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       *       *       *       *       *


ON

GERMINAL SELECTION

AS A

SOURCE OF DEFINITE VARIATION

BY

AUGUST WEISMANN

TRANSLATED FROM THE GERMAN BY
THOMAS J. McCORMACK

       *       *       *       *       *

SECOND EDITION

       *       *       *       *       *

CHICAGO

THE OPEN COURT PUBLISHING COMPANY.

LONDON AGENTS:
KEGAN PAUL, TRENCH, TRÜBNER & CO., LTD.
1902.

       *       *       *       *       *

COPYRIGHT BY
THE OPEN COURT PUBLISHING CO.
1896

       *       *       *       *       *


{3}

PREFACE.

The present paper was read in the first general meeting of the
International Congress of Zoölogists at Leyden on September 16, 1895.
Several points, which for reasons of brevity were omitted when the paper
was read, have been re-embodied in the text, and an Appendix has been added
where a number of topics receive fuller treatment than could well be
accorded to them in a lecture. The address was first printed in _The
Monist_ for January, 1896, and afterwards in a German pamphlet.

The basal idea of the essay--the existence of Germinal Selection--was
propounded by me some time since,[1] but it is here for the first time
fully set forth and tentatively shown to be the necessary complement of the
process of selection. Knowing this factor, we remove, it seems to me, the
patent contradiction of the assumption that the general fitness of
organisms, or the adaptations _necessary_ to their existence, are produced
by _accidental_ variations--a contradiction which formed a serious
stumbling-block to the theory of selection. Though still assuming that the
_primary_ variations are "accidental," I yet hope to have demonstrated that
an interior mechanism exists which compels them to go on increasing in a
definite direction, the moment selection intervenes. _Definitely directed
{4} variation exists_, but not predestined variation, running on
independently of the life-conditions of the organism, as Naegeli, to
mention the most extreme advocate of this doctrine, has assumed; on the
contrary, the variation is such as is elicited and controlled by those
conditions themselves, though indirectly.

In basing my proof of the doctrine of Germinal Selection on the fundamental
conceptions of my theory of heredity, a few words of justification are
necessary, owing to the fact that the last-mentioned theory has been widely
and severely assailed since its first emergence into light and even
repudiated as absolutely futile and erroneous.

In the first place, many critics have characterised it as a "pure creation
of the imagination." And to a certain extent it is such, as every theory
is. But is it on that account necessarily wrong? Can not its fundamental
ideas still be quite correct, and it itself therefore perfectly justified
as a means of further progress?

Surely my critics cannot be ignorant of the prominent part which
imagination has recently played in the exactest of all natural
sciences--physics? Are they unaware that the English physicist Maxwell
"constructed from liquid vortices and friction-pulleys enclosed in cells
with elastic walls, a wonderful mechanism, which served as a mechanical
model for electromagnetism"?[2] He hoped "that further research in the
domain of theoretical electricity would be promoted rather than hindered by
such mechanical {5} fictions." And so it actually happened, for Maxwell
found by means of them "the very equations, whose singular and almost
incomprehensible power Hertz has so beautifully portrayed in his lecture on
the relations between light and electricity." "Maxwell's formulæ were the
direct outcome of his mechanical models." "These ideal mechanisms"--so
relates Boltzmann in the same interesting essay--"were at first widely
ridiculed, but gradually the new ideas worked their way into all fields.
They were themselves more convenient than the old hypotheses. For the
latter could be maintained only in the event of everything's proceeding
smoothly; whereas now little inconsistencies were fraught with no peril,
for no one can take amiss a slight hitch in a mere analogy.--Ultimately
Maxwell's ideas were philosophically generalised as the theory that all
knowledge consists in the disclosure of analogies."

But not only does it seem that there is little appreciation among
biologists for the scientific import of imagination, they also appear to
have little sense for the significance of theory. It is a favorite attitude
nowadays to look upon theory as a sort of superfluous ballast, as a
worthless survival from the epoch of decrepit "nature-philosophies." People
pronounce with pride the miscomprehended utterance of Newton, _Hypotheses
non fingo_, and place the value of the slightest new fact infinitely higher
than that of "the most beautiful theory."[3] And yet theory originally {6}
fashions science out of facts and is the indispensable precondition of
every important scientific advance.

Heinrich Hertz,[4] the discoverer of electric undulations, had the same
thought in mind when he said: "We form inward representations or constructs
of outward objects, so constituted that the results that follow logically
and necessarily from the constructs are in turn always constructs of the
results flowing naturally and necessarily from the objects." "These
constructs or mental images copied after familiar objects possessed of
familiar properties, so constituted that from their manipulation effects
result similar to those which we observe in the objects to be explained.
Experience teaches us that the requirements here made can be fulfilled and
that consequently such 'correspondences' between reality and the supposed
images [or, as Hertz says, between nature and mind] actually exist. Having
succeeded in extracting from the accumulated experience of the past,
representative images or constructs fulfilling all these necessary
requirements, we can then reproduce by them in a short space of time, as we
might by models, results that in the outward world require a long space of
time for their actualisation or can be produced only through our personal
intervention," etc.

{7}

Such representative models, or constructs, now, in my theory of heredity,
are the _determinants_, which may be conceived as indefinitely fashioned
packages of units (biophores) which are set into activity by definite
impressions and put a distinctive stamp upon some small part of the
organism, on some cell or group of cells, evoking definite phenomena
somewhat as a piece of fireworks when lighted produces a brilliant sun, a
shower of sparks, or the glowing characters of a name.

The _ids_, also, are such representative models, and may be compared to a
definitely ordered but variously compounded aggregate of fireworks, in
which the single pieces are so connected as to go off in fixed succession
and to produce a definite resultant phenomenon like a complete inscription
surrounded by a hail of fire and glowing spheres.

Owing to the greater complexity of the phenomena in biology we can never
hope to reach the same distinctness in our constructs and models as in
physics, and the attempt to derive from them mathematical formulæ by the
independent development of which research could be continued, would at
present be utterly fruitless. In the meantime it seems preferable to have
some sort of adequate model to which the imagination can always resort and
with which it can easily operate, rather than to have to revert, in
considering every special problem of heredity, to the mutual actions of the
molecules of living substance and outward agents--processes which we know
only in their roughest outlines. Or is any one presumptuous enough to
believe we can infer from our slight knowledge of the chemical and physical
constitution of the germs of a trout and a salmon the real cause {8} of the
one's becoming a trout and of the other's becoming a salmon?

