facie_, whether some form
of development conformable to law is here to be found, but it soon
becomes manifest that such is certainly the case in a great measure.
In all species the young caterpillars are differently marked to
the adults, and in many the markings change with each of the five
stages of growth indicated by the four ecdyses, this gradational
transformation of the markings being a “development” in the true sense
of the word, _i.e._, an origination of the complex from the simple,
the development of characters from those previously in existence,
and never an inconstant, unconnected series of _per saltum_ changes.
This development of the markings in individuals very well reveals
their phyletic development, since there can be no doubt but that we
have here preserved to us in the ontogeny, as I shall establish more
fully further on, a very slightly altered picture of the phyletic
development. The latter can have been but slightly “falsified” in these
cases, although it is indeed considerably abbreviated, and that in very
different degrees; to the greatest extent in those species which are
most advanced in their phyletic development, and to the least extent
in those which are less advanced. From this the value of being able to
compare a large number of species with respect to their ontogeny will
appear. Unfortunately, however, this has only been possible to a very
limited extent.

The youngest larval stages are those which are of the most importance
for revealing the phyletic development, because they make us
acquainted with the markings of the progenitors of the existing
species. For these investigations it is therefore in the first place
necessary to obtain fertile eggs. Female _Sphingidæ_, however, do not
generally lay eggs in confinement,[66] or at most only a very small
number. In the case of many species (_Deilephila Galii_, _D. Lineata_,
_D. Vespertilio_, _D. Hippophaës_) I have for this reason unfortunately
been unable to observe the entire development, and such observations
would in all probability have given especially valuable information.

I was certainly successful in finding the young larvæ of some of the
above as well as of other species on their food-plants, but even in
the most favourable instances only individuals of the second stage
and generally older. When, however, notwithstanding this imperfection
of the materials, and in spite of the important gaps thus inevitably
caused in these series of observations, it has nevertheless been
possible to form a picture, on the whole tolerably complete, of the
phyletic development of the Sphinx-markings, this well indicates what
a fertile field is offered by the investigation of this subject, and
will, I trust, furnish an inducement to others, not only to fill up the
various gaps in the small family of the _Sphingidæ_, but also to treat
other Lepidopterous families in a similar manner. Such an investigation
of the _Papilionidæ_ appears to me to be especially desirable; not only
of the few European but also of the American and Indian species. We
know practically nothing, of the youngest stages of the _Papilio_ larvæ
from this point of view. No entomological work gives any description of
the form and marking of the newly hatched larvæ, even in the case of
our commonest species (_Papilio Machaon_ and _P. Podalirius_), and I
believe that I do not go too far when I assert that up to the present
time nobody has observed them at this early stage.[67] When, however,
we consider that in these young caterpillars we have preserved to us
the parent-form, extinct for centuries, of the existing species of
_Papilio_, it must assuredly be of the greatest interest to become
accurately acquainted with them, to compare them with the earliest
stages of allied species, and to follow the gradual divergence of the
succeeding stages in different directions, thus forming a picture of
the phyletic development of an evolving group. In the course of such
observations numerous collateral results would doubtless come out.
Investigations of this kind, whether conducted on this or on any other
group, would, above all, show the true systematic affinities of the
forms, _i.e._, their _genealogical_ affinities, and that in a better
way than could be shown by the morphology of the perfect insects or
the adult caterpillars alone. If I am diffident in founding these
conclusions upon the development of the Sphinx-markings treated of
in the present essay, this arises entirely from a knowledge of the
imperfections in the basis of facts. If however, through the united
labours of many investigators, the individual development of all the
species of _Sphingidæ_ now existing should at some future period be
clearly laid before us, we should then not only have arrived at a
knowledge of the relative ages of the different species, genera and
families, but we should also arrive at an explanation of the nature of
their affinities.

It is erroneous to assert that Classification has only to take
form-relationship into consideration; that it should and can be
nothing else than the expression of form-relationship. The latter
is certainly our only measure of blood-relationship, but those who
maintain the assertion that form- and blood-relationship are by no
means always synonymous, are undoubtedly correct. I shall in a future
essay adduce facts which leave no doubt on this point, and which prove
at the same time that modern systematists--especially in the order
Lepidoptera--have always endeavoured--although quite unconsciously--to
make the blood-relationship the basis of their classification. For
this reason alone, larvæ and pupæ would have an important bearing upon
the establishment of systematic groups, although certainly in a manner
frequently irregular.

It must be admitted that so long as we are able to compare the species
of one group with those of another _in one form only_, we are often
unable to ascertain the blood-relationship.[68] In such cases we can
only determine the latter from the form-relationship, and as these
are not always parallel, any conclusion based on a single form must
be very unsound. If, for instance, butterflies emerged from the egg
directly, without passing through any larval stage, a comparison of
their resemblances of form would alone be of systematic value; we
should unite them into groups on the ground of these resemblances
only, and the formation of these groups would then much depend upon
the weight assigned to this or that character. We might thus fall into
error, not only through a different valuation of characters but still
more because two species of near blood-relationship frequently differ
from one another in form to a greater extent than from other species.
We should have no warrant that our conception of the form-relationship
expressed the genealogical connection of the species. But it would
be quite different if every species presented itself in two or three
different forms. If in two species or genera the butterflies as well
as the larvæ and pupæ exhibited the same degree of form-relationship,
the probability that this expressed also the blood-relationship would
then be exceedingly great. Now this agreement certainly does not always
occur, and when these different stages are related in form in unequal
degrees, the problem then is to decide which of these relationships
expresses the genealogy. This decision may be difficult to arrive at
in single cases, since the caterpillar may diverge in form from the
next blood-related species to a greater extent than the butterfly, or,
conversely, the butterfly may diverge more widely from its nearest
blood-related species than the caterpillar.

For such cases there remains the developmental history of the
caterpillar, which will almost always furnish us to a certain extent
with information respecting the true genealogical relationship of the
forms, because it always reveals a portion of the phyletic (ancestral)
development of the species. If we see two species of butterflies
quite dissimilar in form of wing and other characters, we should be
inclined, in spite of many points of agreement between them, to place
them in entirely different genera. But should we then find that not
only did their adult larvæ agree in every detail of marking, but also
that the entire phyletic development of these markings, as revealed
by the ontogeny of the larvæ, had taken precisely the same course in
both species, we should certainly conclude that they possessed a near
blood-relationship, and should place them close together in the same
genus. Such an instance is afforded by the two Hawk-moths, _Chærocampa
Elpenor_ and _C. Porcellus_, as will appear in the course of these
investigations. These two species were placed by Walker in different
genera, the form-relationship of the imagines being thus correctly
represented, since _Porcellus_ (imago), is indeed more nearly related
in form to the species of the genus _Pergesa_, Walker, than to those
of the genus _Chærocampa_.[69] Nevertheless, these species must remain
in the same genus, as no other arrangement expresses their degree of
blood-relationship.

An intimate knowledge of the development-stages of caterpillars thus
offers, even from a systematic point of view, an invaluable means of
judging the degree of blood-relationship, and from this standpoint we
must regard the study of the caterpillar as of more importance than
that of the perfect insect. Certainly all groups would not be so rich
in information as the _Sphingidæ_, or, as I am inclined to believe, the
_Papilionidæ_, since all families of caterpillars do not possess such a
marked and diversified pattern, nor do they present such a varied and
characteristic bodily form. The representation of the true, _i.e._, the
blood-relationship, and through this the formation of natural groups
with any completeness, can certainly only be looked for when we are
intimately acquainted with the different stages of development of the
larvæ of numerous species in every group, from their emergence from
the egg to their period of pupation. The genealogical relationship of
many forms at present of doubtful systematic position would then be
made clear. This investigation, however, could not be the work of a
single individual; not only because the materials for observation are
too great, but, above all, because they are spread over too wide a
field. It is not sufficient to study the European types only--we should
endeavour to learn as much as possible of the Lepidoptera of the whole
world. But such observations can only be made on the spot. Why should
it not be possible to trace the development from the egg, even under
a tropical sky, and to devote to breeding and observing, a portion of
that time which is generally spent in mere collecting? I may perhaps be
able to convince some of the many excellent and careful observers among
entomologists, that beyond the necessary and valuable search for new
forms, there is another field which may be successfully worked, viz.,
the precise investigation of the development of known species.

The first portion of the present essay consists of the determination
of this development in those species of _Sphingidæ_ which have been
accessible to me. Seven genera are successively treated of, some
completely, and others only in some of their stages; and thus I have
sought to present a picture of the course of development of the
markings in each genus, by comparing the species with each other, and
with allied forms in cases where the young stages were unknown. In
this portion, as far as possible, the facts only have been given, the
working up of the latter into general conclusions upon the development
of marking being reserved for the second portion. A complete separation
of facts from generalizations could not, however, be carried out; it
appeared convenient to close the account of each genus with a summary
of the results obtained from the various species.

After having established that the markings of the Sphinx-caterpillars
had undergone an extremely gradual phyletic development, conformable to
law, in certain fixed directions, it appeared desirable to investigate
the causes of the first appearance of these markings, as well as
of their subsequent development. The question as to the biological
significance of marking here presented itself in the first place
for solution, and the third section is devoted to this subject. If
it is maintained that marking is of no importance to the life of
the insect, or that it is so only exceptionally, and that it is in
reality, as it appears to be, a character of purely morphological,
_i.e._, physiological, insignificance, then its striking phylogenetic
development conformable to law cannot be explained by any of the
known factors of species transformation, and we should have to
assume the action of an innate transforming power. In the present
investigations, this subject in particular has been extensively treated
of, and not only the markings of Sphinx-caterpillars, but also those
of caterpillars in general, have been taken into consideration. The
results arrived at are indeed quite opposed to this assumption--marking
is shown to be a character of extreme importance to the life of the
species, and the admission of a phyletic vital force must, at least
from the present point of view, be excluded. This leads to the fifth
section, in which I have attempted to test certain objections to the
admission of a “phyletic vital force.” The sixth section finally gives
a summary of the results obtained.

I may now add a few explanations which are necessary for understanding
the subsequent descriptions. It was found impossible to avoid the
introduction of some new technicalities for describing the various
elements of larval markings, especially as the latter had to be
treated of scientifically. I have therefore chosen the simplest and
most obvious designations, all of which have already been employed by
various authors, but not in any rigorously defined sense. I understand
by the “dorsal line” that which runs down the middle of the back; the
lines above and below the spiracles will be respectively distinguished
as the “supra-” and “infra-spiracular” lines, and the line between the
dorsal and spiracular as the “subdorsal line.” The distinction between
“ring-spots” and “eye-spots” will be made manifest in the course of the
investigation. A glance at any of the existing descriptions of larvæ
will show how necessary it was to introduce a precise terminology.
Even when the latter is exact as far as it goes, the want of precise
expressions not only makes the descriptions unnecessarily long, but it
also considerably increases the difficulty of comparing one species
with another, since we can never be sure whether the same designation
applies to the same homologous character. For instance, when the
larva of _Chærocampa Elpenor_ is said to have “a light longitudinal
line on the sides of the thoracic segments,” this statement is indeed
correct; but it is not apparent whether the line is above or below,
and consequently it does not appear whether it is the equivalent
of the longitudinal line “on the sides” of the segments in other
species. If, however, it is said that this line is “_subdorsal_ on
the thoracic segments, and on the eleventh abdominal segment,” it is
thereby indicated that we have here a residue of the same marking
which is found completely developed in many other Sphinx-larvæ,
and indeed in the young stages of this same species. The mode of
describing caterpillars hitherto in vogue is in fact unscientific;
the descriptions have not been made with a view to determining the
_morphology_ of the larvæ, but simply to meet the practical want of
being able to readily identify any species that may be found: even for
this purpose, however, it would have been better to have employed a
more precise mode of description.




I.

ONTOGENY AND MORPHOLOGY OF SPHINX-MARKINGS.


THE GENUS CHÆROCAMPA, DUPONCHEL.

Although by no means in favour of the excessive subdivision of genera,
I am of opinion that Ochsenheimer’s genus _Deilephila_ has been
correctly separated by Duponchel into the two genera _Chærocampa_ and
_Deilephila_, _sensû strictiori_. Such a division may appear but little
necessary if we examine the perfect insects only; but the developmental
history of the caterpillars shows that there is a wide division between
the two groups of species, these groups however being branches of one
stem.


CHÆROCAMPA ELPENOR, LINN.

Some captured females laid single eggs sparsely on grass, wood,
and especially on the tarlatan with which the breeding-cage was
covered. The eggs are nearly spherical, but somewhat compressed, of a
grass-green colour, a little lighter, and somewhat larger (1.2 millim.)
than those of _Deilephila Euphorbiæ_. During the development of the
embryo the eggs first became yellowish-green, and finally yellowish.


_First Stage._

The young caterpillars are four millimeters in length, and immediately
after hatching are not green, but of a yellowish-white opalescent
colour, the large and somewhat curved caudal horn being black. The
caterpillars were so transparent that under a low magnifying power the
nervous, tracheal, and alimentary systems could be beautifully seen.
As soon as the larvæ began to feed (on _Epilobium parviflorum_) they
became green in consequence of the food appearing through the skin, but
the latter also gradually acquired a dark green colour (Pl. IV., Fig.
17). All the specimens (some twenty in number) were exactly alike, and
showed _no trace of marking_.


_Second Stage._

The first ecdysis occurred after 5-6 days, the length of the
caterpillars being from nine to ten millimeters. After this first moult
they appeared of a shining green, the horn, which was black during the
first stage, becoming a little red at the base, while a fine white
subdorsal line extended from the horn to the head (Fig. 18). The head
and legs were green; the divisions between the segments appeared as
fine light rings, and the entire upper surface of the segments was also
crossed by fine transverse rings, as was also the case in the first
stage.

At the beginning of the present stage no trace of the eye-spots could
be detected; but a few days after the first moult it was observed that
the white subdorsal line was no longer straight on the fourth and fifth
segments, but had become curved upwards into two small crescents. The
latter soon stood out more strongly, owing to the filling up of their
concavities with darker green. These are the first rudiments of the
eye-spots (Figs. 19 and 30). A very fine white line now connected the
spiracles (infra-spiracular line), and could be traced from the last
segment to the head. This line takes no further part in the subsequent
development of the markings, but disappears in the following stage. The
blood-red colour of the base of the black caudal horn is retained till
the fifth stage, and then also disappears.

Before the second moult, which occurs after another period of 5-6
days, the caterpillars, which were about 1.3 centimeters in length,
had assumed their characteristic tapering, slug-like form. I did not
notice that the larvæ at this stage possessed the power of withdrawing
the three foremost segments into the two succeeding ones, as is so
frequently to be observed in the adults; neither were these two
segments so strikingly enlarged as they are at an earlier period.


_Third Stage._

After the second ecdysis the marking and colouring only undergo change
with respect to the eye-spots. The concavities of the crescent-shaped
portions of the subdorsal line become black,[70] the remainder of this
line at the same time losing much of its whiteness, and thus becoming
less distinct, whilst the crescents assume the appearance of small
eye-spots (Fig. 20). During this stage the curved, crescent-formed
portions become prepared for complete separation from the remainder
of the subdorsal line; and just before the third moult the eye-spots
become sharply defined both in front and behind, whilst the black
ground-colour curves upwards, and the white spots gradually become
lenticular and commence to enlarge (Fig. 21).


_Fourth Stage._

The third moult takes place after another interval of 5-6 days, the
eye-spots then becoming very prominent. The white nucleus of the front
spot is kidney-shaped, and that of the hind spot egg-shaped; whilst
the black ground-colour extends as a slender border upwards along the
sides of the spots, but does not completely surround them till towards
the end of the present stage (Fig. 21). The central portion of the
white spots at the same time becomes of a peculiar violet-brown colour
inclining to yellow above, the peripheral region alone remaining pure
white.

Of the subdorsal line only traces are now to be recognized, and these
are retained, with almost unchanged intensity, sometimes into the last
stage, remaining with the greatest persistence on the three front and
on the penultimate segments, whilst on those containing the eye-spots,
_i.e._, the fourth and fifth, not a trace remains. At the present stage
the peculiar mingling of colours becomes apparent over the whole of
the upper surface; the green is no longer uniform, but a mixture of
short and gently sinuous, dark green striations on a lighter ground
now appear. On the sides of the caterpillar these stripes, which are
at first indistinct, but become more strongly pronounced in the next
stage, are arranged obliquely on the spiracles, with the lower portions
directed forwards.


_Fifth Stage._

The fourth moult occurs 7-8 days after the third, the caterpillar being
4-5 centimeters in length. Whilst all the specimens hitherto observed
were with one exception light green, they now mostly changed their
colour and became dark brown. In one case only did the brown colour
appear in the previous (fourth) stage. The striations previously
mentioned appear as dull and interrupted dirty yellow streaks, the same
dirty yellow colour showing itself continuously on the sides of the
four front segments. Of the subdorsal line only a distinct trace is
now to be seen on the eleventh and on the three front segments, whilst
on the third segment the formation of another eye-spot commences to
be plainly perceptible by a local deposition of black (Fig. 23). This
third spot does not, however, become completely developed, either in
this or in the last stage, but the subdorsal line remains continuous
on the three front segments. Among other changes at this stage, there
occurs a considerable shortening of the caudal horn, which at the same
time loses its beautiful black and red colours and becomes brownish.

The two large eye-spots have now nearly attained complete development.
The kidney-shaped white spot has become entirely surrounded by black;
and on the brown, red, and yellow tints present in this spot during the
last stage, a nearly black spot has been developed--the pupil of the
eye (Fig. 33). In order to establish a definite terminology for the
different portions of the eye-spot, I shall designate the pupil as the
“nucleus,” the light ground on which the pupil stands as the “mirror,”
and the black ground which surrounds the mirror as the “ground-area.”

In this fifth stage the larva attains a length of six centimeters,
after which the fifth moult takes place, the caterpillar becoming ready
for pupation in the sixth stage. No striking changes of colouring or
marking occur after the present stage, but only certain unimportant
alterations, which are, however, of the greatest theoretical interest.


_Sixth Stage._

In this stage the eye-like appearance of the spots on the front
segments becomes still more distinct than in the fifth stage; at the
same time these spots repeat themselves on all the other segments from
the fifth to the eleventh, although certainly without pupils, and
appearing only as diffused, deep black spots, of the morphological
significance of which, however, there cannot be the least doubt. They
are situated in precisely the same positions on the 5-11 segments as
those on the third and fourth--near the front, and above and below
the subdorsal line. A feeble indication of the latter can often be
recognized (Fig. 23).

In all dark brown specimens the repeated spots can only be detected in
a favourable light, and after acquiring an intimate knowledge of the
caterpillar; but in light brown and green specimens they appear very
sharply defined.

There is one other new character which I have never observed at an
earlier period than the sixth stage, viz. the small dots which
appear in pairs near the posterior edge of segments 5-11. These dots
cannot have been developed from the subdorsal line, as they are
situated higher than the latter. Their colour varies according to the
ground-colour of the caterpillar, but it is always lighter, being light
green in green specimens, dull yellow in those that are light brown,
and grey in the blackish-brown caterpillars. These “dorsal spots,” as
I shall term them, are chiefly of interest because they are present in
_Chærocampa Porcellus_, in which species they appear one stage earlier
than in _C. Elpenor_.


CHÆROCAMPA PORCELLUS, LINN.

Females captured on the wing, laid in the breeding-cage single eggs of
a light green colour, spheroidal in form, and very similar to those of
_C. Elpenor_.


_First Stage._

The caterpillars on first hatching measure 3.5 millimeters in length,
and are of a uniform light green colour, with a fine white transverse
line on the posterior edge of each segment, precisely similar to that
which appears in the second stage of _C. Elpenor_. They resemble the
latter species still further in showing a fine white subdorsal line,
which can easily be recognized by the naked eye (Fig. 24). Although
the adult larva is distinguished from all the other known species of
_Chærocampa_ by the absence of a caudal horn, a distinct but very small
one is nevertheless present at this first stage, and is indeed retained
throughout the entire course of development, but does not increase
further in size, and thus gradually becomes so small in proportion to
the size of the caterpillar that it may be entirely overlooked.

The first moult takes place after 4-5 days.


_Second Stage._

The blue-green coloration remains unchanged; but a somewhat darker
green dorsal line becomes apparent down the middle of the back (the
dorsal vessel?), and the subdorsal line now becomes very broad and pure
white, being much more conspicuous than in any stage of _C. Elpenor_
(Fig. 25). The tapering of the three front segments occurs at this
stage, and oblique, dark green striations on a lighter ground stand out
distinctly on the spiracles. As with _C. Elpenor_, the first traces of
the future eye-spots appear during the second stage; not in the present
case as a curvature of the subdorsal line, but as a spot-like widening
of the latter, of a brighter white than the somewhat greenish colour of
the remainder of the line.


_Third Stage._

After the second moult, the formation of the dark “ground-area” of the
eye-spots commences by the appearance of a little brown on the under
edge of the foremost of the white spots, this coloration gradually
increasing in extent and in depth. At the same time both spots become
more sharply distinguishable from the subdorsal line, which becomes
constantly greener (Fig. 27). The brown colour soon grows round the
white of the front eye-spot, which becomes so far perfected; whilst
the completion of the hind spot is effected slowly afterwards. The
formation of the eye-spots does not therefore proceed any more rapidly
in this species than in _C. Elpenor_.

At the end of the present stage the length of the caterpillar is about
four centimeters; the ground colour is still sea-green; the subdorsal
line is much diminished, completely fading away at its lower edge, but
remaining sharply defined above, against the green ground-colour (Fig.
26).


_Fourth Stage._

After the third moult all the caterpillars (5) became brown, this
change occurring therefore one stage earlier than is generally the case
with _C. Elpenor_. In single instances the brown colour appeared in the
third stage. The subdorsal line had disappeared from all the segments
but the three first and the last. The eye-spots now rapidly attained
complete development; they contained a black pupil, and gave the insect
a truly repulsive appearance when, on being threatened by danger, it
drew in the front segments, and expanded the fourth (Fig. 28). The
eye-spots of the fifth segment are much less developed than in _C.
Elpenor_; they remain small, and are not readily detected. On the other
hand, there now appear on all the segments with the exception of the
last, just as in the sixth stage of _C. Elpenor_, distinct rudiments of
eye-spots, which present the appearance of irregular, roundish, black
spots on the front borders of the segments, at the height of the former
subdorsal line. In this latter region the black pigment is disposed as
a longitudinal streak, and to this a median line is added, the whole
forming a marking which perhaps makes the caterpillar appear still more
alarming to its foes. This marking is, however, only to be distinctly
recognized on the three first segments. The “dorsal spots” mentioned in
the case of _C. Elpenor_ then appear very distinctly on segments 5-11.

The caterpillars continued to feed for eleven days after the third
moult, at the end of which period the fourth moult took place, but
without the occurrence of any change of marking. The larvæ then buried
themselves, the complete development having taken 28-29 days.

The development of the _Porcellus_ caterpillar was twice followed;
in 1869 in twelve, and in 1874 in five specimens. In no case did I
obtain caterpillars which remained green throughout the entire course
of development, although this colour is stated in the books to occur
occasionally in these larvæ; neither have I been able to find any
figure of an adult green specimen, so that it must in the meantime be
admitted that such specimens, if they occur at all, are exceptional
instances.[71] The theoretical bearing of this admission will appear
later on.


RESULTS OF THE DEVELOPMENT OF CHÆROCAMPA ELPENOR AND C. PORCELLUS;
COMPARISON OF THESE WITH THE OTHER KNOWN SPECIES OF CHÆROCAMPA.

The first stage of _Elpenor_ shows that the most remote ancestor of
the genus possessed no kind of marking, but was uniformly green. At a
later period, the white longitudinal stripe which I have designated
the “subdorsal line” made its appearance, and at a still later period
this line vanished, with the exception of a few more or less distinct
remnants, whilst, at the same time, from certain portions of it, the
eye-spots of the fourth and fifth segments became developed. After the
perfecting of the eye-spots, weak repetitions of the latter appeared as
black spots on all the segments except the last.

In _Porcellus_ the caterpillar emerges from the egg with the subdorsal
line, the first stage of _Elpenor_ being omitted. From this fact we may
venture to conclude that _Porcellus_ is the younger species, or, what
comes to the same thing, that it has further advanced in development.
The whole subsequent history of _Porcellus_ agrees with this view,
its course of development being essentially but a repetition of the
phenomena displayed by _Elpenor_, and differing only in one point, viz.
that all new characters make their appearance one stage earlier than
in the latter species. This is the case with the transformation of the
green into a brown ground-colour; with the repetition of the eye-spots
on the remaining segments in the form of suffused black spots; and
with the appearance of the light “dorsal spots.” Only the eye-spots
themselves appear, and the snout-like tapering of the front segments
occurs in the same stage as in _Elpenor_, _i.e._ the second.

From these data alone, we may venture to infer the occurrence of four
chief stages in the phyletic development of the genus. The first stage
was simply green, without any marking; the second showed a subdorsal
line; the third, eye-spots on the third and fourth segments; and
the fourth stage showed a repetition of the eye-spots, although but
rudimentary, on all the remaining segments with the exception of the
twelfth.

Now if we compare the other known species of _Chærocampa_ larvæ with
the above, we shall arrive at the interesting conclusion that all these
species can be arranged in three groups, which correspond exactly with
the three last phyletic stages as just deduced from the ontogeny of _C.
Elpenor_ and _Porcellus_.

Of the genus _Chærocampa_,[72] over fifty species have been
described,[73] of which the larvæ of only fifteen are known in the
form which they possess at the last ontogenetic stage.

GROUP 1.--I can furnish but little information with respect to this
group. The first species with which I became acquainted was _Chærocampa
Syriaca_,[74] of which I saw two blown caterpillars in Staudinger’s
collection, and which I have figured in Pl. IV., Fig. 29. The larva
is green, and has the short oblique stripes over the legs common to
so many species of _Chærocampa_, the only marking besides these being
a simple white subdorsal line, without any trace of eye-spots. This
species exactly corresponds therefore with the second ontogenetic
stage of _C. Elpenor_ and _Porcellus_. The account of the species,
both in the larval and perfect state, is unfortunately so imperfect,
that we cannot with certainty infer the age of the two caterpillars
from their size. If the moth were of the same size as _Elpenor_, then
the caterpillar figured, having a length of 5.3 centimeters, would not
be in the last but in the penultimate stage, and it remains doubtful
whether it may not acquire eye-spots in the last stage.

That species exist, however, which in their last stage correspond to
the second stage of _Elpenor_, is shown by two of the forms belonging
to Walker’s genus _Darapsa_, which was founded on the characters
of the imagines only. Ten species of this genus are given in Gray’s
catalogue, the adult larva of two of these being known through the
excellent figures of Abbot and Smith.[75] These two caterpillars
possess the characteristic tapering form in a very marked degree;
one is figured in the attitude so often assumed by our species of
_Chærocampa_ on the approach of danger, the three front segments
being withdrawn into the fourth. (Fig. 34, Pl. IV., is copied from
this Plate). There are no eye-spots either in _D. Myron_ or _D.
Chœrilus_,[76] but only a broad white subdorsal line; underneath which,
and to a certain extent proceeding from it, there are oblique white
stripes, precisely similar to those which meet the subdorsal line in
the third stage of _C. Porcellus_.[77]

GROUP 2.--This group contains numerous species which, like our native
_C. Elpenor_ and _Porcellus_, show eye-spots on the fourth and fifth
segments, whilst these markings are absent, or at most only present in
traces, on the remainder. To this section there belong, besides the
two species mentioned, five others, viz. in Europe, _C. Celerio_ and
_Alecto_ (not certainly known?);[78] in India, _C. Nessus_, Drury, and
_Lucasii_, Boisduval;[79] and an unnamed species from Port Natal.

In the species belonging to this group the subdorsal line may be more
or less retained. Thus, _C. Celerio_, according to Hübner’s figure,
has a broad yellow line extending from the horn to the sixth segment,
whilst it is completely absent on the three front segments. In the
unnamed species from Port Natal[80] the subdorsal line extends to the
front edge of the fifth segment, and on the fourth segment only is
there a perfect eye-spot, whilst on the succeeding segments traces of
such markings can be recognized as dark spots similar to those in
_Elpenor_ and _Porcellus_. The transition to the third group is through
another unnamed species from Mozambique,[81] in which rather large
eye-spots have become developed on the fourth and fifth segments and
these are followed by a subdorsal line, which only appears distinctly
at certain places. On this broken subdorsal line, and not completely
separated from it, there are small, roundish eye-spots, situated near
the front edge of each segment; these being, therefore, a somewhat more
perfect repetition of the front eye-spots.[82]

GROUP 3.--In the species of this group the eye-spots are repeated on
all the segments. I am acquainted with seven such _Chærocampa_ larvæ,
of which _C. Bisecta_, Horsfield,[83] shows some affinity to the
foregoing group, since the eye-spots on segments 6-11 have not yet
attained full perfection. In _C. Odenlandiæ_, Fabr.,[84] and in _C.
Alecto_ from India,[85] the eye-spots appear to be perfectly alike on
all the segments; whilst in _C. Acteus_, Cram.,[86] and in the North
American _C. Tersa_[87] (Pl. IV., Fig. 35) they are smaller on the
other segments than on the fourth; and in _C. Celerio_, Linn., from
India,[88] the size of the spots diminishes from the head to the tail.

In this group also the subdorsal line is retained in a very variable
degree. In some species it appears to have completely vanished (_C.
Acteus_, _Celerio_); in others it is present as a light stripe
extending along all the segments (_C. Alecto_); whilst in others it
is retained as a broad white stripe, which extends only to the fourth
segment (_C. Tersa_, Fig. 35). In species possessing eye-spots, the
subdorsal line is thus a very variable character. It is, however, an
interesting fact that even in the present group, which has made the
greatest step forward, the subdorsal line is of general occurrence,
because the eye-spots in all these species may have almost a similar
development to those of _Elpenor_ and _Porcellus_. The ontogeny of the
tropical species would alone give a definite reply on this point, but
unfortunately we are not acquainted with any of the young forms, so
that we can but presume that some of them at least would show only in
the first stage the simple subdorsal line without eye-spots; that in
the second stage the primary pairs of eye-spots would be formed on the
fourth and fifth segments, whilst the transference of these spots to
the remaining segments would take place in the last stage.

The foregoing assumption is based immediately on the ontogeny of
_Elpenor_ and _Porcellus_; it is supported by the considerable size
attained by the eye-spots in many species of the third group, and
would receive additional confirmation by observations on the Indian
_C. Celerio_, supposing that Horsfield’s statements do not arise from
a confusion of species. This skilful observer, who was the first to
breed systematically a large number of tropical larvæ, has given a
figure of the Indian caterpillar of _C. Celerio_, according to which
this species possesses eye-spots on all the segments from the fourth to
the tenth. The European form of this same species has eye-spots only on
segments four and five, a fact which does not appear to have been known
to Horsfield, as no mention of it is made in his notice of the Indian
species. If the caterpillar figured is really that of _Celerio_, which
I consider to be by no means improbable, not only is it thus shown that
in the species of the third group the ocelli on the hind segments have
a secondary origin through a repetition of the primary ones of the
front segments, but we can also establish that the same species in two
different regions may arrive at two different phyletic stages.

If, finally, we sum up the facts taught by the ontogeny of the two
German species, and the adult forms of the other species, we can form
therefrom a tolerably complete picture of the course of development of
the genus _Chærocampa_. Of the four phyletic stages indicated by the
ontogeny of _Elpenor_ and _Porcellus_, three still form the terminus
of the development of existing species. The great differences among
the caterpillars of this genus can be very simply explained on the
view that they stand at different levels of phyletic development; some
species having remained far behind (Group 1), others having advanced
further (Group 2), and others having reached the highest point of
development (Group 3). The fact that the species of the third group
are only tropical accords well with this view, since many facts prove
that phyletic development proceeds more rapidly in the tropics than in
temperate climates.

The striking markings of the _Chærocampa_ larvæ may, in brief, be
stated to originate from a local transformation of two portions of the
subdorsal line into eye-spots, and the subsequent transference of
these two primary ocelli to the other segments. The eye-spots always
originate on segments four and five, and from these the transference
mostly occurs backwards, although in certain cases it takes place at
the same time forwards. Herein, _i.e._ in the origin of the eye-spots,
there lies a great distinction between the genus _Chærocampa_ and the
genus _Deilephila_, with which it was formerly associated, and in which
the origin of a very similar kind of marking can be traced to quite
another source.


THE GENUS DEILEPHILA, OCHSENHEIMER.

I am acquainted with the caterpillars of nine European and one North
American species, these differing in marking to such a wonderful extent
that they appear to offer at first sight but little hope of being able
to trace them to a common form. These ten species can be separated,
according to their markings, into five groups, which I will briefly
define before entering upon their ontogeny.

The first group consists of three species, and comprises the commonest
and most widely-ranging of all the European species, _Deilephila
Euphorbiæ_, as well as _D. Dahlii_ from Sardinia and Corsica, and _D.
Nicæa_, a species of very restricted range, which appears to occur only
in one small district on the French coast of the Mediterranean. These
three species agree in marking to the extent of their possessing in the
adult form two rows of ring-spots on each side, whilst the subdorsal
line is completely absent.

The second group, consisting also of three species, shows a great
resemblance to _Euphorbiæ_, but has only one row of ring-spots.
It contains _D. Vespertilio_, _D. Galii_, and the Algerian _D.
Mauritanica_.

For the third group I only know one representative, _D. Livornica_,
Esp., which possesses a single row of ring-spots connected by a
subdorsal line.

Another group is composed of _D. Zygophylli_, which occurs on the
shores of the Caspian Sea, and the North American _D. Lineata_; these
species possessing a strongly marked subdorsal line, associated with
more or less distinct ring-spots, which I shall designate as “open
rings,” because their black border does not intersect the subdorsal
line, but has the form of an arch above and below it.

In the last group, represented by _D. Hippophaës_, which occurs at the
foot of the Alps (Wallis), and southward as far as Andalusia, there is
only a broad subdorsal line, generally without any trace of a row of
spots.

The important differences of marking displayed by these five groups
are not in any way accidental, but they represent different stages
of phyletic development; or, in other words, the five groups are of
different ages, the first (_Euphorbiæ_, &c.) being the youngest, and
the last (_Hippophaës_) the oldest of the genus.

According to their phyletic age, the groups follow each other in
inverse order, the first being _Hippophaës_, the second that of
_Zygophylli_, the third that of _Livornica_, the fourth that of
_Galii_, and the fifth and youngest that of _Euphorbiæ_. Only in this
last am I acquainted with the _complete_ development of one species,
for which reason I commence with this group, thus proceeding from the
youngest to the oldest forms, instead of taking the more natural course
from the simplest and oldest to the youngest and most complicated.


DEILEPHILA EUPHORBIÆ, LINN.

Some captured females were at once placed in an enclosure about the
size of a small sitting-room. It was evident that they did not feel
quite at home under these conditions, frequently beating their heads
and wings against the tarlatan, but some of them nevertheless laid eggs
at the base of the leaves of _Euphorbia Cyparissias_. The eggs much
resemble those of _Chærocampa Elpenor_, being spheroidal in form, but
rather smaller, and of a somewhat darker green. They were laid in small
clusters composed sometimes of as many as seven, the single eggs being
placed near together, but never touching, and seldom at the point of
the leaf, but generally near the end of a twig, where young shoots are
in close proximity. During the embryonic development the eggs become
coloured, first yellow and partly blackish, and finally completely
black.


_First Stage._

The young caterpillars (Fig. 37, Pl. V.) immediately after hatching
measure four millimeters in length; they are at first rather light,
but in the course of half-an-hour they are seen by the naked eye to
become of a deep velvety black; later, on increasing in size, they
again become paler, appearing of a greenish-black, and subsequently
blackish-green. On further increasing in size (Fig. 38), they are
blackish-green, with the horn, head, legs, and a crescent-shaped
chitinous plate on the back of the prothorax black. There are also on
the last segment a double and two single black chitinous plates. Of the
later marking of the caterpillar there is scarcely anything present.
The spiracles appear as white spots, and on each segment there are
a number (mostly ten) of small warts, each of which emits a single
bristle.

When the young larvæ have attained a length of seven millimeters they
are olive-green, and do not contrast so brilliantly with the green of
the _Euphorbia_ leaves as before; neither do they as yet possess any
markings.


_Second Stage._

The first ecdysis occurs after five days, and with this there appears
quite suddenly a very complicated pattern. The ground-colour is now a
light yellowish-green (Fig. 39), and on each of the twelve segments,
near the front border, there is a pure white round spot in the
middle of a large black transverse spot. I shall designate these, in
accordance with the nomenclature employed for _Chærocampa_, as the
white “mirrors” on black “ground-areas,” both together constituting
“ring-spots,” as distinguished from “eye-spots” proper, in which a
“nucleus,” the pupil of the eye, is also added. In many, but not in all
specimens, very distinct traces of a subdorsal line can be seen as a
light whitish stripe connecting the white spots. The horn, the thoracic
and prolegs, and some spots on the head, are black.

The caterpillars remain unaltered till after four days, when, having a
length of 17 millimeters, the second moult takes place, bringing with
it changes quite as great as those which occurred with the first.


_Third Stage._

The caterpillar now assumes the shagreened appearance which it
possesses in the adult state. Small white warts are arranged in rows
from the dorsal to the spiracular line, and again underneath this line
on the abdominal legs. These dots are not only of value as a character
for differentiating the genera _Deilephila_ and _Chærocampa_, but they
also play a part in the peculiar spot-marking which will be shown later
on. The ground-colour of the caterpillar is now light green (Fig. 40),
replaced by black on certain parts. From the black “ground-area” of the
ring-spots, two black triangles extend towards the posterior borders of
the segments, but usually without reaching them.

The ring-spots are not essentially changed, although it may be observed
that in most specimens the shagreen-dots under each ring-spot are
somewhat larger, and stand closer together than in other places. In
the following stage they become fused into a second white “mirror,” so
that two ring-spots stand one above the other, their black ground-areas
meeting. The formation of the second ring-spot sometimes takes place in
the present stage (Fig. 42).

The subdorsal line has now completely vanished, whilst the spiracular
line[89] appears as a broad stripe above the legs. The horn is yellow
with a black point, and the black spots on the head have increased in
size.