The fact is, we can make no show of accounting for the complex phenomena of
heredity with mere _material_ units; we can never reach these phenomena
from below, but must begin farther up and make the assumption of _vital_
units and _hereditary_ units, if there is to be any advance in this field.

It is undoubtedly a splendid aim which the newly founded science of
developmental mechanics has set itself of laying bare the entire causal
line leading from the egg to the finished organism; yet, however much we
may wish to see the success of this plan realised, we cannot disguise the
fact that little or nothing is to be accomplished by it in the settlement
of the problems of heredity. It is impossible to suspend the study of
heredity until this mechanics is completed, and even if we could it would
help us little, for the riddles of heredity are not concealed in the
ontogenesis of types, or, to give an example, in the developmental history
of man _as a race_, but in the ontogenesis of _individuals_, in that of a
_definite and particular_ man. This last ontogenesis exhibits the phenomena
of variation, of reversion, of the predominance of the one or the other
parent, etc., and no one is likely to believe that inductive evolutional
inquiry alone will ever afford us knowledge of these minute and delicate
processes, which, in their bearing on the total resultant development,
phylogenesis, are after all the most important of all.

There is, accordingly, no choice left. If we are really bent on
scientifically investigating the question of heredity, we are obliged
perforce to form from the observed facts of heredity a highly detailed and
{9} elaborate theory, on the basis of which we can propound new questions,
which will give rise in turn to new facts, and thus will exercise a
retroactive influence on the theory, improving and transforming it.

This is precisely what I have sought to accomplish by my theory of
Germ-plasm, as I stated in the Preface to the book bearing that name. It
was never intended as a theory of life, nor, indeed, primarily, as a theory
of evolution, but first and above all as a theory of heredity. I cannot
understand, therefore, the animadversion, that my theory in no way furthers
our insight into the mechanics of development. That is not its purpose; in
fact, it takes the ultimate physical and chemical processes which make up
the vital processes for granted; and inevitably it is constrained to do so.
Its aim is to put into our hands a serviceable formula by means of which we
can go on working in the field of heredity at any rate, and, if I am not
mistaken, also in that of evolution. To me, at least, the newest results of
developmental mechanics do not seem so widely at variance with the theory
of determinants as might appear at first sight; so far as I can see, they
can be quite readily made to harmonise with the theory, provided only the
initial stage of the disintegration of the germ-plasm in the determinant
groups be not invariably placed at the beginning of the process of
segmentation, but be transferred according to circumstances to a subsequent
period. The exact state of things cannot as yet be determined, so long as
the mass of facts is still in constant flux.

In any event I still hold fast to the hope which I expressed in the Preface
to my _Germ-plasm_, that despite the unavoidable uncertainties in its
foundation my theory would yet prove more than a mere work {10} of
imagination, and that the future would find in it some durable points which
would outlive the mutations of opinion. It is possible that one of these
durable gains is my much impugned idea of determinants, and in fact not
only will the present essay be made to rest on this idea, but it will also
defend it on new grounds, although primarily only as a representation of
something which we do not as yet exactly know, but which still exists and
on which we can reckon, leaving it to the future to decide the greater or
less resemblance of our hypothetical construct to nature.

The real aim of the present essay is to rehabilitate the principle of
selection. If I should succeed in reinstating this principle in its
emperilled rights, it would be a source of extreme satisfaction to me; for
I am so thoroughly convinced of its indispensability as to believe that its
demolition would be synonymous with the renunciation of all inquiry
concerning the causal relation of vital phenomena. If we could understand
the adaptations of nature, whose number is infinite, only upon the
assumption of a teleological principle, then, I think, there would be
little inducement to trouble ourselves about the causal connexion of the
stages of ontogenesis, for no good reason would exist for excluding
teleological principles from this field. Their introduction, however, means
the ruin of science.

  AUGUST WEISMANN.

  FREIBURG, Nov. 18, 1895.

       *       *       *       *       *


{11}

GERMINAL SELECTION.

       *       *       *       *       *

Numerous and varied are the objections that have been advanced against the
theory of selection since it was first enunciated by Darwin and
Wallace--from the unreasoning strictures of Richard Owen and the acute and
thoughtful criticisms of Albert Wigand and Nägeli to the opposition of our
own day, which contends that selection cannot create but only reject, and
which fails to see that precisely through this rejection its creative
efficacy is asserted. The champions of this view are for discovering the
motive forces of evolution in the _laws_ that govern organisms--as if the
norm according to which an event happens were the event itself, as if the
rails which determine the direction of a train could supplant the
locomotive. Of course, from every form of life there proceeds only a
definite, though extremely large, number of tracks, _the possible
variations_, whilst between them lie stretches without tracks, _the
impossible variations_, on which locomotion is impossible. But the actual
travelling of a track is not performed by the track, but by the locomotive,
and on the other hand, the choice of a track, the decision whether the
destination of the train shall be Berlin or Paris, is not made by the
locomotive, the cause of the variation, but by the driver of the
locomotive, who directs the engine on the right track. In the theory of
selection the engine-driver is represented by utility, for with utility
rests the decision {12} as to what particular variational track shall be
travelled. The cogency, the irresistible cogency, as I take it, of the
principle of selection is precisely its capacity of explaining why fit
structures always arise, and that certainly is the great problem of life.
Not the fact of change, but the _manner_ of the change, whereby all things
are maintained capable of life and existence, is the pressing question.

It is, therefore, a very remarkable fact, and one deserving of
consideration, that to-day (1895), after science has been in possession of
this principle for something over thirty years and during this time has
steadily and zealously busied itself with its critical elaboration and with
the exact determination of its scope, that now the estimation in which it
is held should apparently be on the decrease. It would be easy to enumerate
a long list of living writers who assign to it a subordinate part only in
evolution, or none at all. One of our youngest biologists speaks without
ado of the "pretensions of the refuted Darwinian theory, so called,"[5] and
one of the oldest and most talented inquirers of our time, a pioneer in the
theory of evolution, who, unfortunately, is now gone to his rest, Thomas
Huxley, implicitly yet distinctly intimated a doubt regarding the principle
of selection when he said: "Even if the Darwinian hypothesis were swept
away, evolution would still stand where it is." Therefore, he, too,
regarded it as not impossible that this hypothesis should disappear from
among {13} the great explanatory principles by which we seek to approach
nearer to the secrets of nature.

I am not of that opinion. I see in the growth of doubts regarding the
principle of selection and in the pronounced and frequently bitter
opposition which it encounters, a transient depression only of the wave of
opinion, in which every scientific theory must descend after having been
exalted, here perhaps with undue swiftness, to the highest pitch of
recognition. It is the natural reaction from its overestimation, which is
now followed by an equally exaggerated underestimation. The principle of
selection was not overrated in the sense of ascribing to it too much
explanatory efficacy, or of extending too far its sphere of operation, but
in the sense that naturalists imagined that they perfectly understood its
ways of working and had a distinct comprehension of its factors, which was
not so. On the contrary, the deeper they penetrated into its workings the
clearer it appeared that something was lacking, that the action of the
principle, though upon the whole clear and representable, yet when
carefully looked into encountered numerous difficulties, which were
formidable, for the reason that we were unsuccessful in tracing out the
actual details of the individual process, and, therefore, in _fixing_ the
phenomenon as it actually occurred. We can state in no single case how
great a variation must be to have selective value, nor how frequently it
must occur to acquire stability. We do not know when and whether a desired
useful variation really occurs, nor on what its appearance depends; and we
have no means of ascertaining the space of time required for the fulfilment
of the selective processes of nature, and hence cannot calculate the exact
number of such {14} processes that do and can take place at the same time
in the same species. Yet all this is necessary if we wish to follow out the
precise details of a given case.

But perhaps the most discouraging circumstance of all is, that in scarcely
a single actual instance in nature can we assert whether an observed
variation is useful or not--a drawback that I distinctly pointed out some
time ago.[6] Nor is there much hope of betterment in this respect, for
think how impossible it would be for us to observe all the individuals of a
species in all their acts of life, be their habitat ever so limited--and to
observe all this with a precision enabling us to say that this or that
variation possessed selective value, that is, was a decisive factor in
determining the existence of the species.

In many cases we can reach at least a probable inference, and say, for
example, that the great fecundity of the frog is a property having
selective value, basing our inference on the observation that in spite of
this fertility the frogs of a given district do not increase.

But even such inferences offer only a modicum of certainty. For who can say
precisely how large this number is? Or whether it is on the increase or on
the decrease? And besides, the exact degree of the fecundity of these
animals is far from being known. Rigorously viewed, we can only say that
great fecundity must be advantageous to a much-persecuted animal.

And thus it is everywhere. Even in the most indubitable cases of
adaptation, as, for instance, in that of the striking protective coloring
of many butterflies, {15} the sole ground of inference that the species
upon the whole is adequately adapted to its conditions of life, is the
simple fact that the species is, to all appearances, preserved
undiminished, and the inference is not at all permissible that just this
protective coloring has selective value for the species, that is, that if
it were lacking, the species would necessarily have perished.

It is not inconceivable that in many species today these colorings are
actually unnecessary for the preservation of the species, that they
formerly were, but that now the enemies which preyed on the resting
butterflies have grown scarce or have died out entirely, and that the
protective coloring will continue to exist by the law of inertia[7] only
for a short while till panmixia or new adaptations shall modify it.

Discouraging, therefore, as it may be, that the control of nature in her
minutest details is here gainsaid us, yet it were equivalent to sacrificing
the gold to the dross, if simply from our inability to follow out the
details of the individual case we should renounce altogether the principle
of selection, or should proclaim it as only subsidiary, on the ground that
we believe the protective coloring of the butterfly is not a protective
coloring, but a combination of colors inevitably resulting from internal
causes. The protective coloring remains a protective coloring whether at
the time in question it is or is not necessary for the species; and it
arose as protective coloring--arose not because it was a constitutional
necessity of the animal's organism that here a red and there a white,
black, or yellow spot should be produced, but because it was {16}
advantageous, because it was necessary for the animal. There is only one
explanation possible for such patent adaptations and that is selection.
What is more, no other natural way of their originating is conceivable, for
we have no right to assume teleological forces in the domain of natural
phenomena.

I have selected the example of the butterfly's wing, not solely because it
is so widely known, but because it is so exceedingly instructive, because
we are still able to learn so much from it. It has been frequently asserted
that the color-patterns of the butterfly's wings have originated from
internal causes, independently of selection and conformably to inward laws
of evolution. Eimer has attempted to prove this assertion by establishing
in a division of the genus Papilio the fact that the species there admit of
arrangement in series according to affinity of design. But is a proof that
the markings are modified in definite directions during the course of the
species's development equivalent to a definite statement as to the _causes_
that have produced these gradual transformations? Or, is our present
inability to determine with exactness the biological significance of these
markings and their modifications, a proof that the same have no
significance whatever? On the contrary, I believe it can be clearly proved
that the wing of the butterfly is a tablet on which nature has inscribed
everything she has deemed advantageous to the preservation and welfare of
her creatures, and nothing else; or, to abandon the simile, that these
color-patterns have not proceeded from inward evolutional forces, but are
the result of selection. At least in all places where we do understand
their biological significance these patterns are constituted and
distributed over the wing exactly as utility would require. {17}

I do not pledge myself, of course, to give an explanation of every spot and
every line on a wing. The inscription is often a very complicated one,
dating from remote and widely separated ages; for every single existing
species has inherited the patterns of its ancestral species and that again
the patterns of a still older species. Even at its origin, therefore, the
wing was far from being a _tabula rasa_, but was a closely written and
fully covered sheet, on which there was no room for new writing until a
portion of the old had been effaced. But other parts were preserved, or
only slightly modified, and thus in many cases gradually arose designs of
almost undecipherable complexity.

I should be far from maintaining that the markings arose unconformably to
law. Here, as elsewhere, the dominance of law is certain. But I take it,
that the laws involved here, that is, the physiological conditions of the
variation, are without exception subservient to the ends of a higher
power--utility; and that it is utility primarily that determines the kind
of colors, spots, streaks and bands that shall originate, as also their
place and mode of disposition. The laws come into consideration only to the
extent of conditioning the quality of the constructive materials--the
variations, out of which selection fashions the designs in question. And
this also is subject to important restrictions, as will appear in the
sequel.

The meaning of formative laws here is that definite spots on the surfaces
of the wings are linked together in such a manner by inner, invisible
bonds, as to represent the same spots or streaks, so that we can predict
from the appearance of a point at one spot the appearance of another
similar point at another, and {18} so on. It is an undoubted fact that such
relations exist, that the markings frequently exhibit a certain symmetry,
that--to use the words of the most recent observer on this subject,
Bateson[8]--a meristic representation of equivalent design-elements occurs.
But I believe we should be very cautious in deducing laws from these facts,
because all the rules traceable in the markings apply only to small groups
of forms and are never comprehensive nor decisive for the entire class or
even for the single sub-class of diurnal butterflies, in fact, often not so
for a whole genus. All this points to special causes operative only within
this group.