_Fourth Stage._

The third moult, which again occurs after four days, is not accompanied
by such important changes. The green ground-colour has now completely
disappeared, and is replaced by a dull black. The caterpillars are now,
as also in the previous stage, extremely variable. Thus, for example,
a triangular patch of the green ground-colour may be retained on the
posterior edge of the segments (Fig. 41), those specimens which possess
this character generally having their markings retarded in development,
as shown by the absence of the second “mirror” of the ring-spots.

In Fig. 41 the shagreen-dots from which this second “mirror” is
subsequently formed, are distinctly larger than the others, and on the
eleventh segment two of them have already coalesced.


_Fifth Stage._

After another period of four days, the fourth moult takes place. The
marking remains the same, but the colours become more vivid; the
brick-red of the head, horn, dorsal line and legs, changing into a
fiery red. The spiracular line, formerly green alternating with yellow,
generally becomes resolved into a row of reddish-yellow spots. Ten days
later the caterpillar (8.5 centimeters in length), ceases to feed, and
prepares for pupation.

In this last stage also there is great variability of colour, but
although each particular character is subject to fluctuation, the
individuals of the same brood show but little variation among
themselves.[90] Thus, the dorsal line is sometimes black, and sometimes
red, or again, this colour interrupted with black, so that only small
red spots mark its course. The head may be entirely red, or this
colour mixed with black. On the under side of the caterpillar, red
generally predominates, but in some specimens this is replaced by
black. The ground-colour is also variable, being generally a shining
brownish-black, but sometimes dull coaly black. The shagreen-dots
are sometimes white and sometimes yellow, and the “mirrors” of the
ring-spots are also often yellowish.

The most interesting variation, however, appears to me to be the
following:--In many specimens from Kaiserstuhl (Breisgau), the red
was unusually vivid, and was not limited to the ordinary places, but
occupied also the triangles on the posterior edges of the segments
(Fig. 44), which are green in the third and fourth stages (Fig. 42).
This variety has also been figured by Hübner. In one individual (Fig.
43), the under ring-spots were wanting, whilst the upper ones possessed
a beautiful red nucleus fading away anteriorly, and showing the first
step in the formation of a complete eye-spot.

I cannot positively assert that a fifth moult occurs in the last ten
days, although I am very doubtful whether this is the case. It is
certain, however, that some time before pupation, and whilst the larva
is still feeding, the striking colours fade out, and become replaced
chiefly by black.

The ontogeny of this species is obviously but a very incomplete
representation of its phyletic development. This is at once apparent
from the large gap between the first and second stages. It is not
possible that a row of ring-spots can have arisen suddenly; in all
probability they have been developed from a subdorsal line, which in
_Euphorbiæ_ is now only indicated in the second stage by a faint line.
This conjecture is raised to a certainty when we call in the aid of the
remaining species of _Deilephila_.


DEILEPHILA NICÆA, DE PRUNNER.

I only know this species from blown larvæ in Staudinger’s collection,
and Duponchel’s figure, of which Fig. 51, Pl. VI. is a copy. The adult
insect possesses two perfectly separated rows of ring-spots. Duponchel
figures also two younger stages, of which the youngest is probably the
third stage. The larva is 18 millimeters in length, of a leaf-green
colour, and shows no trace of a subdorsal line, but possesses the two
rows of ring-spots, which only differ from those of the succeeding
stages in the green colour of the “mirror.”


DEILEPHILA DAHLII, TREITSCHKE.

I am familiar with numerous specimens in various stages, collected in
Sardinia by Dr. Staudinger, and preserved by inflation.

The first stage is blackish, and shows no kind of marking; thus
agreeing with the corresponding stage of _Euphorbiæ_. The second stage
is unfortunately not represented in Staudinger’s collection.

The third stage shows a row of ring-spots, which are, however,
connected by a very distinct and sharply defined subdorsal line. In the
fourth stage a second row of (under) ring-spots is added, whilst the
subdorsal line generally at the same time disappears.

The caterpillar remains unchanged during the fifth stage, when it shows
a great resemblance in marking to _Euphorbiæ_; neither does it appear
to differ essentially from this species in colour, so far as can be
judged from preserved specimens and single figures (in Duponchel and
Hübner). I have, moreover, seen several larvæ in the last stage, and
the subdorsal could be distinctly recognized as a broad light stripe.

Of the four groups, the second (that of _Galii_), appears to me to be
of but very little importance, as I shall now proceed to show from the
development of _D. Vespertilio_.


DEILEPHILA VESPERTILIO, FABRICIUS.

Hitherto I have unfortunately been unable to obtain fertile eggs of
this species, so that I can say nothing about the first stage. The
latter would have been of interest, not only because of the marking,
but also because of the presence of a residual caudal horn.

I am likewise only acquainted with the end of the second stage, having
found, at the end of June 1873, a single caterpillar on _Epilobium
Rosmarinifolium_, just previous to its second ecdysis. In the case of
such young caterpillars, however, the new characters which appear in
the succeeding stage are generally perceptible through the transparent
chitinous skin at the end of the preceding stage, so that the markings
of the insect are thus caused to change. The caterpillar found was
about 16 millimeters long, and of a beautiful smooth and shining
grass-green (Fig. 13). A broad white subdorsal line extended from the
first to the penultimate segment, from which the horn was completely
absent. On close inspection the first traces of the ring-spots could
be detected near the anterior edge of each segment as feeble, round,
yellow, ill-defined spots, situated on the subdorsal line itself (Fig.
13). On the first segment only there is no spot, and here no ring-spot
is afterwards formed. Besides these markings, there was only to be seen
a yellowish-white spiracular line.

This solitary specimen unfortunately buried itself before the moult
for which it had prepared itself had occurred; but this ecdysis is
associated with a very important transformation. This statement is
founded on a blown specimen in Staudinger’s collection; it is only
18 millimeters in length, but already shows the later grey colouring
in place of the beautiful green. In this, the third stage, the broad
white subdorsal line bears on each segment a red spot enclosed between
black crescents above and below (Fig. 49 A). In the fourth stage,
during which I have seen many living caterpillars, the subdorsal line
is still distinctly present in some individuals (Fig. 14), but the
spots (“mirrors”) are now completely surrounded by a narrow black
ring (“ground-area”), which sharply separates them from the subdorsal
line (Fig. 49 B). In the fifth stage this ring becomes a somewhat
irregularly formed black “ground-area,” whilst the subdorsal line
completely vanishes (Figs. 51 and 49 C). The mirrors are white, but
generally have a reddish nucleus, which obviously corresponds to the
primary yellow spots from which the whole development of the ring-spots
originates. This character is, however, sometimes absent; and many
other variations also occur in the earlier stages, all of which can be
easily explained as cases of arrested, or retarded development. Thus,
the subdorsal line often disappears earlier, and is only present in the
fourth stage as a feeble light stripe.


DEILEPHILA GALII, FABRICIUS.

The markings of this species appear to be developed in a precisely
similar manner to those of _D. Vespertilio_. The adult larva, as in
the last species, shows no trace of a subdorsal line. A row of large
black spots, each having an irregular round, yellowish-white nucleus,
is situated on an olive-green, blackish-brown, brown, or dirty yellow
ground. I have, unfortunately, also in this case been unable to procure
fertile eggs. There is, however, one figure of a caterpillar, 2.5
centimeters long, by Hübner, which is of a light green colour, and has
five longitudinal lines; one dorsal, two subdorsal, and a spiracular
line. The subdorsal is white, and bears in the place of the ring-spots
small red dots, whilst the line itself is bordered with black where the
red spots are situated. Hübner has probably figured the third stage,
so that we may venture to conclude that in the second stage there is
a subdorsal line either quite free from spots, or only showing such
feeble rudiments as are to be seen in the second stage of _Vespertilio_.

I found two specimens in the fourth stage in the Upper Engadine.
One of these (Fig. 45) was already of a dark, blackish-green
ground-colour[91] with a broad, greenish-white subdorsal line sharply
defined throughout its entire length, and containing ring-spots of a
sulphur-yellow with an orange-red nucleus; the black “ground-area” did
not encroach upon the subdorsal line, but was confined to two faint
crescents situated above and below the “mirror.” Only the two foremost
“mirrors” (on the second and third segments) were without nuclei.

The remaining peculiarities of coloration are shown in the figure. I
may here only point out the shagreening present on the sides and a
portion of the under surface.

The specimen figured was 3.3 centimeters long; a second example
measured 2.8 centimeters in length, and was essentially similar, but
showed that a considerable amount of variability must prevail at this
stage of development. It was pitchy black, with a very indistinct
subdorsal line and a few ring-spots, the “mirrors” of which were also
sulphur-yellow, with the orange-red nucleus. The shagreening was quite
as strong as in the first specimen, the dots being yellow instead
of white. It is specially to be observed, because of its important
theoretical bearing, that in this larva the ring-spots were absent on
the three front segments, and on the fourth only, a faint indication
of one could be perceived. In the caterpillar figured the ring-spots
increase also in distinctness from the tail to the head.


_Fifth Stage._

The two specimens just mentioned, after moulting, acquired the
well-known markings of the adult caterpillar already briefly described
above. The fifth is the last stage.

The larva is known to occur in several variations, Rösel having figured
it in three forms; light green, olive-green, and dirty yellow. It has
not been since considered worth the trouble to attend to the subject of
caterpillar coloration. Thus, Wilde,[92] in his well-known work, takes
no notice of Rösel’s observation, but simply describes the caterpillar
of _Galii_ as “blackish olive-green.”

Having had an opportunity of observing twenty-five adult specimens of
this somewhat scarce species at one time, I am able to state that it
is not in this instance di- or polymorphism, but a case presenting a
great degree of variability, with which we have to deal. There are
not several sharply-defined types of coloration; but the extremes are
connected by numerous intermediate forms. The extreme forms, however,
certainly preponderate.

I have never met with Rösel’s light green form; neither was there
a dark green specimen among the twenty-five mentioned, and I
only know this variety from single individuals, found at a former
period. Among the twenty-five caterpillars; all gradations of colour
occurred, from pitchy black to light clay-yellow, and even to an
almost whitish-yellow; some were brownish-black, others of a beautiful
chestnut-brown, and others yellowish brown, dark clay-yellow, or
brownish-red. Out of twenty-one specimens of which the ground-colours
were noted, there were nine black, nine clay-yellow, and three brown;
each of the three groups again showing various minor modifications of
colour. The other colours also varied somewhat. Thus, the “mirrors”
were sometimes white, sometimes strong yellow, and occasionally they
also contained a reddish nucleus.

The variations in the shagreening were especially interesting, inasmuch
as these appeared to have a striking connection with the general
colouring of the caterpillar. Black specimens seldom show such sparse
shagreening as that represented in Pl. V., Fig. 46, but are generally
thickly scattered with large shagreen-dots right up to the dorsal line
(Fig. 47, Pl. VI.), then strikingly resembling the adult larva of _D.
Euphorbiæ_. The light ochreous-yellow individuals, on the other hand,
were sometimes entirely without shagreening (Fig. 48, Pl. VI.), being
smooth, and much resembling the light ochreous-yellow or yellowish-red
caterpillar of _D. Nicæa_ (Fig. 51, Pl. VI.). I have never seen a
caterpillar of _Galii_ which showed traces of the subdorsal line in
the last stage, nor have I ever met with one which possessed a second
row of “mirror” spots; so that retrogression or a sudden advance in
development does not appear to occur.

Of the North African _D. Mauritanica_, which likewise belongs to the
_Galii_ group, I have not been able to obtain specimens or figures
of the younger stages. The adult caterpillar is very similar to that
of _Euphorbiæ_, but differs in the absence of the second row of
ring-spots. For this reason it must be regarded as a retarded form at
an older stage of phyletic development.

I now proceed to the _Livornica_ group.


DEILEPHILA LIVORNICA, ESPER.

This, the only European species here to be considered, possesses
almost the same markings as _Galii_ in its fourth stage, _i.e._, a
subdorsal line with interpolated ring-spots. The species is known to
be rare, and I have not been able to obtain living specimens, but I
have examined several blown larvæ, all of which agree in having the
ring-spots sharply distinct from the whitish subdorsal line, so that
the latter is thereby interrupted. Figures of the adult larva are given
in the works of Hübner, Boisduval, and Duponchel. In most specimens
the ground-colour is brown, although Boisduval[93] also figures a
light green specimen; from which it may be inferred, from analogy with
_Galii_ and _Vespertilio_, that the first stages are green. In Dr.
Staudinger’s collection there is a young larva, probably in the fourth
stage, the ground-colour of which is light ash-grey. The dorsal and
subdorsal lines are white, the latter showing in the positions where
the ring-spots subsequently appear, small white “mirrors” with red
nuclei, exactly corresponding to the stage of _Vespertilio_ represented
in Fig. 49 A, Pl. VI. The “mirrors” are nothing more than dilatations
of the subdorsal line, which is not therefore interrupted by them. The
black “ground-area” does not surround the “mirrors” completely, but
borders them only above and below, and is much more strongly developed
above, extending in this direction to the dorsal line.

The fourth group comprises the two species _D. Lineata_, Fabr., and _D.
Zygophylli_, Ochs., the former being the North American representative
of our _D. Livornica_, but differing in remaining permanently at the
fourth stage of this last species. I am acquainted with _D. Lineata_
only through the figure of the adult larva given by Abbot and Smith,
which figure, judging from the position and form of the spots, I
am compelled to believe is not quite correct, notwithstanding the
excellence of the other illustrations. The ground-colour of the
caterpillar is green; the subdorsal yellow, bordered with black,
slightly curved, arched lines, which nowhere interrupt its continuity.
This North American species appears therefore to be an older form than
our _Livornica_.


DEILEPHILA ZYGOPHYLLI, OCHSENHEIMER.

This species, which is the next allied form to _D. Lineata_, is
an inhabitant of Southern Russia. I have seen four specimens of
the caterpillar in Dr. Staudinger’s collection, three of which are
certainly in the last ontogenetic stage. The ground-colour appears
ash-grey, ash-brown, or blackish with whitish granulations. A broad
white subdorsal line extends to the base of the black caudal horn, this
line in one specimen appearing at first sight not to possess a trace of
spot rudiments (Fig. 50). On closer investigation, however, there could
be observed, in the same position where the ring-spots stand in the
other species of _Deilephila_, small black crescents above and below
the subdorsal line. In other specimens the white subdorsal line had
also become expanded in these positions into distinct spots; indeed, in
one individual light white mirror-spots, bordered above and below by
black crescents, stood on the subdorsal line (Fig. 50 A).

It is thus in this distinguishing character that the caterpillar is
extremely variable, and we may suppose either that this species is now
in a state of transition to a higher stage of phyletic development, or
else that the ring-spots were formerly more strongly developed, and
are now degenerating. The developmental history of the larva could
alone decide which of these two views is correct. There would be no
difficulty in procuring materials for this purpose if one of the
numerous and zealous Russian naturalists would take up the subject.


DEILEPHILA HIPPOPHAËS, ESPER.

This is the only representative of the fifth and oldest group known to
me. The moth resembles _D. Euphorbiæ_ to the extent of being sometimes
confounded with it, a circumstance which is made the more remarkable by
the fact that the caterpillars are so completely different.

The adult larva of this local moth has been made known by the figures,
more or less exact, in the works of Hübner, Boisduval, and Duponchel.
Wilde also gives a description of it, although from a foreign source.
I will not here delay myself by criticizing the different descriptions
and figures; they are partly correct, partly inexact, and sometimes
altogether erroneous; they were of no avail for the question which here
primarily concerns us, and new observation had to be undertaken.

I have been able to compare altogether about forty caterpillars,
thirty-five of which were living. All these specimens possessed nearly
the same greyish-green ground-colour, and most of them had exactly
the simple marking as represented, for instance, in Hübner’s figure,
_i.e._, a rather broad greenish-white subdorsal line, somewhat faded
at the edges, and without a trace of spots on any of the segments
with the exception of the eleventh, on which there was a yellowish,
black-bordered mirror-spot, with a broad, diffused, vivid orange-red
nucleus. Specimens also occur, and by no means uncommonly, in which no
other markings are to be seen than those mentioned; there were nine
among twenty-eight examples compared from this point of view.

In many other individuals of this species small red spots appear on
the subdorsal line, exactly in the positions where the ring-spots are
situated in the other species of the genus (Fig. 60), so that these
spots are thus repetitions of the single ring-spot--a fact which must
appear of the greatest interest in connection with the development
of the markings throughout the whole genus. But this is not all, for
again in other specimens, these red spots stand on a large yellow
“mirror,” and in one individual (Fig. 59), they had become developed
into well-formed ring-spots through the addition of a black border. We
have thus presented to us in one and the same stage of a species, the
complete development of ring-spots from a subdorsal line.

These facts acquire a still greater interest, as showing how new
elements of marking are produced. The spots on the subdorsal line
decrease from the posterior to the anterior segments, so that they
must undoubtedly be regarded as a repetition or transference of the
ring-spot previously developed on the eleventh segment. I will now
proceed to furnish proofs in support of this statement.

I have never met with any specimens having ring-spots on all the
segments--in the most prominent instances these spots were present
on segments 10-5. This was the case in three out of the twenty-eight
caterpillars minutely examined. On all these segments, however, the
ring-spots were not equally developed, but increased in perfection
from the posterior towards the anterior segments. In the larva
represented in Fig. 59 for example, there is a completely developed
ring-spot on segment 10, which, although possessing but a feeble black
“ground-area,” is still distinctly bordered; on segment 9 this border
is less sharp, and not so dark, and it is still less sharp and much
lighter on segments 8 and 7, whilst it has completely disappeared from
segment 6, the yellow “mirror” having at the same time lost in size. On
segment 5, only two small contiguous reddish spots, the first rudiments
of the nucleus,[94] can be recognized on close inspection.

Specimens in which the spots extend from the eleventh to the seventh
segment are of more frequent occurrence, five having been found
among the twenty-eight. In these the spots diminish anteriorly in
size, perfection, and intensity of colour. Still more frequently (in
eleven specimens) are the ring-spots or their rudiments restricted to
the tenth and ninth segments, the spot on the latter being without
exception less developed than that on the former segment.

An anteriorly progressing formation of ring-spots thus undoubtedly
occurs, the spots generally diminishing in perfection very suddenly
towards the front segments; and specimens, such as that represented in
Fig. 60, Pl. VII., in which traces of ring-spots are to be seen on all
the segments from the tenth to the fifth, are of rare occurrence.

From what elements of marking are these _secondary_ ring-spots
resulting from transference developed? They do not, as in the case
of the _primary_ eye-spots of the _Chærocampinæ_, originate in the
separation of one portion of the subdorsal line, and the subsequent
formation of this detached spot into a “mirror;” but they arise from
the formation of a nucleus, first one and then two of the shagreen-dots
on the subdorsal line acquiring a yellowish or reddish colour (Fig.
61, Pl. VII., segments 6 and 7). The ground on which these two spots
are situated then becomes yellow (Fig. 61, Pl. VII., segment 8), and
a more or less distinct black border, having the form of two small
crescents, is afterwards formed. At a later period these two crescents
and also the two primary nuclei coalesce, producing a ring-spot which,
as in Fig. 61, Pl. VII., segment 9, can be distinctly resolved into two
portions.

It certainly cannot be denied that these facts may also be
theoretically interpreted in a reverse sense. We might interpret
the phenomena in this case, as also in that of _D. Zygophylli_, as
a gradual disappearance from the front towards the hind segments of
ring-spots formerly present, a view which could only be refuted by the
ontogeny of the species. I have not been fortunate enough to procure
eggs of _D. Hippophaës_, so that the younger stages are unknown to me.
Among my caterpillars, however, there were two in the fourth stage of
development, but these did not show ring-spots on all the segments, as
we should expect on the above view; on the contrary, no trace of such
spots could be seen on any of the segments with the exception of the
eleventh, on which there was a ring-spot less perfectly developed than
in the last stage.

In this fourth stage the larva of _D. Hippophaës_ is of a lighter green
(Fig. 58), the subdorsal yellowish with sharp boundaries, and the
infra-spiracular line pure white, as in the next stage. The shagreening
is present, but none of the shagreen-dots are red or reddish, and no
trace of a ring-spot can be detected on the subdorsal line with the
exception of that on the eleventh segment. In this last position this
line is somewhat widened, and a long, diffused, rose-red spot can there
be recognized upon it (Fig. 58 A). The black “ground-area” present
in the fifth stage is as yet absent, and the spot is not so sharply
separated anteriorly from the subdorsal line as it becomes later.

From these observations we might venture to expect that in the third
stage of _Hippophaës_, the subdorsal line would also be free from this
spot on the eleventh segment, and it is possible that in the second
stage this line is itself absent.


THE GENUS DEILEPHILA: SUMMARY OF FACTS AND CONCLUSIONS.

Regarding only the _adult_ larvæ of the species of _Deilephila_,
these represent in their five groups, five stages in the phyletic
development of the genus; but if we also take into consideration the
developmental history, two more stages must be added, viz., that in
which the caterpillar possesses no particular marking, as was found to
be the case in the first stage of the development of _D. Euphorbiæ_
and _D. Dahlii_; and a second stage with a subdorsal line, but without
any ring-spot formations. Seven stages of phyletic development must
therefore be distinguished.

_Stage 1._--No species with entire absence of marking in the adult form
now occurs.

_Stage 2._--A subdorsal, accompanied by a spiracular line, extends from
the caudal horn to the first segment. This also no longer forms the
final stage of the ontogeny, but is, however, undoubtedly retained in
the second stage of several species (_D. Vespertilio_, _Livornica_,
_Lineata_, and perhaps also _Galii_).

_Stage 3._--The subdorsal line bears a ring-spot on the penultimate
segment; the other markings as in the last stage. _D. Hippophaës_ only
belongs to this stage, a small number of specimens, however, showing
a transition to the following stage by the transference of ring-spots
from the posterior to the anterior segments.

_Stage 4._--Open ring-spots appear on the subdorsal line on all the
segments from the eleventh to the first. _D. Zygophylli_ and the North
American _D. Lineata_ belong here.

_Stage 5._--Closed ring-spots are situated on the subdorsal line. Of
the known species, only _D. Livornica_ concludes its development at
this phyletic stage.

_Stage 6._--A single row of ring-spots replaces the subdorsal line. _D.
Galii_, _Vespertilio_, and _Mauritanica_ represent this stage at the
conclusion of their ontogeny.[95]

_Stage 7._--A double row of ring-spots. Only _D. Dahlii_, _Euphorbiæ_,
and _Nicæa_ attain to this highest stage of _Deilephila_ marking, the
two first species in the fourth stage, and _Nicæa_ in the third stage
of its ontogeny.

Although our knowledge of the history of the development of the
individual species is still so fragmentary, we may conclude with
certainty that the development of the markings has been uniform
throughout--that it has proceeded in the same manner in all species.
All the species appear to be making for the same goal, and the
question thus arises whether there may not be an innate force
urging their phyletic development. The rigorous examination of this
conception must be reserved for a later section. Here, as we are
only occupied essentially in establishing facts, it must be remarked
that retrogression has never been observed. The young larval forms
of a species never show the markings of a later phyletic stage than
the older larval forms; the development takes the same course in all
species, only making a greater advance in the same direction in some
than in others.

Thus, _Nicæa_ and _Euphorbiæ_ have advanced to the seventh phyletic
stage, _Zygophylli_ and _Hippophaës_ only to the third, and some
specimens of _Zygophylli_ to the fourth. But at whatever phyletic
stage the ontogeny of a species may terminate, the young larval
stages always display the older phyletic stages. Thus, _Galii_ in its
last ontogenetic stage reaches the _sixth_ phyletic stage; in its
penultimate stage it reaches the _fifth_ phyletic stage; and in its
third stage; the _fourth_ phyletic stage is represented, so that little
imagination is required to anticipate that in the second stage the
_third_ or _second_ phyletic stage would be pictured.

If we tabulate the development of the various species, indicating the
ontogenetic stages by Arabic numerals, and the stages of the phylogeny
which are reached in each stage of the ontogeny by Roman numerals, we
obtain a useful synopsis of the series of developments, and, at the
same time, it shows how many gaps still remain to be filled up in order
to complete our knowledge even of this small group of species.


TABLE OF DEVELOPMENT OF THE SPECIES OF DEILEPHILA.

  +-----------------+---------+---------+---------+---------+---------+
  |   Deilephila.   | Ontogeny| Ontogeny| Ontogeny| Ontogeny| Ontogeny|
  |                 | Stage 1.| Stage 2.| Stage 3.| Stage 4.| Stage 5.|
  +-----------------+---------+---------+---------+---------+---------+
  |  1. Hippophaës  |    ?    |    ?    |    ?    |   III.  | III.-IV.|
  |  2. Zygophylli  |    ?    |    ?    |    ?    |    ?    | III.-IV.|
  |  3. Lineata     |    ?    |    ?    |    ?    |    ?    |   IV.   |
  |  4. Livornica   |    ?    |    ?    |    ?    |   IV.   |    V.   |
  |  5. Galii       |    ?    |    ?    |   IV.   |    V.   |   VI.   |
  |  6. Vespertilio |    ?    | II. (?) |   IV.   |    V.   |   VI.   |
  |  7. Mauritanica |    ?    |    ?    |    ?    |    ?    |   VI.   |
  |  8. Dahlii      |    I.   |    ?    |   VI.   |   VII.  |  VII.   |
  |  9. Euphorbiæ   |    I.   |    V.   |   VI.   |   VII.  |  VII.   |
  | 10. Nicæa       |    ?    |    ?    |  VII.   |   VII.  |  VII.   |
  +-----------------+---------+---------+---------+---------+---------+

From this very incomplete table we perceive that, in certain instances,
the stages can be represented as a continuous series of phyletic
steps, as in the case of _D. Galii_; that in others certain steps
may be omitted, as with _D. Euphorbiæ_, in which grade I. of stage 1
is immediately followed by grade V. in stage 2. In reality the gap
caused by this omission is still greater than would appear, as grade
V. is only indicated, and not actually reached, the subdorsal not
being present as a sharply-defined line, but only as a faint stripe.
The suppression of phyletic steps increases with the advancement in
phyletic development. The higher the step to which a species finally
attains, the greater is the tendency of the initial stages to be
compressed, or omitted altogether.

From what has thus far been seen with respect to the development of
_D. Hippophaës_, there may be drawn what to me appears to be a very
important conclusion, viz. that the ring-spots of _Deilephila_ first
originated on the segment bearing the caudal horn, and were then
gradually transferred as secondary spots to the preceding segments.
Complete certainty would be given to this conclusion by a knowledge
of the young forms of other phyletically retarded species, especially
those of the American _D. Lineata_, and perhaps also those of
_Zygophylli_ and _Livornica_. The few observations on the development
of _D. Galii_ already recorded give support to this view, since
the absence of ring-spots on the three front segments in the young
caterpillar (one instance), or their less perfect formation on these
segments (second instance), indicates a forward transference of the
spots.

If the foregoing view be accepted, there follows from it a fundamental
difference between the development of the genera _Chærocampa_ and
_Deilephila_. In the former the formation of the eye-spots proceeds
from a subdorsal line, but they first appear on two of the front
segments, and are then transferred to the _posterior_ segments. In
_Deilephila_, on the other hand, a single ring-spot is formed on the
penultimate segment bearing the caudal horn, and this is repeated on
the _anterior_ segments by secondary transference. With respect to
the origination of the ring-spot also, there is a distinction between
this genus and _Chærocampa_, inasmuch as the first step towards the
eye-formation in the latter consists in the separation of a curved
portion of the subdorsal line, whilst in _Deilephila_ the nuclear
spot first seems to originate and the separation of the mirror-spot
from the subdorsal line appears to occur secondarily. It is difficult
here to draw further conclusions, since the first appearance of the
primary ring-spot has not yet been observed, and no more certain
inference respecting the history of the formation of the _primary_
ring-spots can be drawn from the manner in which the _secondary_
ring-spots are formed. Because in _Hippophaës_ the formation of the
secondary ring-spots begins with the red coloration of one or two
shagreen-dots, it does not follow that the primary spot on the eleventh
segment also originated in this manner; and this is not without
importance when we are concerned with the causes which underlie the
formation of ring-spots. In _Chærocampa_ also, the formation of the
primary eye-spots appears to differ from that of the secondary--in the
latter the black “ground-area” first appearing, and in the former the
“mirror-spot.” The secondary eye-spots certainly remain rudimentary in
this last genus, so that the evidence in support of this conclusion is
thus much weakened; but it must be admitted that we are here on ground
still too uncertain to admit of wider conclusions being based thereon.

As a final result of the investigation, we may advance the opinion
that the existing species of the genus _Deilephila_ have reached five
different phyletic stages, and that their very different external
appearance is explained by their different phyletic ages; the
appearance from these caterpillars of moths so extremely similar, can
otherwise be scarcely understood.

It may appear almost unnecessary to bring forward additional proofs in
support of this interpretation of the facts, but in a field where the
data are so scanty, no argument which can be drawn from them should be
considered as superfluous. The variations which occasionally occur
in the larvæ, however, to a certain extent furnish a proof of the
correctness of the theoretical interpretation offered.

When, in the ontogeny of these species, we actually see before us
a series of stages of phyletic development, we must admit that
ordinary reversion may occur, causing an adult caterpillar to show
the characters of the young. Forms reverting to an earlier phyletic
stage must, on the whole, occur but seldom, as this stage is removed
further back in the ontogeny. Thus, indications of the subdorsal line
must occur but rarely in the _adult_ larvæ of _Euphorbiæ_, and still
less frequently in _Nicæa_, whilst they must be expected to be of more
common occurrence in _Vespertilio_, and also, as has already been
seen, in _Dahlii_. In this last species, as also in _Vespertilio_,
the completely-developed subdorsal line is still present in the third
stage, whilst it is possessed by _Euphorbiæ_ only in the second stage,
and then in a rudimentary condition.

The state of affairs may in fact be thus described: Among several
hundred adult larvæ of _Dahlii_ found in Sardinia by Dr. Staudinger,
there were some which did not actually possess a distinct subdorsal
line, but in place thereof, and as its last indication, a feeble light
stripe. One of Dr. Staudinger’s caterpillars showed also a distinct
line between the closed eye-spots. In the last stage of _Vespertilio_
this line appears still more frequently, whilst in _Euphorbiæ_ it is
extremely rare, and when present it only appears as a faint indication.
This is the case with one of the specimens figured in Hübner’s work as
an “aberration,” and also with one in Dr. Staudinger’s collection. Of
_Nicæa_ I have at most seen only eight specimens, none of which showed
any trace of the long-vanished subdorsal line.

It must be expected that any ontogenetic stage would most readily
revert to the preceding phyletic stage, so that characters present
in the preceding stage are consequently those which would most
commonly arise by reversion. This postulate of the theory also finds
confirmation in the facts. Caterpillars which, when full grown,
belong to the _seventh_ phyletic stage, _e.g._ _D. Euphorbiæ_, not
unfrequently show variations corresponding to the _sixth_ stage,
_i.e._ only one instead of two rows of ring-spots--the upper and
first-appearing series. On the other hand, forms reverting to the
_fifth_ phyletic stage (ring-spots with connecting subdorsal line)
occur but very rarely. I have never met with such cases in adult
living caterpillars of _D. Euphorbiæ_, although in one instance such
a larva was found in the fourth ontogenetic stage; but the strikingly
dark, brownish subdorsal line which connected the otherwise perfectly
developed ring-spots, completely disappeared in the fifth stage of the
ontogeny. Those larvæ which, in the adult state, belong to the _sixth_
phyletic stage, not unfrequently show the characters of the _fifth_
stage more or less developed, as, for example, _D. Vespertilio_.[96]


THE GENUS SMERINTHUS, LATREILLE.

The caterpillars of this genus are very similar in appearance, and all
possess extremely simple markings. The occurrence of numerous stages
of development of these markings is thus excluded, and the study of the
ontogeny therefore promised to furnish less information concerning the
phyletic development of the genus than in the case of the preceding
genera. This investigation has nevertheless also yielded interesting
results, and the facts here recorded will be found of value in likewise
throwing light on the causes which have produced the markings of
caterpillars.

I shall commence, as in former cases, with the developmental history.
I have easily been able to obtain fertile eggs of all the species of
_Smerinthus_ known to me. Impregnated females laid large numbers of
eggs in confinement, and also bred females of the commoner species can
readily be made to copulate, when pinned, and exposed in a suitable
place in the open air. A male soon appears under these circumstances,
and copulation is effected as readily as though the insect were not
fastened in the way indicated.


SMERINTHUS TILIÆ, LINN.[97]

The light green eggs are nearly spherical, and after fourteen days
(beginning of July) the young larvæ emerge. These are also of a light
green colour, and are conspicuous for the great length of the caudal
horn, which is nearly half as long as the body. This horn is likewise
of a light green at first, but becomes dark violet in the course of an
hour. No trace of any markings can be detected at this stage.

As soon as the caterpillars are hatched they commence to nibble the
empty egg shells; then they run about with great activity, and after
several hours take up their position on the largest vein on the under
side of the lime leaves, where they remain for a long period. In this
situation they have the same form and colour as the leaf-vein, and
are very difficult to discover, which would not be the case if they
reposed obliquely or transversely to the vein. In about 4-5 days the
caterpillars undergo their first moult, and enter upon the second
stage. On each side of the segments 11-4, there now appear seven
oblique whitish stripes on a somewhat darker green ground; these
slope in the direction of the caudal horn. Owing to the transparency
of the skin, a dark green dorsal line appears in the position of the
underlying dorsal vessel, the green contents of the alimentary canal
being distinctly visible through the absence of adipose matter in the
tissues. The larvæ possess also a fine whitish subdorsal line, which
extends from the horn to the head. The horn at this stage becomes black
with a yellowish red base.

In the third stage, which occurs after six or seven days, the oblique
stripes appear darker, and the subdorsal line disappears.


_Fourth Stage._

After another period of 4-5 days the third moult takes place, and there
now commences a dimorphism which will perhaps be better designated
as variability, since the two extremes are connected by transitional
forms. The majority of the larvæ have, as in the preceding stage, pure
white oblique stripes, but many of them possess a blood-red spot on
the anterior side of the stripes, this spot showing all gradations
in size and depth of colour between maximum development and a mere
trace. Special interest attaches to these spots, as they are the first
rudiments of the coloured border of the oblique stripes which occurs in
so many _Sphinx_ caterpillars.

In the fifth stage--the last of the larval development--the red spots
become more strongly pronounced. Among eighty caterpillars from one
brood there were about twenty without any red whilst the remainder
were ornamented with more or less vivid blood-red spots, often large
and irregular in form. In some specimens the spots had become drawn
out into lines,[98] forming a coloured edge to the oblique white
stripes, similar to that possessed by the larva of _Sphinx Ligustri_.
The caterpillar is thus represented in many figures, but generally
the coloured stripe is made too regular, as in reality it is always
irregularly defined above, and never so sharp and even as in _Sphinx
Ligustri_. The character is here obviously not yet perfected, but is
still in a state of development.


SMERINTHUS POPULI, LINN.

From green spherical eggs there emerged larvæ 6.5 millimeters in length
without any markings. They were of a light greenish-white, the large
head and long caudal horn being of the same colour. The posterior
boundary of the segments appears as a light shining ring (Pl. VI. Fig.
55).

The characteristic markings of the genus appear on the following day
without the occurrence of any moult: seven oblique white stripes arise
from near the dorsal line, and extend along the sides in a direction
parallel to that of the horn. On the three front segments they are
represented only by three small white spots (Fig. 56). The caterpillar
likewise possesses a marking of which the adult species of the genus
retain only a trace, viz., a well-developed, pure white subdorsal line,
which is crossed by the six anterior oblique stripes, and uniting with
the upper part of the seventh extends to the caudal horn.

I long believed that the markings described were first acquired in
the second stage, as I was possessed with the generally accepted idea
that the changes of form and colour in insects could only occur at the
period of ecdysis. I at first thought that the moult had escaped my
notice, and I was only undeceived by close observation of individual
specimens.


_Second Stage._

The first moult took place after five days, the larvæ being 1.4
centimeters in length. Only unimportant changes of marking are
connected therewith. The subdorsal line loses much in thickness and
definition, and the first and last of the oblique stripes become
considerably broader than the intermediate ones (Fig. 57). The green
ground colour and also the stripes acquire a yellowish hue.

On the other hand, there occur changes in form. The head, which was at
first rounded, becomes of the characteristic triangular shape, with the
apex upwards, common to all the species of the genus, and at the same
time acquires two white lines, which unite above at the apex of the
angle. The shagreening of the skin now also takes place, and the red
spot at the base of the horn is formed.

There appears to be at this stage a general tendency for the suffusion
of red, the thoracic legs also becoming of this colour.


_Third Stage._

The second moult occurs after six or eight days, the marking only
changing to the extent of the subdorsal line becoming still more
indistinct. This line can now only be distinctly recognized on the
three front segments in a few individuals, whilst in the majority it is
completely absent. Sometimes the ferruginous red spots on the oblique
stripes now appear, but this character is not completely developed till
the fifth stage. Out of about ninety bred specimens in which I followed
the entire development, only one possessed such spots, and these were
situated on both sides of the sixth segment.


_Fourth Stage._

The third moult, which takes place after another period of six days, is
not associated with any change of marking.

In this stage also I observed in one specimen (not the one just
mentioned) the ferruginous spots, and again only on the sixth segment.
On account of the theoretical conclusions which may be drawn from this
localization of the spots--supposing it to be of general occurrence--it
becomes of importance to institute observations with different broods,
so as to investigate their first appearance, frequency, and local
limitation. It appears to me very probable that, with respect to
frequency and time of appearance, there would be great differences,
since, in the last stage, it is just this character which shows a great
variability. It would be more remarkable if it should be established
that the first appearance of the spots was always limited to a certain
segment; and there would then be a great analogy with the first
appearance of the eye-spots in _Chærocampa_ and the ring-spots in
_Deilephila_.