If internal laws controlled the marking on butterflies' wings, we should
expect that some general rule could be established, requiring that the
upper and under surfaces of the wings should be alike, or that they should
be different, or that the fore wings should be colored the same as or
differently from the hind wings, etc. But in reality all possible kinds of
combinations occur simultaneously, and no rule holds throughout. Or, it
might be supposed that bright colors should occur only on the upper surface
or only on the under surface, or on the fore wings or only on the hind
wings. But the fact is, they occur indiscriminately, now here, now there,
and no one method of appearance is uniform throughout all the species. But
the fitness of the various distributions of colors is apparent, and the
moment we apply the principle of utility we know why in the diurnal
butterflies the upper surface alone is usually variegated and the under
surface protectively colored, or why in the nocturnal {19} butterflies the
fore wings have the appearance of bark, of old wood, or of a leaf, whilst
the hind wings, which are covered while resting, alone are brilliantly
colored. On this theory we also understand the exceptions to these rules.
We comprehend why Danaids, Heliconids, Euploids, and Acracids, in fact all
diurnal butterflies, offensive to the taste and smell, are mostly brightly
marked and equally so on both surfaces, whilst all species not thus exempt
from persecution have the protective coloring on the under surface and are
frequently quite differently colored there from what they are on the upper.

In any event, the supposed formative laws are not obligatory. Dispensations
from them can be issued and are issued _whenever utility requires it_.
Indeed, so far may these transgressions of the law extend, that in the very
midst of the diurnal butterflies is found a genus, the South American
Ageronia, which, like the nocturnal butterfly, shows on the entire _upper_
surface of both wings a pronounced bark-coloration, and concerning which we
also know (and in this respect it is an isolated genus and differs from
almost all other diurnal butterflies), that it spreads out its wings when
at rest like the nocturnal butterfly, and does not close them above it as
its relatives do. Therefore, entirely apart from cases of mimicry, which
after all constitute the strongest proof, the facts here cited are alone
sufficient to remove all doubt that not inner necessities or so-called
formative laws have painted the surface of the butterflies' wings, but that
the conditions of life have wielded the brush.

This becomes more apparent on considering the details. I have remarked that
the usually striking colorations of exempt butterflies, as of the
Heliconids, {20} are the same on both the upper and the lower surfaces of
the wings. Possibly the expression of a law might be seen in this fact, and
it might be said, the coloration of the Heliconids _runs through_ from the
upper to the under surface. But among numerous imitators of the Heliconids
is the genus Protogonius, which has the coloration of the Heliconids on its
upper surface, but on its lower exhibits a magnificent leaf-design. During
flight it appears to be a Heliconid and at rest a leaf. How is it possible
that two such totally different types of coloration should be combined in a
single species, if any sort of _inner_ rigorous necessity existed,
regulating the coloration of the two wing-surfaces? Now, although we are
unable to prove that the Protogonius species would have perished unless
they possessed this duplex coloration, yet it would be nothing less than
intellectual blindness to deny that the butterflies in question are
effectively protected, both at rest and during flight, _that their
colorations are adaptive_. We do not know their primitive history, but we
shall hardly go astray if we assume that the ancestors of the Protogonius
species were forest-butterflies and already possessed an under surface
resembling a leaf. By this device they were protected when at rest.
Afterwards, when this protection was no longer sufficient, they acquired on
their upper surface the coloration of the exempt species with which they
most harmonised in abode, habits of life, and outward appearance.

At the same time it is explained why these butterflies did not acquire the
coloration of the Heliconids on the under surface. The reason is, that in
the attitude of repose they were already protected, and that in an
admirable manner. {21}

That _exempt_ diurnal butterflies should be colored on the upper and under
surfaces alike, and should never resemble in the attitude of repose their
ordinary surroundings, is intelligible when we reflect that it is a much
greater protection to be despised when discovered than to be well, or very
well, but never absolutely, protected from discovery.

It has been so often reiterated that diurnal butterflies, as a rule, are
protectively colored on the under surfaces, that one has some misgivings in
stating the fact again. And yet the least of those who hold this to be a
trivial commonplace know how strongly its implications militate against the
inner motive and formative forces of the organism, which are ever and anon
appealed to. No less than sixty-two genera are counted today in the family
of diurnal butterflies known as the Nymphalidæ. Of these by far the largest
majority are sympathetically colored underneath, that is, they show in the
posture of rest the colorings of their usual environment. In a large number
of the species belonging to this group the entire surface of the hind wings
possesses such a sympathetic coloration, as does also the distant apex of
the fore wings. Why? The reason is obvious. This part only of the fore wing
is visible in the attitude of repose. Here, then,--as a zealous opponent of
the theory of selection once exclaimed,--there is undoubted "correlation"
between the coloring of the surface of the hind wing and of the apex of the
fore wing. Correlation is unquestionably a fine word, but in the present
instance it contributes nothing to the understanding of the problem, for
there are near relatives and often species of the same genera in which this
correlation is not restricted to the apex of the {22} fore wings, but
extends to a third or even more of their wings, and these species are also
in the habit of drawing back their wings less completely in the state of
rest, thus rendering a larger portion of them visible. There are species,
too, like the forest-butterflies of South America just mentioned, the
Protogonius, Anæa, Kallima species, etc., which have nearly the _whole_ of
the under surfaces of their fore wings marked according to the same pattern
with their hind wings, and these butterflies when at rest hold their fore
wings free and uncovered by their hind wings. Where are the formative laws
in such cases?

Or, perhaps some one will say: "The covering by the hind wings hinders the
formation of scales on the wing, or impedes the formation of the colors in
the scales." Such a person should examine one of these species. He will
find that the scales are just as dense on the covered as on the uncovered
surface of the wing, and in many species, for example, in Katagramma, the
scales of the covered surface are colored most brilliantly of all.

But the facts are still more irresistible, when we consider _special
adaptations_; for example, the imitation of leaves, which is so often
cited. It is to be noted, first, that this sort of imitation is by no means
restricted to a few genera, still less to a few species. All the numerous
species of the genus Anæa, which are distributed over the forests of
tropical South America, exhibit this imitation in pronounced and varied
forms, as do likewise the American genera Hypna and Siderone, the Asiatic
Symphaedra, the African Salamis, Eurypheme, etc. I have observed
fifty-three genera in which it is present in one, several, or in many
species, but there are many others. {23}

These genera, now, are by no means all so nearly allied that they could
have inherited the leaf-markings from a common ancestral form. They belong
to different continents and have probably for the most part acquired their
protective colorings themselves. But one resemblance they have in
common--they are all _forest-butterflies_. Now what is it that has put so
many genera of forest-butterflies and no others into positions where they
could acquire this resemblance to leaves? Was it directive formative laws?
If we closely examine the markings by which the similarity of the leaf is
determined, we shall find, for example, in Kallima Inachis, and Parallecta,
the Indian leaf-butterflies, that the leaf-markings are executed _in
absolute independence of the other uniformities governing the wing_.

From the tail of the wing to the apex of the fore wings runs with a
beautiful curvature a thick, doubly-contoured dark line accompanied by a
brighter one, representing the midrib of the leaf. This line cuts the
"veins" and the "cells" of the wing in the most disregardful fashion, here
in acute and here in obtuse angles, and in absolute independence of the
regular system of divisions of the wing, which should assuredly be the
expression of the "formative law of the wing," if that were the product of
an internal directive principle. But leaving this last question aside, this
much is certain with regard to the markings, that they are dependent, not
on an _internal_, but on an _external_ directive power.

Should any one be still unconvinced by the evidence we have adduced, let
him give the leaf-markings a closer inspection. He will find that the
midrib is composed of two pieces of which the one belongs to the {24} hind
wing and the other to the fore wing, and that the two fit each other
exactly when the butterfly is in the attitude of repose, but not otherwise.
Now these two pieces of the leaf-rib do not begin on corresponding spots of
the two wings, but on absolutely non-identical spots. And the same is also
true of the lines which represent the lateral ribs of the leaf. These lines
proceed in acute angles from the rib; to the right and to the left in the
same angle, those of the same side parallel with each other. Here, too, no
relation is noticeable between the parts of the wings over which the lines
pass. The venation of the wing is utterly ignored by the leaf-markings, and
its surface is treated as a _tabula rasa_ upon which anything conceivable
can be drawn. In other words, we are presented here with a _bilaterally
symmetrical_ figure engraved on a surface which is essentially _radially
symmetrical_ in its divisions.

I lay unusual stress upon this point because it shows that we are dealing
here with one of those cases which cannot be explained by mechanical, that
is, by natural means, unless natural selection actually exists and is
actually competent to create new properties; for the Lamarckian principle
is excluded here _ab initio_, seeing that we are dealing with a formation
which is only passive in its effects; the leaf-markings are effectual
simply by their existence and not by any function which they perform; they
are present in flight as well as at rest, during the absence of danger, as
well as during the approach of an enemy.

Nor are we helped here by the assumption of _purely internal motive
forces_, which Nägeli, Askenasy, and others have put forward as supplying a
_mechanical_ force of evolution. It is impossible to regard the {25}
coincidence of an Indian butterfly with the leaf of a tree now growing in
an Indian forest as fortuitous, as a _lusus naturæ_. Assuming this
seemingly mechanical force, therefore, we should be led back inevitably to
a teleological principle which produces adaptive characters and which must
have deposited the directive principle in the very first germ of
terrestrial organisms, so that after untold ages at a definite time and
place the illusive leaf-markings should be developed. The assumption of
pre-established harmony between the evolution of the ancestral line of the
tree with its pre-figurative leaf, and that of the butterfly with its
imitating wing, is absolutely necessary here--a fact which I pointed out
many years ago,[9] but which is constantly forgotten by the promulgators of
the theory of internal evolutionary forces.

For the present I leave out of consideration altogether the question as to
the conceivable extent of the sphere of operation of natural selection; I
am primarily concerned only with elucidating the process of selection
itself, wholly irrespective of the comprehensiveness or limitedness of its
sphere of action. For this purpose it is sufficient to show, as I have just
done, _that cases exist wherein all natural explanations except that of
selection fail us_. But let us now see how far the principle of selection
will carry us in the explanation of such cases--natural selection, I mean,
as it was formulated by Darwin and Wallace.

There can be no doubt but the leaf-markings readily admit of production in
this manner, slowly and with a gradual but constant increase of fidelity,
provided a single condition is fulfilled: _the occurrence of the {26} right
variations at the right place_. But just here, it would seem, is the
insurmountable barrier to the explanatory power of our principle, for who,
or what, is to be our guarantee that dark scales shall appear at the exact
spots on the wing where the midrib of the leaf must grow? And that later
dark scales shall appear at the exact spots to which the midrib must be
prolonged? And that still later such dark spots shall appear at the places
whence the lateral ribs start, and that here also a definite acute angle
shall be accurately preserved, and the mutual distances of the lateral ribs
shall be alike and their courses parallel? And that the prolongation of the
median rib from the hind wing to the fore wing shall be extended exactly to
that spot where the fore wing is not covered by the hind wing in the
attitude of repose? And so on.

If I could go more minutely into this matter, I should attempt to prove
that the markings, as I have just assumed, have not arisen suddenly, but
were perfected very, very gradually; that in one species they began on the
fore wing and in another on the hind wing; and that in many they never
until recently proceeded beyond one wing, in other species they went only a
little way, and in only a few did they spread over the entire surface of
both wings.

That these markings advanced slowly and gradually, but with marvelous
accuracy, is no mere conjecture. But it follows that the right variations
at the right places must never have been wanting, or, as I expressed it
before: _the useful variations were always present_. But how is that
possible in such long extensive lines of dissimilar variations as have
gradually come to constitute markings of the complexity here presented?
Suppose that the useful colors had not {27} appeared at all, or had not
appeared at the right places? It is a fact that in constant species, that
is, in such as are not in process of transformation, the variations of the
markings are by no means frequent or abundant. Or, suppose that they had
really appeared, but occurred only in individuals, or in a small percentage
of individuals?

Such are the objections raised against the theory of selection by its
opponents, and put forward as insurmountable obstacles to the process. Nor
are such objections relevant only in the case of protective colorings; they
are applicable in all cases where the process of selection is concerned.
Take the case of instincts that are called into action only once in life,
as, for example, the pupal performances of insects, the artificial
fabrication of cocoons, etc. How is it that the useful variations were
always present here? And yet they must have been present, if such
complicated spinning instincts could have taken their rise as are
observable in the silk-worm, or in the emperor-moth. And they have been
developed, and that in whole families, in forms varying in all species, and
in every case adapted to the special wants of the species.