_Fifth Stage._

The adult caterpillar does not differ in marking to any considerable
extent from the preceding stages. The first and last stripes do not
appear larger than the intermediate ones, as the latter now increase
in size. Many specimens were entirely without red spots; in others
they were present, but were small and inconspicuous, whilst in others
again there were two spots, one above the other, of a vivid ferruginous
red, these coalescing in some cases, and thus forming one spot of a
considerable size. I have never seen these spots formed into a regular,
linear, coloured border to the white oblique stripes--as occasionally
happens in _Tiliæ_--either in living specimens, blown larvæ, or in
figures.


SMERINTHUS OCELLATUS, LINN.

The green eggs much resemble those of _Populi_, as also do the newly
hatched caterpillars, which, as in the case of this last species, are
entirely without markings. As with _Populi_, the markings are formed in
the course of the first stage, and are distinctly visible before the
first moult. The long caudal horn is of a red colour.

After two to three days the caterpillars moult, their length then being
one centimeter; the seven beautiful oblique white stripes, and the fine
white subdorsal line, are more strongly pronounced, the latter becoming
broader in front. They differ from _Populi_ in having the oblique
stripes united in the dorsal line.

The second moult occurs after another three days, and brings no
important change; only the fine subdorsal line becoming somewhat
fainter. Neither is the third moult, which takes place four days later,
associated with the appearance of any essentially new character. The
oblique stripes remain as before, but their upper portions now stand
on a somewhat darker green ground-colour, whilst the subdorsal line
vanishes, leaving distinct traces only on the three or four front
segments.

The fourth moult follows after a period of seven days, and my bred
larvæ underwent scarcely any alteration in marking. Only small
differences in coloration became perceptible in the head and horn,
these changing to bluish. Specimens occur, although but rarely, which
show in this last stage red spots in the vicinity of the oblique
stripes, just in the same manner as with _Populi_, in which species,
however, they occur more commonly. I only once found an adult larva of
_Ocellatus_ possessing reddish-brown spots above and below the oblique
stripes,[99] exactly as in one of the specimens figured by Rösel.[100]

In this stage also there remains almost always on the three to six
front segments, a more or less distinct residue of the subdorsal, which
extends backwards from the head as a whitish line intersecting the
foremost oblique stripes. (Fig. 70, Pl. VII.)


RESULTS OF THE DEVELOPMENTAL HISTORY OF SMERINTHUS TILIÆ, POPULI AND
OCELLATUS.

From the meagre materials furnished by these three obviously nearly
related species, we may at least conclude that, with respect to
marking, three stages of development can be distinguished:--(1) Simple
(green) coloration without marking; (2) subdorsal lines crossed by
seven pairs of oblique stripes; (3) more or less complete absence of
the subdorsal lines, the oblique stripes remaining, and showing a
tendency to become edged with a red border.

Which of the three species is the oldest I will not attempt to decide.
If we might venture to form any conclusion from the frequency of the
red spots, _Tiliæ_ would be the youngest, _i.e._, the species which
has made the farthest advance. But this does not agree with the fact
that the oblique stripes appear somewhat later in this species. Both
these distinctions are, however, too unimportant to enable us to
build certain conclusions on them. Neither does a comparison of the
adult larvæ with other species of _Smerinthus_ furnish any further
information of importance.

Of the genus _Smerinthus_, Latr., thirty species were catalogued by
Gray,[101] of which I am only acquainted with the larvæ of eight (five
European, and three North American). None of these in the last stage
possess a complete subdorsal line together with oblique stripes.
Neither, on the other hand, do any of them show a more advanced stage
of development in having the red spots constantly formed into coloured
border-stripes. We must therefore admit that they have all reached
nearly the same stage of phyletic development. On turning to the
doubtfully placed genus _Calymnia_, Boisduval, which is represented in
Gray by only one species, figured by Westwood[102] as a _Smerinthus_,
we first meet with an older stage of development of the genus.

The adult caterpillar of _C. Panopus_, from the East Indies, possesses,
in addition to the oblique stripes, a completely developed subdorsal
line,[103] and thus corresponds to the first stage of _S. Populi_.
This species may possibly retain in its ontogeny a stage in which
the oblique stripes are also absent, whilst the subdorsal line is
present. From the early disappearance of the subdorsal line in the
species of _Smerinthus_, we may venture to conclude that this character
appeared at an early stage of the phylogeny, whilst the oblique stripes
represent a secondary form of marking, as shall be further established
subsequently.[104]


THE GENUS MACROGLOSSA, OCHSENHEIMER.

The adult larvæ of five species are known, and to these I can now
add a sixth. In Gray the genus contains twenty-six species.[105] I
cannot find any figures or descriptions of the young stages of these
caterpillars, and I have myself only observed the complete ontogeny of
one species.

By placing a captured female _M. Stellatarum_ in a capacious
breeding-cage, in the open air, I was enabled to procure eggs. The
moth hovered about over the flowers, and laid its small, grass-green,
spherical eggs (partly when on the wing), singly, on the leaves, buds,
and stalks of _Galium Mollugo_. Altogether 130 were obtained in three
days.[106]


_First Stage._

After about eight days the caterpillars emerge. They are only two
millimeters in length, and are at first yellowish, but soon become
green, set with small single bristles, and they possess a short
greenish caudal horn, which afterwards becomes black. The head is
greenish-yellow. The young larvæ are entirely destitute of marking.
(Pl. III., Fig. 1).


_Second Stage._

The first moult takes place after four days, the caterpillar now
acquiring the marking which it essentially retains to pupation.

Fine white subdorsal and spiracular lines appear, and at the same time
a dark green dorsal line, which, however, does not arise from the
deposition of pigment, as is generally the case, but from a division in
the folds of the fatty tissue along this position. (Fig. 2, Pl. III.)

The colour is now dirty green in all specimens, the skin being finely
shagreened.


_Third Stage._

The second moult, occurring after another period of four days, does not
bring any change of marking, the colour only becoming somewhat darker.
Length, twelve millimeters.


_Fourth Stage._

The third moult (after another four days) likewise brings only a
change of colouring, which is of such a nature that the caterpillar
becomes dimorphic. At the same time that peculiar roughening of the
skin takes place which, in the case of _Chærocampa_, was designated as
“shagreening.” The colour is now light grass-green in some specimens,
and dark green in others; in these last the subdorsal line is edged
above with dark brown, and the spiracles are also of this colour.
Length, seventeen millimeters.


_Fifth Stage._

Four days later, after the fourth ecdysis, the dimorphism becomes a
polymorphism. Five chief types can be distinguished:--

_Variety I._--Light green (Fig. 7, Pl. III.); dorsal line,
blackish-green, strongly marked; subdorsal line broad, pure white,
edged above with dark green; spiracular line, chrome-yellow; horn,
black, with yellow tip and blue sides. Spiracles, blackish-brown, with
narrow yellow border; legs, and extremities of prolegs, vermilion-red.

_Variety II._--Blackish-brown (Fig. 6, Pl. III.); head and prothorax,
yellowish-brown; markings the same as above.

_Variety III._--Blackish-green or greenish-black (Figs. 10 and
11, Pl. III.); subdorsal line with blackish-green border above,
gradually passing into a light green ground-colour; spiracular line,
chrome-yellow; head and prothorax, greenish-yellow.

_Variety IV._--Light green (Figs. 4 and 12, Pl. III.); dorsal line
quite feeble; subdorsal broad, only faintly edged with dark green;
subspiracular line, faint yellowish; head and prothorax, green.

_Variety V._--Brownish-violet (Fig. 8, Pl. III.); the black dorsal line
on a reddish ground either narrow or broad.

From these five varieties we see that the different types do not
stand immediately next to one another; they are, in fact, connected
by numerous transitional forms, the ground-colour varying greatly,
being dark or light, yellowish or bluish. (Compare Figs. 4, 5, 7, and
12.) The markings remain the same in all, but may be of very different
intensities. The dorsal line is often only very feebly indicated, and
the subdorsal line is frequently but faintly edged; the latter is
also sometimes deep black above and bordered rather darkly beneath,
the sides then being of a dark green, often with blackish dots on
the yellow spiracular line (Fig. 5, Pl. III.), this likewise being
frequently edged with black. Only the horn and legs are alike in all
forms. The green ground-colour passes into blackish-green, greenish or
brownish-black, and again, from reddish-brown to lilac (Fig. 3), this
last being the rarest colour.

The designation “polymorphism” may here appear very inapplicable,
since we have no sharply distinct forms, but five very variable
ground-colours connected by numerous intermediate modes of coloration.
Should, however, the term “variability” be suggested, I am in
possession of an observation which tends to show that the different
colours have to a certain extent become fixed. I found a brown
caterpillar, the five front segments of which were light green on
the left side, and the fifth segment brown and green mixed (Fig. 9,
Pl. III.). Such parti-coloration can evidently only appear where we
have contending characters which cannot become combined; just as in
the case of hermaphrodite bees, where one half of a segment is male
and the other half female, the two characters never becoming fused so
as to produce a truly intermediate form.[107] From this observation,
I conclude that some of the chief varieties of _Stellatarum_ have
already become so far removed from one another that they must be
regarded as intermediate fixed forms, the colours of which no longer
become fused together when they occur in one individual, but are
developed in adjacent regions. Other facts agree with this conclusion.
Thus, among the 140 adult larvæ which I bred from the batch of eggs
above mentioned, the transition forms were much in the minority. There
were forty-nine green and sixty-three brown caterpillars, whilst only
twenty-eight were more or less transitional.

On these grounds I designate the phenomenon as “polymorphism,” although
it may not yet have reached, as such, its sharpest limits. This would
be brought about by the elimination of the intermediate forms.[108]

Immediately before pupation, all the caterpillars, both green and
brown, acquire a lilac coloration. The fifth stage lasts seven days,
and the whole larval development twenty-three days, the period from
the deposition of the eggs to the appearance of the moth being only
thirty-one days.

I have treated of the polymorphism of _Stellatarum_ in detail, not
only because it has hitherto remained unknown, and an analysis of such
cases has been completely ignored,[109] but more particularly because,
it appears to me, that important conclusions can be drawn therefrom.
Moreover, such an extreme multiplicity of forms is interesting, since,
so far as I know, polymorphism to this extent has not been observed in
any insect.

The theoretical bearing of this polymorphism will be treated of
subsequently. It is not in any way connected with a more advanced
development of the markings, since _M. Stellatarum_ shows in this
respect a very low state of development. This species displays only
two stages:--(1), complete absence of all markings; and (2), a simple
subdorsal, with dorsal and spiracular lines. We must therefore admit
that the phyletic development of the markings has for a long time
remained at a standstill, or, what expresses the same thing, that the
marking which the adult larva now possesses is extremely old.

In order to complete my observations on _M. Stellatarum_, I now add
some remarks on the pupa, the colour variations of which it appeared
of importance to investigate, owing to the extraordinary variability
of the caterpillar. The pupa varies but very slightly; the ochreous
yellow ground-colour sometimes passes into reddish, and sometimes
into greenish; the rather complicated blackish-brown marking of
streaky lines is very constant, especially on the wing portions, being
at most only more or less strongly pronounced. The minute colour
variations of the pupa therefore have no connection with the colour
of the caterpillar, both green and brown larvæ furnishing sometimes
reddish-yellow and sometimes greenish-yellow pupæ.

The comparison of _M. Stellatarum_ with the other known species of the
genus, brings scarcely any addition to our knowledge of the phyletic
development. Thus, the two European species of which the caterpillars
are known, viz. _M. Fuciformis_ and _Bombyliformis_,[110] show
essentially the same markings as _Stellatarum_, the chief element being
a well-developed subdorsal line. The Indian _M. Gilia_, Herrich-Schäf.,
possesses also this line,[111] and, together with the East Indian _M.
Corythus_, Walk.,[112] has oblique stripes in addition; the stripes
do not, however, cross this line, but commence underneath it, and
probably originated at a later period than the subdorsal line. Should
this be the case, we must regard _M. Corythus_ as representing a
later phyletic stage. According to Duponchel’s figures, in both _M.
Fuciformis_ and _Bombyliformis_ small oblique stripes (red) occur near
the spiracles, but these have nothing to do with the oblique stripes
of _M. Gilia_ just mentioned, as they run in a contrary direction. Of
the two European species, I have only seen the living caterpillar of
_Fuciformis_, and this possessed no oblique stripes.

To these five species I am now enabled to add a sixth, viz.
_Macroglossa Croatica_,[113] a species inhabiting Asia Minor and
Eastern Europe, of which a specimen and notice were kindly forwarded
to me by Dr. Staudinger. The adult caterpillar much resembles that of
_M. Stellatarum_ in form and marking, but the subdorsal line appears
much less distinctly defined, and the dorsal and spiracular lines seem
to be entirely absent. The colour is generally green, but varies to
red, and the subdorsal is more distinct and sharper in the young than
in the adult larva. The markings of this species do not therefore in
any way surpass those of _Stellatarum_, but are, on the contrary, much
simpler.[114]


THE GENUS PTEROGON, BOISD.[115]

Although I am acquainted with only a small portion of the developmental
history of a single species of this genus, I will here proceed to
record this fragment, since, taken in connection with two other
species, it appears to me sufficient to determine, at least broadly,
the direction of development which this genus has taken.


PTEROGON ŒNOTHERÆ, FABR.

The adult larva, as made known by many, and for the most part good
figures, has very complicated markings, which do not seem derivable
from any of the elements of marking in the _Sphingidæ_ hitherto
considered. I was therefore much surprised at finding a young
caterpillar of this species, only twelve millimeters in length, of
a light green colour, without any trace of the subsequent latticed
marking, and with a broad white subdorsal line extending along all
the twelve segments. (Pl. VII., Fig. 63). Judging from the size and
subsequent development, this caterpillar was probably in the third
stage.

The same colouring and marking remained during the following (fourth)
stage; but in the position occupied by the caudal horn in other
_Sphingidæ_, there could now be observed the rudiment of a future
ocellus in the form of a round yellowish spot (Pl. VII., Fig. 64). The
subdorsal line disappears suddenly in the fifth stage, when the larva
becomes dark green (rarely) or blackish-brown; the latticed marking
and the small oblique stripes are also acquired, together with the
beautifully developed eye-spots, consisting of a yellow mirror with
black nucleus and ground-area (Pl. VII., Fig. 65).

The North American _Pterogon Gauræ_ and _P. Abboti_[116] also show
markings precisely similar to those of this European species in the
adult state; but in the two former the markings are of special interest
as indicating the manner in which the primary Sphinx-marking has become
transformed into that of the apparently totally different adult _P.
Œnotheræ_. _P. Gauræ_ is green, with a complicated latticed marking,
which closer observation shows to arise from the dorsal line being
resolved into small black dots, whilst the subdorsal line is broken up
into black, white-bordered triangles. This caterpillar therefore gives
fresh support to the remarkable phenomenon that the animals as well as
the plants of North America are phyletically older than the European
fauna and flora, a view which also appeared similarly confirmed by
_Deilephila Lineata_, the representative form of _D. Livornica_. In
entire accordance with this is the fact that the larva of _P. Gauræ_ is
without the eye-spot on the eleventh segment, and instead thereof still
shows the original although small caudal horn. The perfect insect also
resembles our _P. Œnotheræ_ in colour and marking, but not in the form
of the wings.

That the caterpillars of the genus _Pterogon_ originally possessed the
caudal horn we learn from _P. Gorgoniades_, Hübn.,[117] a species
now inhabiting south-east Russia, and for a knowledge of which I am
indebted to Dr. Staudinger’s collection. There are in this about
eight blown specimens, from 3.7 to 3.9 centimeters in length, which
show a marking, sometimes on a red and sometimes on a green ground,
which unites this species with the young form of _P. Œnotheræ_, viz.,
a broad white subdorsal line, extending from the small caudal horn
to the head. In addition to this, however, the caterpillar possesses
an extraordinarily broad white red-bordered infra-spiracular line, a
fine white dorsal stripe, and a similar line between the subdorsal and
spiracular, _i.e._ a supra-spiracular line.

The caterpillars in Staudinger’s collection, notwithstanding their
small size, all belong to the last stage, as the moth itself does not
measure more than 2.6 centimeters in expanse, and is therefore among
the smallest of the known _Sphingidæ_. This species has therefore in
the adult condition a marking very similar to that of _Œnotheræ_ when
young--it bears to _Œnotheræ_ the same relationship that _Deilephila
Hippophaës_ does to _D. Euphorbiæ_, only in the present case the
interval between the two species is greater. _Gorgoniades_ is obviously
a phyletically older species, as we perceive from the marking and
from the possession of a horn. We certainly do not yet know whether
_Œnotheræ_ possesses a horn in its earliest stages, although in all
probability it does so; in any case the ancestor of _Œnotheræ_ had a
horn, since the closely allied _P. Gauræ_ now possesses one.

We thus see that also in the genus _Pterogon_ the marking of the
caterpillars commences with a longitudinal line formed from the
subdorsal; an infra-spiracular or also a supra-spiracular line
(_Gorgoniades_) being added. A latticed marking is developed from the
linear marking by the breaking up of the latter into spots or small
patches, which finally (in _Œnotheræ_) become completely independent,
their connection with the linear marking being no longer directly
perceptible.


THE GENUS SPHINX, LINN.

Of this genus (in the narrow sense employed by Gray) I have only been
able, in spite of all trouble, to obtain fertile eggs of one species.
The females cannot be induced to lay in confinement, and eggs can only
be obtained by chance.

I long searched in vain the literature of this subject for some account
of the young stages of these caterpillars, and at length found, in a
note to Rösel’s work, an observation of Kleemann’s on the young forms
of _Sphinx Ligustri_, which, although far from complete, throws light
on certain points.

From a female of _S. Ligustri_ Kleemann obtained 400 fertile eggs. The
caterpillars on emerging are “at first entirely light yellowish-green,
but become greener after feeding on the fresh leaves;” the horn is also
at first light green, and then becomes “darker.” The young larvæ spin
webs, by which they fasten themselves to the leaves of their food-plant
(this, so far as I know, has not been observed in any species of
_Sphingidæ_). They moult four times, the border round the head and the
purple stripes appearing after the third moult, these stripes “having
previously been entirely white.” The ecdyses follow at intervals
of about six days, increasing to about ten days after the fourth
moult.[118]

From this short account we gather that in the third stage the marking
consists of seven oblique white stripes, which acquire coloured edges
in the fourth stage, a fact which I have myself frequently observed.
On the most important point Kleemann’s observations unfortunately give
no information--the presence or absence of a subdorsal line in the
youngest stages. That he does not mention this character, can in no way
be considered as a proof of its actual absence. I am rather inclined to
believe that it is present in the first, and perhaps also in the second
stage. There occur, however, species of the genus _Sphinx_ (_sensû
strictiori_) which possess a subdorsal line when young, as I think may
be certainly inferred from the fact that the remains of such a line
are present in the adult larva of _S. Convolvuli_.

This conclusion becomes still more certain on comparing the markings
with those of a nearly allied genus; without such comparison the
separation of the genus _Macrosila_, Boisd., from _Sphinx_ is scarcely
justifiable. If to these two genera we add _Dolba_, Walk., and
_Acherontia_, Ochs., we must be principally struck with the great
similarity in the markings, which often reaches to such an extent that
the differences between two species consist entirely in small shades of
colour, while the divergence of the moths is far greater.

Of the genera mentioned, I am acquainted altogether with fourteen
species of caterpillars:--_Macrosila Hasdrubal_, _Rustica_,[119] and
_Cingulata_;[119] _Sphinx Convolvuli_, _Ligustri_, _Carolina_,[119]
_Quinquemaculata_,[119] _Drupiferarum_,[119] _Kalmiæ_,[119] and
_Gordius_;[119] _Dolba Hylæus_;[119] _Acherontia Atropos_, _Styx_,[120]
and _Satanas_.[120] With one exception all these caterpillars possess
oblique stripes of the nature of those of the _Smerinthus_ larvæ,
and most of them are without any trace of a subdorsal line; one
species--the North American _M. Cingulata_--has a completely developed
subdorsal; and the typical European species, _S. Convolvuli_, has a
rudimentary subdorsal line. The ground-colour in most of these species
is of the same green as that of the leaves of their food-plants; some
are brown, _i.e._ earth-coloured, and in these the markings do not
appear so prominently; others again possess very striking colours (_A.
Atropos_), the oblique stripes in these cases being very vivid. Only
_M. Hasdrubal_[121] separates itself completely from this system of
classification, since this species is deep black with narrow yellow
rings, the horn and last segment being red.

The large and most striking caterpillar of _M. Hasdrubal_ is the same
which Wallace has made use of for his theory of the brilliant colours
of caterpillars. The explanation of the origin of this widely divergent
mode of marking could only be furnished by the ontogeny, in which
one or another of the older phyletic stages will certainly have been
preserved.

Strictly speaking the same should be said of the other
species--nevertheless their comparison with the so similarly marked
_Smerinthinæ_, together with the circumstance that in certain species
a subdorsal line can be traced, makes it appear correct to suppose
that here also the subdorsal was the primary marking, this line being
subsequently entirely replaced by the oblique stripes. The _Sphinginæ_
would therefore be a younger group than the _Smerinthinæ_, a conclusion
which is borne out by the fact that in the former the oblique stripes
have reached a higher development, being always of two, and sometimes
even of three colours (_S. Drupiferarum_, white, red, black), whilst in
the species of _Smerinthus_ they only occasionally possess uniformly
coloured borders.


THE GENUS ANCERYX, BOISD.

Although this genus is not admitted into most of the European
catalogues--the solitary European species representing it being
referred to the genus _Sphinx_, Linn.[122]--its separation from
_Sphinx_ appears to me to be justified, not because of the striking
differences presented by the moths, but because the caterpillars,
judging from the little we know of them, likewise show a similar degree
of difference.

I have frequently succeeded in obtaining fertile eggs of _Anceryx
Pinastri_ and I will now give the developmental history of this
caterpillar, which has already been figured with great accuracy in
Ratzeburg’s excellent work on forest insects. Rösel was acquainted
with the fact that the “pine moth” laid its eggs singly on the needles
of the pine in June and July, and he described them as “yellowish,
shining, oval, and of the size of a millet seed.”

On emerging, the caterpillars are six millimeters in length, of a light
yellow colour, the head shining black with a yellow clypeus. The caudal
horn, which is forked at the tip, is also at first yellowish, but soon
becomes black. No particular marking is as yet present, but a reddish
stripe extends along the region of the dorsal vessel, and the course of
the spiracles is also marked by an orange-red line. (Fig. 53, A & B,
Pl. VI.)

As soon as the young larvæ are filled with food they acquire a greenish
streak. The first moult occurs after four days, and immediately after
this there is still an absence of distinct markings, with the exception
of a greenish-white spiracular line. In the course of some hours,
however, the original light green ground-colour becomes darker, and at
the same time a sharp, greenish-white subdorsal line appears, together
with a parallel line extending above the spiracles, which, in _Pterogon
Gorgoniades_, has already been designated as the “supra-spiracular.”
The dorsal line is absent: the head is light green, with two narrow
blackish-brown lines surrounding the clypeus; the horn and thoracic
legs are black; claspers, reddish green; length, twelve to thirteen
millimeters. (Fig. 54.)


_Third Stage._

After another period of four days the second moult occurs, neither
colour nor marking being thereby affected. Only the horn, now no longer
forked, becomes brownish with a black tip. The young caterpillars
are now, as before, admirably adapted to the pine needles, on which
they feed by day, and from which they can only be distinguished with
difficulty.


_Fourth Stage._

The third moult also brings no essential change. The ground-colour and
marking remain the same, only the spiracles, which were formerly dull
yellowish, are now of a vivid brick-red. The horn becomes yellowish-red
at the base.


_Fifth Stage._

The marking is only completely changed in the fifth and last stage.
A broad reddish-brown dorsal line replaces the subdorsal, more or
less completely. The supra-spiracular line also becomes broken up
into numerous short lengths, whilst the green ground-colour in some
specimens becomes more or less replaced by a brownish shade extending
from the back to the sides. Horn, black; the upper part of the first
segment with a corneous plate, similar to that of the _Deilephila_
larvæ.

This stage is very variable, as shown by the figures in various works.
The variations arise on the one hand from the struggle between the
green ground-colour and the reddish-brown extending from above, and,
on the other hand, from a more or less complete disappearance of the
associated longitudinal lines. The latter are sometimes completely
retained, this being the case in a caterpillar figured by Hübner
(_Sphinges_, III., _Legitimæ_ C, b), where both the subdorsal and
supra-spiracular lines are continuous from segment 11 to segment 1, an
instance which may perhaps be regarded as a reversion to the primary
form.

The entire change of the marking from the fourth to the fifth stage
depends upon the fact that the young larvæ resemble the _needles_ of
the pine, whilst the adults are adapted to the _branches_. I shall
return to this later.

The ontogeny of _A. Pinastri_ makes us acquainted with three different
forms of marking: (1) simple coloration without marking; (2) a
marking composed of three pairs of parallel longitudinal lines; (3) a
complicated marking, arising from the breaking up of the last and the
addition of a darker dorsal line.

Of the fourteen species placed by Gray in the genus _Anceryx_, I find,
in addition to the one described, notices of only two caterpillars:--

_A. Coniferarum_,[123] a North American species, lives on _Pinus
Palustris_, and was figured by Abbot and Smith. Colour and marking very
similar to _A. Pinastri_.

_A. Ello_, Linn.,[124] according to the authority of Mérian, is
described by Clemens[125] as dark brown, “with a white dorsal line,
and irregular white spots on the sides.” It lives on a “species of
_Psidium_ or _Guava_.”

Most of the species of _Anceryx_ appear to live on _Coniferæ_, to which
they show a general and decided adaptation. In the absence of decisive
information, I partly infer this from the names, as _Anceryx Juniperi_
(Africa). It has long been known that in our _A. Pinastri_ the mixture
of brown and fir-green, interspersed with conspicuous irregular light
yellowish and white spots, causes the adult larva to present a very
perfect adaptation to its environment. Of this caterpillar Rösel
states:--“After eating it remains motionless, and is then difficult
to see, because it is of the same colour as its food, since its brown
dorsal line has almost the colour of the pine twigs; and who is not
familiar with the fact that beneath the green needles there is also
much yellow to be found?”

This adaptation to the needles and twigs obviously explains why this
caterpillar in the adult condition is so far removed from those of the
genus _Sphinx_, while the moths are so nearly related that they were
only separated as a distinct genus when we became acquainted with a
large number of species.




II.

CONCLUSIONS FROM PHYLOGENY.


The considerations previously set forth are entirely based on Fritz
Müller’s and Haeckel’s view, that the development of the individual
presents the ancestral history _in nuce_, the ontogeny being a
condensed recapitulation of the phylogeny.

Although this law is generally true--all recent investigations
on development having given it fresh confirmation--it must not
be forgotten that this “recapitulation” is not only considerably
abbreviated, but may also be “falsified,” so that a searching
examination into each particular case is very desirable.

The question thus arises, in the first place, as to whether the
markings of caterpillars, so distinct at the different stages of
growth, are actually to be regarded as residual markings inherited
from the parent-form; or whether their differences do not depend upon
the fact that the caterpillar, in the course of growth, is exposed to
different external conditions of life, to which it has adapted itself
by assuming a different guise.

The former is undoubtedly the case. It can by no means be denied that
the conditions of life in young caterpillars are _sometimes_ different
to those of the adults. It will, in fact, be shown later on, that in
certain cases the assumption of a new guise at an advanced age actually
depends upon adaptation to new conditions of life; but as a rule, the
external conditions remain very similar during the development of the
larva, as follows from the fact that a change of food-plant never takes
place.[126] We should therefore rather expect a complete similarity
of marking throughout the entire larval period, instead of the great
differences which we actually observe.

Different circumstances appear to me to show that the markings of
young larvæ are only exceptionally due to a new adaptation, but that
as a rule they depend upon heredity. In the first place, there is
the fact that closely allied species, exposed to precisely similar
external conditions, as, for instance, _Chærocampa Elpenor_ and
_Porcellus_, possess exactly the same markings when young, these
markings nevertheless appearing at different stages of growth. Thus,
the subdorsal line first appears in _Elpenor_ in the second stage,
whilst in _Porcellus_ it is present during the first stage. If this
line were acquired by the young larva for adapting it at this age to
special conditions of life, it should appear in both species at the
same stage. Since this is not the case, we may conclude that it is
only an inherited character derived from the adult ancestor of the two
species, and now relegated to the young stages, being (so to speak),
pushed further back in one species than in the other.

But the strongest, and, as it appears to me, the most convincing proof
of the purely phyletic significance of the young larval markings, is
to be found in the striking regularity with which these are developed
in a similar manner in all allied species, howsoever different may
be their external conditions of life. In all the species of the
_Chærocampa_ group (the genera _Chærocampa_ and _Deilephila_) the
marking--no matter how different this may be in later stages--arises
from the simple subdorsal line. This occurs even in species which live
on the most diverse plants, and in which the markings can be of no
biological importance as long as the larvæ are so small as to be only
visible through a lens, and where there can be no possible imitation
of leaf-stalks or veins, the leaves and caterpillars being so very
distinct.

Moreover, when in the _Macroglossinæ_ (the genera _Macroglossa_,
_Pterogon_, and _Thyreus_) we see precisely the same simple marking
(the subdorsal) line retained throughout all the stages in two genera,
whilst in the _Smerinthinæ_ this line vanishes at a very early stage,
and in the _Sphinginæ_ is only present in traces, we can give but
one explanation of these facts. We have here a fragmentary series
representing the phyletic development of the Sphinx-markings, which
latter have arisen from one original plan--the simple subdorsal
line--and have then undergone further development in various
directions. As this subsequent development advanced, the older phyletic
stages would always be relegated to younger ontogenetic stages, until
finally they would be but feebly represented even in the youngest stage
(_D. Euphorbiæ_), or else entirely eliminated (most of the species of
the genus _Sphinx_). I believe that no other sufficient explanation of
these facts can be adduced. Granting that the correctness of the above
views can no longer be doubted, we may now take up the certain position
that the ontogeny of larval markings reveals their phylogeny, more or
less completely, according to the number of phyletic stages omitted,
or, in some exceptional cases, falsified. In other words, the ontogeny
of larval markings is a more or less condensed and occasionally
falsified recapitulation of the phylogeny.

Considering this to be established, we have next to deal with the
uniformity of the developmental phenomena, from which we may then
attempt to trace out the inciting causes underlying this development.

The law, or, perhaps better, the line of direction followed by the
development, is essentially the following:--

1. The development commences with a state of simplicity, and advances
gradually to one of complexity.

2. New characters first make their appearance in the last stage of the
ontogeny.

3. Such characters then become gradually carried back to the earlier
ontogenetic stages, thus displacing the older characters, until the
latter disappear completely.

The first of these laws appears almost self-evident. Whenever we
speak of development, we conceive a progression from the simple to
the complex. This result therefore does nothing but confirm the
observation, that we have actually here before us a development in
the true sense of the word, and not simply a succession of different
independent conditions.

The two following laws, on the other hand, lay claim to a greater
importance. They are not now enunciated for the first time, but were
deduced some years ago by Würtemberger[127] from a study of the
ammonites. In this case also the new characters predominate in the
later periods of life, and are then transferred back to the younger
ontogenetic stages in the course of phyletic development. “The change
in the character of the shell in ammonites, first makes itself
conspicuous in the last chamber; but in the succeeding generations
this change continually recedes towards the beginning of the spiral
chambers, until it prevails throughout the greater part of the
convolutions.”

In the same sense must also be conceived the case which Neumayr and
Paul have recently made known respecting certain forms of _Melanopsis_
from the West Sclavonian _Paludina_ bed. In _M. Recurrens_ the last
convolutions of the shell are smooth, this being a new character; the
small upper convolutions, however, are delicately ribbed, as is also
the case with the last convolution of the immediate progenitor. The
embryonic convolutions again are smooth, and the author believes (on
other grounds) that the more remote progenitor possessed a smooth shell.

In this case therefore, and in that of the ammonites, every shell
to a certain extent proclaims the ancestral history of the species;
in one and the same shell we find different phyletic stages brought
into proximity. The markings of caterpillars do not offer similar
facilities; nevertheless I believe that by their means we are led
somewhat further, and are able to enter more deeply into the causes
underlying the processes of transformation, because we can here observe
the living creature, and are thus enabled to study its life-history
with more precision than is possible with a fossil species.

When, in 1873, I received Würtemberger’s memoir, I was not only struck
with the agreement of his chief results with those which I had arrived
at by the study of larval markings, but I was almost as much astonished
at the great difference in the interpretation of the facts. The latter
indicate the gradual backward transference of a new character from the
latest to the earlier ontogenetic stages. Without further confirmation
Würtemberger assumes that it is to a certain extent self-evident
that the force producing this backward transference is the same as
that which, according to his view, first called forth the character
in question in the last stage, viz., natural selection. “Variations
acquired at an advanced age of the organism may, when advantageous,
be inherited by the succeeding generations, in such a manner that they
always appear a little earlier than in the preceding generations.”

It is certainly theoretically conceivable that a newly acquired
character, when also advantageous to the earlier stages, might be
gradually transferred to these stages, since in this case those
individuals in which this character appeared earliest would have the
greatest chance of surviving. In the case of the development of larval
markings, however, there are facts which appear to me to show that such
backward transference of a new character is, in a certain measure,
independent of the principle of utility, and that it must therefore
be referred to another cause--to the innate law of growth which rules
every organism.

When, in the larva of _C. Elpenor_, we perceive that the two eye-spots
which are first formed on the fourth and fifth segments appear
subsequently on the other segments as faint traces of no biological
value whatever, we cannot explain this phenomenon by natural selection.
We should rather say that in segmented animals there is a tendency for
similar characters to be repeated on all the segments; and this simply
amounts to the statement, that an innate law of growth is necessary for
the repetition of such newly acquired characters.

The existence of such a law of growth, acting independently of natural
selection, may therefore be considered as established, and indeed
cannot be disputed (Darwin’s “correlation of growth”). In the present
case it appears to me that an innate law of this kind, determining
the backward transference of new characters, is deducible from the
instances already quoted in another sense, viz., from the fact that in
many cases characters which are decidedly advantageous to the adult
are transferred to the younger stages, where they are at most of but
indifferent value, and can certainly be of no direct advantage. This is
the case with the oblique stripes of _Smerinthus_, which, in the adult
larvæ, resemble the leaf ribs, as will be shown more fully later on,
and, in conjunction with the green coloration, cause these caterpillars
to be very difficult of detection on their food-plants. The insects
are easily overlooked, and can only be distinctly recognized on close
inspection.

Now these oblique stripes appear, in all the _Smerinthus_ caterpillars
known to me, in the second, and sometimes even in the first stage,
_i.e._ in larvæ of from 0.7 to 1 centimeter in length. The stripes
are here much closer together than the ribs of any of the leaves of
either willow, poplar, or lime, and can therefore have no resemblance
to these leaves. The young caterpillars are certainly not rendered more
conspicuous by the oblique stripes, since they can only be recognized
on close inspection. It is for this reason that the stripes have not
been eliminated by natural selection.

The remarkable phenomenon of the backward transference of newly
acquired characters may therefore be formulated as follows:--Changes
which have arisen in the later ontogenetic stages have a tendency to
be transferred back to the younger stages in the course of phyletic
development.

The facts of development already recorded furnish numerous proofs
that this transference occurs gradually, and step by step, taking the
same course as that which led to the first establishment of the new
character in the final ontogenetic stage.

Did this law not obtain, the ontogeny would lose much of the interest
which it now possesses for us. It would then be no longer possible,
from the ontogenetic course of development of an organ or of a
character, to draw a conclusion as to its phylogeny. If, for instance,
the eye-spots of the _Chærocampa_ larvæ, which must have been acquired
at a late age, were transferred back to the younger ontogenetic stages
in the course of phyletic development, as eye-spots already perfected,
and not showing their rudimentary commencement as indentations of the
subdorsal line, the phenomenon would then give us no information as to
the manner of their formation.

It is well known to all who have studied the developmental history of
any group of animals, that no organ, or no character, however complex,
appears suddenly in the ontogeny; whereas, on the other hand, it
appears certain that new, or more advanced, but simpler characters,
predominate in the last stage of development. We are thus led to the
following modification of the foregoing conclusion:--Newly acquired
characters undergo, as a whole, backward transference, by which means
they are to a certain extent displaced from the final ontogenetic stage
by characters which appear later.

This must be a purely mechanical process, depending on that innate law
of growth, the action of which we may observe without being able to
explain fully. Under certain conditions the operation of this law may
be prevented by natural selection. Thus, for instance, if the young
caterpillars of _Anceryx Pinastri_ have not acquired the characteristic
marking of the adults, it is probably because they are better protected
by their resemblance to the green pine-needles than they would be if
they possessed the pattern of the larger caterpillars in their last
stage.

The backward transference of newly acquired characters may also
possibly be accelerated when these characters are advantageous to the
younger stages; but this transference takes place quite independently
of any advantage if the characters are of indifferent value, being then
entirely brought about by innate laws of growth.

That new characters actually predominate in the last stage of the
ontogeny, may also be demonstrated from the markings of caterpillars.
It is, of course, not hereby implied, that throughout the whole animal
kingdom new characters can only appear in the last ontogenetic stage.
Haeckel is quite correct in maintaining that the power of adaptation of
an organism is not restricted to any particular period. Under certain
circumstances transformations may occur at any period of development;
and it is precisely insects undergoing metamorphosis that prove this
point, since their larvæ differ so widely from their imagines that the
earlier stages may be completely disguised. It is here only signified
that, with respect to the development of caterpillars, new characters
first appear in the adult. The complexity of the markings, which so
frequently increases with the age of the caterpillar, can scarcely
bear any other interpretation than that the new characters were always
acquired in the last stage of the ontogeny. In certain cases we are
able, although with some uncertainty, to catch Nature in the act of
adding a new character.

I am disposed to regard the blood-red or rust-red spots which occur
in the last stage of the three species of _Smerinthus_ larvæ in the
neighbourhood of the oblique stripes as a case in point. It has
already been shown that these red spots must be regarded as the first
rudiments of the linear coloured edges which reach complete development
in the genus _Sphinx_. In some specimens of _Smerinthus Tiliæ_ the
spots coalesce so as to form an irregular coloured edge to the oblique
stripes. In _S. Populi_ they occur in many individuals, but remain
always in the spot stage; whilst _S. Ocellatus_ is but seldom, and _S.
Quercus_ appears never to be spotted.

The spots both of _S. Tiliæ_ and _Populi_ certainly do not show
themselves exclusively in the fifth (last) stage, but also in the
fourth, and sometimes in _Populi_ even as early as the third stage,
from which we might be disposed to conclude that the new character did
not first appear in the last stage. But the majority of the spotted
individuals first acquire their spots in the fifth stage, and only a
minority in the fourth; so that their occasional earlier appearance
must be ascribed to the backward transference of a character acquired
in the fifth stage. Moreover, the fourth and fifth stages of the
caterpillars are closely analogous both in size, mode of life, and
marking, and are therefore analogous with reference to the environment,
so that it is to be expected that new characters, when depending on
adaptation, would be rapidly transferred from the fifth stage to the
fourth.[128] We should thus have a case of the acceleration by natural
selection, of processes determined by innate causes. Why changes should
predominate in the last stage, is a question closely connected with
that of the causes of larval markings in general, and may therefore
be investigated later.

But if we here assume in anticipation that all new markings depend
on adaptation to the conditions of life, and arise through natural
selection, it will not be difficult to draw the conclusion that such
new characters must prevail in the last stage. There are two conditions
favouring this view; the size of the insect, and the longer duration
of the last stage. As long as the caterpillar is so small as to be
entirely covered by a leaf, it only requires a good adaptation in
colour in order to be completely hidden; independently of which, it is
also possible that many of its foes do not consider it worth attacking
at this stage. The last stage, moreover, is of considerably longer
duration than any of the four preceding ones; in _Deilephila Euphorbiæ_
this stage lasts for ten days, whilst the remaining stages have a
duration of four days; in _Sphinx Ligustri_ the last stage also extends
over ten days, and the others over six days.

In its last stage, therefore, a caterpillar is for a longer period
exposed to the danger of being discovered by its foes; and since, at
the same time, its enemies become more numerous, and its increased size
makes it more easy of detection, it is readily conceivable that a
change in the conditions of life, such, for instance, as removal to a
new food-plant, would bring about the adaptation of the _adult larva_
as its chief result.

I shall next proceed to show how far the assumption here made--that all
markings depend on natural selection--is correct.




III.

BIOLOGICAL VALUE OF MARKING IN GENERAL.


Having now described the development of larval markings, so far as
possible from their external phenomena, and having traced therefrom
the underlying law of development, I may next proceed to the main
problem--the attempt to discover those deeper inciting causes which
have produced marking in general.

The same two contingencies here present themselves as those which
relate to organic life as a whole; either the remarkably complex and
apparently incomprehensible characters to which we give the name of
markings owe their origin to the direct and indirect gradual action
of the changing conditions of life, or else they arise from causes
entirely innate in the organism itself, _i.e._ from a phyletic vital
force. I have already stated in the Introduction why the markings of
caterpillars appear to me such particularly favourable characters for
deciding this question, or, more precisely, why these characters, above
any others, appear to me to render such decision more easily possible;
repetition is here therefore unnecessary.

The whole of the present investigation had not been planned when
I joined with those who, from the first, admitted the omnipotence
of natural selection as an article of faith or scientific axiom. A
question which can only be solved by the inductive method cannot
possibly be regarded as settled, nor can further evidence be considered
unnecessary, because the first proofs favour the principle. The
admission of a mysteriously working phyletic power appears very
unsatisfactory to those who are striving after knowledge; the existence
of this power, however, is not to be considered as disproved,
because hundreds of characters can be referred to the action of
natural selection, and many others to that of the direct action of
the conditions of life. If the development of the organic world is
to be considered as absolutely dependent on the influence of the
environment, not only should we be able here and there to select at
pleasure characters which appeared the most accessible for elucidating
this point, but it becomes in the first place necessary to attempt
to completely refer _all_ characters belonging to any particular
group of phenomena, however small this group might be, to known
transforming factors. We should then see whether this were possible,
or whether there would remain residual phenomena not explicable by
known principles and compelling us to admit the existence of a force
of development innate in the organism. In any case the “phyletic vital
force” can only be got rid of by a process of elimination--by proving
that all the characters generally occurring throughout the group of
phenomena in question, must be attributed to other causes, and that
consequently nothing remains for the action of the supposed phyletic
vital force, which would in this manner be negatived, since we cannot
infer the presence of a force if the latter exerts no action whatever.

I shall here attempt such an investigation of the group of phenomena
displayed by larval markings, with special reference to those of the
_Sphingidæ_. The alternatives upon which we have to decide are the
following:--Are the markings of caterpillars purely morphological
characters, produced entirely by internal causes? or, are they simply
the response of the organism to external influences?

The solution of these questions will be arrived at by seeking to refer
all the markings present to one of the known transforming factors, and
the success or failure of this attempt will give the required decision.
The first question to be attacked is obviously this,--whether the
Sphinx-markings are actually, as they appear at first sight, purely
morphological characters. If it can be shown that all these markings
were originally of biological value, they must be attributed to the
action of natural selection.

Did I here at once proceed to establish the biological value of larval
markings--and especially of those of the _Sphingidæ_--so as to arrive
in this manner at a conclusion as to their dependence upon natural
selection, it would be impossible to avoid the consideration of the
total coloration of the caterpillars, since the marking frequently
consists only of a local strengthening of the colour, and cannot be
comprehended without coming to this understanding. The action of the
markings also often appears to be opposed to that of the colouring,
making the caterpillar again conspicuous; so that the two factors must
necessarily be considered together. I shall therefore commence the
investigation with colour in general, and then proceed to treat of
marking.




IV.

BIOLOGICAL VALUE OF COLOUR.


The general prevalence of protective colouring among caterpillars has
already been so frequently treated of that it is not here my intention
to recall particular instances. In order to judge of the effect of
marking, however, it will be well to bear in mind that these insects,
being generally defenceless and thus requiring protection, have
acquired the most diverse means of rendering themselves in some measure
secure from their foes.

The sharp spines which occur on the caterpillars of many butterflies
(_Vanessa_, _Melitæa_, _Argynnis_), and the hairs on those of
many moths, serve for protective purposes. Among other means of
protection--although in a different sense--we have in all the species
of the great family of the _Papilionidæ_ the strikingly coloured
(yellowish red) odour-emitting tentacles concealed near the head,
and suddenly protruded for terrifying foes; and likewise the forked
horn at the tail of the caterpillars of the genus of moths _Harpyia_,
the tentacles of which can be suddenly protruded in a similar
manner. Adaptive colours and forms combined with certain habits[129]
are, however, much more common than defensive weapons. Thus, the
caterpillars of the _Noctuæ_ belonging to the genus _Catocala_ and its
allies, feed only at night on the green leaves of various forest-trees;
by day they rest in crevices of the bark on the tree trunk, which they
resemble so perfectly in the colour of their peculiar glossy dull grey
or brownish skin beset with small humps, that only sharp eyes can
detect them, even when we are familiar with their habits.[130]

The striking resemblance of many moths to splinters of wood is well
known, and to this is added a habit which helps their disguise, viz.,
that of remaining stiff and motionless on the approach of danger,
just like a splinter projecting from the branch.[131] Among the moths
coming under this category may be mentioned _Cucullia Verbasci_, and
particularly those of the genus _Xylina_, which, when at rest, closely
resemble a broken splinter of wood in the colour and marking of their
fore wings, and when touched, have a habit of drawing in their legs and
falling without opening their wings as though dead.

That simple adaptive colouring prevails widely among caterpillars
is shown by the large number of green species.[132] It may be fairly
said that all caterpillars which possess no other means of protection
or defence are adaptively coloured. These facts are now well known;
so also is the explanation of the varied and striking colours of many
caterpillars given by Wallace.[133] There is, however, novelty in the
proof contained in the foregoing descriptions of larval development,
as to the manner in which the di- and polymorphism of caterpillars
can be explained from the external phenomena which they present,
these phenomena being well adapted for showing the great importance
of protective colouring to the larvæ. We have here presented a
double adaptation, although not quite of the nature of that which I
formerly admitted on hypothetical grounds.[134] In the first place,
from the developmental history there results the conclusion that all
Sphinx-larvæ which, in the adult state, are di- or polymorphic, are
unicolorous when young. Thus, the caterpillars of _Chærocampa Elpenor_
all remain green till the fourth stage, when they mostly become
light or dark brown, and only very seldom retain their green colour.
_Chærocampa Porcellus_ behaves in a precisely similar manner; as also
does _Pterogon Œnotheræ_, which inhabits the same localities, and is
found on the same food-plant, but is not very closely related to the
_Chærocampa_. In this species also (_P. Œnotheræ_) the brown is more
common than the green form in the adult state, both varieties showing a
complicated marking. The young larvæ possess only a light green colour,
and a pure white subdorsal line as the only marking; they are so well
adapted to the leaves of their food-plants, _Epilobium Hirsutum_,
and _E. Rosmarinifolium_, that they can only be detected with great
difficulty. After the third moult they become brown, and can be easily
seen when at rest on their food-plant.

Now in all known caterpillars brown colours are adaptive, sometimes
causing a resemblance to the soil, and at others to dead leaves or
branches. As soon, therefore, as the caterpillars have attained a
considerable size, they remain concealed by day.[135] The truth of this
observation not only appears from various entomological notes, but I
have frequently convinced myself of its accuracy. I well remember from
the earliest times that _C. Elpenor_, especially when the larva is
adult, always rests by day among the dead branches and leaves of its
shrub-like food-plant, _Epilobium Hirsutum_; and even when this species
lives on the low-growing _Epilobium Parviflorum_, it conceals itself
by day on the ground, among the tangled leaves and branches. I have
observed that _Sphinx Convolvuli_ has a precisely similar habit, for
which reason it is difficult to obtain, even in localities where it
occurs very commonly.

In the neighbourhood of Basle I once found at mid-day a brown
caterpillar of _Pterogon Œnotheræ_ on an isolated dead branch
of _Epilobium Rosmarinifolium_, and I was informed by Herr
Riggenbach-Stähelin--a collector of great experience who accompanied
me--that these caterpillars always rest (by day) on withered plants as
soon as they become brown, but before this change they are only to be
found on green plants.

Thus, it cannot well be doubted that the change of colour is associated
with a change in the habits of life, and the question arises as to
which has been the primary change.

If the view here entertained, that the later brown coloration is
adaptive, be correct, the species must have first acquired the habit of
concealing itself by day on the ground and among dead herbage, before
the original green colour could have been changed into brown by natural
selection. This must represent the actual facts of the case.

Nearly allied species which at an advanced age are not dimorphic, but
are darkly coloured in all individuals, are especially calculated
to throw some light on this point. For instance, the caterpillar of
_Deilephila Vespertilio_, which comes under this denomination, is light
green when young, and rests both by day and night on the leaves of the
plant on which it feeds. As soon as it acquires its dark colour--after
the third moult--it changes its habits, concealing itself by day on the
ground and feeding only by night. For this reason collectors prefer
seeking for it in the evening, or with a lantern by night.

The most instructive case, however, is that of _Deilephila Hippophaës_,
in which no change of colour is associated with age, the caterpillar,
throughout its whole life, remaining of a greyish green, which exactly
matches the colour of the leaves of its food-plant, _Hippophae
Rhamnoides_. Nevertheless this species also possesses the habit of
feeding only at night as soon as it has attained to a considerable
size, hiding itself by day at the root of its food-plant. Collectors
expressly state that this larva can scarcely be found by day, and
recommend that it should be sought for at night with a lantern.

From the foregoing facts and considerations it may fairly be concluded,
that the habit of hiding by day, possessed by these and other allied
caterpillars, was acquired when they resembled the leaves in colour,
and that the adaptation to the colour of the soil, or dead foliage and
withered branches, ensued as a secondary consequence.

But why have these caterpillars acquired such a habit, since they
appear to be perfectly protected by their resemblance in colour to
the green leaves? The answer to this question is easily given when we
consider in which species this habit generally occurs.

Does the habit prevail only among the species of the one genus
_Deilephila_, and in all the species of this genus? This is by no
means the case, since, on the one hand, many species of _Deilephila_,
such as _D. Euphorbiæ_, _Galii_, _Nicæa_, and _Dahlii_, do not possess
the habit, and, on the other hand, it occurs in species of other
genera, such as _Macroglossa Stellatarum_, _Sphinx Convolvuli_, and
_Acherontia Atropos_.

The habit in question must therefore be the result of certain external
conditions of life common to all those species which rest by day. The
mode of life common to them all is that they do not live on trees with
large leaves or with thick foliage, but on low plants or small-leaved
shrubs, such as the Sea Buckthorn.[136] I believe I do not err when
I attribute the habit possessed by the adult larvæ, of concealing
themselves by day, to the fact that the green colour is protective only
so long as they are small--or, more precisely speaking, as long as
their size does not considerably exceed that of a leaf or twig of their
food-plant. When they become considerably larger, they must become
conspicuous in spite of their adaptive colour, so that it would then be
advantageous for them to conceal themselves by day, and to feed only
by night. This habit they have acquired, and still observe, even when
the secondary adaptation to the colour of the soil, &c., has not been
brought about. We learn this from _D. Hippophaës_, which remains green
throughout its whole larval existence; and no less from the green forms
of the adult larvæ of _Sphinx Convolvuli_, _Chærocampa Elpenor_, and
_Porcellus_, all of which conceal themselves by day in the same manner
as their brown allies.

It may be objected that there are Sphinx-larvæ--instances of which I
have myself adduced--which live on low small-leaved plants, and which
nevertheless do not hide themselves by day. This is the case with the
spurge-feeding _D. Euphorbiæ_, so common in many parts of Germany. This
caterpillar must, however, be classed with those which, on account
of their distastefulness, or for other reasons to be subsequently
considered, are rejected by birds and other larger foes, and which, as
Wallace has shown, derive advantage from being coloured as vividly as
possible. I shall return to this subject later, when treating of the
biological value of special markings.

On the other hand, it is readily conceivable that, from the conditions
of life of caterpillars living on trees or shrubs with dense foliage,
the habit of resting by day and descending from the tree for
concealment would not have been acquired. Such larvæ are sufficiently
protected by their green colour among the large and numerous leaves;
and I shall have occasion to show subsequently that their markings
increase this protective resemblance.

The di- or polymorphism of the larvæ of the _Sphingidæ_ does not
therefore depend upon a _contemporaneous_ double adaptation, but
upon the replacement of an old protective colour by a new and better
one, and therefore upon a _successive_ double adaptation. The adult
caterpillars of _C. Elpenor_ are not sometimes brown and sometimes
green because some individuals have become adapted to leaves and others
to the soil, but because the anciently inherited green has not yet
been completely replaced by the newly acquired brown coloration, some
individuals still retaining the old green colour.

When, in another place,[137] I formerly stated “that a species can
become adapted in this or that manner to given conditions of life, and
that by no means can only one best adapted form be allowed for each
species,” this statement is theoretically correct speaking generally,
but not in its application to the present class of cases. A comparison
with one another of those caterpillars which repose by day, distinctly
shows that they all possess a tendency to abandon the green and assume
a dull colour, but that this process of replacement has advanced
further in some species than in others. It will not be without interest
to follow this operation in some detailed cases, since we may thus
obtain an insight into the processes by which polymorphism has arisen,
as well as into the connection between this phenomenon and simple
variability.

In _D. Hippophaës_ the process has either not yet commenced, or is
as yet in its first rudiments. If we may trust the statements of
authors, together with the ordinary green form there occurs, rarely,
a silver-grey variety, which may be regarded as the beginning of a
process of colour substitution. Among thirty-five living specimens of
this scarce species which I was able to procure, the grey form did not
occur, neither have I found it in collections.

In _Macroglossa Stellatarum_ we see the transforming process in full
operation. A large number of individuals (about thirty-five per cent.)
are still green; the number of dark-coloured individuals reaches
forty-six per cent., these, therefore, preponderating; whilst between
the two extremes there are about nineteen per cent. of transition
forms, showing all possible shades between light green and dark
blackish-brown or brownish-violet, and even, in solitary individuals,
pure violet (See Figs. 3-12, Pl. III.). The relatively small number
of the intermediate forms, taken in connection with the fact that all
the 140 specimens employed in my investigation were obtained from one
female, leads to the conclusion that these forms owe their existence to
cross-breeding. It would be superfluous to attempt to prove this last
conclusion with reference to the before-mentioned case, in which a
caterpillar was streaked with brown and green (Fig. 9, Pl. III.).

The process of transformation, as already mentioned, advances in such
a manner that the intermediate forms diminish relatively to the dark
individuals. This is found to be the case with _Sphinx Convolvuli_,
and almost to the same extent with _Chærocampa Elpenor_, in both of
which species the green caterpillars are the rarest.[138] Forms truly
intermediate in colour between green and brown no longer occur, but
apparently only different shades of light and dark brown, passing into
brownish-black.

The process has again made a further advance in _Chærocampa Porcellus_
and _Celerio_ as well as in _Pterogon Œnotheræ_. In all these species
the green form occurs,[139] but so rarely that very few collectors have
seen it. The brown form has therefore in these cases nearly become the
predominant type, and the solitary green specimens which occasionally
occur, may be regarded as reversions to an older phyletic stage.

_Deilephila Livornica_ appears to have reached a similar stage, but the
caterpillar of this species has been so imperfectly observed, that it
is difficult to determine, even approximately, the relative proportion
of the brown to the green individuals. I have only seen one of the
latter in Dr. Staudinger’s collection (Compare Fig. 62, Pl. VII.).

In _Deilephila Vespertilio_, _Euphorbiæ_, _Dahlii_, _Mauritanica_,
_Nicæa_, and _Galii_, the green form has completely disappeared. The
blackish olive-green colour shown by many caterpillars of the two last
species, can be considered as a faint retention of the light green
colour which they formerly possessed, and which they both show at the
present time in their young stages.

Beginning with the appearance of single darker individuals, we pass
on in the first place to a greater variability of colouring, and
from this, by the greater diminution of the intermediate forms,
to polymorphism; the complete extermination of these forms ending
in dimorphism. The whole process of transformation has been thus
effected:--As the new colouring always prevailed over the old, the
latter was at length completely displaced, and the caterpillars, which
were at first simply variable, became polymorphic and then dimorphic,
finally returning to monomorphism.

We thus see the process of transformation still going on, and no
doubt can arise as to its inciting causes. When a character can with
certainty be ascribed to adaptation, we can explain its origin in no
other way than by the action of natural selection. If, as I believe,
it can and has been shown, not only that caterpillars in general
possess adaptive colours, but that these colours can change during the
lifetime of one and the same species, in correspondence with external
conditions, we must certainly gain a very high conception of the power
which natural selection exerts on this group of living forms.[140]




V.

BIOLOGICAL VALUE OF SPECIAL MARKINGS.


The following questions now present themselves: Have the markings
of caterpillars any biological value, or are they in a measure only
sports of nature? Can they be considered as partially or entirely the
result of natural selection, or has this agency had no share in their
production?

The problem here offers itself more distinctly than in any other group
of living forms, because it presents an alternative without a third
possibility. In other words, if it is not possible to show that larval
markings have a distinct biological significance, there remains only
for their explanation the assumption of a phyletic force, since the
direct action of the environment is insufficient to account for such
regularity of development throughout a series of forms. The explanation
by sexual selection is excluded _ab initio_, since we are here
concerned with larvæ, and not with reproductive forms.[141]

The biological significance of marking--if such significance it
possess--will be most easily investigated by examining whether species
with similar markings have any conditions of life in common which would
permit of any possible inference as to the significance of the markings.

Among the _Sphingidæ_ we find four chief forms of marking; (1) complete
absence of all marking; (2) longitudinal stripes; either a simple
subdorsal or this together with a spiracular and dorsal line; (3)
oblique stripes; (4) eye-spots and ring-spots, single, paired, or in
complete rows.

Now if we consider in which species these four kinds of marking are of
general occurrence, not only in the small group of the _Sphingidæ_
but in the whole order Lepidoptera, we shall arrive at the following
results:--

1. _Complete absence of marking_, so common in the larvæ of
other insects, such as the Coleoptera, is but seldom found among
Lepidopterous caterpillars.

To this category belong all the species of _Sesiidæ_ (the genera
_Sesia_, _Trochilia_, _Sciapteron_, _Bembecia_, &c.), the larvæ of
which, without exception, are of a whitish or yellowish colour, and
live partly in the wood of trees and shrubs and partly in the shoots
of herbaceous plants. Subterranean larvæ also, living at the roots
of plants, such as _Hepialus Humuli_ at the roots of hop, and _H.
Lupulinus_ at those of _Triticum Repens_, possess neither colour nor
marking. These, like the foregoing, are yellowish-white, evidently
because they are deprived of the influence of light.[142] The larvæ of
certain small moths, such as _Tortrix Arbutana_ and _Pomonana_, which
live in fruit, and many case-bearing _Tineina_, are likewise without
marking and devoid of bright colour, being generally whitish. Many
of the small caterpillars which feed exteriorly are also--so far as my
experience extends--without definite markings, these being among the
most minute, such as the greenish leaf-mining species of _Nepticula_.
It is among the larger species that we first meet with longitudinal
and oblique stripes. Eye-spots do not occur in any of these larvæ, a
circumstance of the greatest importance for the biological significance
of this character, as will be shown subsequently. The small size of
the caterpillars cannot be the sole cause of the absence of such
eye-spots, since in young _Smerinthus_ caterpillars one centimeter
long, the oblique stripes are beautifully developed, and the larvæ of
many of the smaller moths considerably exceed this size. The surface of
these caterpillars therefore, _i.e._, the field on which markings are
displayed, is not absolutely too small for the development of such a
character.

Besides the larvæ of the Micro-lepidoptera and of those species living
in the dark, there is also a complete absence of marking in the young
stages of many caterpillars. Thus, all the _Sphingidæ_ of which I have
been able to observe the development, show no markings immediately
after emergence from the egg; in many they appear very soon, even
before the first moult, and, in other species, after this period.

2. _The second category of markings, longitudinal stripes_, is very
widely distributed among the most diverse families. This character
is found among the larvæ of butterflies, _Sphingidæ_, _Noctuæ_,
Micro-lepidoptera, &c., but in all these groups it is absent in many
species. This last fact is opposed to the view that this character
is purely morphological, and leads to the supposition that it may
have a biological value, being of service for the preservation of the
individual, and therefore of the species.

I find that such marking is of service, stripes extending
longitudinally along the upper surface of the caterpillar generally
making the latter less conspicuous. This, of course, does not hold
good under all circumstances, since there are many species with very
striking colours which possess longitudinal stripes. Let us consider,
however, a case of adaptive colouring, such as a green caterpillar,
which, on this account only, is difficult to see, since it accords
with the colour of the plant on which it lives. If it is a small
caterpillar, _i.e._, if its length and thickness do not considerably
exceed that of the parts of its food-plant, it can scarcely be better
concealed--stripes would hardly confer any special advantage unless the
parts of the plant were also striped. But the case is quite different
if the caterpillar is considerably larger than the parts of the plant
(leaves, stalks, &c.). The most perfect adaptive colouring would not
now prevent it from standing out conspicuously as a larger body, among
the surrounding parts of the plants. It must be distinctly advantageous
therefore to such a caterpillar to be striped, since these markings
to a certain extent divide the large body into several longitudinal
portions--they no longer permit it to be seen as a whole, and thus act
more effectively than mere assimilative colouring in causing it to
escape detection. This protection would be the more efficacious if the
stripes resembled the parts of the plant in colour and size, such, for
instance, as the lines of light and shadow produced by stalks or by
long and sharp-edged leaves.

If this view be correct, we should expect longitudinal stripes to be
absent in the smallest caterpillars, and to be present more especially
in those species which live on plants with their parts similarly
disposed, _i.e._, on plants with numerous thin, closely-growing stalks
and grass-like leaves, or on plants with needle-shaped leaves.

It has already been mentioned that the smallest species are devoid of
longitudinal striping. The larvæ of the Micro-lepidoptera show no such
marking, even when they do not live in the dark, but feed either on the
surface or in superficial galleries of the leaves (_Nepticula_, &c.),
in which they must be exposed to almost as much light as when living
on the surface. The fact that the subdorsal line sometimes appears in
very young Sphinx-larvæ is explained, as has already been shown, by the
gradual backward transference of adaptational characters acquired in
the last stage of development.

It can easily be demonstrated that longitudinally striped caterpillars
mostly live on plants, of which the general appearance gives the
impression of a striped arrangement. We have only to consider in
connection with their mode of life, any large group of adaptively
coloured species marked in this manner. Thus, among the butterflies,
nearly all the _Satyrinæ_ possess larvæ conspicuously striped--a fact
which is readily explicable, because all these caterpillars live on
grasses. This is the case with the genera _Melanargia_, _Erebia_,
_Satyrus_, _Pararge_, _Epinephele_, and _Cænonympha_, no species of
which, so far as the larvæ are known, is without longitudinal stripes,
and all of which feed on grasses. It is interesting that here also,
as in certain _Sphingidæ_, some species are brown, _i.e._, adapted to
the soil, whilst the majority are green, and are therefore adapted to
living grass. Just as in the case of the _Sphingidæ_ also, the brown
species conceal themselves by day on the earth, whilst some of the
green species have likewise acquired this habit. I have already shown
how this habit originates from the increasing size of the growing
larva, which would otherwise become too conspicuous, in spite of
adaptive colour and marking. A beautiful confirmation of this view is
found in the circumstance that only the largest species of _Satyrus_,
such as _S. Proserpinus_, _Hermione_, _Phædrus_, &c., possess brown
caterpillars. I should not be surprised if a more exact investigation
of these species, which have hitherto been but seldom observed,
revealed in some cases a dimorphism similar to that of the _Sphingidæ_;
and I believe that I may venture to predict that the young stages
of all these brown larvæ--at present quite unknown--are, as in the
last-named group, green.

Besides the _Satyrinæ_, most of the larvæ of the _Pierinæ_ and
_Hesperidæ_ possess longitudinal stripes, which are generally less
strongly pronounced than in the former subfamily. Some of the _Pierinæ_
live on _Cruciferæ_, of which the narrow leaves and thin leaf- and
flower-stalks present nothing but a linear arrangement; other species
of this group, however, feed on _Leguminosæ_ (_Lathyrus_, _Lotus_,
_Coronilla_, _Vicia_), and some few on broad-leaved bushes (_Rhamnus_).
This last fact may appear to be opposed to the theory; but light
lateral stripes, such for example, as those possessed by _Gonepteryx
Rhamni_, can never be disadvantageous, and may be of use, even on large
leaves, so that if we consider them as an inherited character, there
is no reason for natural selection to eliminate them. In the case of
caterpillars living on vetch, clover, and other _Leguminosæ_, it must
not be forgotten that, although their food-plants do not present any
longitudinal arrangement of parts, they always grow among grasses, the
species feeding on such plants always resting between grass stems, and
very frequently on the grass itself, so that they can have no better
protective marking than longitudinal stripes. The striping of the
_Hesperidæ_ larvæ, which partly feed on grasses but mostly on species
of _Leguminosæ_, can be explained in a similar manner.

It is not here my intention to go through all the groups of Lepidoptera
in this manner. The instances adduced are quite sufficient to prove
that longitudinal stripes occur wherever we should expect to find them,
and that they really possess the biological significance which I have
ascribed to them. That these markings are occasionally converted into
an adaptive imitation of certain special parts of a plant, is shown
by the larvæ of many moths, such for example as _Chesias Spartiata_,
which lives on broom (_Spartium Scoparium_), its longitudinal stripes
deceptively resembling the sharp edges of the stems of this plant.[143]

3. _Oblique striping._ Can the lilac and white oblique stripes on the
sides of a large green caterpillar, such as those of _Sphinx Ligustri_;
or the red and white, or white, black, and red stripes of _Smerinthus
Tiliæ_ and _Sphinx Drupiferarum_ respectively, be of any possible use?
Have we not here just one of those cases which clearly prove that such
a character is purely morphological, and worthless for the preservation
of the individual? Does not Nature occasionally sport with purposeless
forms and colours; or, as it has often been poetically expressed, does
she not here give play to the wealth of her phantasy?

At first sight this indeed appears to be the case. We might
almost doubt the adaptive importance of the green ground-colour
on finding coloured stripes added thereto, and thus--as one might
suppose--abolishing the beneficial action of this ground-colour, by
making the insect strikingly conspicuous. But this view would be
decidedly incorrect, since oblique stripes are of just the same
importance as longitudinal stripes. The former serve to render the
caterpillar difficult of detection, by making it resemble, as far as
possible, a leaf; they are imitations of the leaf-veins.

Nobody who is in the habit of searching for caterpillars will
doubt that, in cases where the oblique stripes are simply white or
greenish-white, it is extremely difficult to see the insect on its
food-plant, _e.g._ _S. Ocellatus_ on _Salix_; not only because it
possesses the colour of the leaves, but no less because its large body
does not present an unbroken green surface, which would bring it into
strong contrast with the leaves, and thus arrest the attention. In the
case of the species named, the coloured area of the body is divided by
oblique parallel stripes, just in the same manner as a willow leaf.
In such instances of course we have not presented to us any special
imitation of a leaf with all its details--there is not a perfect
resemblance of the insect to a leaf, but only an arrangement of lines
and interspaces which does not greatly differ from the division of a
leaf by its ribs.

That this view is correct is shown by the occurrence of this form
of marking. It is on the whole rare, being found, besides in
many _Sphingidæ_, in isolated cases in various families, but is
always confined to those larvæ which live on ribbed leaves, and
never occurring in species which feed on grasses or on trees with
needle-shaped leaves. This has already been shown with respect to
the _Sphingidæ_, in which the oblique stripes are only completely
developed in the subfamilies _Smerinthinæ_ and _Sphinginæ_. The species
of _Smerinthus_ all live on trees such as willows, poplars, lime,
oak, &c., and all possess oblique stripes. The genus _Anceryx_ also
belongs to the _Sphinginæ_, and these caterpillars, as far as known,
live on trees with needle-shaped leaves. The moths of this last genus
are very closely allied to the species of _Sphinx_, not only in form
and colour, but also in many details of marking. The larvæ are however
different, this distinction arising entirely from their adaptation
to needle-shaped leaves, the _Sphinx_ caterpillars being adapted
to ordinary foliage. The species of _Anceryx_, as has been already
shown, are brown mixed with green, and never possess even a trace of
the oblique stripes, but have a latticed marking, consisting of many
interrupted lines, which very effectively serves to conceal them among
the needles and brown bark of the _Coniferæ_.

Of the _Sphinginæ_ living on plants with ordinary foliage, not a single
species is without oblique stripes. I am acquainted with ten species of
caterpillars and their respective food-plants, viz. _Sphinx Carolina_,
_Convolvuli_, _Quinquemaculata_, _Prini_, _Drupiferarum_, _Ligustri_;
_Macrosila Rustica_ and _Cingulata_; _Dolba Hylæus_ and _Acherontia
Atropos_.

Besides among the _Sphinginæ_, oblique stripes occur in the larvæ of
certain butterflies, viz. _Apatura Iris_, _Ilia_, and _Clytie_, all of
which live on forest trees (aspen and willows), and are excellently
adapted to the leaves by their green colour. In addition to these, I am
acquainted with the larvæ of some few moths, viz. of _Aglia Tau_ and
_Endromis Versicolora_, both of which also live on forest trees.

Oblique stripes also occasionally occur in the smaller caterpillars
of _Noctuæ_, _Geometræ_, and even in those of certain _Pyrales_, in
all of which they are shorter and differently arranged. In these cases
also, my theory of adaptation holds good, but it would take us too far
if I attempted to go more closely into them. I will here only mention
the extraordinary adaptation shown by the caterpillar of _Eriopus
Pteridis_. This little Noctuid lives on _Pteris Aquilina_; it possesses
the same green colour as this fern, and has double oblique white
stripes crossing at a sharp angle on each segment, these resembling the
lines of _sori_ of the fern-frond so closely, that the insect is very
difficult to perceive.

After all these illustrations it can no longer remain doubtful that
the oblique stripes of the _Sphingidæ_ are adaptive. But how are
the coloured edges bordering these stripes in so many species to be
explained?

I must confess that I long doubted the possibility of being able to
ascribe any biological value to this character, which appeared to me
only conspicuous, and not protective. Cases may actually occur in which
the brightly coloured edges of the oblique stripes make the caterpillar
conspicuous--just in the same manner as any marking may bring about a
conspicuous appearance by presenting a striking contrast of colour.
I am acquainted with no such instance, however. As a rule, in all
well-adapted caterpillars, considering their colour in its totality,
this is certainly not the case. The coloured edges, on the contrary,
enhance the deceptive appearance by representing the oblique shadows
cast by the ribs on the under-side of the leaf; all these caterpillars
rest underneath the leaves, and never on the upper surface.

This explanation may, perhaps, at first sight appear far-fetched,
but if the experiment be made of observing a caterpillar of _Sphinx
Ligustri_ on its food-plant, not immediately before one’s eyes in a
room, but at a distance as under natural conditions, it will be found
that the violet edges do not stand out brightly, but show a colour
very similar to that of the shadows playing about the leaves. The
coloured edges, in fact, produce a more effective breaking up of the
large green surface of the caterpillar’s body, than whitish stripes
alone. Of course if the insect was placed on a bare twig in the sun, it
would be easily visible at a distance; the larva never rests in such
a position, however, but always in the deep shadow of the leaves, in
which situation the coloured edges produce their peculiar effect. It
may be objected that the oblique white stripes, standing simply on
a dark green ground-colour, would produce the same effect, and that
my explanation therefore leaves the bright colouring of these edges
still unaccounted for. I certainly cannot say why in _Sphinx Ligustri_
these edges are lilac, and in _S. Drupiferarum_, _S. Prini_, and
_Dolba Hylæus_ red, nor why they are black and green in _Macrosila
Rustica_, and blue in _Acherontia Atropos_. If we knew exactly on what
plants these caterpillars fed originally, we might perhaps indulge in
comparing with an artistic eye the shadows playing about their leaves,
seeing in one case more red, and in another more blue or violet. The
coloured stripes of the _Sphingidæ_ must be regarded as the single
strokes of a great master on the countenance of a human portrait.
Looked into closely, we see red, blue, or even green spots and strokes;
but all these colours, conspicuous when close, disappear on retreating,
a general effect of colour being then produced, which cannot be
precisely described by words.

Quite in accordance with this explanation, we see caterpillars with
the brightest coloured stripes concealing themselves in the earth by
day, and betaking themselves to their food-plants only in the dusk of
the evening or dawn of morning and even during the night; _i.e._ in a
light so faint that feeble colours would produce scarcely any effect.
The bright blue of _Acherontia Atropos_, for example, would give the
impression of oblique shadows without any distinctive colour.

It is precisely the case of this last caterpillar, which formerly
appeared to me to present insurmountable difficulties to the
explanation of the coloured stripes by adaptation, and I believed that
this insect would have to be classed with those species which are
brightly coloured because they are distasteful, and are avoided by
birds. But although we have no experiments on this point, I must reject
this view. Unfortunately, we know scarcely anything of the ontogeny
of this caterpillar; but we know at least that the young larvæ (stage
four) are greener than the more purely yellow ones of the fifth stage
(which, however, are also frequently green), and we know further that
some adults are of a dark brownish-grey, without any striking colours.
From analogy with the dimorphism of the species of _Chærocampa_ and
_Sphinx_, fully considered previously, it must therefore be concluded
that in this case also, a new process of adaptation has commenced--that
the caterpillar is becoming adapted to the soil in and on which it
conceals itself by day.[144] An insect which acquires undoubted
protective colours cannot, however, be classed with those which possess
an immunity from hostile attacks.

That the coloured edges are correctly explained as imitations of the
oblique shadows of the leaf-ribs, may also be proved from another point
of view. Let us assume, for the sake of argument, that these coloured
stripes are not adaptive, and that they have not been produced by
natural selection, but by a hypothetical phyletic force. We should
then expect to see them appear at some period in the course of the
phyletic development--perhaps at first only in solitary individuals,
then in several, and finally in all; but we certainly could not expect
that at first single, irregular, coloured spots should arise in the
neighbourhood of the oblique white stripes--that these spots should
then multiply, and fusing together, should adhere to the white stripes,
so as to form an irregular spot-like edge, which finally becomes formed
into a straight, uniformly broad stripe. The phyletic development of
the coloured edges takes place, however, in such a manner, the species
of _Smerinthus_, as has already been established, showing this with
particular distinctness. In _S. Tiliæ_ the course of development can
be followed till the somewhat irregular red border is formed. In the
species of _Sphinx_ this border has become completely linear. It is
very possible that the ontogeny of _S. Ligustri_ or _Drupiferarum_
would reveal the whole process, although it may also be possible that
owing to the contraction of the development, much of the phylogeny is
already lost.

I have now arrived at the consideration of the last kind of marking
which occurs in the _Sphingidæ_, viz.:--

4. _Eye-spots and Ring-spots._--These markings, besides among the
_Sphingidæ_, are found only in a very few caterpillars, such as certain
tropical _Papilionidæ_ and _Noctuæ_. I know nothing of the conditions
of life and habits of these species, however, and without such
knowledge it is impossible to arrive at a complete explanation.

With Darwin, I take an eye-spot to be “a spot within a ring of another
colour, like the pupil within the iris,” but to this central spot
“concentric zones” maybe added. In the _Chærocampa_ larvæ and in
_Pterogon Œnotheræ_, in which complete ocelli occur, there are always
three zones--a central spot, the pupil, or, as I have called it, the
“nucleus;” then a light zone, the “mirror;” and, surrounding this
again, a dark zone (generally black), the “ground-area.”

As ring-spots I will consider those ocelli which are without the
nucleus (pupil), and which are not therefore, strictly speaking,
deceptive imitations of an eye, but present a conspicuous light spot
surrounded by a dark zone.

Between these two kinds of markings there is, however, no sharp
boundary, and morphologically they can scarcely be separated. Species
with ring-spots sometimes have nuclei, and ocellated larvæ in some
cases possess only a pale spot instead of a dark pupil. I deal here
with the two kinds separately, because it happens that they appear
in two distinct genera, in each of which they have their special
developmental history. Ring-spots originate in a different position,
and in another manner than eye-spots; but it must not, on this
account, be assumed without further inquiry, that they are called
into existence by the same causes; they must rather be investigated
separately, from their origin.