Particularly striking is the proof afforded of this constant presence of
the useful variations by cases where we meet with the development of highly
special adaptations that are uncommon even for the group of organisms
concerned. Such a case, for example, is the apparatus designed for the
capture of small animals and their digestion, found in widely different
plants and widely separated families. On the other hand, very common
adaptations, such as the eyes of animals, show distinctly that in all cases
where it was necessary, the useful variations for the formation of {28} an
eye were presented, and were presented further exactly at spots at which
organs of vision could perform their best work: thus, in Turbellaria and
many other worms that live in the light, at the anterior extremity of the
body and on the dorsal surface; in certain mussels, on the edge of the
mantle; in terrestrial snails, on the antennæ; in certain tropical marine
snails inhabiting shallow waters, on the back; and in the chitons even on
the dorsal surface of the shell!

But even taking the very simplest cases of selection, it is impossible to
do without this assumption, that the useful variations are always present,
or that _they always exist in a sufficiently large number of individuals
for the selective process_. You know the thickness and power of resistance
of the egg-shells of round-worms. The eggs of the round-worms of horses
have been known to continue their course of development undisturbed even
after they had been thrown into strong alcohol and all other kinds of
injurious liquids--much to the vexation of the embryologists, who wished to
preserve a definite stage of development and sought to kill the embryo at
that stage. Indeed, think of the result, if in the course of their
phylogenesis stout and resistant variations of egg-shells had not been
presented in these worms, or had not always been presented, or had not been
presented in every generation and not in sufficient quantities.

The cogency of the facts is absolutely overpowering when we consider that
practically no modification occurs _alone_, that every primary modification
brings in its train secondary ones, and that these induce forced
modifications in many parts of the body, frequently of the most
diversified, or even self-contradictory, forms. Recently Herbert Spencer
has drawn {29} fresh attention to these secondary modifications, which must
always occur in harmony with the primary one, and has, as he thinks,
advanced in this set of facts, a convincing disproof of the contention that
such coadaptive modifications of numerous cofunctioning parts can rest on
natural selection. Now, although I deem his conclusion precipitate, yet the
very fact of a simultaneous, functionally concordant, yet essentially
diversified modification of numerous parts, points conclusively to the
circumstance that _something is still wanting to the selection of Darwin
and Wallace, which it is obligatory on us to discover, if we possibly can_,
and without which selection as yet offers no complete explanation of the
phyletic processes of transformation. There is a hidden secret to be
unriddled here before we can obtain a satisfactory insight into the
phenomena in question. _We must seek to discover why it happens that the
useful variations are always present._

Herbert Spencer appealed to Lamarck's principle for the explanation of
coadaptation, and it is certain that functional adaptation is operative
during the individual life, and that it compensates in a certain measure
the inequalities of the inherited constitutions. I shall not repeat what I
have said before on this subject, nor maintain, in refutation of Spencer's
contention, that functional adaptation is itself nothing more than the
efflux of _intra-biontic_ selective processes, as Spencer himself once
suggested in a prophetic moment, but which it was left for Wilhelm Roux to
introduce into science as "the struggle of the parts" of organisms.[10] I
shall only remark that if functional adaptations were themselves
inheritable, this would still be insufficient {30} for the explanation of
coadaptation, for the reason that precisely similar coadaptive
modifications occur in _purely passively_ functioning parts, in which,
consequently, modification _by_ function is excluded. This is the case with
the skeletal parts of Articulata; e. g., it is true of their articular
surfaces with their complex adaptations to the most varied forms of
locomotion. In all these cases the ready-made, hard, unalterable, chitinous
part is _first_ set into activity; consequently its adaptation to the
function must have been _previously_ effected, independently of that
function. These joints, and divers other parts, accordingly, have been
developed in the precisest manner for the function, and the latter could
have had no direct share in their formation. When we consider, now, that it
is impossible that every one of the numerous surfaces, ridges, furrows, and
corners found in a single such articulation, let alone in all the
articulations of the body, should hold in its hands the power of life and
death over individuals for untold successions of generations, the fact is
again unmistakably impressed upon our attention that the conception of the
selective processes which has hitherto obtained is insufficient, that the
root of the process in fact lies deeper, that it is to be found in the
place where it is determined what variations of the parts of the organism
shall appear--namely _in the germ_.

The phenomena observed in the _stunting_, or _degeneration_, _of parts
rendered useless_, point to the same conclusion. They show distinctly that
ordinary selection which operates by the removal of entire persons,
_personal selection_, as I prefer to call it, cannot be the only cause of
degeneration; for in most cases of degeneration it cannot be assumed that
slight individual {31} vacillations in the size of the organ in question
have possessed selective value. On the contrary, we see such retrogressions
affected apparently _in the shape of a continuous evolutionary process
determined by internal causes_, in the case of which there can be no
question whatever of selection of persons or of a survival of the fittest,
that is, of individuals with the smallest rudiments.

It is this consideration principally that has won so many adherents for the
Lamarckian principle in recent times, particularly among the
paleontologists. They see the outer toes of hoofed animals constantly and
steadily degenerating through long successions of generations and species,
concurrently with the re-enforcement of one or two middle toes, which are
preferred or are afterwards used exclusively for stepping, and they believe
correctly enough that these results should not be ascribed to the effects
of personal selection alone. They demand a principle which shall effect the
degeneration by internal forces, and believe that they have found it in
functional adaptation.[11] {32} On this last point, now, I believe, they
are mistaken, be they ever so strongly convinced of the correctness of
their view and ever so aggressive and embittered in their defence of it.

Recently, an inquirer of great caution and calmness of judgment, Prof. C.
Lloyd Morgan, has expressed the opinion that the Lamarckian principle must
at least be admitted as a working hypothesis. But with this I cannot agree,
at least as things stand at present. A working hypothesis may be false, and
yet lead to further progress; that is, it may constitute an advance to the
extent of being useful in formulating the problem and in illuminating paths
that are likely to lead to results. But it seems to me that a hypothesis of
this kind has performed its services and must be discarded the moment it is
found to be at hopeless variance with the facts. If it can be proved that
precisely the same degenerative processes also take place in such
superfluous parts as have only _passive_ and not active functions, as is
the case with the _chitinous parts of the skeleton of Arthropoda_, then it
is a demonstrated fact, that the cessation of functional action is not the
efficient cause of the process of degeneration. At once your legitimate
working hypothesis is transformed into an illegitimate dogma--illegitimate
because it no longer serves as a guide on the path to knowledge but {33}
blocks that path. For the person who is convinced he has found the right
explanation is not going to seek for it.