Eye-spots are possessed by the genera _Chærocampa_ and _Pterogon_;
ring-spots by the genus _Deilephila_. In accordance with the data
furnished by the above-given developmental histories, the origination
of these markings in the two genera may be thus represented:--

In the genera named, eye-spots and ring-spots are formed by the
transformation of single portions of the subdorsal line.

In _Chærocampa_ the primary ocelli originate on the fourth and fifth
segments by the detachment of a curved portion of the subdorsal, this
fragment becoming the “mirror,” and acquiring a dark encircling zone
(“ground-area”). The nucleus (pupil) is added subsequently.

In _Deilephila_ we learn from the development of _D. Hippophaës_, that
the primary annulus arises on the segment bearing the caudal horn (the
eleventh) by the deposition of a red spot on the white subdorsal line,
which is somewhat enlarged in this region. The formation of a dark
“ground-area” subsequently occurs, and with this, at first the partial,
and then the complete, detachment of the mirror-spot from the subdorsal
line takes place.

In both genera the spots arise at first locally on one or two segments,
from which they are transferred to the others as a secondary
character. In _Chærocampa_ this transference is chiefly backwards, in
_Deilephila_ invariably forwards.

We have now to inquire whether complete eye-spots--such as those of the
_Chærocampa_ larvæ--have any significance at all, and whether they are
of biological importance. It is clear at starting, that these spots do
not belong to that class of markings which make their possessors more
difficult of detection; they have rather the opposite effect.

We might thus be disposed to class ocellated caterpillars with those
“brightly coloured” species which, like the _Heliconinæ_ and _Danainæ_
among butterflies, possess a disgusting taste, and which to a certain
extent bear the signal of their distastefulness in their brilliant
colours. But even if I had not found by experiment that our native
_Chærocampa_ larvæ were devoured by birds and lizards, and that they
are not therefore distasteful to these insect persecutors, from the
circumstance that these caterpillars are all protectively coloured,
it could have been inferred that they do not belong to this category.
It has been found that all adaptively coloured caterpillars are
eaten, and one and the same species cannot possibly be at the same
time inconspicuously (adaptively) and conspicuously coloured; the one
condition excludes the other.

What other significance can eye-spots possess than that of making the
insects conspicuous? Had we to deal with sexually mature forms, we
should, in the first place, think of the action of sexual selection,
and should regard these spots as objects of taste, like the ocelli
on the feathers of the peacock and argus-pheasant. But we are here
concerned with larvæ, and sexual selection is thus excluded.

The eye-spots must therefore possess some other significance, or else
they are of no importance at all to the life of the insect, and are
purely “morphological characters;” in which case, supposing this could
be proved, they would owe their existence exclusively to forces innate
in the organism itself--a view which very closely approaches the
admission of a phyletic vital force.

I am of opinion, however, that eye-spots certainly possess a biological
value as a means of terrifying--they belong to that numerous class of
characters which occur in the most diverse groups of animals, and which
serve the purpose of making their possessors appear as alarming as
possible.

The caterpillars of the _Sphingidæ_ are known to behave themselves in
different manners when attacked. Some species, such, for instance,
as _Sphinx Ligustri_ and _Smerinthus Ocellatus_, on the approach of
danger assume the so-called Sphinx attitude; if they are then actually
seized, they dash themselves madly to right and left, by this means not
only attempting to get free, but also to terrify their persecutor.
This habit frequently succeeds with men, and more especially with
women and children; perhaps more easily in these cases than with their
experienced foes, birds.

The ocellated _Chærocampa_ larvæ behave differently. They remain quiet
on being attacked, and do not put on a Sphinx-like attitude, but only
withdraw the head and three small front segments into the large fourth
segment, which thus becomes much swollen, and is on this account taken
for the head of the insect by the inexperienced.[145] Now the large
eye-spots are situated on the fourth segment, and it does not require
much imagination to see in such a caterpillar an alarming monster with
fiery eyes, especially if we consider the size which it must appear to
an enemy such as a lizard or small bird. Fig. 28 represents the larva
of _C. Porcellus_ in an attitude of defence, although but imperfectly,
since the front segments can be still more withdrawn.

These facts and considerations do not, however, amount to scientific
demonstration, and I therefore made a series of experiments, in order
to determine whether these caterpillars did actually frighten small
birds. The first experiment proved but little satisfactory. A jay,
which had been domesticated for years, to which I threw a caterpillar
of _Chærocampa Elpenor_, did not give the insect any time for
manœuvring, but killed it immediately by a strong blow with its bill.
This bird had been tame for years, and was in the habit of pecking at
everything thrown to him. Perhaps a wild jay (_Garrulus Glandarius_)
would have treated the insect differently, but it is hardly possible
that such a large and courageous bird would have much respect for
our native caterpillars. I now turned to wild birds. A large brown
_Elpenor_ larva was placed in the food-trough of an open fowl-house
from which the fowls had been removed. A flock of sparrows and
chaffinches (_Fringilla Domestica_ and _Cœlebs_) soon flew down from
the neighbouring trees, and alighted near the trough to pick up stray
food in their usual manner. One bird soon flew on to the edge of the
trough, and was just about to hop into it when it caught sight of the
caterpillar, and stood jerking its head from side to side, but did not
venture to enter. Another bird soon came, and behaved in a precisely
similar manner; then a third, and a fourth; others settled on the perch
over the trough, and a flock of ten or twelve were finally perched
around. They all stretched their heads and looked into the trough, but
none flew into it.

I now made the reverse experiment, by removing the caterpillar and
allowing the birds again to assemble, when they hopped briskly into the
trough.

I often repeated this experiment, and always with the same result. Once
it could be plainly seen that it was really fear and not mere curiosity
that the birds showed towards the caterpillar. The latter was outside
the trough amongst scattered grains of food, so that from one side it
was concealed by the trough. A sparrow flew down obliquely from above,
so that at first it could not see the caterpillar, close to which it
alighted. The instant it caught sight of the insect, however, it turned
in evident fright and flew away.

Of course these experiments do not prove that the larger insectivorous
birds are also afraid of these caterpillars. Although I have not
been able to experiment with such birds, I can certainly prove that
even fowls have a strong dislike to these insects. I frequently
placed a large _Elpenor_ larva in the poultry yard, where it was soon
discovered, and a fowl would run hastily towards it, but would draw
back its head just when about to give a blow with the bill, as soon
as it saw the caterpillar closely. The bird would now run round the
larva irresolutely in a circle--the insect in the meantime assuming
its terrifying attitude--and stretching out its head would make ten
or twenty attempts to deal a blow with its bill, drawing back again
each time. All the cocks and hens acted in a similar manner, and it
was often five or ten minutes before one particularly courageous bird
would give the first peck, which would soon be followed by a second
and third, till the caterpillar, appearing palatable, would finally be
swallowed.

These experiments were always made in the presence of several persons,
in order to guard myself against too subjective an interpretation of
the phenomena; but they all invariably considered the conduct of the
birds to be as I have here represented it.[146]

If it be admitted that the ocelli of caterpillars are thus means of
exciting terror, the difficulty of their occurring in protectively
coloured species at once vanishes. They do not diminish the advantage
of the adaptive colouring, because they do not make the caterpillars
conspicuous, or at least any more easily visible at a distance,
excepting when the insects have assumed their attitude of alarm. But
these markings are of use when, in spite of protective colouring, the
larva is attacked by an enemy. The eye-spots accordingly serve the
caterpillar as a second means of defence, which is resorted to when the
protective colouring has failed.

By this it must not be understood that the ocelli of the _Chærocampa_
larvæ invariably possess only this, and no other significance for the
life of the insect. Every pattern can be conceived to render its
possessor in the highest degree conspicuous by strongly contrasted
and brilliant colouring, so that it might be anticipated that perfect
eye-spots in certain unpalatable species would lose their original
meaning, and instead of serving for terrifying become mere signals of
distastefulness. This is perhaps the case with _Chærocampa Tersa_ (Fig.
35), the numerous eye-spots of which make the insect easily visible.
Without experimenting on this point, however, no certain conclusion
can be ventured upon, and it may be equally possible that in this case
the variegated ocelli with bright red nuclei resemble the blossoms of
the food-plant (_Spermacoce Hyssopifolia_).[147] I here mention this
possibility only in order to show how an inherited form of marking,
even when as well-defined and complicated as in the present case, may,
under certain circumstances, be turned in quite another direction
by natural selection, for the benefit of its possessor. Just in the
same manner one and the same organ, such, for instance, as the limb
of a crustacean, may, in the course of phyletic development, perform
very different functions--first serving for locomotion, then for
respiration, then for reproduction or oviposition, and finally for the
acquisition of food.

I now proceed to the consideration of the biological value of
incomplete eye-spots, or, as I have termed them, ring-spots. Are these
also means of terrifying, or are they only signals of distastefulness?

I must at the outset acknowledge that on this point I am able to offer
but a very undecided explanation. The decision is only to be arrived
at by experiments conducted with each separate species upon which
one desires to pronounce judgment. It is not here legitimate to draw
analogical inferences, and to apply one case to all, since it is not
only possible, but very probable, that the biological significance
of ring-spots changes in different species. Nothing but a large
series of experiments could completely establish this. Unfortunately
I have hitherto failed in obtaining materials for this purpose. I
would have deferred the publication of this essay for a year, could
I have foreseen with certainty that such materials would have been
forthcoming in sufficient quantity during the following summer; but
this unfortunately depends very much upon chance, and I believed that
a preliminary conclusion would be preferable to uncertainty. Perhaps
some entomologist to whom materials are more easily accessible, may, by
continuing these experiments, accomplish this object.

The experiments hitherto made by other observers, are not sufficient
for deciding the question under consideration. Weir,[148] as is
well known, showed that certain brightly coloured and conspicuous
larvæ were refused by insectivorous birds; and Butler[149] proved the
same for lizards and frogs. These experiments are unfortunately so
briefly described, that in no case is the species experimented with
mentioned by name, so that we do not know whether there were any Sphinx
caterpillars among them.[150] I have likewise experimented in this
direction with lizards, in order to convince myself of the truth of
the statement that (1) there are caterpillars which are not eaten on
account of their taste, and (2) that such larvæ possess bright colours.
I obtained positive, and on the whole, very decided results. Thus,
the common orange and blue striped caterpillars of _Bombyx Neustria_
enjoyed complete immunity from the attacks of lizards, whilst those of
the nearly allied _Eriogaster Lanestris_ and _L. Pini_ were devoured,
although not exactly relished. That the hairiness is not the cause
of their being unpalatable, is shown by the fact that _L. Pini_ is
much more hairy than _B. Neustria_. The very conspicuous yellow and
black ringed caterpillar of _Euchelia Jacobææ_ gave also most decided
results. I frequently placed this insect in a cage with _Lacerta
Viridis_, but they would never even notice them, and I often saw the
caterpillars crawl over the body, or even the head of the lizards,
without being snapped at. On every occasion the larvæ remained for
several days with the lizards without one being ever missed. The
reptiles behaved in a precisely similar manner with respect to the moth
of _E. Jacobææ_, not one of which was ever touched by them. The yellow
and black longitudinally striped caterpillars of _Pygæra Bucephala_
were also avoided, and so were the brightly coloured larvæ of the
large cabbage white (_Pieris Brassicæ_), which when crushed give a
disagreeable odour. This last property clearly shows why lizards reject
this species as distasteful. Both caterpillar and butterfly possess
a blood of a strong yellow colour and oily consistency, in which,
however, I could not detect such a decided smell as is emitted by that
of the _Heliconinæ_ and _Danainæ_.[151]

I next made the experiment of placing before a lizard a caterpillar
as much as possible like that of _E. Jacobææ_. Half grown larvæ of
_Bombyx Rubi_ likewise possess golden yellow (but narrower) transverse
rings on a dark ground, and they are much more hairy than those of
_E. Jacobææ_. The lizard first applied its tongue to this caterpillar
and then withdrew it, so that I believed it would also be avoided;
nevertheless it was subsequently eaten. The caterpillars of _Saturnia
Carpini_ were similarly devoured in spite of their bristly hairs,
and likewise cuspidate larvæ (_Dicranura Vinula_), notwithstanding
their extraordinary appearance and their forked caudal horn.[152]
These lizards were by no means epicures, but consumed large numbers
of earth-worms, slugs, and great caterpillars, and once a specimen of
the large and powerfully biting Orthopteron, _Decticus Verucivorus_.
Creatures which possessed a strongly repugnant odour were, however,
always rejected, this being the case with the strongly smelling
beetle, _Chrysomela Populi_, as also with the stinking centipede,
_Iulus Terrestris_, whilst the inodorous _Lithobius Forficatus_ was
greedily eaten. I will call particular attention to these last facts,
because they favour the supposition that with rejected caterpillars a
disgusting odour--although perhaps not always perceptible by us--is the
cause of their being unpalatable.

Striking colours are of course only signals of distastefulness, and the
experiment with _Bombyx Rubi_ shows that the lizards were from the
first prejudiced against such larvæ, the prejudice only being overcome
on actually trying the specimen offered. A subsequent observation
which I made after arriving at this conclusion, is most noteworthy.
After the lizard had learnt by experience that there might be not only
distasteful caterpillars (_E. Jacobææ_), but also palatable ones banded
with black and yellow (_B. Rubi_), it sometimes tasted the _Jacobææ_
larvæ, as if to convince itself that the insect was actually as it
appeared to be, viz., unpalatable!

A striking appearance combined with a very perceptible and penetrating
odour is occasionally to be met with, as in the caterpillar of the
common Swallow-tail, _Papilio Machaon_. I have never seen a lizard
make the slightest attempt to attack this species. I once placed two
large specimens of this caterpillar in the lizard vivarium, where they
remained for five days, and finally pupated unharmed on the side of the
case.

I have recorded these experiments, although they do not thus far
relate to Sphinx-caterpillars, with the markings of which we are
here primarily concerned, because it appeared to me in the first
place necessary to establish by my own experiments that signals of
distastefulness did occur in caterpillars.

I now come to my unfortunately very meagre experience with _Deilephila_
larvæ, with only two species of which have I been able to experiment,
viz., _D. Galii_ and _Euphorbiæ_.

The first of these was constantly rejected. Two large caterpillars, one
of the black and the other of the yellow variety, were left for twelve
hours in the lizard vivarium, without being either examined or touched.
It thus appears that _D. Galii_ is a distasteful morsel to lizards;
and the habits of the caterpillar are quite in accordance with this,
since it does not conceal itself, but rests fully exposed by day on a
stem, so that it can scarcely escape being detected. It is almost as
conspicuous as _D. Euphorbiæ_.

I was much surprised to find, however, that this last species was not
rejected by lizards. On placing a large caterpillar, six to seven
centimeters long, in the vivarium, the lizard immediately commenced
to watch it, and as soon as it began to crawl about, seized it by the
head, and, after shaking it violently, commenced to swallow it. In
spite of its vigorous twisting and turning, the insect gradually began
to disappear, amidst repeated shakings; and in less than five minutes
was completely swallowed.[153] With regard to lizards, therefore, the
prominent ring-spots of this larva are not effective as a means of
alarm, nor are they considered as a sign of distastefulness.

Unfortunately I have not hitherto been able to make any experiments
with birds. It would be rash to conclude from the experience with
lizards that ring-spots were of no biological value. There is scarcely
any one means of protection which can render its possessor secure
against _all_ its foes. The venom of the most poisonous snakes does
not protect them from the attack of the secretary bird (_Serpentarius
Secretarius_) and serpent eagle (_Spilornis Cheela_); and the adder,
as is well known, is devoured by hedgehogs without hesitation. It
must therefore be admitted that many species which are protected by
distastefulness, may possess certain foes against which this quality
is of no avail. Thus, it cannot be said that brightly coloured
caterpillars, which are not eaten by birds and lizards, are also spared
by ichneumons. It is readily conceivable therefore, that the larva of
_D. Euphorbiæ_ may not be unpalatable to lizards, because they swallow
it whole; whilst it is perhaps distasteful to birds, because they must
hack and tear in order to swallow it.

From these considerations it still appears most probable to me that
_D. Euphorbiæ_, and the nearly allied _D. Dahlii_ and _Mauritanica_,
bear conspicuous ring-spots as signs of their being unpalatable to the
majority of their foes. The fact that these species feed on poisonous
_Euphorbiaceæ_, combined with their habit of exposing themselves openly
by day, so as to be easily seen at a distance, may perhaps give
support to this view. As these insects are not protectively coloured,
this habit would long ago have led to their extermination; instead
of this, however, we find that in all situations favourable to their
conditions of life they are among the commonest of the _Sphingidæ_.

Thus, _D. Euphorbiæ_ occurs in large numbers both in South and North
Germany (Berlin); and Dr. Staudinger informs me that in Sardinia the
larvæ of _D. Dahlii_ were brought to him by baskets full.

But if the conspicuous ring-spots (combined of course with the other
bright colours) may be regarded as signals of distastefulness in many
species of _Deilephila_, this by no means excludes the possibility that
in some species these markings play another part, and are effective as
a means of alarm. It even appears conceivable to me that in one and the
same caterpillar they may play both parts against different foes, and
it would certainly be of interest to confirm or refute this supposition
by experiment.

In the light yellow variety of the caterpillar of _D. Galii_ the
ring-spots may serve as means of alarm, and still more so in that of
_D. Nicæa_, the resemblance of which to a snake has struck earlier
observers.[154]

In those species of _Deilephila_ which conceal themselves by day, the
ring-spots cannot be considered as signals of distastefulness, and they
must therefore have some other meaning. As examples of this class may
be mentioned _D. Vespertilio_, which is protectively coloured both in
the young and in the adult stages; and likewise _D. Hippophaës_, in
which this habit of concealment is associated with adaptive colouring.
In the case of the first-named species, it appears possible that the
numerous large ring-spots may serve to alarm small foes, but the
truth of this supposition could only be decided by experiment. In _D.
Hippophaës_, on the other hand, such an interpretation must be at once
rejected, since most individuals possess but a single ring-spot, which
shows no resemblance whatever to an eye.

I long sought in vain for the meaning of this ring-spot, the
discovery of which would in this particular case be of the greatest
value, because we have here obviously the commencement of the whole
development of ring-spots before us--the initial stage from which the
marking of all the other species of _Deilephila_ has proceeded.

I believe that I have now found the correct answer to this riddle, but
unfortunately at a period of the year when I am unable to prove it
experimentally. I consider that the ring-spots are crude imitations of
the berries of the food-plant. The latter are orange-red, and exactly
of the same colour as the spots; the agreement in colour between the
latter and the berries is quite as close as that between the leaves and
the general colouring of the caterpillar. I know of no species which
more closely resembles the colour of the leaves of its food-plant,
the dark upper side and light under side corresponding in the leaves
and caterpillars. The colour of the _Hippophae_ is not an ordinary
green, but a grey-green, which shade also occurs, although certainly
but rarely, in the larvæ. I may expressly state that I have repeatedly
shown to people as many as six to eight of the large caterpillars on
one buckthorn branch, without their being able at once to detect them.
It is not therefore mere supposition, but a fact, that this species
is protected by its general colouring. At first the orange-red spots
appear rather to diminish this protection--at least when the insects
are placed on young shoots bearing no berries. But since at the same
time when the berries become red (end of July and the beginning of
August) the caterpillars are in their last stage of development (_i.e._
possess red spots), it appears extremely probable that these spots are
vague representations of the berries. For the same reason that these
caterpillars have acquired the habit of feeding only at dusk and during
the morning twilight, or at night, and of concealing themselves by
day, it must be advantageous for them to have the surface of their
large bodies not only divided by white stripes, but also interrupted
in yet another manner. How could this be better effected than by two
spots which, in colour and position, represent the grouping of the red
berries on the branches? When feeding, the insect always rests with
the hind segments on a branch, the front segments only being more or
less raised and held parallel to the leaves; the red spots thus always
appear on the stem, where the berries are likewise situated. It might
indeed be almost supposed that the small progress which the formation
of secondary ring-spots on the other segments has made up to the
present time, is explicable by the fact that such berry-like spots on
other portions of the caterpillar would be rather injurious than useful.

It may, however, be asked how an imitation of red berries, which are
eaten by birds just as much as other berries, can be advantageous to a
caterpillar, since by this means it would rather attract the attention
of its enemies?

Two answers can be given to this. In the first place, the berries are
so numerous on every plant that there is but a very small chance of the
smaller and less conspicuous berry-spots catching the eye of a bird
before the true berries; and, secondly, the latter, although beginning
to turn red when the caterpillars are feeding, do not completely
ripen till the autumn, when the leaves are shed, and the yellowish-red
clusters of berries can be seen at a distance. The caterpillar,
however, pupates long before this time.

I have considered this case in such detail because it appears to me
of special importance. It is the only instance which teaches us that
the rows of ring-spots of the _Deilephila_ larvæ proceed from one
original pair--the only instance which permits of the whole course of
development being traced to its origin. Were it possible to arrive at
the causes of the formation of these spots, their original or primary
significance would thereby be made clear.

I will now briefly summarise the results of the investigation of the
biological value of the _Deilephila_ ring-spots.

In the known species of the genus now existing these spots have
different meanings.

In some species (certainly in _Galii_, and probably in _Euphorbiæ_
and _Mauritanica_) the conspicuous ring-spots serve as signals of
distastefulness for certain enemies (not for all).

In a second group of species they serve as a means of alarm, like the
eye-spots of the _Chærocampa_ larvæ (_Nicæa_? light form of _Galii_?).

Finally, in a third group, of which I can at present only cite
_Hippophaës_, they act as an adaptive resemblance to a portion of a
plant, and enhance the efficacy of the protective colouring.


5. _Subordinate Markings._--If, from the foregoing
considerations, it appears that the three chief elements of the
Sphinx-markings--longitudinal and oblique stripes, and spot
formations--are not purely morphological characters, but have a very
decided significance with respect to their possessors, there should be
no difficulty in referring the whole of the markings of the _Sphingidæ_
to the action of natural selection, supposing that these three kinds of
marking were the only ones which actually occurred.

In various species, however, there appear other patterns, which I have
comprised under the term “subordinate markings,” some of which I will
select, for the purpose of showing the reasons which permit of their
being thus designated.

I ascribe to this category, for example, that fine network of dark
longitudinal streaks which often extends over the whole upper side
of the caterpillar, and which is termed the “reticulation.” This
character is found chiefly in the adult larvæ of _Chærocampa_, being
most strongly pronounced in the brown varieties: it occurs also in
_Deilephila Vespertilio_, _Pterogon Œnotheræ_, and _Sphinx Convolvuli_.
As far as I know, it is only associated with adaptive colours, and
indeed occurs only in those caterpillars which rest periodically at
the base of their food-plants among the dead leaves and branches. I do
not consider this reticulation to be a distinct imitation, but only as
one of the various means of breaking up the large uniform surface of
the caterpillar so as to make it present inequalities, and thus render
it less conspicuous. There can be no doubt as to the dependence of this
character upon natural selection.

There is, however, a second group of markings, which must be referred
to another origin. To this group, for instance, belong those light dots
in _Chærocampa Porcellus_ and _Elpenor_ which have been termed “dorsal
spots.” I know of no other explanation for these than that they are the
necessary results of other new formations, and depend on correlation
(Darwin), or, as I may express it, they are the result of the action of
the law governing the organization of these species.

As long as we are confined to the mere supposition that the character
in question may be the outward expression of an innate law of growth,
it is permissible to attempt to show that a quite similar formation in
another species depends upon such a law.

Many of the dark specimens of _Sphinx Convolvuli_ show whitish dots
on segments six to eleven, one being situated on the front edge of
each of these segments, at the height of the completely vanished
subdorsal line (Fig. 52). These spots vary much in size, lightness,
and sharpness of definition. Now it might be difficult to attribute
any biological significance to this character, but its origin becomes
clear on examining light specimens in which the oblique white stripes
are distinct on the sides and the subdorsal line is retained at least
on the five or six anterior segments. It can then be seen that the
spots are located at the points of intersection of the subdorsal and
the oblique stripes (Fig. 16, Pl. III.), and they can accordingly be
explained by the tendency to the deposition of light pigment being
twice as great in these positions as in other portions of the two
systems of light lines. Light spots are thus formed when the lines
which cross at these points are partially or completely extinct
throughout their remaining course.

A marking is therefore produced in this case by a purely innate law of
growth--by the superposition of two ancient characters now rudimentary.
Many other unimportant details of marking must be regarded as having
been produced in a similar manner, although it may not be possible to
prove this with respect to every minute spot and stripe. The majority
of “subordinate markings” depend on the commingling of inherited, but
now meaningless, characters with newly acquired ones.

It would be quite erroneous to attribute to natural selection only
those characters which can be demonstrated to still possess a
biological value in the species possessing them. They may be equally
due to heredity. Thus, it is quite possible that the faint and
inconspicuous ring-spots of _Deilephila Vespertilio_ are now valueless
to the life of the species--they may be derived from an ancestral form,
and have not been eliminated by natural selection simply because they
are harmless. I only mention this as a hypothetical case.

In the case of markings of the second class, _i.e._ oblique stripes,
a transference to later phyletic stages can be demonstrated, although
the stripes thereby lose their original biological value. Thus, the
_Chærocampa_ larvæ, when they were green throughout their whole life
and adapted to the leaves, appear to have all possessed light oblique
stripes in imitation of the leaf-ribs. All the species of the older
type of colouring and marking, such as _Chærocampa Syriaca_ (Fig. 29)
and _Darapsa Chœrilus_ (Fig. 34), and also the light green young forms
of _C. Elpenor_ (Fig. 20), and _Porcellus_ (Figs. 25 and 26), show
these oblique stripes. In these last species the foliage imitation
is abandoned at a later stage, and a dark brown, or blackish-brown,
ground-colour acquired. Nevertheless the oblique stripes do not
disappear, but show themselves--in the fourth stage especially, and
sometimes in the fifth--as distinct dirty yellow stripes, although not
so sharply defined as in the earlier stages. These persistent stripes,
in accordance with their small biological value, are very variable,
since they are only useful in so far as they help to break up the
large surface presented by the caterpillar, and are of no value as
imitations of surrounding objects.

The oblique stripes of _Sphinx Convolvuli_ offer a precisely similar
case; and it may be safely predicted that the young forms of this
species would possess sharply defined light oblique stripes, since
more or less distinct remnants of these markings occur in all the
adult larvæ, and especially in the green form. The entire pattern of
this caterpillar depends essentially on the commingling of characters
persisting from an earlier period, _i.e._ of residues of the subdorsal
and oblique stripes, both these markings being extraordinarily
variable. The black reticulation was added to the ground-colour as
a new means of adaptation, this character appearing only in the
phyletically younger brown form, and being entirely absent, or only
faintly indicated, in the older green variety.




VI.

OBJECTIONS TO A PHYLETIC VITAL FORCE.


It has been shown in the previous section that the three elements
composing the markings of the Sphinx-larvæ originally possessed a
distinct significance with respect to the life of the species, and
that they were by this means called into existence. It has likewise
been shown, that in most of the species which possess these characters
at the present time they still have a decided, although sometimes a
different use, for their possessors, so that from this point of view no
objection can be raised to their being considered as having arisen by
natural selection.

On looking at the phenomenon as a whole, however, certain instances
occur which appear quite irreconcilable with this view.

The most formidable objection is offered by the genus _Deilephila_.
The row of ring-spots which nearly all the existing species have more
or less developed, has arisen from a simple subdorsal line. It would
not, therefore, be surprising if a species were discovered which
possessed this line without any ring-spots as its only marking. If
_D. Hippophaës_ were thus marked, there would be no objection to the
theoretical assumption that this[155] was the ancestor of the other
species. It would then be said that ring-spots were first developed
in a later species by natural selection, and that they had been
transmitted to all succeeding and younger species.

Certain individuals of _D. Hippophaës_, however, possess small
ring-spots, some of which are well developed on several segments.
In this species the row of ring-spots is therefore comprised in the
development. The remaining species, which are much younger phyletically
than _Hippophaës_, could not have inherited their ring-spots from the
latter, since this species itself only possesses them occasionally,
and, so to speak, in a tentative manner. The spots would therefore
appear to have arisen spontaneously in this species, and independently
of those in the other species. But if this were the case, how should
we be able to prove that in the other species also the ring-spots did
not arise independently; and if, moreover, a large number of species
showed the same character without its being referable to inheritance
from a common ancestor, how could this be otherwise explained than as
the result of a force innate in these species and producing similar
variations? But this is nothing but Askenasy’s “fixed direction of
variation”--_i.e._, a phyletic vital force.

The only escape from this difficulty is perhaps to be found in proving
that _D. Hippophaës_ formerly possessed ring-spots, and that these
have been subsequently either partially or completely lost, so that
their occasional appearance in this species would therefore depend upon
reversion. The ontogeny, however, teaches us that this is not the case,
since the young caterpillar does not possess a greater number of more
distinct ring-spots, but wants them altogether with the exception of a
red spot on the eleventh segment, which is, however, much fainter than
in the last stage.

This last-mentioned fact contains the solution of the problem. The
premises from which this reasoning set out were all incorrect--the one
red spot on the eleventh segment is likewise a ring-spot, and indeed
the most important one of all, being primary, or the first to come
into existence. Now all specimens, without exception, possess this
first ring-spot, which is useful, and has therefore been called forth
by natural selection; it is not inherited, but newly acquired by this
species; at least, if the explanation of these spots which I have
previously offered is correct.

The primary pair of spots may have been transferred from this to later
species by heredity; and since, in all segmented animals there is a
tendency for the peculiarities of one segment to be repeated on the
others, this repetition must have occurred with greater frequency and
more completely in the later species--the more so if the process were
favoured by natural selection, _i.e._ if the row of ring-spots which
originated in this manner could in any way be turned to the use of the
species.

In _Hippophaës_ itself there must also be a tendency to the formation
of secondary ring-spots, and indeed in a number of specimens we
actually see series of such ring-spots, the latter being present in
varying numbers, and in very different states of development. The fact
that the ring-spots have not become a constant and well-developed
character, is simply explained by the circumstance that as such they
would have endangered the existence of the species.

In this case there is therefore no necessity for assuming a phyletic
vital force. The ring-spots of the genus _Deilephila_ rather furnish
us with an excellent explanation of a fact which might otherwise have
been adduced in support of a phyletic vital force, viz., the strict
uniformity in the development of larval markings.

Before I had been led to the discovery, by the study of the marking and
development of _Hippophaës_, that the spots of the genus _Deilephila_
originated on one segment only, from which they were transferred
secondarily to the others, this astonishing regularity appeared to me
an incomprehensible problem, which could only be solved by assuming
a phyletic vital force. If it be attempted, for the ten species here
considered, to construct a genealogical tree based on the supposition
that it is the _rows of spots_ which have been inherited in cases
where they occur, and not the _mere tendency to their production_
by the transference of the one originally inherited primary spot to
the remaining segments, the attempt will fail. The greater number of
the species would have to be arranged in one row, since one species
always bears a perfected form of marking, which appears in the young
stages of the following species. But it is very improbable that nine
different species, derived directly the one from the other, would
contemporaneously survive.[156] One species, _D. Vespertilio_, could
not be inserted at all in the genealogical tree, since it wants one
character which occurs in all the other species, viz., the caudal horn,
which is absent even in the third stage, and must therefore have been
lost at a very early period of the phyletic development, so that we
may consider it to be on this account genetically allied to the oldest
known form. But the markings of this larva pass through precisely the
same stages of development as do those of the other species. Now if
the ring-spots were inherited as such, the existence of a hornless
species with ring-spots would be an insoluble riddle, and would favour
the admission of parallel developmental series, which again could be
scarcely otherwise explained than by a “fixed direction of variation.”
We have here one of that class of cases which the supporters of a
phyletic vital force have already so often made use of in support of
their view.

The explanation of such a case--_i.e._ its reference to known causes of
species transformation--is never easy, and is indeed impossible without
a precise knowledge of the ontogeny of many species, as well as of the
original significance of the characters in question. In the case of the
_Deilephila_ larvæ, however, such knowledge is still wanting. It is
true that they present us with parallel developmental series, but these
do not depend on an unknown phyletic force--the parallelism can be
referred to the action of the imperfectly known laws of growth innate
in segmented organisms. Because the characters of one segment have
a tendency to repeat themselves on the others, from one parent-form
possessing ring-spots on one segment only, there may have proceeded
several developmental series, all of which developed rows of such spots
independently of each other.

From these considerations we may venture to construct the following
genealogical tree:--

[Illustration: POSSIBLE GENEALOGICAL TREE OF THE GENUS DEILEPHILA

The circles indicate the phyletic stages IV.-VIII.; the eighth is only
reached by _Nicæa_, and is distinguished from the seventh chiefly by
the ontogeny, in the third stage of which the seventh phyletic stage is
reached, whilst in _Euphorbiæ_ and _Dahlii_ this stage is reached in
the fourth ontogenetic stage. The phyletic stages indicated by queries
are extinct, and only known through the ontogeny of existing species.
It must be understood that this pedigree expresses only the ideal and
not the actual relations of the species to one another. Thus, it is
possible that _Hippophaës_ is not the parent-form, but an unknown or
extinct species, which must, however, have possessed the same marking,
and so on.]

Four parallel series here proceed from the parent-form _Hippophaës_;
there may have been five, or possibly only three, but the incomplete
state of our knowledge of the ontogeny does not permit of any certain
conclusion. For the point under consideration this is, however, quite
immaterial. The distance from the central point (the parent-form)
indicates the grade of phyletic development which the respective
species have at present reached.

There is another case which is no less instructive, because it reveals,
although in a somewhat different manner, the action of a law of growth
innate in the organism itself, but which can nevertheless by no means
be regarded as equivalent to a phyletic vital force. I refer to the
coloured edges of the oblique stripes which occur in most of the
species of the genus _Sphinx_. It has already been insisted upon in
a previous section, that the mode in which this character originates
negatives the assumption of a phyletic force, because these coloured
edges are gradually built up out of irregularly scattered spots. There
is no occasion for a “developmental force” to grope in the dark; if
such a power exists, we should expect that it would add new characters
to old ones with the precision of a master workman.

If, however, the coloured edges certainly depend on natural selection,
this agency causing the scattered spots to coalesce and become linear,
we have here the proof that such spots first arose in a precisely
similar manner in several species, quite independently of one
another--that, in fact, a “fixed direction of variation” in a certain
sense exists.

In three species of _Smerinthus_-larvæ, red spots appear towards
the end of the ontogeny; in _S. Populi_ and _Ocellatus_ in only a
minority of individuals, and always separate (not coalescent), and in
_S. Tiliæ_ in a majority of specimens, the spots frequently becoming
fused into one large, single, longish marking. These three species
cannot have inherited the spots from a common ancestor, since they are
absent in the younger ontogenetic stages, or occur only exceptionally,
becoming larger and more numerous in the last stage; they obviously
form a character which must be considered as a case of “anticipated
development.”

How is it then that three species vary independently of each other
in an analogous manner? I know of no other answer to this question
than that similar variations must necessarily arise from similar
physical constitutions--or, otherwise expressed, the three species have
inherited from an unknown parent species, devoid of spots, not this
last character itself, but a physical constitution, having a tendency
to the formation of red spots on the skin.[157] The case offers many
analogies to that of the colour varieties of _Lacerta Muralis_,
to which Eimer[158] briefly calls attention in his interesting
communications on the blue lizard of the Faraglioni Rocks at Capri.
The South Italian lizards, although having differently formed skulls,
show the same brilliantly coloured varieties as those of North Italy;
and Eimer believes that these parallel variations in widely separated
localities, some of which have long been isolated, must be referred
to a tendency towards fixed directions of variation innate in the
constitution of the species.

I long ago insisted[159] that it should not be forgotten that natural
selection is, in the first place, dependent upon the variations which
an organism offers to this agency, and that, although the number of
possible variations may be very great for each species, yet this
number is by no means to be considered as literally infinite. For
every species there may be _impossible_ variations. For this reason I
am of opinion that the physical nature of each species is of no less
importance in the production of new characters than natural selection,
which must always, in the first place, operate upon the results of this
physical nature, _i.e._ upon the variations presented, and can thus
call new ones into existence.

It requires but a slight alteration of the definition to make out of
this “restricted” or “limited variability,” which is the necessary
consequence of the physical nature of each species, a “fixed direction
of variation” in the sense of a phyletic vital force. Instead of--the
_Smerinthus_-larvæ show a tendency to produce red spots on the skin, it
is only necessary to say--these larvæ tend to produce red borders to
the oblique stripes. The latter statement would, however, be incorrect,
since the red borders first arose by the coalescence of red spots
through the action of natural selection. It is not even correct to say
that _all_ the species of _Smerinthus_ show this tendency to produce
spots, since this character does not seem to occur either in _S.
Quercus_ or _S. Tremulæ_.

The distinction between the two modes of conception will become clear
if we ask, as an example, whether those _Chærocampa_-larvæ which do not
at present possess eye-spots will subsequently acquire these markings,
supposing that they maintain their existence on the earth for a
sufficient period?

The supporters of a “fixed direction of variation” would answer this
question in the affirmative. Ocelli constitute a character which occurs
in nearly all the species of the group--they are the goal towards
which the phyletic force is urging, and which must sooner or later
be reached by each member of the group. On the other hand, I cannot
express so decidedly my own opinion, viz., that such complicated
characters as the many-coloured oblique stripes or eye-spots are never
the results of purely internal forces, but always arise by the action
of natural selection, _i.e._ by the combination of such minute and
simple variations as may present themselves. It may be replied that the
formation of eye-spots in those species which are at present devoid of
them, cannot indeed be considered impossible, but that they would only
appear if the constitution of these species had a tendency to give rise
to the production of darker spots on the edge of the subdorsal line,
and if at the same time, the possession of eye-spots would be of use to
the caterpillar under its special conditions of life.

The condition of affairs would be quite different if we were simply
concerned with the transference of a character from one segment
on which it was already present, to the remaining segments. The
transference would, in this case, result from causes purely innate in
the organism--from the action of laws of equilibration or of growth
(correlation), and the external conditions of life would play only a
negative part, since they might prevent the complete reproduction of
a character, such, for example, as eye-spots, on all the segments,
in cases where it was disadvantageous to the species. The fact that
our species of _Chærocampa_ have only faint indications, and not a
completely-developed eye-spot, on the remaining segments, may perhaps
be explained in this manner. It is conceivable that the two pairs
of ocelli on the front segments are more effective as a means of
alarm than if the insects were provided with two long rows of such
markings; but nothing can be stated with certainty on this point until
experiments have been made with caterpillars having rows of eye-spots.

The question raised above--whether the species of _Chærocampa_ at
present devoid of eye-spots are to be expected to acquire this
character in the course of their further phyletic development--brings
with it another point, which cannot be here passed over.

If the _utility_ of the four kinds of markings in their perfected form
is demonstrated, their origination through natural selection is not,
strictly speaking, thereby proved. It must also be shown that the first
rudiments of these characters were also of use to their possessors. The
question as to the utility of the “initial stages” of useful characters
must here be set at rest.

In the case of markings such as longitudinal and oblique stripes, it is
quite evident that the initial stages of these simple characters do not
differ greatly from the perfected marking, but this is certainly not
the case with eye- and ring-spots. The most light is thrown upon this
question by the latter, because a species which has remained at the
initial stage of the formation of ring-spots here presents itself for
examination, viz. _Deilephila Hippophaës_.

I have attempted to show that the orange-red spots, which, as a rule,
adorn only the eleventh segment, enhance the adaptive colouring of this
caterpillar by their resemblance to the berries of the sea-buckthorn,
whilst the general surface resembles the leaves in colour. If this be
admitted, the origination of these spots by natural selection offers no
difficulty, since a smaller spot, or one of a fainter red, must also be
of some use to its possessor.

This case is of importance, as showing that a “change of function” may
occur in markings, just as it does in certain organs among the most
diverse species of animals, in the course of phyletic development. The
spots which in _Hippophaës_ are imitations of red berries, in species
which have further advanced phyletically play quite another part--they
serve as means of alarm, or signals of distastefulness.

It appears to me very improbable, however, that the perfect ocelli of
the _Chærocampa_-larvæ have also undergone such a “functional change”
(Dohrn). I rather believe that the first rudiments of these markings
produced the same effect as that which they now exercise, viz.,
terror. We are certainly not so favourably circumstanced in this case
in knowing a species which shows the initial steps of this character
in its last stage of life; but in the initial steps which the second
stage of certain species present, we see preserved the form under which
the eye-spots first appeared in the phylogeny, and from this we are
enabled to judge with some certainty of the effect which they must have
produced at the time.

In the ontogeny of _C. Elpenor_ and _Porcellus_ we see that a small
curvature of the subdorsal line first arises, the concavity of which
becomes filled with darker green, and soon afterwards with black; the
upwardly curved piece of the subdorsal then becomes detached and more
completely surrounded by black. The white fragment of the subdorsal
which has become separated, in the next place broadens, and a black
(dark) pupil appears in its centre.

Now the first rudiments of the eye-spot certainly appear very
insignificant in a caterpillar two centimeters long, but we must
not forget that in the ancestors of the existing _Chærocampa_-larvæ
this character appeared in the adult state. If we conceive the
curvature of the white subdorsal with the underlying dark pigment
to be correspondingly magnified, its importance as a means of alarm
can scarcely be denied, particularly when we consider that this
marking stands on the enlarged fourth segment, which alone invests
the caterpillar with a singular, and, to smaller foes, an alarming
appearance. We know that in the case of those _Chærocampa_-larvæ which
possess no eye-spots, the distension of this segment is employed
against hostile attacks. (See the illustration of _Darapsa Chærilus_,
Pl. IV., Fig. 34.) Those markings which even only remotely resembled an
eye must, in such a position, have increased the terrifying action. On
these grounds I believe that it may be safely admitted, that this kind
of marking possessed the same significance in its initial stages as
it now does when fully perfected. No functional change has here taken
place.

Among all the facts brought together in the first section I only
know of one group of phenomena which at least permit of an attempt
to refer them to a phyletic vital force. This is the occurrence of
dark ground-colours in adult larvæ which are of light colours in
their young condition. I have already attempted to show that in
the _Chærocampa_-larvæ this change of colour depends on a double
adaptation, the young caterpillars being adapted to the green
colour of the plant and the adults to the soil and dead leaves.
This interpretation appears the more correct when we find the same
process, viz. the gradual replacement of the original green by brown
colours, among species of widely different genera, which, with the
dark colouring, possess the necessarily correlated habit of hiding
themselves by day when in the adult condition. This is the case with
_Sphinx Convolvuli_, _Deilephila Vespertilio_, and _Acherontia Atropos_.

Thus far all has been easily explicable by natural selection; but
when we also see a “tendency” to acquire a dark colour in the course
of development, in those species which neither conceal themselves
nor are adaptively coloured, but are very conspicuously marked--and
if, further, it can be shown that these species, such for instance
as _Deilephila Galii_, actually possess immunity from the attacks of
foes,--how can this tendency to the formation of a dark colour be
otherwise explained than by the admission of a phyletic vital force
urging the variations in this direction?

Nevertheless I believe that also on this point an appeal to unknown
forces can be dispensed with. In the first place, dark ground-colours
can be of use to a species otherwise than as means of adaptation. In
_D. Galii_, as well as in _D. Euphorbiæ_, the light ring-spots appear
rather at their brightest on the pitchy-black ground; and if this
caterpillar must (_sit venia verbo!_) become conspicuous, this purpose
would be best attained by acquiring a dark ground-colour, such as that
of _D. Euphorbiæ_.

The tendency, apparently common to all these _Sphingidæ_, to acquire
a dark colour with increasing age, depends therefore on two quite
distinct adaptations--first, in species sought by enemies, on an
adaptation to the colour of the soil; and secondly, in species rejected
by foes, on the endeavour to produce the greatest possible contrast of
colour.

Moreover, the supposition from which this last plea for a vital
force set out is not universally correct, since there are species,
such for instance as _D. Nicæa_, which never acquire a dark colour;
and in _D. Galii_ also, although all the individuals abandon the
protective green of the young stages, they by no means all acquire a
dark hue in exchange for this colour; many individuals in their light
ochreous-yellow colouring rather strikingly resemble the snake-like
caterpillar of _D. Nicæa_.




VII.

PHYLETIC DEVELOPMENT OF THE MARKINGS OF THE SPHINGIDÆ: SUMMARY AND
CONCLUSION.


If, from the form possessed by many of the caterpillars of the
_Sphingidæ_ on their emergence from the egg, we may venture to draw
a conclusion concerning the oldest phyletic stage, these larvæ were
originally completely destitute of marking. The characteristic caudal
horn must be older than the existing markings, since it is present in
the younger stages (except in cases where it is altogether wanting),
and is generally even larger than at a later age.

There is, however, further evidence that there were once Sphinx-larvæ
without any markings. Such a species now exists. I do not mean the
boring caterpillars of the _Sesiidæ_,[160] which live in the dark,
and are therefore colourless, but I refer to a large larva (over six
centimeters long) preserved in spirit in the Berlin Museum,[161] which,
from its form, belongs to the _Smerinthus_ group. It possesses a caudal
horn, and on the whole upper surface is covered with short and sparsely
scattered bristles, such as occur in the _Sesiidæ_. The colour of this
unknown insect appears to have been light green, although it now shows
only a yellowish shade. Every trace of marking is absent, and it thus
corresponds exactly with the youngest stages of the majority of the
existing Sphinx-larvæ--even to the short bristles sparsely scattered
over the whole upper surface of its body. We have therefore, so to
speak, a living fossil before us, and it would be of great interest to
ascertain its history.

All the data furnished by the developmental history go to show that
of the three kinds of markings which occur in the _Sphingidæ_, viz.,
longitudinal and oblique stripes and spots, the first is the oldest.
Among the species which are ornamented with oblique stripes or spots
there are many which are longitudinally striped in their young stages,
but the reverse case never occurs--young larvæ never show spots or
oblique stripes when the adult is only striped longitudinally.

The first and oldest marking of the caterpillars of the _Sphingidæ_ was
therefore the longitudinal striping, or, more precisely speaking, the
subdorsal, to which dorsal and spiracular lines may have been added.
That this second stage of phyletic development has also been preserved
in existing species has already been sufficiently shown; the greater
portion of one group, the _Macroglossinæ_, has indeed remained at this
stage of development.

From the biological value which must be attributed to this kind of
marking, its origination by natural selection presents no difficulty.
The first rudiments of striping must have been useful, since they must
have broken up the large surface of the body of the caterpillar into
several portions, and would thus have rendered it less conspicuous to
its enemies.

Thus it is not difficult to perceive how a whole group of genera could
have made shift with this low grade of marking up to the present time.
Colour and marking are not the only means of offence and defence
possessed by these insects; and it is just such simply-marked larvæ as
those of the _Macroglossinæ_ which have the protective habit of feeding
only at night, and of concealing themselves by day. Moreover, under
certain conditions of life the longitudinal stripes may be a better
means of protection, even for a Sphinx-larva, than any other marking;
and all those species in which this pattern is retained at the present
time live either among grasses or on _Coniferæ_.

It cannot be properly said that the second form of marking--the
oblique stripes--has been developed out of the first. If these had
arisen by the transformation of the longitudinal stripes, the two
forms could not exist side by side. This is the case, however, both
in certain species in the adult state (_Calymnia Panopus_[162]), as
well as in others during their young stages (most beautifully seen
in _Smerinthus Populi_, Fig. 56). Various facts tend to show that
the oblique stripes appeared in the phyletic development _later_
than the longitudinal lines. In the first place they appear later
than the latter in the ontogeny of certain species. This is the case
with _Chærocampa Elpenor_ and _Porcellus_, in which, however, they
certainly do not reach a high state of development. Then again, the
longitudinal lines disappear completely in the course of the ontogeny,
whilst the oblique stripes alone maintain their ground. Thus, the
subdorsal line vanishes at a very early stage, with the exception of
a small residue,[163] in all native species of _Smerinthus_. I have
already attempted to show that new characters are only acquired _in the
last stage_, and that if still newer ones are then added, the former
disappear from the last stage, and are transferred back to a younger
one. _Characters vanish therefore from a stage in the same order as
they were acquired._

Finally, among the genera with longitudinal stripes (_e.g.
Macroglossa_) we know certain species which, when at an advanced
age, possess oblique stripes (_M. Fuciformis_), although these slant
in a direction opposite to those of most of the other larvæ of the
_Sphingidæ_. These are, however, always species which differ from their
allies in their mode of life, not feeding on grasses or low plants, but
on large-leaved shrubs. If we were able to ascertain the ontogeny of
these species, we should find that the oblique stripes appeared late in
life, as has already been shown in the case of _Pterogon Œnotheræ_.

If it be asked why the longitudinal lines were first formed, and then
the oblique stripes, it may be replied that the physical constitution
of these caterpillars would be more easily able to give rise to simple
longitudinal lines than to complicated oblique stripes crossing their
segments.[164] It may perhaps also be suggested that the oldest
_Sphingidæ_ lived entirely on low plants among grasses, and in the
course of time gradually took to shrubs and trees. At the present time
the majority of the Sphinx-larvæ still live on low plants, and but few
on trees, such caterpillars generally belonging to certain special
genera.

The character of oblique stripes becomes perfected by the addition
of coloured edges, the latter, as is self-evident, having been added
subsesequently.

The third chief constituent of the Sphinx-markings, _i.e._ the
spots--whether perfect ocelli or only ring-spots--in two of the special
genera here considered, arise on the subdorsal, where they are either
deposited (_Deilephila_), or built up from a fragment of this line
(_Chærocampa_). That these markings can, however, also originate
independently of the subdorsal, is shown by the ocellus of _Pterogon
Œnotheræ_, situated on the segment bearing the caudal horn. In this
case, however, the ontogeny teaches us that the spot also succeeds the
subdorsal, so that we can state generally that all these spot-markings
are of later origin than the longitudinal striping.

The question as to the relative ages of the oblique stripes and the
spot-marking does not admit of a general answer. In some cases (_C.
Elpenor_ and _Porcellus_) the oblique stripes disappear when the ocelli
reach complete development, and we may therefore venture to conclude
that in these cases the former appeared earlier in the phylogeny.
But it is very probable that oblique stripes arose independently at
different periods, just as longitudinal lines occur irregularly in
quite distinct families. It would be a great error if we were to
ascribe the possession of oblique stripes solely to descent from a
common ancestor. The oblique markings found on certain species of
_Macroglossa_ (_M. Corythus_ from India) have not been inherited from
a remote period, but have been independently acquired by this or by
some recent ancestral species. They have nothing to do _genetically_
with the oblique stripes which occur in some species of _Chærocampa_
(_e.g._ in _C. Nessus_, from India), or with those of the species of
_Smerinthus_ and _Sphinx_. They depend simply on analogous adaptation
(Seidlitz[165]), _i.e._ on adaptation to an analogous environment.

The case is similar with the spot-markings. I have already shown that
under certain conditions ring-spots may assume the exact appearance of
eye-spots by the formation of a nucleus in the “mirror,” such as occurs
occasionally in _Deilephila Euphorbiæ_ (Fig. 43), more frequently
in _D. Galii_, and as a rule in _D. Vespertilio_. Nevertheless,
these markings arise in quite another manner to the eye-spots of the
_Chærocampinæ_, with which they consequently have no genetic relation;
the two genera became separated at a time when they neither possessed
spot-markings. Further, in _Pterogon Œnotheræ_ we find a third kind
of spot-marking, which is most closely allied to the ocelli of the
_Chærocampa_-larvæ, but is situated in quite another position, and must
have originated in another manner, and consequently quite independently
of these eye-spots.

It can also be readily understood why the first and second elements of
the markings of the _Sphingidæ_ should be mutually exclusive, and not
the second and third or the first and third.

A light longitudinal line cutting the oblique stripes, considerably
diminishes that resemblance to a leaf towards which the latter have a
tendency, and it is therefore only found in cases where an adaptive
marking can be of no effect on account of the small size of the
caterpillar, _i.e._ in quite young stages. (See, for instance, Fig.
56, the first stage of _S. Populi_.) At a later period of life the
old marking must give way to the new, and we accordingly find that
the subdorsal line vanishes from all the segments on which oblique
stripes are situated, and is only retained on the anterior segments
where the latter are wanting. In some few cases both elements of
marking certainly occur together, such as in _Calymnia Panopus_ and
_Macroglossa Corythus_; but the oblique stripes are, under these
circumstances, shorter, and do not extend above the subdorsal line, and
in _Darapsa Chœrilus_ even become fused into the latter.[166]

In certain cases there may also be a special leaf structure imitated
by the longitudinal lines, but on the whole the latter diminish the
effect of the oblique stripes; and we accordingly find that not only
has the subdorsal disappeared from those segments with oblique stripes,
but that most larvæ with this last character are also without the
otherwise broad spiracular and dorsal lines. This is the case with all
the species of _Smerinthus_[167] known to me, as well as with all the
species of the genera _Sphinx_, _Dolba_, and _Acherontia_.

Oblique stripes and spot-markings are not, however, necessarily
mutually exclusive in their action, and we also find these in certain
cases united in the same larva, although certainly never in an equal
state of perfection. Thus, _Chærocampa Nessus_[168] possesses strongly
marked oblique stripes, but feebly developed ocelli; and, on the other
hand, _Chærocampa Elpenor_ shows strongly developed eye-spots, but the
earlier oblique stripes are at most only present as faint traces. This
is easily explained by the mode of life. These caterpillars--at least
such of them as are perfectly known--do not live on plants with large,
strongly-ribbed leaves, and are even in the majority of individuals
adapted to the colour of the soil; the oblique stripes have therefore
in these cases only the significance of rudimentary formations.

That the first and third forms of markings also are not always mutually
prejudicial in their action is shown by the case of _Chærocampa
Tersa_, in which the eye-spots certainly appear to possess some
other significance than as a means of causing terror. In most of the
_Chærocampa_-larvæ the subdorsal line disappears in the course of the
phylogeny, and it can be understood that the illusive appearance of the
eye-spots would be more perfect if they did not stand upon a white
line.

If we consider the small number of facts with which I have here been
able to deal, the result of these investigations will not be deemed
unsatisfactory. It has been possible to show that each of the three
chief elements of the markings of the _Sphingidæ_ have a biological
significance, and their origin by means of natural selection has thus
been made to appear probable. It has further been possible to show
that the first rudiments of these markings must also have been of
use; and it thus appears to me that their origin by means of natural
selection has been proved to demonstration. Moreover, it has not been
difficult to understand the displacement of the primary elements of the
markings by secondary characters added at a later period, as likewise
an essential effect of natural selection. Finally, it has been possible
to explain also the subordinate or accessory elements of the markings,
partly by the action of natural selection, and partly as the result of
markings formerly present acting by correlation.

From the origin and gradual evolution of the markings of the
_Sphingidæ_ we may accordingly sketch the following picture:--

The oldest Sphinx-larvæ were without markings; they were probably
protected only by adaptive colouring, and a large caudal horn, and by
being armed with short bristles.

Their successors, through natural selection, became longitudinally
striped; they acquired a subdorsal line extending from the horn to the
head, as well as a spiracular, and sometimes also a dorsal, line. The
caterpillars thus marked must have been best hidden on those plants
in which an arrangement of parallel linear parts predominated; and we
may venture to suppose that at this period most of the larvæ of the
_Sphingidæ_ lived on or among such plants (grasses).

At a later period oblique stripes were added to the longitudinal
lines, the former (almost always) slanting across the seven hindmost
segments from the back towards the feet in the direction of the
caudal horn. Whether these stripes all arose simultaneously, or, as
is more probable, whether only one at first appeared, which was then
transferred to the other segments by correlation assisted by natural
selection, cannot, at least from the facts available, at present be
determined.

On the whole, as the oblique stripes became lengthened towards the
back, the longitudinal lines disappeared, since they injured the
deceptive effect of the stripes. In many species also there were formed
dark or variegated coloured edges to the oblique stripes, in imitation
of the shadow lines cast by the leaf-ribs.

Whilst one group of _Sphingidæ_ (_Sphinx_, _Smerinthus_) were thus
striving to make their external appearance approximate more and more to
that of a ribbed leaf, others of the longitudinally striped species
became developed in another manner.

Some of the latter lived indeed on bush-like leaved plants, but no
oblique stripes were developed, because these would have been useless
among the dense, narrow, and feebly-ribbed leaves of the food-plants.
These caterpillars, from the earlier markings, simply retained the
longitudinal lines, which, combined with a very close resemblance to
the colour of the leaves, afforded them a high degree of protection
against discovery. This protection would also have been enhanced if
other parts of the food-plant, such as the berries (_Hippophaës_), were
imitated in colour and position in such a manner that the large body
of the caterpillar contrasted still less with its environment. In this
way the first ring-spot probably arose in some species on only one--the
penultimate segment.

As soon as this first pair of ring-spots had become an established
character of the species, they had a tendency to become repeated
on the other segments, advancing from the hind segments towards
the front ones. Under certain conditions this repetition of the
ring-spots might have been of great disadvantage to the species, and
would therefore have been as far as possible prevented by natural
selection (_Hippophaës_); in other cases, however, no disadvantage
would have resulted--the caterpillar, well adapted to the colour of its
food-plant, would not have been made more conspicuous by the small
ring-spots, which might thus have become repeated on all the segments
(_Zygophylli_). In cases like the two latter, striking colours must
have been eliminated when inherited from an immediate ancestor; but on
this point nothing can as yet be said with certainty.

In other cases the repetition of the ring-spots with strongly
contrasted colours was neither prejudicial nor indifferent, but could
be turned to the further advantage of the species. If a caterpillar fed
on plants containing acrid juices (_Euphorbiaceæ_) which, by permeating
its alimentary system, rendered it repulsive to other animals, the
ring-spots commencing to appear (by repetition) would furnish an
easy means for natural selection to adorn the species with brilliant
colours, which would protect it from attack by acting as signals of
distastefulness.

But if the dark spots stood on a light ground (_Nicæa_), they would
present the appearance of eyes, and cause their possessors to appear
alarming to smaller foes.

From the developmental histories and biological data at present before
us, it cannot with certainty be said which of these two functions of
the ring-spots was first acquired in the phylogeny, but we may perhaps
suppose that their significance as a means of causing alarm was arrived
at finally.

It may also be easily conceived that as the ring-spots became more and
more complicated, they would occasionally have played other parts,
being fashioned once again in these stages into imitations of portions
of plants, such as a row of berries or flower-buds. For this, however,
there is as yet no positive evidence.

As the ring-spots became detached from the subdorsal line out of which
they had arisen, the latter disappeared more and more completely from
the last ontogenetic stage, and receded towards the younger stages of
life of the caterpillar--it became _historical_. This disappearance
of the subdorsal may also be explained by the fact that the original
longitudinal stripe imitating the linear arrangement of leaves would
become meaningless, even if it did not always diminish the effect of
the ring-spots. But characters which have become worthless are known
in the course of time to become rudimentary, and finally to disappear
altogether. I do not believe that disuse alone causes such characters
to vanish, although in the case of active organs it may have a large
share in this suppression. With markings it cannot, however, be a
question of use or disuse--nevertheless they gradually disappear as
soon as they become meaningless. I consider this to be the effect of
the arrest of the controlling action of natural selection upon these
characters (suspension of the so-called “conservative adaptation,”
Seidlitz). Any variations may become of value if the character
concerned is met with in the necessary state of fluctuation. That this
process of extinction does not proceed rapidly, but rather with extreme
slowness, is seen in the ontogeny of several species of _Deilephila_,
which retain the now meaningless subdorsal line through a whole series
of stages of life.

In another group of Sphinx-larvæ with longitudinal stripes, an eye-spot
became developed independently of the subdorsal line, in the position
of the caudal horn, which has here vanished with the exception of a
small knob-like swelling. This character--which we now see perfected in
_Pterogon Œnotheræ_--undoubtedly serves as a means of causing terror;
but whether the incipient stages possessed the same significance,
cannot be decided in the isolated case offered by the one species of
the genus _Pterogon_ possessing this marking.

In a third group of longitudinally striped caterpillars, the younger
genus _Chærocampa_, eye-spots were developed directly from portions of
the subdorsal line, at first only on the fourth and fifth segments. It
can be here positively asserted that this character served as a means
of alarm from its very commencement. It is certainly for this reason
that we see the subdorsal line in the immediate neighbourhood of the
spots disappear at an early stage, whilst it is retained on the other
segments for a longer period. A portion of the younger (tropical)
species of this group then developed similar, or nearly similar, ocelli
on the remaining segments by correlation; and it may now have occurred
that in solitary cases the eye-spots acquired another significance
(_C. Tersa_?), becoming of use as a disguise by resembling berries or
flower-buds. It is also conceivable that the eye-spots may in other
cases have been converted into a warning sign of distastefulness.

In all those larvæ which possessed purely terrifying markings,
however, not only was the original protective colouring preserved,
but in most of them this colour gradually became replaced by a
better one (adaptation of the adult larva to the soil). The oblique
stripes imitating the leaf-ribs also are by no means lost, but are
almost always present, although but feebly developed, and often only
temporarily.

The pattern formed by the oblique stripes may also be retained, even
with perfect adaptation to the soil, and may be converted to a new use
by losing its sharpness, and, instead of imitating definite parts of
plants, may become transformed into an irregular and confused marking,
and thus best serve to represent the complicated lights and shadows,
stripes, spots, &c., cast on the ground under low-growing plants from
between the stems and dead leaves.

Just as in the case of ocellated species where caterpillars without
eye-spots may retain and newly utilize their older markings, so larvæ
having oblique stripes with the most diversely coloured edges may show
the same markings in allied (younger?) species, both in a rudimentary
and in a transformed condition. These markings may thus contribute to
the formation of a latticed or reticulated pattern. Even the oldest
marking, the subdorsal line, may still play a part, since its remnants
cause certain portions of the complicated pattern to appear more
strongly marked (_S. Convolvuli_). Finally, when an adaptation to a
changing environment intersected by lights and shadows is required,
new markings may be here added as in other cases, viz., dark streaks
extending over the light surface of the whole caterpillar.

In concluding this essay, I may remark that, with respect to the
wide and generally important question which gave rise to these
investigations, a clearer and simpler result has been obtained than
could have been expected, considering the complexity of the characters
requiring to be traced to their causes, as well as our still highly
imperfect knowledge of ontogenetic and biological facts.

For a long time I believed that it was not possible to trace all the
forms of marking and their combinations to those causes which are
known to produce transformation; I expected that there would be an
inexplicable residue.

But this is not the case. Although it cannot yet be stated at first
sight with certainty in every single instance how far any particular
element of marking may have a biological value in the species
possessing it, nevertheless it has been established that each of
the elements of marking occurring in the larvæ of the _Sphingidæ_
originally possessed a decided biological significance, which was
produced by natural selection.

In the case of the three chief elements of the markings of the
_Sphingidæ_, it can be further shown that not only the initial stages
but also their ultimate perfection--the highest stages of their
development, are of decided advantage to their possessors, and have
a distinct biological value, so that the gradual development and
improvement of these characters can be traced to the action of natural
selection.

But although natural selection is the factor which has called into
existence and perfected the three chief forms and certain of the
subsidiary markings, in the repetition of the local character on the
other segments, as well as in the formation of new elements of marking
at the points of intersection of older characters now rudimentary, we
can recognize a second factor which must be entirely innate in the
organism, and which governs the uniformity of the bodily structure
in such a manner that no part can become changed without exerting a
certain action on the other parts--an innate law of growth (Darwin’s
“correlation”).

Only once during the whole course of the investigations was it for an
instant doubtful whether a phyletic vital force did not make itself
apparent, viz., in the red spots accompanying the oblique stripes
in several _Smerinthus_-larvæ. Closer analysis, however, enabled us
to perceive most distinctly the wide gulf that separates “analogous
variation” from the mystic phyletic vital force. Nothing further
remains therefore for the action of this force in respect to the
marking and colouring of the _Sphingidæ_, since several even of the
subordinate markings can be traced to their causes, only the “dorsal
spots” of our two native species of _Chærocampa_ having been referred
to correlation without decided proof. From the temporary inability
to explain satisfactorily such an insignificant detail, no one will,
however, infer the existence of such a cumbrous power as a phyletic
vital force.

The final result to which these investigations have led us is
therefore the following:--The action of a phyletic vital force cannot
be recognized in the marking and colouring of the _Sphingidæ_; the
origination and perfection of these characters depend entirely on the
known factors of natural selection and correlation.




=II.=

ON PHYLETIC PARALLELISM IN METAMORPHIC SPECIES.


INTRODUCTION.

In the previous essay I attempted to trace a whole group of apparently
“purely morphological” characters to the action of known factors of
transformation, to explain them completely by these factors, and in
this manner I endeavoured to exclude the operation of an internal power
inciting change (phyletic vital force).

In this second study I have attempted to solve the problem as to
whether such an innate inciting power can be shown to exist by
comparing the forms of the two chief stages of metamorphic species, or
whether such a force can be dispensed with.

Nobody has as yet apparently entertained the idea of testing this
question by those species which appear in the two forms of larva
and imago (insects), or, expressed in more general terms, by those
species the individuals of which successively possess quite different
forms (metamorphosis), or in which the different forms that occur
are distributed among different individuals alternating with and
proceeding from one another (alternation of generation). Nevertheless,
it is precisely here that quite distinct form-relationships would be
expected according as the development of the organic world depended
on a phyletic vital force, or was simply the response of the specific
organism to the action of the environment.

Assuming the first to be the case, there must have occurred, and must
still occur, what I designate “phyletic parallelism,” _i.e._ the two
stages of metamorphic species must have undergone a precisely parallel
development--every change in the butterfly must have been accompanied
or followed by a change in the caterpillar, and the systematic groups
of the butterflies must be also found in a precisely corresponding
manner in a systematic grouping of the caterpillars. If species are
able to fashion themselves into new forms by an innate power causing
periodic change, this re-moulding cannot possibly affect only one
single stage of development--such as the larva only--but would rather
extend, either contemporaneously or successively, to all stages--larva,
pupa, and imago: each stage would acquire a new form, and it might
even be expected that each would change to the same extent. At least,
it cannot be perceived why a purely internal force should influence
the development of one stage more than that of another. The larvæ
and imagines of two species must differ from one another to the
same extent, and the same must hold good for the larvæ and imagines
of two genera, families, and so forth. In brief, a larval system
must completely coincide with the system based entirely on imaginal
characters, or, what amounts to the same thing, the form-relationships
of the larvæ must correspond exactly with the form-relationships of the
imagines.

On the other hand, the condition of affairs must be quite different
if an internal power causing phyletic remodelling does not exist, the
transformation of species depending entirely on the action of the
environment. In this case dissimilarities in the phyletic development
of the different stages of life must be expected, since the temporary,
and often widely deviating, conditions of life in the two stages can
and must frequently influence the one stage whilst leaving the other
unacted upon--the former can therefore undergo remodelling while the
latter remains unchanged.[169]

By this means there would arise an unequal difference between the two
stages of two species. Thus, the butterflies, supposing these to have
become changed, would bear a more remote form-relationship to each
other than the caterpillars, and the differences between the former
(imagines) would always be greater than that between the larvæ if the
butterflies were, at several successive periods, affected by changing
influences whilst the larvæ continued under the same conditions and
accordingly remained unaltered. The two stages would not coincide
in their phyletic development--the latter could not be expressed by
parallel lines, and we should accordingly expect to find that there
was by no means a complete congruity between the systems founded on
the larval and imaginal characters respectively, but rather that the
caterpillars frequently formed different systematic groups to the
butterflies.[170]

Accordingly, the problem to be investigated was whether in those
species which develope by means of metamorphosis, and of which the
individual stages exist under very different conditions of life, a
complete phyletic parallelism was to be found or not. This cannot
be decided _directly_ since we cannot see the phyletic development
unfolded under our observation, but it can be established _indirectly_
by examining and comparing with each other the form-relationships
of the two separate stages--by confronting the larval and imaginal
systematic groups. If the phyletic development has been parallel and
perfectly equal, so also must its end-results--the forms at present
existing--stand at equal distances from one another; larval and
imaginal systems must coincide and be _congruent_. If the course of
the phyletic development has not been parallel, there must appear
inequalities--incongruences between the two systems.

I am certain that systematists of the old school will read these
lines with dismay. Do we not regard it as a considerable advance in
taxonomy that we have generally ceased to classify species simply
according to one or to some few characters, and that we now take
into consideration not merely the last stage of the development (the
imago), but likewise the widely divergent young stages (larva and
pupa)? And now shall it not be investigated whether caterpillars and
butterflies do not form quite distinct systems? In the case of new
species of butterflies of doubtful systematic position was not always
the first question:--what is the nature of the caterpillars? and did
not this frequently throw light upon the relationships of the imago?
Assuredly; and without any doubt we have been quite correct in taking
the larval structure into consideration. But in so doing we should
always keep in mind that there are two kinds of relationship--form- and
blood-relationship--which might possibly not always coincide.

It has hitherto been tacitly assumed that the degree of relationship
between the imagines is always the same as that between the larvæ,
and if blood-relationship is spoken of this must naturally be the
case, since the larva and the imago are the same individual. In all
groups of animals we have not always the means of deciding strictly
between form- and blood-relationship, and must accordingly frequently
content ourselves by taking simply the form-relationship as the
basis of our systems, although the latter may not always express the
blood-relationship. But it is exactly in the case of metamorphic
species that there is no necessity for, nor ought we to remain
satisfied with, this mode of procedure, since we have here two kinds
of form-relationship, that of the larvæ and that of the imagines, and,
as I have just attempted to show, it is by no means self-evident that
these always agree; there are indeed already a sufficient number of
instances to show that such agreement does not generally exist.

This want of coincidence is strikingly shown in a group of animals
widely remote from the Insecta, viz. the Hydromedusæ, the systematic
arrangement of which is quite different according as this is based on
the polypoid or on the medusoid generation. Thus, the medusoid family
of the oceanic Hydrozoa springs from polypites belonging to quite
different families, and in each of these polypoid families there are
species which produce _Medusæ_ of another family.

Similarly, the larvæ of the Ophiuroidea (_Pluteus_-form) among the
Echinodermata are not the most closely related in form to those of
the ordinary star-fishes, but rather to the larvæ of quite a distinct
order, the sea-urchins.

I will not assert that in these two cases the dissimilarity in the
form-relationship, or, as I may designate it, the incongruence
of the morphological systems, must depend on an unequal rate of
phyletic development in the two stages or generations, or that this
incongruence can be completely explained by the admission of such an
unequal rate of development: indeed it appears to me probable that,
at least in the _Ophiureæ_, quite another factor is concerned--that
the form-relationship to the larvæ of the sea-urchins does not depend
upon blood-relationship, but on convergence (Oscar Schmidt), _i.e._ on
adaptation to similar conditions of life. These two cases, however,
show that unequal form-relationship of two stages may occur.

From such instances we certainly cannot infer off-hand that a phyletic
force does not exist; it must first be investigated whether and to
what extent such dissimilarities can be referred to unequal phyletic
development and, should this be the case, whether deviations from a
strict congruence of the morphological systems are not compatible
with the admission of an internal transforming power. _That a certain
amount_ of influence is exerted by the environment on the course of the
processes of development of the organic world, will however be acceded
to by the defenders of the phyletic vital force. It must therefore be
demonstrated that deviations from complete congruence occur, which,
from their nature or magnitude, are incompatible with the admission of
innate powers, and, on the other hand, it must likewise be attempted to
show that the departures from this congruence as well as the congruence
itself can be explained without admitting a phyletic vital force.

In the following pages I shall attempt to solve this question for the
order Lepidoptera, with the occasional assistance of two other orders
of insects. Neither the Echinodermata nor the Hydromedusæ are at
present adapted to such a critical examination; the number of species
in these groups of which the development has been established with
certainty is still too small, and their biological conditions are
still to a great extent unknown. In both these respects they are far
surpassed by the Lepidoptera. In this group we know a large number
of species in the two chief stages of their development and likewise
more or less exactly the conditions under which they exist during each
of these phases. We are thus able to judge, at least to a certain
extent, what changes in the conditions of life produce changes of
structure. Neither in the number of known species of larvæ, nor in the
intimate knowledge of their mode of life, can any of the remaining
orders of insects compete with the Lepidoptera. There is no Dipterous
or Hymenopterous genus in which ten or more species are so intimately
known in the larval stage that they can be employed for the purposes
of morphological comparison. Who is able to define the distinctions
between the life-conditions of the larvæ of twenty different species
of _Culex_ or of _Tipula_? The caterpillars of closely allied species
of Lepidoptera, on the other hand, frequently live on different
plants, from which circumstance alone a certain difference in the
life-conditions is brought about.

The chief question which the research had to reply to was the
following:--Does there exist a complete phyletic parallelism among
Lepidoptera or not? or, more precisely speaking:--Can we infer, from
the form-relationships which at present exist between larvæ on the one
hand and imagines on the other, an exactly parallel course of phyletic
development in both stages; or do incongruences of form-relationship
exist which point to unequal development?

Before I proceed to the solution of this question it is indispensable
that one point should be cleared up which has not been hitherto touched
upon, but which must be settled before the problem can be formally
stated in general terms. Before it can be asked whether larvæ and
imagines have undergone a precisely parallel development, we must
know whether unequal development is possible--whether there does not
exist such an intimate structural relationship between the two stages
that every change in one of these must bring about a change in the
other. Were this the case, every change in the butterfly would cause
a correlative change in the caterpillar, and _vice versâ_, so that an
inequality of form-relationship between the larvæ on one hand and the
imagines on the other would be inconceivable--systems based on the
characters of the caterpillars would completely coincide with those
based on the characters of the butterflies and we should arrive at a
false conclusion if we attributed the phyletically parallel development
of the two stages to the existence of an internal phyletic force,
whilst it was only the known factor, correlation, which caused the
equality of the course of development.

For these reasons it must first be established that the larva and
imago are not respectively fixed in form, and the whole of the first
section will therefore be devoted to proving that the two stages change
independently of one another. Conclusions as to the causes of change
will then be drawn, and these will corroborate from another side a
subsequent inquiry as to the presence or absence of complete congruence
in the two morphological systems. The two questions the answers to
which will be successively attempted are by no means identical,
although closely related, since it is quite conceivable that the first
may be answered by there being no precise correlation of form, or only
an extremely small correlation, between the caterpillar and the imago,
whilst, at the same time, it would not be thereby decided whether
the phyletic development of the two stages had kept pace uniformly
or not. A perfect congruence of morphological relationships could
only take place if transformations resulted from an internal power
instead of external influences. The question:--Does there exist a fixed
correlation of form between the two stages? must therefore be followed
by another:--Do the form-relationships of the two stages coincide or
not--has their phyletic development been uniform or not?




FOOTNOTES


[1] A most minute and exact description of the newly hatched larva of
_Chionobas Aëllo_ is given by the American entomologist, Samuel H.
Scudder. Ann. Soc. Ent. de Belgique, xvi., 1873.

[2] I am aware that this certainly cannot be said of philosophers
like Lotze or Herbert Spencer; but these are at the same time both
naturalists and philosophers.

[3] “Über die Artrechte des _Polyommatus Amyntas_ und _Polysperchon_.”
Stett. ent. Zeit. 1849. Vol. x. p. 177-182. [In Kirby’s “Synonymic
Catalogue of Diurnal Lepidoptera” _Plebeius Amyntas_ is given as a
synonym and _P. Polysperchon_ as a var. of _P. Argiades_ Pall. R.M.]

[4] “Die Arten der Lepidopteren-Gattung _Ino_ Leach, nebst einigen
Vorbemerkungen über Localvarietäten.” Stett. ent. Zeit. 1862. Vol.
xxiii. p. 342.

[5] [Eng. ed. W. H. Edwards has since pointed out several beautiful
cases of seasonal dimorphism in America. Thus _Plebeius Pseudargiolus_
is the summer form of _P. Violacea_, and _Phyciodes Tharos_ the summer
form of _P. Marcia_. See Edwards’ “Butterflies of North America,”
1868-79.]