I can understand perfectly well the hesitation that has prevailed on this
point in many minds, from their having seen _one_ aspect of the facts more
distinctly than the other. From this sceptical point of view Osborn has
drawn the following perfectly correct conclusion: "If acquired variations
are transmitted, there must be some unknown principle in heredity; if they
are not transmitted, there must be some unknown factor in evolution."[12]

Such in fact is the case and I shall attempt to point out to you what this
factor is. My inference is a very simple one: if we are forced by the facts
on all hands to the assumption that the useful variations which render
selection possible are always present, then _some profound connection must
exist between the utility of a variation and its actual appearance_, or, in
other words, _the direction of the variation of a part must be determined
by utility_, and we shall have to see whether facts exist that confirm our
conjecture.

The facts do indeed exist and lie before our very eyes, despite their not
having been recognised as such before. All _artificial selection_ practised
by man rests on the fact that by means of the selection of individuals
having a given character slightly more pronounced than usual, there is
gradually produced a general augmentation of this character, which
subsequently reaches a point never before attained by any individual {34}
of this species. I shall choose an example which seems to me especially
clear and simple because only one character has been substantially modified
here. The long-tailed variety of domestic cock, now bred in Japan and
Corea, owes its existence to skilful selection and not at all to the
circumstance that at some period of the race's history a cock with
tail-feathers six feet in length suddenly and spasmodically appeared. At
the present day even, as Professor Ishikawa of Tokio writes me, the
breeders still make extraordinary efforts to increase the length of the
tail, and every inch gained adds considerably to the value of the bird. Now
nothing has been done here whatever except always to select for purposes of
breeding the cocks with the longest feathers; and in this way alone were
these feathers, after the lapse of many generations, prolonged to a length
far exceeding every previous variation.

I once asked a famous dove-fancier, Mr. W. B. Tegetmeier of London, whether
it was his opinion that by artificial selection alone a character could be
augmented. He thought a long time and finally said: "It is without our
power to do anything if the variation which we seek is not presented, but
once that variation is given, then I think the augmentation can be
effected." And that in fact is the case. If cocks had never existed whose
tail-feathers were a little longer than usual the Japanese breed could
never have originated; but as the facts are, always the cocks with the
longest feathers were chosen from each generation, and these only were
bred, and thus a hereditary augmentation of the character in question was
effected, which would hardly have been deemed possible.

Now what does this mean? Simply that the {35} hereditary diathesis, the
constitutional predisposition (_Anlage_) of the breed was changed in the
respect in question, and our conclusion from this and numerous similar
facts of artificial selection runs as follows: _by the selection alone of
the plus or minus variations of a character is the constant modification of
that character in the plus or minus direction determined._ Obviously the
hereditary _diminution_ of a part is also effected by the simple selection
of the individuals in each generation possessing the smallest parts, as is
proved, for example, by the tiny bills and feet of numerous breeds of
doves. We may assert, therefore, in general terms: a definitely directed
progressive variation of a given part is produced by continued selection in
that definite direction. This is no hypothesis, but a direct inference from
the facts and may also be expressed as follows: _By a selection of the kind
referred to the germ is progressively modified in a manner corresponding
with the production of a definitely directed progressive variation of the
part._

In this general form the proposition is not likely to encounter opposition,
as certainly no one is prepared to uphold the view that the germ remains
unchanged whilst the products proceeding from it, its descendants, are
modified. On the contrary, all will agree when I say that the germ in this
case must have undergone modifications, and that their character must
correspond with the modifications undergone by its products. Thus far,
then, we find ourselves, not on the ground of the hypothesis that has been
lately so much maligned, but on the ground of facts and of direct
inferences from facts. But if we attempt to pierce deeper into the problem,
we are in need of the hypothesis. {36}

The first and most natural explanation will be this--that through selection
the zero-point, about which, figuratively speaking, the organ may be said
to oscillate in its plus and minus variations, is displaced upwards or
downwards. Darwin himself assumed that the variations oscillated about a
mean point, and the statistical researches of Galton, Weldon, and others
have furnished a proof of the assumption. If selection, now, always picks
out the plus variations for imitation, perforce, then, the mean or
zero-point will be displaced in the upward direction, and the variations of
the following generation will oscillate about a higher mean than before.
This elevation of the zero-point of a variation would be continued in this
manner until the total equilibrium of the organism was in danger of being
disturbed.

There is involved here, however, an assumption which is by no means
self-evident, that every advancement gained by the variation in question
constitutes a new centre for the variations occurring in the following
generation. _That this is a fact_, is proved by such actual results of
selection as are obtained in the case of the Japanese cock. But the
question remains, Why is this the fact?

Now here, I think, my theory of determinants gives a satisfactory answer.
According to that theory every independently and hereditarily variable part
is represented in the germ by a _determinant_, that is by a determinative
group of vital units, whose size and power of assimilation correspond to
the size and vigor of the part. These determinants multiply, as do all
vital units, by growth and division, and necessarily they increase rapidly
in every individual, and the more rapidly the greater the quantity of the
germinal cells {37} the individual produces. And since there is no more
reason for excluding irregularities of passive nutrition, and of the supply
of nutriment in these minute, microscopically invisible parts, than there
is in the larger visible parts of the cells, tissues, and organs,
consequently the descendants of a determinant can never all be exactly
alike in size and capacity of assimilation, but they will oscillate in this
respect to and fro about the maternal determinant as about their
zero-point, and will be partly greater, partly smaller, and partly of the
same size as that. In these oscillations, now, the material for further
selection is presented, and in the inevitable fluctuations of the nutrient
supply I see the reason why every stage attained becomes immediately the
zero-point of new fluctuations, and consequently why the size of a part can
be augmented or diminished by selection without limit, solely by the
displacement of the zero-point of variation as the result of selection.

We should err, however, if we believed that we had penetrated to the root
of the phenomenon by this insight. There is certainly some other and
mightier factor involved here than the simple selection of persons and the
consequent displacement of the zero-point of variation. It would seem,
indeed, as if in one case, _videlicet_, in that of the Japanese cock, the
augmentation of the character in question were completely explained by this
factor _alone_. In fact, in this and similar cases we cannot penetrate
deeper into the processes of variation, and therefore cannot say _a priori_
whether other factors have or have not been involved in the augmentation of
the character in question--other characters, that is, than the simple
displacement of the zero-point. There is, however, another class of
phyletic modifications, which point {38} unmistakably to the conclusion
that the displacement of the zero-point of variation by personal selection
is not and cannot be the only factor in the determination and
accomplishment of the direction of variation. I refer to _retrogressive
development_, the gradual degeneration of parts or characters that have
grown useless, the gradual disappearance of the eye in cave-animals, of the
legs in snakes and whales, of the wings in certain female butterflies, in
short, to that entire enormous mass of facts comprehended under the
designation of "rudimentary organs."