[6] [Eng. ed. I learn by a written communication from Dr. Speyer that
two Geometræ, _Selenia Tetralunaria_ and _S. Illunaria_ Hüb., are
seasonally dimorphic. In both species the winter form is much larger
and darker.] [_Selenia Lunaria_, _S. Illustraria_, and some species of
_Ephyra_ (_E. Punctaria_ and _E. Omicronaria_) are likewise seasonally
dimorphic. For remarks on the case of _S. Illustraria_ see Dr. Knaggs
in Ent. Mo. Mag., vol. iii. p. 238, and p. 256. Some observations on
_E. Punctaria_ were communicated to the Entomological Society of London
by Professor Westwood in 1877, on the authority of Mr. B. G. Cole. See
Proc. Ent. Soc. 1877, pp. vi, vii. R.M.]

[7] [In 1860 Andrew Murray directed attention to the disguising colours
of species which, like the Alpine hare, stoat, and ptarmigan, undergo
seasonal variation of colour. See a paper “On the Disguises of Nature,
being an inquiry into the laws which regulate external form and colour
in plants and animals.” Edinb. New Phil. Journ., Jan. 1860. In 1873 I
attempted to show that these and other cases of “variable protective
colouring” could be fairly attributed to natural selection. See Proc.
Zoo. Soc., Feb. 4th, 1873, pp. 153-162. R.M.]

[8] [A phenomenon somewhat analogous to seasonal change of protecting
colour does occur in some Lepidoptera, only the change, instead of
occurring in the same individual, is displayed by the successive
individuals of the same brood. See Dr. Wallace on _Bombyx Cynthia_,
Trans. Ent. Soc. Vol. v. p. 485. R.M.]

[9] “Über den Einfluss der Isolirung auf die Artbildung.” Leipzig,
1872, pp. 55-62.

[10] [Mr. A. R. Wallace maintains that the obscurely coloured females
of those butterflies which possess brightly coloured males have been
rendered inconspicuous by natural selection, owing to the greater
need of protection by the former sex. See “Contributions to the
Theory of Natural Selection,” London, 1870, pp. 112-114. It is now
generally admitted that the underside of butterflies has undergone
protectional adaptation; and many cases of local variation in the
colour of the underside of the wings, in accordance with the nature of
the soil, &c., are known. See, for instance, Mr. D. G. Rutherford on
the colour-varieties of _Aterica Meleagris_ (Proc. Ent. Soc. 1878, p.
xlii.), and Mr. J. Jenner Weir on a similar phenomenon in _Hipparchia
Semele_ (_loc. cit._ p. xlix.) R.M.]

[11] [The fact that moths which, like the Geometræ, rest by day with
the wings spread out, are protectively marked on the _upper_ side,
fully corroborates this statement. R.M.]

[12] “Über die Einwirkung verschiedener, während der
Entwicklungsperioden angewendeter Wärmegrade auf die Färbung und
Zeichnung der Schmetterlinge.” A communication to the Society of
Natural Science of Steiermark, 1864.

[13] See Exp. 9, Appendix I.

[14] See Exp. 11, Appendix I.

[15] See Exps. 4, 9, and 11, Appendix I.

[16] It seems to me very necessary to have a word expressing whether a
species produces one, two, or more generations in the year, and I have
therefore coined the expression _mono-_, _di-_, and _polygoneutic_ from
γονεύω, I produce.

[17] [Eng. ed. In the German edition, which appeared in 1874, I was
not able to support this hypothesis by geographical data, and could
then only ask the question “whether in the most northern portion of
its area of distribution, appears in two or only in one generation?”
This question is now answered by the Swedish Expedition to the Yenisei
in 1876. Herr Philipp Trybom, one of the members of this expedition,
observed _A. Levana_ at the end of June and beginning of July, in
the middle of Yenisei, in 60°-63° N. (Dagfjärilar från Yenisei in
Översigt ap k. Vertensk. Akad. Förhandlingon, 1877, No. 6.) Trybom
found _Levana_ at Yenisk on June 23rd, at Worogova (61° 5´) on July
3rd, at Asinova (61° 25´) on July 4th, at Insarowa (62° 5´) on July
7th, and at Alinskaja (63° 25´) on July 9th. The butterflies were
especially abundant at the beginning of June, and were all of the
typical _Levana_ form. Trybom expressly states, “we did not find a
single specimen which differed perceptibly from Weismann’s Figs. 1 and
2 (‘Saison-Dimorphismus’ Taf. I.).”

The Swedish expedition soon left the Yenisei, and consequently was not
able to decide by observations whether a second generation possessing
the _Prorsa_ form appeared later in the summer. Nevertheless, it may be
stated with great probability that this is not the case. The districts
in which _Levana_ occurs on the Yenisei have about the same isotherm as
Archangel or Haparanda, and therefore the same summer temperature. Dr.
Staudinger, whose views I solicited, writes to me:--“In Finnmark (about
67° N.) I observed no species with two generations; even _Polyommatus
Phlæas_, which occurs there, and which in Germany has always two, and
in the south, perhaps, three generations, in Finnmark has only one
generation. A second generation would be impossible, and this would
also be the case with _Levana_ in the middle of Yenisei. I certainly
have _Levana_ and _Prorsa_ from the middle of Amur, but _Levana_ flies
there at the end of May, and the summers are very warm.” The middle of
Amur lies, moreover, in 50° N. lat., and therefore 10°-13° south of the
districts of the Yenisei mentioned.

It must thus be certainly admitted that on the Yenisei _A. Levana_
occurs only in the _Levana_ form, and that consequently this species
is at the present time, in the northernmost portion of its area of
distribution, in the same condition as that in which I conceive
it to have been in mid Europe during the glacial period. It would
be of the greatest interest to make experiments in breeding with
this single-brooded _Levana_ from the Yenisei, i.e., to attempt to
change its offspring into the _Prorsa_ form by the action of a high
temperature. If this could not be accomplished it would furnish a
confirmation of my hypothesis than which nothing more rigorous could be
desired.]

[18] See Exp. 10, Appendix I.

[19] When Dorfmeister remarks that hibernating pupæ which, at an
early stage “were taken for development into a room, or not exposed
to any cold, gave dwarfed, weakly and crippled,” or otherwise damaged
butterflies, this is entirely attributable to the fact that this able
entomologist had neglected to supply the necessary moisture to the
warm air. By keeping pupæ over water I have always obtained very fine
butterflies.

[20] [For other remarkable cases of sexual dimorphism (not _antigeny_
in the sense used by Mr. S. H. Scudder, Proc. Amer. Acad., vol. xii.
1877, pp. 150-158) see Wallace “On the Phenomena of Variation and
Geographical Distribution, as illustrated by the Papilionidæ of the
Malayan Region,” Trans. Linn. Soc., vol. xxv. 1865, pp. 5-10. R.M.]

[21] [Eng. ed. Dimorphism of this kind has since been made known: the
North American _Limenitis Artemis_ and _L. Proserpina_ are not two
species, as was formerly believed, but only one. Edwards bred both
forms from eggs of _Proserpina_. Both are single-brooded, and both have
males and females. The two forms fly together, but _L. Artemis_ is much
more widely distributed, and more abundant than _L. Proserpina_. See
“Butterflies of North America,” vol. ii.]

[22] [Eng. ed. Edwards has since proved experimentally that by the
application of ice a large proportion of the pupæ do indeed give rise
to the var. _Telamonides_. He bred from eggs of _Telamonides_ 122 pupæ,
which, under natural conditions, would nearly all have given the var.
_Marcellus_. After two months’ exposure to the low temperature there
emerged from August 24th to October 16th, fifty butterflies, viz.
twenty-two _Telamonides_, one intermediate form between _Telamonides_
and _Walshii_, eight intermediate forms between _Telamonides_ and
_Marcellus_ more nearly related to the former, six intermediate forms
between _Telamonides_ and _Marcellus_, but more closely resembling the
latter, and thirteen _Marcellus_. Through various mishaps the action of
the ice was not complete and equal. See the “Canadian Entomologist,”
1875, p. 228. In the newly discovered case of _Phyciodes Tharos_ also,
Edwards has succeeded in causing the brood from the winter form to
revert, by the application of ice to this same form. See Appendix II.
for a _résumé_ of Edwards’ experiments upon both _Papilio Ajax_ and
_Phyciodes Tharos_. R.M.]

[23] Thus from eggs of _Walshii_, laid on April 10th, Edwards obtained,
after a pupal period of fourteen days, from the 1st to the 6th of June,
fifty-eight butterflies of the form _Marcellus_, one of _Walshii_, and
one of _Telamonides_.

[24] [The word ‘Amixie,’ from the Greek ἀμιξία, was first adopted
by the author to express the idea of the prevention of crossing by
isolation in his essay “Über den Einfluss der Isolirung auf die
Artbildung,” Leipzig, 1872, p. 49. R.M.]

[25] [Eng. ed. In 1844, Boisduval maintained this relationship of the
two forms. See Speyer’s “Geographische Verbreit. d. Schmetterl.,” i. p.
455.]

[26] According to a written communication from Dr. Staudinger, the
female _Bryoniæ_ from Lapland are never so dusky as is commonly the
case in the Alps, but they often have, on the other hand, a yellow
instead of a white ground-colour. In the Alps, yellow specimens are not
uncommon, and in the Jura are even the rule.

[27] [According to W. F. Kirby (Syn. Cat. Diurn. Lepidop.), the species
is almost cosmopolitan, occurring, as well as throughout Europe, in
Northern India (var. _Timeus_), Shanghai (var. _Chinensis_), Abyssinia
(var. _Pseudophlæas_), Massachusetts (var. _Americana_), and California
(var. _Hypophlæas_). In a long series from Northern India, in my own
collection, all the specimens are extremely dark, the males being
almost black. R.M.]

[28] [Eng. ed. From a written communication from Dr. Speyer, it appears
that also in Germany there is a small difference between the two
generations. The German summer brood has likewise more black on the
upper side, although seldom so much as the South European summer brood.]

[29] [Assuming that in all butterflies similar colours are produced by
the same chemical compounds. R.M.]

[30] [Mr. H. W. Bates mentions instances of local variation in colour
affecting many distinct species in the same district in his memoir “On
the Lepidoptera of the Amazon Valley;” Trans. Linn. Soc., vol. xxiii.
Mr. A. R. Wallace also has brought together a large number of cases
of variation in colour according to distribution, in his address to
the biological section of the British Association at Glasgow in 1876.
See “Brit. Assoc. Report,” 1876, pp. 100-110. For observations on the
change of colour in British Lepidoptera according to distribution see
papers by Mr. E. Birchall in “Ent. Mo. Mag.,” Nov., 1876, and by Dr.
F. Buchanan White, “Ent. Mo. Mag.,” Dec., 1876. The colour variations
in all these cases are of course not _protective_ as in the well-known
case of _Gnophos obscurata_, &c. R.M.]

[31] See Figs. 10 and 14, 11 and 15, Plate I.

[32] “On the Origin and Metamorphoses of Insects,” London, 1874.

[33] I at first thought of designating the two forms of cyclical or
homochronic heredity as ontogenetic- and phyletic-cyclical heredity.
The former would certainly be correct; the latter would be also
applicable to alternation of generation (in which actually two or
more phyletic stages alternate with each other) but not to all those
cases which I attribute to heterogenesis, in which, as with seasonal
dimorphism, a series of generations of _the same_ phyletic stage
constitute the point of departure.

[34] When Meyer-Dürr, who is otherwise very accurate, states in his
“Verzeichniss der Schmetterlinge der Schweiz,” (1852, p. 207), that the
winter and summer generations of _P. Ægeria_ differ to a small extent
in the contour of the wings and in marking, he has committed an error.
The characters which this author attributes to the summer form are
much more applicable to the female sex. There exists in this species a
trifling sexual dimorphism, but no seasonal dimorphism.

[35] P. C. Zeller, “Bemerkungen über die auf einer Reise nach Italien
und Sicilien gesammelten Schmetterlingsarten.” Isis, 1847, ii.-xii.

[36] “Isoporien der europäischen Tagfalter.” Stuttgart, 1873.

[37] [Trans. Linn. Soc., vol. xxv. 1865, p. 9. R.M.]

[38] It is certainly preferable to make use of the expression
“metagenesis” in this special sense instead of introducing a new one.
As a general designation, comprehending metagenesis and heterogenesis,
there will then remain the expression “alternation of generation,” if
one does not prefer to say “cyclical propagation.” The latter may be
well used in contradistinction to “metamorphosis.”

[39] _Loc. cit._ chap. iv.

[40] The idea that alternation of generation is derived from
polymorphism (not the reverse, as usually happens; i.e. polymorphism
from alternation of generation) is not new, as I find whilst correcting
the final proof. Semper has already expressed it at the conclusion of
his interesting memoir, “Über Generationswechsel bei Steinkorallen,”
&c. See “Zeitschrift f. wiss. Zool.” vol. xxii. 1872.

[41] See my essay “Über den Einfluss der Isolirung auf die Artbildung.”
Leipzig, 1872.

[42] [In the case of monogoneutic species which, by artificial
‘forcing,’ have been made to give two generations in the year, it has
generally been found that the reproductive system has been imperfectly
developed in the second brood. A minute anatomical investigation of the
sexual organs in the two broods of seasonally dimorphic insects would
be of great interest, and might lead to important results. R.M.]

[43] “Grundzüge der Zoologie.” 2nd ed. Leipzig, 1872. Introduction.

[44] With reference to this subject, see the discussion by the Belgian
Entomological Society, Brussels, 1873.

[45] P. E. Müller, “Bidrag til Cladocerners Fortplantingshistorie,”
1868.

[46] Sars, in “Förhandlinger i Videnskabs Selskabet i Christiania,”
1873, part i.

[47] [Eng. ed. Recent researches on alternation of generation in
the Daphniacea have convinced me that _direct_ action of external
conditions does not in these cases come into consideration, but only
_indirect_ action.]

[48] See my memoir, “Über Bau und Lebenserscheinungen der _Leptodora
hyalina_,” Zeitschrift f. wiss. Zool., vol. xxiv. part 3, 1874.

[49] Stettin. entom. Zeit., vol. xviii. p. 83, 1857.

[50] Compt. Rend., vol. lxxvii. p. 1164, 1873.

[51] [“Accidental” in the sense of our being in ignorance of the laws
of variation, as so frequently insisted upon by Darwin. R.M.]

[52] [Eng. ed. Since this was written I have studied the ornamental
colours of the _Daphniidæ_; and, as a result, I no longer doubt that
sexual selection plays a very important part in the marking and
colouring of butterflies. I by no means exclude both transforming
factors, however; it is quite conceivable, on the contrary, that a
change produced directly by climate may be still further increased by
sexual selection. The above given case of _Polyommatus Phlæas_ may
perhaps be explained in this manner. That sexual selection plays a
part in butterflies, is proved above all by the odoriferous scales and
tufts of the males discovered by Fritz Müller.] [For remarks on the
odours emitted by butterflies and moths, see Fritz Müller in “Jena.
Zeit. f. Naturwissen.,” vol. xi. p. 99; also “Notes on Brazilian
Entomology,” Trans. Ent. Soc. 1878, p. 211. The odoriferous organs
of the female _Heliconinæ_ are fully described in a paper in “Zeit.
f. Wissen. Zool.,” vol. xxx. p. 167. The position of the scent-tufts
in the sphinx-moths is shown in Proc. Entom. Soc. 1878, p. ii. Many
British moths, such as _Phlogophora meticulosa_, _Cosmia trapezina_,
&c. &c., have tufts in a similar position. The fans on the feet of
_Acidalia bisetata_, _Herminia barbalis_, _H. tarsipennalis_, &c., are
also probably scent organs. A large moth from Jamaica, well known to
possess a powerful odour when alive (_Erebus odorus_ Linn.), has great
scent-tufts on the hind legs. For the application of the theory of
sexual selection to butterflies, see, in addition, to Darwin’s “Descent
of Man,” Fritz Müller in “Kosmos,” vol. ii. p. 42; also for January,
1879, p. 285; and Darwin in “Nature,” vol. xxi. January 8th, 1880, p.
237. R.M.]

[53] Nägeli, “Entstehung und Begriff der naturhistorischen Art,”
Munich, 1865, p. 25. The author interprets the facts above quoted in a
quite opposite sense, but this is obviously erroneous.

[54] See my essay, “Über den Einfluss der Isolirung auf die
Artbildung.” Leipzig, 1872.

[55] [Eng. ed. In the summer of 1877, Dr. Hilgendorf again investigated
the Steinheim fossil shells, and found his former statements to be
completely confirmed. At the meeting of the German Naturalists and
Physicists at Munich, in 1877, he exhibited numerous preparations,
which left no doubt that the chief results of his first research were
correct, and that there have been deposited a series of successively
derived species together with their connecting intermediate forms.]

[56] See my essay, “Über die Berechtigung der Darwin’schen Theorie.”
Leipzig, 1868.

[57] I expressly insist upon this here, because the notice of
Askenasy’s thoughtful essay which I gave in the “Archiv für
Anthropologie” (1873) has frequently been misunderstood.

[58] The experiments upon _Papilio Ajax_ and _Phyciodes Tharos_,
described in this Appendix, were made by Mr. W. H. Edwards (see his
“Butterflies of North America;” also the “Canadian Entomologist,” vol.
vii. p. 228-240, and vol. ix. p. 1-10, 51-5, and 203-6); and I have
added them, together with some hitherto unpublished results, to Dr.
Weismann’s Essay, in order to complete the history of the subject of
seasonal dimorphism up to the present time.--R.M.

[59] This is a striking illustration of the diversity of individual
constitution so frequently insisted on by Dr. Weismann in the foregoing
portion of this work.

[60] The reader who wishes to acquire a detailed knowledge of the
different varieties of this butterfly, of which a very large number
are known, must consult the plates and descriptions in Edwards’
“Butterflies of North America,” vol. ii.

[61] Mr. Edwards has shown also that _Argynnis Myrina_ can lay fertile
eggs when but a few hours out of the chrysalis. Canad. Ent., September,
1876, vol. viii. No. 9.

[62] Mr. Edwards remarks that the habit of becoming lethargic is of
great service to a digoneutic species in a mountain region where it is
exposed to sharp changes of temperature. “If the fate of the species
depended on the last larval brood of the year, and especially if the
larvæ must reach a certain stage of growth before they were fitted to
enter upon their hibernation, it might well happen that now and then an
early frost or a tempestuous season would destroy all the larvæ of the
district.”

[63] Compare this with Weismann’s remarks, pp. 19-22, and 53.

[64] See Canad. Ent., vol. ix. p. 69.

[65] Figures of the different forms of this species are given in vol.
i. of Edward’s “Butterflies of North America.”

[66] Only the species of _Smerinthus_ can be made to lay eggs regularly
in confinement; _Macroglossa Stellatarum_ laid a number in a large
gauze-covered breeding-cage; the species of _Deilephila_ could not be
induced to lay more than single ones in such a cage. From species of
_Chærocampa_ also I never obtained but a few eggs, and from _Sphinx_
and _Acherontia_ never more than single ones.

[67] [Eng. ed. Since the appearance of the German edition of this
work, numerous descriptions of the young stages of caterpillars have
been given, but in all cases without representing the relationship of
the forms.] [In the excellent figures of larvæ at various stages of
growth, given in some of the more recent works on Lepidoptera, there
will be found much material which may be regarded as a contribution to
the field of research entered on by the author in the present essay,
_i.e._ the ontogeny and comparative morphology of larval markings,
although it is much to be regretted that the figures and descriptions
have not been given from this point of view. In his “Butterflies of
North America,” for example, W. H. Edwards figures the young as well
as the adult larvæ of species of _Apatura_, _Argynnis_, _Libythea_,
_Phyciodes_, _Limenitis_, _Colias_, _Papilio_, &c. Burmeister, in his
recently published “Lépidoptères de la République Argentine,” figures
the young stages of species of _Caligo_, _Opsiphanes_, _Callidryas_,
_Philampelus_, &c. Messrs. Hellins and Buckler have figured and
described the early stages of large numbers of the caterpillars of
British Lepidoptera, but their figures remain unpublished. The larvæ
of many of our native species belonging to the genera _Liparis_,
_Tæniocampa_, _Epunda_, _Cymatophora_, _Calocampa_, &c., are dull when
young, but become brightly coloured at the last moult. Such changes
of colour are probably associated with some change, either in the
habits or in the environment; and a careful study of the ontogenetic
development of such species in connection with their life-history would
furnish results of great value to the present inquiry. The same remarks
apply to those _Noctuæ_ larvæ which are brightly coloured in their
young stages, and become dull when adult.

Among other papers which may be considered as contributions to
the present subject, I may mention the following:--In 1864 Capt.
Hutton published a paper, “On the Reversion and Restoration of the
Silkworm, Part II.” (Trans. Ent. Soc. 1864, p. 295), in which he
describes the various stages of development of several species of
_Bombycidæ_. In 1867 G. Semper published accounts of the early stages
of several Sphinx-larvæ (“Beiträge zur Entwicklungsgeschichte einiger
ostasiatischer Schmetterlinge,” Verhandl. k.k. Zoolog.-botan. Gesell.
in Wien, vol. xvii.). The question as to the number of claspers in
young _Noctuæ_ larvæ has been raised in notes by Dr. F. Buchanan White
(“Ent. Mo. Mag.,” vol. v. p. 204) and B. Lockyer (“Entomologist,” 1871,
p. 433). A valuable paper, “On the Embryonic Larvæ of Butterflies,”
was published in 1871 by S. H. Scudder (“Ent. Mo. Mag.,” vol. viii. p.
122). For remarks on the development of the larva of _Papilio Merope_,
see J. P. Mansel Weale in Trans. Ent. Soc., 1874, p. 131, and Pl. I.;
also this author on the young stages of the larva of _Gynanisa Isis_,
Trans. Ent. Soc., 1878, p. 184. For an account of the development of
the larvæ of certain North American species of _Satyrus_, see W. H.
Edwards in the “Canadian Entom.,” vol. xii. p. 21. Mr. P. H. Gosse’s
recent description of the newly hatched caterpillar of _Papilio
Homerus_ (Proc. Ent. Soc. 1879, p. lv), furnishes a good illustration
of the value of studying the ontogeny. The natural affinities of
the _Papilionidæ_ were at one time much disputed, some systematists
placing this family at the head of the Lepidoptera, and others
regarding them as being more closely allied to the moths. Mr. Gosse’s
observation tends to confirm the latter view, now generally received by
Lepidopterists, since he states that the larva in question “suggests
one of the great _Saturniadæ_, such as _Samia Cecropia_.” Mr. Scudder,
in the paper above referred to, adopts an analogous argument to show
the close relationship between the _Papilionidæ_ and _Hesperidæ_. R.M.]

[68] [Mr. A. G. Butler has recently furnished a good illustration of
the danger of classifying Lepidoptera according to the affinities of
the perfect insects only, in his paper, “On the Natural Affinities of
the Lepidoptera hitherto referred to the Genus _Acronycta_ of authors,”
Trans. Ent. Soc. 1879, p. 313. If the author’s views are ultimately
accepted, the species at present grouped under this genus will be
distributed among the _Arctiidæ_, _Liparidæ_, _Notodontidæ_, and
_Noctuæ_. Mr. Butler’s determination of the affinities of the species
supposed to belong to the genus mentioned, is based chiefly upon a
comparative examination of the larvæ, and this is far more likely to
show the true blood-relationship of the species than a comparison of
the perfect insects only. A study of the comparative ontogeny can alone
give a final answer to this question. R.M.]

[69] [In his recent revision of the _Sphingidæ_, Mr. A. G. Butler
(Trans. Zoo. Soc., vol. ix. part x.) retains Walker’s arrangement. R.M.]

[70] The deposition of black pigment may commence immediately before
ecdysis.

[71] [Mr. Herbert Goss states (Proc. Ent. Soc. 1878, p. v.) that
according to his experience, the green and brown varieties of _C.
Porcellus_ (erroneously printed as _Elpenor_ in the passage referred
to) are about equally common, the former colour not being in any way
confined to young larvæ. Mr. Owen Wilson in his recent work, “The Larvæ
of British Lepidoptera and their food-plants,” figures (Pl. VIII.,
Figs. 3 and 3a) the two forms, both apparently in the adult state.
During the years 1878-79, my friend, Mr. J. Evershed, jun., took five
of these full-grown larvæ in Surrey, one of these being the green
variety. In order to get more statistics on this subject, I applied
this year (1880) to Messrs. Davis of Dartford, who informed me that
among 18-20 adult caterpillars of _Porcellus_ in their possession,
there was only one green specimen. R.M.]

[72] I unite the genera _Pergesa_ and _Darapsa_ of Walk. with
_Chærocampa_, Dup.; the first appears to me to be quite untenable,
since it is impossible that two species, of which the caterpillars
agree so completely as those of _C. Elpenor_ and _Porcellus_, can
be located in different genera. _Porcellus_ indeed was referred to
the genus _Pergesa_ because of its different contour of wings, an
instance which distinctly shows how dangerous it is to attempt to found
Lepidopterous genera without considering the caterpillars. The genus
_Darapsa_ also appears to me to be of very doubtful value, and in any
case requires further confirmation with respect to the larval forms.

[73] [Mr. A. G. Butler (Trans. Zoo. Soc., vol. ix., part. x., 1876)
gives a list of about eighty-four species of _Chærocampa_, and sixteen
of _Pergesa_, besides numerous other species belonging to several
genera placed between _Chærocampa_ and _Pergesa_. Of _Darapsa_, he
states “that this genus was founded upon most heterogeneous material,
the first three species being referable to Hübner’s genus _Otus_, the
fifth to Walker’s genus _Diodosida_, the sixth and eighth to the genus
_Daphnis_ of Hübner, the seventh, ninth, and tenth to _Chærocampa_ of
Duponchel; there therefore remains only the fourth species, allied to
_Chærocampa_, but apparently sufficiently distinct.” The species still
retained in the genus _Darapsa_ is _D. rhodocera_, Wlk., from Haiti.
R.M.]

[74] [_Otus Syriacus_ of Butler’s revision. R.M.]

[75] Abbot and Smith. “The Natural History of the rarer Lepidopterous
Insects of Georgia, collected from the observations of John Abbot, with
the plants on which they feed.” London, 1797, 2 vols. fol.

[76] [_Otus Chœrilus_ and _O. Myron_ of Butler’s revision. R.M.]

[77] [To this group may also be added _Ampelophaga Rubiginosa_,
Ménétriés, from China and Japan, the caterpillar of which, having the
distinct subdorsal line without any trace of eye-spots, is figured by
Butler (_loc. cit._, Pl. XCI., Fig. 4). This author also gives a figure
of another species belonging to the subfamily _Chærocampinæ_ (Pl. XC.,
Fig. 11), viz. _Acosmeryx Anceus_, Cram., from Amboina, Java, Silhet,
and S. India; the caterpillar is green, with seven oblique yellow
stripes along the sides, and a very conspicuous white subdorsal line
with a red border above. As there are no eye-spots, this species may
be referred to the present group provisionally, although its general
marking is very distinct from that of the _Chærocampa_ group. R.M.]

[78] [Eng. ed. Dr. Staudinger has since obtained the caterpillar of _C.
Alecto_ from Beyrout; it possesses “a very distinct subdorsal line,
and on the fourth segment a beautiful eye-spot, which is repeated with
gradual diminution to segments 7-8”.]

[79] Figured in “A Catalogue of Lepidopterous Insects in the Museum
of the East India Company,” by Thomas Horsfield and Frederick Moore.
London, 1857. Vol. i., Pl. XI.

[80] Figured in Trans. Ent. Soc., New Series, vol. iv., Pl. XIII.

[81] _Ibid._

[82] [The following species figured by Butler (_loc. cit._ Pls. XC.
and XCI.) appear to belong to the second group--_Chærocampa Japonica_,
Boisd., which is figured in two forms, one brown, and the other green.
The former has two distinct ocelli on the fourth and fifth segments,
and a distinct rudiment on the sixth, whilst the subdorsal line extends
from the second eye-spot to the caudal horn, and beneath this line
the oblique lateral stripes stand out conspicuously in dark brown on
a lighter ground. The ocelli are equally well developed on the fourth
and fifth segments in the green variety, the subdorsal line commencing
on the sixth segment, and extending to the caudal horn; there is no
trace of a third eye-spot, nor are there any oblique lateral stripes;
the insect is almost the exact counterpart of _C. Elpenor_ in its
fourth stage. (See Fig. 21, Pl. IV.) _Pergesa Mongoliana_, Butl., is
brown, without a trace of the subdorsal line except on the three front
segments, and with only one large eye-spot on the fourth segment.
_Chærocampa Lewisii_, Butl., from Japan, is likewise figured in two
forms. The brown variety has the subdorsal line on the three front
segments only, distinct ocelli on the fourth and fifth segments, and
gradually diminishing rudiments on the remaining segments. The green
form appears to be transitional between the present and the third
group, as it possesses a distinct, but rudimentary eye-spot on the
third segment, besides the fully developed ones on the fourth and
fifth, and very conspicuous, but gradually decreasing repetitions
of rudimentary ocelli on segments 6-10. To this group may be added
_Chærocampa Aristor_, Boisd., the caterpillar of which is figured by
Burmeister (Lép. Rép. Arg., Pl. XV., Fig. 4) in the characteristic
attitude of alarm, with the front segments retracted, and the ocelli on
the fourth segment prominently exposed. The subdorsal line is present
in this species. Burmeister also figures two of the early stages (Pl.
XV., Fig. 7, A and B), and describes the complete development of
_Philampelus Labruscæ_, another species belonging to the subfamily
_Chærocampinæ_. The earliest stage (3-4 days old) is simple green, with
no trace of any marking except a black spot on each side of the fourth
segment, the position of the future ocelli. A curved horn is present
both in this stage and the following one, during which the caterpillar
is still green, but now has seven oblique red lateral stripes. The
caudal horn is shed at the second moult, after which the colour becomes
darker, the adult larva (figured by Madame Mérian, in her work on
Surinam, pl. 34 and Sepp., pl. 32) being mottled brown. In addition to
the ocellus on the fourth segment, there is another slightly larger
on the eleventh segment, so that this species may perhaps be another
transition to the third group; but our knowledge is still too imperfect
to attempt to generalize with safety. R.M.]

[83] Cat. Lep. Ins. East Ind. Comp., Pl. XIII. [Figured also by Butler
(=_Chæerocampa Silhetensis_, Walker), _loc. cit._ Pl. XCII., Fig. 8.
R.M.]

[84] Cat. Lep. Ins. East Ind. Comp., Pl. XIII. [Figured also by Butler,
_loc. cit._ Pl. XCI., Fig. 1. R.M.]

[85] Horsfield and Moore, _loc. cit._ Pl. X.

[86] _Ibid._ [=_Pergesa Acteus_, Walker. R.M.]

[87] [Figured also by Burmeister, _loc. cit._ Pl. XV., Fig. 3. R.M.]

[88] Horsfield and Moore, _loc. cit._, Pl. XI.

[89] To be accurate this should be designated the infra-spiracular
line; but this term cannot be well applied except in cases where
there is also a supra-spiracular line, as, for instance, in _Anceryx
(Hyloicus) Pinastri_.

[90] Upon this fact obviously depends the statement of that extremely
accurate observer Rösel, that the caterpillar of _Euphorbiæ_ is but
very slightly variable (“Insektenbelustigungen,” Bd. iii. p. 36). I
formerly held the same opinion, till I convinced myself that this
species is very constant in some localities, but very variable in
others. It appears that local influences make the caterpillar variable.

[91] The green is considerably too light in Fig. 45.

[92] “Die Pflanzen und Raupen Deutschlands.” Berlin, 1860, p. 83.

[93] Fig. 62, Pl. VII., is copied from Boisduval.

[94] The fading of the red anteriorly has not been represented in the
figure.

[95] [The caterpillar of _Deilephila Euphorbiarum_, figured by
Burmeister (Lép. Rép. Arg., Pl. XVI, Fig. 1) belongs to this stage.
R.M.]

[96] [In concluding this account of the _Chærocampinæ_ I may call
attention to the following species, which have since been figured by
Burmeister:--_Pachylia Ficus_, Linn. (_loc. cit._ Pl. XIV., Figs. 1
and 2); during the three first stages the larva is uniformly green,
with a yellow subdorsal line, and below this ten oblique yellow
stripes slanting away from the head; after the third moult the colour
completely changes, the whole area of the body being divided into two
distinct portions by the subdorsal line, above which the colour is
red, and underneath of a pale green; the oblique stripes have almost
disappeared; no occelli nor annuli are present. _Pachylia Syces_, Hübn.
(_loc. cit._ Fig. 3); very similar to the last species in its young
stages (figured also by Mérian, Surin. pl. 33). _Philampelus Vitis_,
Linn. (_loc. cit._ Figs. 4 and 5); two stages represented; between
first and second moults green, with oblique paler stripes slanting in
same direction as in _Pachylia_, and each one containing a red streak
surrounding the spiracle. When adult, the ground-colour is yellow above
and green beneath, the whole surface being mottled with deep black and
red transverse markings; the oblique stripes whitish, bordered with
black at their lower extremities (figured also by Mérian, pls. 9 and
39). _Philampelus Anchemolus_, Cram. (_loc. cit._ Pl. XV., Fig. 1;
Mérian, pl. 47); green when young, with seven oblique red stripes; when
adult, uniformly brown, with seven pale yellow lateral markings, the
first four of which are spots, and the remainder broad oblique stripes
slanting forwards. _Philampelus Labruscæ_, (see note 82, p. 195). R.M.]

[97] [_Mimas Tiliæ_ of Butler’s revision. The author states that this
genus is “easily distinguished from _Laothoë_ by the form of the wings,
the outer margin of secondaries deeply excavated below the apex, and
the secondaries narrow and not denticulated.” Here again we have a
clashing of the results arrived at by a study of the ontogeny of the
larvæ, on the one hand, and the founding of genera on the characters of
the imagines only, on the other. Of the three species discussed by Dr.
Weismann, Mr. Butler, following other authors, refers _Tiliæ_ to the
genus _Mimas_, _Populi_ to _Laothoë_, and _Ocellatus_ to _Smerinthus_.
It is to be hoped that when our knowledge of the developmental history
of larvæ is more complete in all groups, a reconciliation between the
results of the biological investigator and the pure systematist will be
brought about, so that a genus may not, as at present, have such very
different values when regarded from these two points of view. R.M.]

[98] The caterpillar is thus figured by Rösel.

[99] [In 1879 Mr. E. Boscher found about thirty full-grown caterpillars
of this species in the neighbourhood of Twickenham, ten to twelve
of which were feeding on _Salix viminalis_, and the remainder, from
a locality not far distant, on _Salix triandra_. The whole of the
specimens taken on the plant first named, had the red-brown spots above
and below the oblique stripes more or less completely developed, as
I myself had an opportunity of observing. In these spotted specimens
the ground-colour was bright yellowish-green, and in the others this
colour was dull whitish-green above, passing into bluish-green below.
Should these observations receive wider confirmation, it would be
fair to conclude that this species is now in two states of phyletic
development, the more advanced stage being represented by the brighter
spotted variety. (See also Proc. Ent. Soc. 1879, p. xliv.). Mr. Peter
Cameron has recently suggested (Trans. Ent. Soc. 1880, p. 69) that the
reddish-brown spots on the _Smerinthus_ caterpillars may serve for
purposes of disguise, as they closely resemble, both in colour and
form, certain galls (_Phytoptus_) of the food-plants of these species.
If this view be admitted, these spots must be considered as a new
character, now being developed by natural selection. The variation in
the ground-colour of the two forms of _S. Ocellatus_ may possibly be
phytophagic, but this can only be decisively settled by a series of
carefully conducted experiments. R.M.]

[100] “Insekten-Belustigungen,” Suppl. Pl. 38, Fig. 40.

[101] “Catalogue of Lepidop.” British Museum. [Butler divides the
subfamily _Smerinthinæ_ into 17 genera, containing 79 species, viz.
_Metamimas_, 2; _Mimas_, 4; _Polyptychus_, 7; _Lophostethus_, 1;
_Sphingonæpiopsis_, 1; _Langia_, 2; _Triptogon_, 23; _Laothoë_, 2;
_Cressonia_, 3; _Paonias_, 2; _Calasymbolus_, 5; _Smerinthus_, 5;
_Pseudosmerinthus_, 2; _Daphnusa_, 4; _Leucophlebia_, 5; _Basiana_, 10;
_Cæquosa_, 1. R.M.]

[102] “Cabinet Orient. Entom.,” p. 13, Pl. VI., Fig. 2. [Butler places
this species doubtfully among the _Sphinginæ_. R.M.]

[103] “Catalogue of the Lepidop. Insects of the E.I. Co.,” by Horsfield
and Moore. Pl. VIII., Fig. 6.

[104] [The larvæ of four other species of this subfamily have since
been made known through Mr. Butler’s figures. _Smerinthus Tatarinovii_,
Ménetriés (_loc. cit._ Pl. XC., Fig. 16), from Japan, is “pale
sea-green, tuberculated with white, with seven lateral, oblique,
crimson-edged white stripes.” There is no trace of the subdorsal line
shown in the figure, so that this species thus appears to be in the
third phyletic stage of development. _Smerinthus Planus_, Walker, from
China (_loc. cit._ Pl XCII., Fig. 11), is “pale green, with white or
yellow lateral stripes.” A trace of the subdorsal line remains on
the front segments, thus showing that the species is in the second
phyletic stage of development. _Triptogon Roseipennis_, Butler, from
Hakodadi (_loc. cit._ Pl. XCI., Fig. 6), is represented as yellow,
with seven oblique white stripes, with large irregular triangular red
spots extending from the anterior edge of the stripes, nearly across
each segment. It is probably in the third phyletic stage. The Indian
_Polyptychus Dentatus_, Cramer (_loc. cit._ Pl. XCI., Fig. 10), is
“bluish-green at the sides, with oblique purple stripes, with a broad,
dorsal, longitudinal, golden-green band, bordered by subtriangular
purple spots, one above each stripe.” The dorsal band is bordered by
coloured stripes, which may be the subdorsal lines; but the position
in which it is figured, and its very different mode of coloration,
make it very difficult to compare satisfactorily with the foregoing
species. The genus _Ambulyx_ is closely allied to the _Smerinthinæ_,
and the two following species may be here mentioned: _A. Gannascus_,
Stoll, figured by Burmeister (_loc. cit._ Pl. XIII., Fig. 5), is green,
with a yellow subdorsal line, and seven oblique white lateral stripes,
edged with red. _A. Liturata_, Butl. (_loc. cit._ Pl. XCI., Fig. 2), is
yellowish-green above, passing into bluish-green below. The subdorsal
is present on the three front segments, and is followed by a row of
white, elongated patches, one on each segment, these being the upper
portions of a row of lateral oblique stripes. The thickened upper
extremities of the latter are edged with red, and their arrangement
is very suggestive of their having arisen from the breaking up of a
subdorsal line. R.M.]

[105] [Butler catalogues 43 species of this genus. R.M.]

[106] The deposition of eggs was accomplished by the insect laying hold
of the point of a twig with its legs during flight, and curving its
abdomen upwards against a leaf, the wings being kept vibrating. The egg
is instantaneously fastened to the leaf. This operation is repeated
from twice to four times successively, the moth then hovering over and
sucking at the flowers for some time. The eggs exactly resemble in
colour the young green buds of _Galium_.

[107] [Figures of a remarkable case of gynandromorphism in a butterfly
(_Cirrochroa Aoris_, Doubl.) have recently been published by Prof.
Westwood (Trans. Ent. Soc. 1880, p. 113). On the right fore- and
hind-wings of a male specimen there are patches of female colouring,
thus bearing out in a very striking manner the above views concerning
the non-fusibility of characters (in this case sexual) which have been
long fixed. Complete (_i.e._ half-and-half) gynandromorphism is not
uncommon in butterflies. R.M.]

[108] [I have long held the opinion that the di- and trimorphism
displayed by certain butterflies has originated through polymorphism
from ordinary variability. I will not here enter into details, but
will only cite a few instances indicating the general direction of the
arguments. The phenomenon to which I refer is that so ably treated of
by Mr. A. R. Wallace (see Part I., p. 32, note 20) and others. One male
has often two or more distinctly coloured females, and in such cases
one form of the female generally resembles the male in colour. Cases
of polymorphic _mimetic_ females may for the present be excluded, in
order to reduce the argument to its greatest simplicity. Thus, in the
case of native species, _Colias Edusa_ has two females, one having the
orange ground-colour of the male, and the other the well-known light
form, var. _Helice_. So, also, _Argynnis Paphia_ has a normal female
and the dark melanic form var. _Valezina_. Numerous other cases might
be mentioned among exotic species; and, looking at the phenomenon as a
whole, it is seen to be one of _gradation_. For instance, our common
“Blues” (_Plebeius Icarus_, _P. Thetis_, &c.) have females showing a
complete gradation between the ordinary blue male and the brown female
coloration. In a large number of specimens of _Callosune Eupompe_
in my cabinet, collected in Arabia by the late J. K. Lord, there is
a completely graduated series of females, varying from individuals
having the scarlet tips of the fore-wings as strongly developed as in
the males, to specimens without a trace of such colouring; and the
same is the case with other species of this and allied genera. In
such instances it is only necessary for the intermediate female forms
to become extinct, in order to have true cases of dimorphism. It is
significant that in 1877, when _Colias Edusa_ appeared in this country
in such extraordinary profusion, large numbers of _intermediate forms_
were captured, these forming an uninterrupted series connecting the
normal female and the var. _Helice_. R.M.]

[109] [Many of our best describers of caterpillars, such as the late
Edward Newman, Messrs. Hellins and Buckler, &c., have described the
various forms of numerous polymorphic species, but not from the point
of view of the comparative morphology and ontogeny of the markings.
R.M.]

[110] [In Butler’s revision both these species are placed in the genus
_Hemaris_. R.M.]

[111] [This species is figured also by Butler (_loc. cit._ Pl. XC.,
Fig. 9), who represents it with seven oblique green lines between the
spiracles and below the subdorsal line. R.M.]

[112] “Cat. E. Ind. Co. Mus.,” Pl. VIII., Fig. 2. [Walker, Lepidop.
Heter. VIII., p. 92, No. 14, 1856; this species is strictly confined to
Java. R.M.]

[113] [Eng. ed. The caterpillar is described and figured by Millière,
“Iconographie des Chenilles et Lépidoptères inédits,” tome iii., Paris,
1869; also in the Annales, Soc. Linn. de Lyon, 1871 and 1873.] [This
sp. = _Hemaris Croatica_, Esper., of Butler’s revision. R.M.]

[114] [The following additional species of the subfamily
_Macroglossinæ_ have been figured by Butler:--_Lophura Hyas_, Walk.
(_loc. cit._ Pl. XC., Figs. 1 and 2), Hong-Kong, Silhet, and Java. The
larva is apparently figured in two stages, the younger being red-brown
with oblique white stripes, and the head and three front segments
green. The larger specimen is green, mottled with red-brown, and no
oblique stripes. In both figures the subdorsal line is indicated. The
whole colouring is very suggestive of protective resemblance. _Hemaris
Hylas_, Linn., from China, Japan, Ceylon, India, Australia, and Africa
(_loc. cit._ Pl. XC., Fig. 4). The upper part of the body is light
blue, and the lower part green, the two areas being separated by a
white subdorsal line bordered above with brown. The dorsal line is
feebly represented. _Macroglossa Belis_, Cram., N. India (_loc. cit._
Pl. XC., Fig. 6), is figured with the ground-colour deep indigo; a
conspicuous white subdorsal, and a yellow spiracular line is present;
on the side of each segment, between the two lines mentioned, there
is a large red spot with a yellow nucleus (? eye-spots), the spots
decreasing in size towards the head and tail; these probably confer
upon this species some special protective advantage. _Macroglossa
Pyrrhosticta_, Butler, China and Japan (_loc. cit._ Pl. XC., Fig.
8), is greenish-white with dorsal and subdorsal lines, and seven
dark oblique stripes along the sides, below the subdorsal line.
Of the foregoing species _Hemaris Hyas_ appears to be in the same
phyletic stage as _M. Stellatarum_ and _M. Croatica_, &c., whilst _M.
Pyrrhosticta_ is probably, together with _M. Corythus_ and _M. Gilia_,
in another and more advanced stage, which is also passed through by
_Lophura Hyas_ in the course of its ontogenetic development. This last
species (adult) and _M. Belis_ may represent phyletic stages still
further advanced. _Caliomma Pluto_, Walk., of which the caterpillar is
figured by Burmeister (_loc. cit._ Pl. XIII., Fig. 1), appears to be
a case of special protective resemblance to a twig or branch of its
food-plant. Figured also by Chavannes; Bull. Soc. Vadoise des Sci.
Nat., Dec. 6th 1854. R.M.]

[115] [Genus _Pterogon_, Boisd., = _Proserpinus_ and _Lophura_ (part).
Butler, _loc. cit._ p. 632. The species above treated of = _Proserpinus
Œnotheræ_, Fabr. R.M.]

[116] [These species = _Thyreus Abboti_ and _Proserpinus Gauræ_
of Butler’s revision. Of the former he states:--“Transformations
described, and larva and imago figured, Am. Ent. ii. p. 123, 1870; the
larva is also figured by Scudder in Harris’s ‘Correspondence,’ Pl.
III., Fig. 1 (1869), and by Packard in his ‘Guide,’ p. 276, Fig. 203.”
R.M.]

[117] [_Proserpinus (Sphinx) Gorgon_, Esp. R.M.]

[118] Rösel, _loc. cit._ vol. iii., p. 26, note.

[119] Figured and described by Abbot and Smith. [_Macrosila (Sphinx)
Cingulata_ is figured also by Burmeister, _loc. cit._ Pl. XII., Fig. 1.
R.M.]

[120] Figured in “Cat. Lep. E. Ind. Co.”

[121] See the figure in Sepp’s Surinam Lepidoptera, P. 3, Pl. CI.,
1848. A specimen in alcohol of the adult caterpillar is in the
Berlin Museum. [The following is the synonymy of the above mentioned
species:--_Macrosila Hasdrubal_, Walk. = _Pseudosphinx (Sphinx)
Tetrio_, Linn.; _M. Cingulata_ = _Protoparce (Sphinx) Cingulata_,
Fabr.; _M. Rustica_ = _Protoparce (Sphinx) Rustica_, Fabr.; _Sphinx
Convolvuli_, Linn. = _Protoparce Convolvuli_; _S. Carolina_, Linn. =
_P. Carolina_; the other species remain in the genera, as given above.
The following additional species of _Sphinginæ_ and _Acherontiinæ_
have been figured by Butler:--_Pseudosphinx Cyrtolophia_, Butl.,
from Madras (_loc. cit._ Pl. XCI., Figs. 11 and 13); _Protoparce
Orientalis_, Butl., from India, China, Java, &c. (Pl. XCI., Fig. 16);
_Diludia Vates_, Butl. from India, &c. (Pl. XCI., Fig. 18); _Nephele
Hespera_, Fabr., from India, Australia, &c. (Pl. XCI., Fig. 20);
_Acherontia Morta_, Hübn., from Java, China, India, &c. (Pl. XCII.,
Fig. 9); and _A. Medusa_, Butl., from nearly the same localities as
the last (Pl. XCII., Fig. 10). Most of these species fall under Dr.
Weismann’s general remarks, so that it is unnecessary to give detailed
descriptions. The most divergent marking is that of _P. Cyrtolophia_,
which has a broad white dorsal line bordered with pink, and two large
pink ovals on the back of the four anterior segments, the hindmost and
larger of these being bisected by the dorsal line. In _N. Hespera_ the
subdorsal line is present on segments 6 to 11 only, and it is highly
significant that the oblique stripes are absent from these segments,
but are present on the anterior segments, where the subdorsal line
fails. With reference to the larva of _A. Atropos_, Mr. Mansel Weale
states (Proc. Ent. Soc. 1878, p. v.) that in S. Africa the ordinary
form feeds generally on _Solanaceæ_, whilst the darker and rarer
variety is found only on species of _Lantana_. The following species
of these subfamilies are figured by Burmeister: _Amphonyx Jatrophæ_
(_loc. cit._ Pl. XI., Fig. 1); _Protoparce (Diludia) Florestan_, Cram.
(Fig. 2); _Sphinx Justiciæ_, Walk. (Fig. 3); _Protoparce (Diludia)
Lichenea_, Walk. (Fig. 4); _Sphinx (Protoparce) Cingulata_, Fabr. (Pl.
XII., Fig. 1); and _Sphinx Cestri_ (Fig. 5). All these species have the
characteristic Sphinx-like markings. _Dilophonota Ello_, Linn. (Pl.
XII., Fig. 2), is greenish-brown with a yellow subdorsal line, and
_D. Hippothöon_ (Fig. 4), yellow with a whitish subdorsal. Neither of
these has oblique stripes. _D. Œnotrus_, Cram. (Fig. 3), has neither
stripes nor subdorsal, but is uniform brown above, passing into green
beneath. _Protoparce Albiplaga_, Walk. (Pl. XIII., Fig. 2, also Mérian,
Pl. III., and Abbot and Smith, I., Pl. XXIV.), pale green with large
yellow, black-bordered patches surrounding the spiracles. _Pseudosphinx
Tetrio_, Linn. (Pl. XIII., Fig. 3), and _P. Scyron_ (Fig. 4) are black
with broad transverse belts, yellow and white respectively, encircling
the middle of each segment. These light bands serve very effectively to
break up the uniform surface of the large bodies of these insects, but
the whole marking is suggestive of distastefulness. R.M.]

[122] [The species referred to is placed by Butler in Hübner’s genus
_Hyloicus_. R.M.]

[123] [= _Ellema Coniferarum_, of Butler’s revision. R.M.]

[124] [= _Dilophonota Ello_ of Butler’s revision. R.M.]

[125] “Synopsis of the North American Sphingides.” Philadelphia, 1859.

[126] [The larvæ of many moths which feed on deciduous trees during
the autumn and hibernate, are stated to feed on low-growing plants in
the spring, before the buds of their food-trees open. On the other
hand, low-plant feeders, such as _Triphæna Fimbria_, &c., are stated
to sometimes feed at night in early spring on the buds of trees.
The habits and ontogeny of these species are of special interest
in connection with the present researches, and are well worthy of
investigation. R.M.]

[127] “Neuer Beitrag zum geologischen Beweise der Darwin’schen
Theorie.” 1873, Nos. 1 and 2. [This principle, in common with many
others which have only been completely worked out of late years, is
foreshadowed by Darwin. Thus, he states when speaking of inheritance
at corresponding periods of life: “I could give a good many cases
of variations (taking the word in the largest sense) which have
supervened at an earlier age in the child than in the parent” (“Origin
of Species,” 1st ed., 1860, p. 444). In the case of inherited diseases
also: “It is impossible to ... doubt that there is a strong tendency to
inheritance in disease at corresponding periods of life. When the rule
fails, the disease is apt to come on earlier in the child than in the
parent; the exceptions in the other direction being very much rarer.”
(“Variation of Animals and Plants under Domestication,” 1st ed., 1868,
vol. ii., p. 83.) R.M.]

[128] [If the reddish-brown spots on the larva of _S. Populi_ have
the protective function assigned to them by Mr. Peter Cameron (Trans.
Ent. Soc. 1880, p. 69), it can be readily understood that they would
be of service to the insect in the fourth stage, and the backward
transference of this character might thus be accelerated by natural
selection, in accordance with the above principles. (See, also,
note 100, p. 241.) R.M.]

[129] [For cases of correlation of habit with protective resemblance
in larvæ, see a paper in “Ann. and Mag. of Nat. Hist.,” Feb., 1878,
pp. 159, 160. Also Fritz Müller on a Brazilian Cochliopod larva,
Trans. Ent. Soc. 1878, p. 223. Mr. Mansel Weale states, with reference
to S. African _Sphingidæ_ (Proc. Ent. Soc. 1878, p. vi.), that many
species when seized “have a habit of doubling up the body, and then
jumping a considerable distance with a spring-like action. This is
especially the case with species having eye-like markings; and it
is probable that if attacked by birds in a hesitating manner, such
species might effect their escape amid the grass or foliage.” Many
of the defensive weapons and habits of larvæ are doubtless means of
protection from ichneumons and other parasitic foes. In the case of
saw-flies, Mr. Peter Cameron has shown (Trans. Ent. Soc. 1878, p. 196)
that the lashing about of the posterior part of the body may actually
frighten away such enemies. The grotesque attitude and spider-like
appearance and movements of the caterpillar of _Stauropus Fagi_ are
considered by Hermann Müller (“Kosmos,” Nov., 1879, p. 123) to be means
of protection from ichneumons. Among the most remarkable means of
defence possessed by larvæ is that of secreting a liquid, which Mr. W.
H. Edwards has shown, in the case of certain North American _Lycænidæ_
(“Canadian Entomologist.” vol. x., 1878, pp. 3-9 and 131-136), to be
attractive to ants, who regularly attend these caterpillars, in the
same manner and for the same purpose as they do our aphides. The mutual
advantage derived by the ants and larvæ was discovered in the case of
_Lycæna Pseudargiolus_. Mr. Edwards states that the _mature_ larva
of this species is singularly free from Hymenopterous and Dipterous
parasites:--“Why this species, and doubtless many other _Lycænæ_,
are thus favoured will, perhaps, in some degree appear from a little
incident to be related. On 20th June, in the woods, I saw a mature
larva on its food-plant; and on its back, facing towards the tail of
the larva, stood motionless one of the larger ants.... At less than
two inches behind the larva, on the stem, was a large ichneumon-fly,
watching its chance to thrust its ovipositor into the larva. I bent
down the stem, and held it horizontally before me, without alarming
either of the parties. The fly crawled a little nearer and rested,
and again nearer, the ant making no sign. At length, after several
advances, the fly turned its abdomen under and forward, thrust out
its ovipositor, and strained itself to the utmost to reach its prey.
The sting was just about to touch the extreme end of the larva, when
the ant made a dash at the fly, which flew away, and so long as I
watched--at least five minutes--did not return. The larva had been
quiet all this time, its tubes out of sight, and head buried in a
flower-bud, but the moment the ant rushed and the fly fled, it seemed
to become aware of the danger, and thrashed about the end of its body
repeatedly in great alarm. But the tubes were not protruded, as I was
clearly able to see with my lens. The ant saved the larva, and it is
probable that ichneumons would in no case get an opportunity to sting
so long as such vigilant guards were about. It strikes me that the
larvæ know their protectors, and are able and willing to reward them.
The advantage is mutual, and the association is friendly always.” Those
who are familiar with Mr. Belt’s description of the standing armies of
ants kept by the “bull’s-horn thorn” (“Naturalist in Nicaragua,” pp.
218-222) and by certain _Cecropiæ_ and _Melastomæ_, will be struck with
the analogy between these and the foregoing case. R.M.]

[130] [The adaptive resemblance is considerably enhanced in _Catocala_
and in _Lasiocampa Quercifolia_ by the row of fleshy protuberances
along the sides of these caterpillars, which enables them to rest
on the tree trunks by day without casting a sharp shadow. The hairs
along the sides of the caterpillar of _Pæcilocampa Populi_ doubtless
serve the same purpose. (See a paper by Sir John Lubbock, Trans. Ent.
Soc. 1878, p. 242; also Peter Cameron, _ibid._, 1880, p. 75.) It is
well known to collectors that one of the best methods of finding the
caterpillars of the _Catocalæ_ is to _feel_ for them by day on the
barks of their respective food-trees, or to beat for them at night.
R.M.]

[131] [See Wallace’s “Contributions to the Theory of Natural
Selection,” 1st ed., p. 62. Also a paper in “Ann. Mag. Nat. Hist.” Feb.
1878, p. 159, for cases in point. Rösel in 1746 mentioned this habit
in _Calocampa Exoleta_. Hermann Müller has recorded many other similar
instances on the authority of Dr. Speyer; see “Kosmos,” Nov., 1879, p.
114. R.M.]

[132] [Andrew Murray called attention to this fact in 1859 (“Edinburgh
New Philos. Journ.,” Jan., 1860, p. 9). This view is also corroborated
by the fact that no internal feeders are green; see note 142, p. 310
and Proc. Zoo. Soc. 1873, p. 159. R.M.]

[133] [Proc. Ent. Soc. March 4th, 1867; and “Contributions to the
Theory of Natural Selection,” 1st ed., pp. 117-122; also Darwin’s
“Descent of Man,” 2nd ed., p. 325. Among the most important recent
additions to the subject of the colours, spines, and odours of
caterpillars, I may call attention to a paper by Fritz Müller
(“Kosmos,” Dec., 1877), the following abstract of which I communicated
to the Entomological Society (Proc. 1878, pp. vi, vii):--“The larvæ of
_Dione Juno_ and _Acræa Thalia_ live gregariously, and are brown in
colour; they are covered with spines, but, being of dull colours, their
spiny protection (which in the case of _D. Juno_ is very imperfect)
would not preserve them unless they were distinguished as inedible at
the right time, and not after being seized, in accordance with the
principles laid down by Mr. Wallace. It is suggested that the social
habits of the larvæ which lead then to congregate in large numbers,
make up for their want of colour, since their offensive odour then
gives timely warning to an approaching enemy. The caterpillars of
_Colænis Julia_ and _Dione Vanillæ_ are equally wanting in bright
colours, but are solitary in their habits, and these species rest
on the under side of the leaf when feeding. On the other hand,
the caterpillars of _Heliconius Eucrate_, _Colænis Dido_, and _C.
Isabella_, which are of solitary habits, and which freely expose
themselves, are very gaudily coloured, and therefore most conspicuous.
As examples of nearly allied larvæ, of which some species are
gregarious and others solitary, Fritz Müller mentions _Morpho_ and
_Brassolis_, which are gregarious; while _Opsiphanes_ and _Caligo_
are solitary. The larva of _Papilio Pompeius_ also is gregarious,
and those of _P. Nephalion_, _P. Polydamas_, and _P. Thoas_ are
solitary.... Fritz Müller sums up his observations by remarking that
those caterpillars which live alone, and lack the bright colouring as
a sign of offensiveness, must hide themselves; as those of _C. Julia_
and _D. Vanillæ_. The spiny covering is much less a protection against
birds than against smaller enemies; and they may, by the protective
habit of living together, diffuse around themselves an offensive
atmosphere, even to man, and thus gradually becoming shorter (as with
_D. Juno_), the spines of these caterpillars become useless, and
finally are altogether dropped.” See also Sir John Lubbock’s “Note on
the Colours of British Caterpillars,” Trans. Ent. Soc. 1878, p. 239.
Mr. Peter Cameron finds (Trans. Ent. Soc. 1880, pp. 71 and 75) that
these remarks are also applicable to the larvæ of certain saw-flies. In
1877 Mr. J. W. Slater published a paper “On the Food of gaily-coloured
Caterpillars” (Trans. Ent. Soc. 1877, p. 205), in which he suggested
that such caterpillars might derive their distasteful qualities from
feeding on plants containing poisonous or otherwise noxious principles.
A much larger number of observations will be required, however, before
this view can be accepted as of general application. A beautiful
illustration of the theory of warning colours is given by Belt in his
“Naturalist in Nicaragua,” p. 321. All the frogs found in the woods
round St. Domingo are, with one exception, protectively coloured; they
are of nocturnal habits, and are devoured by snakes and birds. The
exception was a species of bright red and blue colours, which hopped
about by day and made no attempt at concealment. From these facts
Mr. Belt concluded that this species was inedible, and on trying the
experiment with ducks and fowls this was found to be the case. R.M.]

[134] See the essay “Über den Einfluss der Isolirung auf die
Artbilding.” Leipzig, 1872, p. 22.

[135] [See also preceding note 133, p. 294. R.M.]

[136] [Eng. ed. The habit of hiding by day occurs also in those
caterpillars which resemble the bark of their food-trees. Thus
_Catocala Sponsa_ and _Promissa_ conceal themselves by day in crevices
of the bark, and are, under these circumstances, only found with
difficulty. Dr. Fritz Müller also writes to me that in Brazil the
caterpillars of _Papilio Evander_ rest in this manner in large numbers,
crowded together into dense masses, on the trunks of the orange-trees,
which they resemble in colour.]

[137] “Über den Einfluss der Isolirung auf die Artbildung.” Leipzig,
1872, p. 21.

[138] I am unfortunately not able to give exact numbers showing the
relative proportions of the different forms, since I have never bred
_S. Convolvuli_ from eggs, nor _C. Elpenor_ in sufficient numbers.

[139] [With reference to _C. Porcellus_, see note 71, p. 188. R.M.]

[140] [In the class of cases treated of in the foregoing portions
of this essay, the external conditions remain unaltered during the
lifetime of the caterpillar, but change of habit, and in some cases
of colour, occurs when the insect has attained a size conceivable _à
priori_, and are realized by observation, in which the environment
itself may undergo change during the lifetime of the individual
caterpillar. Thus, in the case of hibernating species, the colour which
is adaptive to the autumnal colours of the foliage of their food-trees
would not assimilate to that of the newly-opened leaves in the spring.
I have already quoted (Proc. Zoo. Soc. 1873, p. 155) as instances
of what may be called “seasonal adaptation,” the larvæ of _Geometra
Papilionaria_, _Acidalia Degenararia_, and _Gnophos Obscurata_, and
many more could be named. These species undergo a change of colour
before or after hibernation, the change being always adaptive to the
environment.

It has long been known that caterpillars which feed on flowers or on
plants of variously-coloured foliage, in some cases partake of the
colour of their food. See, for instance, Dr. L. Möller’s memoir, “Die
Abhängigkeit der Inseckten von ihrer Umgebung,” 1867, and B. D. Walsh
“On Phytophagic Varieties and Phytophagic Species,” Proc. Ent. Soc.
Philadelph., vol. iii., p. 403. In 1865 Mr. R. McLachlan published a
paper entitled “Observations on some remarkable varieties of _Sterrha
Sacraria_, Linn., with general notes on variation in Lepidoptera”
(Trans. Ent. Soc. 1865, p. 453), in which he gave many illustrations
of this phenomenon. The larva of _Heliothis Peltiger_, according to
Mr. Reading’s description (Newman’s “British Moths,” p. 438), is
another case in point. In 1874 a number of instances were published
by Mr. Thomas G. Gentry in a paper entitled “Remarkable Variations in
Coloration, Ornamentation, &c., of certain Crepuscular and Nocturnal
Lepidopterous Larvæ” (“Canadian Entomologist,” vol. vi., p. 85. See
also W. H. Edwards’ description of the summer and autumnal larvæ of
_Lycæna Pseudargiolus_; _Ibid._, vol. x., pp. 12, 13).

The caterpillars of the _Sphingidæ_ appear also in some cases to vary
in a manner very suggestive of phytophagic influences. The observations
upon _S. Ocellatus_ recorded in the previous note (p. 241) may perhaps
be interpreted in this sense. In order to get experimental evidence
upon this subject, I may add that Mr. E. Boscher was good enough at
my request to repeat his observations, and conduct some breeding
experiments during the present year (1880). In the same locality as
that previously mentioned, seven larvæ were found feeding on _Salix
viminalis_, all of which were the bright green spotted variety; and
in the same osier-bed six more were found on another species of
_Salix_, two of these being the bluish-green variety, and the other
four the bright green form. Unless we have here a local race, these
observations, in connection with those of last year, tend to show
that the light green form is associated with _Salix viminalis_. When
found in the natural state feeding on apple, the caterpillar of this
species is generally, perhaps invariably, the bluish-green form. In
order to try the effect of breeding the larvæ _ab ovo_ on distinct
food-plants, a large number of eggs laid by a female _Ocellatus_ in
July were divided into three batches, one being supplied with _Salix
triandra_, another with _S. viminalis_, and the third lot with apple.
The experiment unfortunately failed in great part, owing to most of the
larvæ dying off, three from the third lot only surviving; but these
were all of the bluish-green form, which colour was shown by all the
caterpillars of this batch from their earliest stage. The observation
is thus so far successful, as it goes to support the view that the
variety mentioned is associated with apple (and _S. triandra?_) My
friend Mr. W. J. Argent informs me that he had a number of specimens
of _Sphinx Ligustri_ in his possession this autumn, some of which had
been found on lilac and others on laurestinus, and he states that all
those on the latter plant had the ground-colour distinctly darker than
in those feeding on lilac. I learn also from Mr. W. Davis, of Dartford,
that he found a number of these larvæ this year feeding on ash, and
that they were all differently coloured to those found on lilac or
privet, being of a more greyish-green. Another case of colour-variation
in larvæ is that _Emmelesia Unifasciata_, specimens of which I have
recently had an opportunity of examining, through the courtesy of Mr.
W. Davis. This species feeds on the seeds of a species of _Bartsia_
when the capsules are in various stages of growth, and (omitting
details of marking) those caterpillars found on the green capsules
were green, whilst those on the brown capsules were of a corresponding
colour.

On the whole I am inclined to believe that sufficient importance
has not hitherto been given to phytophagic variability as a
factor in determining larval coloration, and a large field for
experimental investigation here lies open for future work. The
obscure chemico-physiological processes which may perhaps be shown
by such researches to lead to phytophagic variation, cannot, I am
persuaded, produce any great divergence of character if unaided;
but when such causes of variability play into the hands of natural
selection variations of direct _protective advantage_ to the species,
we can easily see that this all-important agency would seize upon and
perpetuate such a power of adaptability to a variable environment. (See
Proc. Zoo. Soc. 1873, p. 158, and “Nature,” vol. xiv., pp. 329 and
330.) R.M.]

[141] [In 1879 Mr. George Francis, of Adelaide, forwarded from the
latter place a number of moths (a species of Anapæa) together with
their larvæ (in alcohol) and cocoons (Proc. Ent. Soc. 1879, p. xvi),
and in an accompanying note he stated that the male larva when living
is of “a bright emerald green, with red and pink markings on the back,
and yellow, black, and white streaks on the sides.” The male larva
is described as being smaller than the female, and as possessing
all the brilliant colours, the latter “having no red markings, but
only white, yellow, and green, with a little black.” I was at first
disposed to think that we might be dealing here with two distinct
species having differently marked larvæ; but Mr. Francis this present
year (1880) forwarded a large number of the living cocoons of this
species, which I separated according to size, and, on the emergence
of the moths (August), I found that all those from the small cocoons
were males, and those from the larger cocoons females. There can be no
doubt, therefore, that we have but one species in this case, the larva
of which presents the remarkable phenomenon of sexual difference of
coloration. As an analogous fact I may here mention the well-known case
of _Orgyia Antiqua_, the larva of which differs in the colour of the
tufts of hair according to sex. R.M.]

[142] [I have already given reasons for suspecting that the colour of
green caterpillars may be due to the presence of chlorophyll (or some
derivative thereof) in their tissues (see Proc. Zoo. Soc. 1873, p.
159). This substance appears to be one of great chemical stability,
and, according to Chautard, who has detected it in an unaltered
state in the tissues of certain leaf-feeding insects by means of its
absorption spectrum (“Comp. Rend.” Jan. 13th, 1873), it resists the
animal digestive processes (Ann. Ch. Phys. [5], iii., 1-56). If this
view should be established by future observations, we must regard the
green colour of caterpillars as having been produced, when protective,
from phytophagic variability by the action of natural selection; and
the absence of colour in internal feeders, above referred to, is only
secondarily due to the exclusion of light, and depends primarily on
the absence of chlorophyll in their food. In connection with this
I may adduce the fact, that some few species of _Nepticula_ (_N.
Oxyacanthella_, _N. Viscerella_, &c.) are green, although they live in
leaf-galleries where this colour can hardly be of use as a protection;
but their food (hawthorn and elm) contains chlorophyll. See also note
130, p. 293. Further investigations in this direction are much needed.
R.M.]

[143] [The same applies to _Pseudoterpna Cytisaria_, also feeding on
broom at the same time of the year. The most striking cases of adaptive
resemblance brought about by longitudinal stripes are to be found among
fir and pine feeders, species belonging to the most diverse families
(_Hyloicus Pinastri_, _Trachea Piniperda_, _Fidonia Piniaria_, &c.,
&c.) all being most admirably concealed among the needle-shaped leaves.
R.M.]

[144] The geographical distribution of the dark form indicates that
in the case of this species also, the form referred to is replacing
the yellow (green) variety. Whilst in the middle of Europe (Germany,
France, Hungary) the dark form is extremely rare, in the south of
Spain this variety, as I learn from Dr. Noll, is almost as common as
the yellow one. I hear also from Dr. Staudinger that in South Africa
(Port Natal) the dark form is somewhat the commoner, although the
golden-yellow and, more rarely, the green varieties, occur there. I
have seen a caterpillar and several moths from Port Natal, and these
all agree exactly with ours. The displacement of the green (yellow)
form by the dark soil-adapted variety, appears therefore to proceed
more rapidly in a warm than in a temperate climate. [Eng. ed. Dr.
Noll writes to me from Frankfort that the caterpillar of _Acherontia
Atropos_ in the south of Spain does not, as with us, conceal itself by
day in the earth, but on the stems underneath the leaves. “At Cadiz, on
the hot, sandy shore, _Solanum violaceum_ grows to a height of three
feet, and on a single plant I often found more than a dozen _Atropos_
larvæ resting with the head retracted. It can easily be understood
why the lateral stripes are blue when one has seen the south European
_Solaneæ_, on which this larva is at home. _Solanum violaceum_ is
scarcely green: violet tints alternate with brown, green, and yellow
over the whole plant, and between these appear the yellow-anthered
flowers, and golden-yellow berries of the size of a greengage. Thus
it happens that the numerous thorns, an inch long, between which the
caterpillar rests on the stem, pass from violet into shades of blue,
red, green, and yellow.”]

[145] [For Mr. J. P. Mansel Weale’s remarks on the habits of certain
ocellated S. African Sphinx-larvæ see note 129, p. 290. R.M.]

[146] [Some experiments with the caterpillar of _C. Elpenor_,
confirming these results, have been made by Lady Verney. See “Good
Words,” Dec. 1877, p. 838. R.M.]

[147] [The eye-spots on _Ch. Nerii_ have thus been supposed by some
observers to be imitations of the flowers of the periwinkle, one of
its food-plants. See, for instance, Sir John Lubbock’s “Scientific
Lectures,” p. 51. R.M.]

[148] “On Insects and Insectivorous Birds,” Trans. Ent. Soc. 1869, p.
21.

[149] _Ibid._, p. 27.

[150] [Messrs. Weir and Butler inform me that they have not
experimented with Sphinx-larvæ. R.M.]

[151] [It appears that the nauseous character of these last butterflies
is to a certain extent retained after death, as I found that in an
old collection which had been destroyed by mites, the least mutilated
specimens were species of _Danais_ and _Euplæa_, genera which are known
to be distasteful when living, and to serve as models for mimicry. See
Proc. Ent. Soc. 1877, p. xii. R.M.]

[152] [This bears out the view expressed in a previous note 129, p.
290, that the grotesque attitude and caudal tentacles are more for
protection against ichneumons than against larger foes. R.M.]

[153] These experiments, as already mentioned above, were not made with
the common German lizard (_Lacerta Stirpium_), but with the large South
European _Lacerta Viridis_.

[154] Thus, Boisduval states of this caterpillar, which in Provence
lives on _Euphorbia esula_ and allied species:--“Its resemblance
to a serpent, and its brilliant colour, permit of its being easily
discovered.” This was written in 1843, long before natural selection
was thought of.

[155] Or some other extinct analogously-marked species.

[156] [See Darwin’s remarks on the struggle for life being most severe
between individuals and varieties of the same species “Origin of
Species,” 6th ed. p. 59. R.M.]

[157] [Compare this with Darwin’s remarks on “analogous variations,”
“Origin of Species,” 6th ed., p. 125. R.M.]

[158] “Zoologische Studien auf Capri. II. Lacerta muralis cærula,
ein Beitrag zur Darwin’schen Lehre.” Leipzig, 1874. [The subject of
colour-variation in lizards has been much discussed in “Nature” since
the publication of the above mentioned essay; see vol. xix., pp. 4, 53,
97, and 122, and vol. xx., pp. 290 and 480. R M.]

[159] “Über die Berechtigung der Darwin’schen Theorie.” Leipzig,
1868. See also the previous essay “On the Seasonal Dimorphism of
Butterflies,” pp. 112-116.

[160] [Mr. A. G. Butler has recently advanced the view that this family
is not allied to the _Sphingidæ_, but is related on the one side to the
_Pyrales_, and on the other to the _Gelechiidæ_. See his paper “On the
Natural Affinities of the Lepidopterous Family _Ægeriidæ_,” Trans. Ent.
Soc. 1878, p. 121. R.M.]

[161] I am indebted to my esteemed colleague, Prof. Gestäcker, for the
knowledge of this specimen.

[162] Cat. Lep. East India Co., Pl. VIII.

[163] Such a residue is distinctly visible in _S. Ocellatus_: see Fig.
70, Pl. VII.

[164] [The question here also suggests itself as to why the _dorsal_
line should not have been the primary longitudinal stripe, seeing
that such a marking is almost naturally produced in many caterpillars
by the food in the alimentary canal; or, in other words, why has not
natural selection taken advantage of such an obvious means of producing
a stripe in cases where it would have been advantageous? In answer
to this I may state, that in large numbers of species the dorsal
line has thus become utilized; but in the case of large caterpillars
resting among foliage, it can be easily seen that light lateral (_i.e._
subdorsal) stripes, are more effective in breaking the homogeneity
of the body than a dorsal line only slightly darker than the general
ground-colour. Lateral lines are in fact visible from _two directions
of space_. If a caterpillar thus marked be placed on a twig, these
lines are visible when we look at the creature’s back or at either
side. That the subdorsal are therefore the primary lines, as shown by
Dr. Weismann’s observations of the ontogeny of many of the _Sphingidæ_,
is quite in harmony with the view of their having been produced by
natural selection. R.M.]

[165] “Die Darwin’sche Theorie. Elf Vorlesungen über die Entstehung der
Thiere und Pflanzen durch Naturzüchtung.” 2nd ed., Leipzig, 1875, p.
195.

[166] [In the following species, already mentioned in previous
notes, the oblique stripes are bounded at their upper extremities by
a conspicuous subdorsal line:--_Acosmeryx Anceus_, Cram.; _Sphinx
Cingulata_, Fabr.; _Pachylia Ficus_, Linn.; _P. Syces_, Hübn. In
_Pseudosphinx Cyrtolophia_, Butl., the oblique white stripes,
beautifully shaded with pink, run into the white pink-bordered dorsal
line, so that when seen from above the markings present the appearance
of the midrib and lateral veins of a leaf, and are probably specially
adapted for this purpose. R.M.]

[167] [The dorsal line as well as the oblique stripes is present in
the caterpillar of _Smerinthus Tartarinovii_, Ménét.; and in _Ambulyx
Gannascus_, Stoll., the oblique stripes are bounded above by a
subdorsal line, as in the species named in the preceding note. R.M.]

[168] Cat. Lep. East India Co., Pl. XI.

[169] [Compare this with Darwin’s “Origin of Species” (1st. ed. p.
440), where it is stated that when an animal, during any part of its
embryonic career, is active, and has to provide for itself, “the
period of activity may come on earlier or later in life; but whenever
it comes on, the adaptation of the larva to its conditions of life is
just as perfect and beautiful as in the adult animal. From such special
adaptations the similarity of the larvæ or active embryos of allied
animals is sometimes much obscured.” R.M.]

[170] [For Fritz Müller’s application of this principle to the case of
certain groups of Brazilian butterflies see Appendix II. to this Part.
R.M.]




Transcriber’s Notes


Punctuation, hyphenation, and spelling were made consistent when a
predominant preference was found in this book; otherwise they were not
changed.

Simple typographical errors were corrected; occasional unbalanced
quotation marks were corrected.

Ambiguous hyphens at the ends of lines were retained.

“Errata” at the end of Volume II have been applied to the relevant
text of this eBook.

This is Volume I of a two-volume set. The Table of Contents for both
volumes is in this one. This Volume I ends on page 400, so references
to pages 401-729 will be found in Volume II.

The Index for both volumes is in Volume II.

Footnotes and references to them have been renumbered into one
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eBook may substitute question marks or other placeholders.

Page 137: “lunules, 1 had 3, 1 had 3, and a 4th” was printed that way.

Page 142: “of the 10 females 8 were changed” was misprinted as “7
were”, which was an arithmetic error; corrected here.

Page 151: “No change was observed” was printed as “charge”; changed
here.

Volume II is available at no charge from Project Gutenberg,
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