I have endeavored on a previous occasion to point out the significance of
the part played in the great process of animate evolution by these
retrogressive growths, and I made at the time the statement that "the
phenomena of retrogressive growth enabled us in a greater measure almost
than those of progressive growth to penetrate to the causes which produce
the transformations of animate nature." Although at that time[13] I had no
inkling of certain processes which today I shall seek to prove the
existence of, yet my statement receives a fresh confirmation from these
facts.

For, in most retrogressive processes _active_ selection in Darwin's sense
plays no part, and advocates of the Lamarckian principle, as above
remarked, have rightly denied that active selection, that is, the selection
of individuals possessing the useless organ in its most reduced state, is
sufficient to explain the process of degeneration. I, for my part, have
never assumed this, {39} and I enunciated precisely on this account the
_principle of panmixia_. Now, although this, as I still have no reason for
doubting, is a perfectly correct principle, which really does have an
essential and indispensable share in the process of retrogression, still it
is not _alone_ sufficient for a full explanation of the phenomena. My
opponents, in advancing this objection, were right, to the extent indicated
and as I expressly acknowledge, although they were unable to substitute
anything positive in its stead or to render my explanation complete. The
very fact of the cessation of control over the organ is sufficient to
explain its _degeneration_, that is, its deterioration, the disharmony of
its parts, but not the fact which actually and always occurs where an organ
has become useless--viz., _its gradual and unceasing diminution continuing
for thousands and thousands of years culminating in its final and absolute
effacement._

If, now, neither the selection of persons nor the cessation of personal
selection can explain this phenomenon, assuredly some other principle must
be the efficient cause here, and this cause I believe I have indicated in
an essay written at the close of last year and only recently published.[14]
I call it _germinal selection_.

The principle in question reposes on the application, made some fifteen
years ago by Wilhelm Roux, of the principle of selection to the _parts_ of
organisms--on the _struggle of the parts_, as he called it. If such a
struggle obtains among organs, tissues, and cells, it must also obtain
between the smallest and for us invisible vital particles, not only between
those of the body-cells, strictly so called, but also between those of the
{40} germinal cells. Roux himself spoke of the struggle of the molecules,
by which he presumably understood the smallest ultimate units of vital
phenomena--elements which De Vries designated pangenes, Wiesner plasomes,
and I _biophores_, after Brücke's ingenious conception[15] of these
invisible entities had been almost totally forgotten, or at least had lain
unnoticed for thirty years. No struggle, as that is understood in the
theory of selection, could take place between real {41} molecules, for
molecules are neither nourished, subject to growth, nor propagated.

The gradual degeneration of organs grown useless may be explained, now, by
the theory of determinants very simply and without any co-operation on the
part of active personal selection, as follows.

Nutrition, it is known, is not merely a passive process. A part is not only
_nourished_ but also actively _nourishes_ itself, and the more vigorously,
the more powerful and capable of assimilation it is. Hence powerful
determinants in the germ will absorb nutriment more rapidly than weaker
determinants. The latter, accordingly, will grow more slowly and will
produce weaker descendants than the former.

Let us assume, now, that a part of the body, say the hinder extremities of
the quadruped ancestors of {42} our common whales, are rendered useless.
Panmixia steps in, _i. e._, selection ceases to influence these organs.
Individuals with large and individuals with small hind legs are equally
favored in the struggle for existence.

From this fact alone would result a degradation of the organ, but of course
it would not be very marked in extent, seeing that the minus variations
which occur are no longer removed. According to our assumption, however,
such minus variations repose on the weaker determinants of the germ, that
is, on such as absorb nutriment less powerfully than the rest. And since
every determinant battles stoutly with its neighbors for food, that is,
takes to itself as much of it as it can, consonantly with its power of
assimilation and proportionately to the nutrient supply, therefore the
unimpoverished neighbors of this minus determinant will deprive it of its
nutriment more rapidly than was the case with its more robust ancestors;
hence, it will be unable to obtain the full quantum of food corresponding
even to its weakened capacity of assimilation, and the result will be that
its ancestors will be weakened still more. Inasmuch, now, as no weeding out
of the weaker determinants of the hind leg by personal selection takes
place on our hypothesis, inevitably the average strength of this
determinant must slowly but constantly diminish, that is, the leg must grow
smaller and smaller until finally it disappears altogether. The
determinants[16] of the useless organ are constantly at {43} a disadvantage
as compared with the determinants of their environment in the germinal
tenement, because no assistance is offered to them by personal selection
after they have once been weakened by a decrease of the passive nutrient
influx. Nor is the degeneration stopped by the uninterrupted crossing of
individuals in sexual propagation, but only slightly retarded. The number
of individuals with weaker determinants must, despite this fact, go on
increasing from generation to generation, so that soon every determinant
that still happens to be endowed with exceptional vigor will be confronted
by a decided overplus of weaker determinants, and by continued crossing
therefore will become more and more impoverished. Panmixia is the
indispensable precondition of the whole process; for owing to the fact that
persons with weak determinants are just as capable of life as those with
strong, owing to the fact that they cannot now, as formerly, when the organ
was still useful, be removed by personal selection, solely by this means is
a further weakening effected in the following generations--in short, only
by this means are the determinants of the useless organ brought upon the
inclined plane, down which they are destined slowly but incessantly to
slide towards their completed extinction.

The foregoing explanation will be probably accepted as satisfactory _in a
purely formal regard_, but it will be objected that, even granting this, it
has not yet been proved to be the correct one. In answer I can of course
adduce nothing except that it is at present the only one that can be given.
It may be that the actual state of things in nature is different, but if it
can be shown that a self-direction of variation merely from the need of it
is at all conceivable by mechanical means, {44} that in itself, it seems to
me, is a decided gain. It must also not be forgotten that some process or
other _must_ take place in the germ-plasm when an organ becomes
rudimentary, and that as the result of it this organ, and only this organ,
must disappear. Now in what shall this process consist, if not in a
modification of the constitution of the germ? And how could the effect of
such a modification be limited only to _one_ organ which was becoming
rudimentary if the modification itself were not a local one? These are
questions which it is incumbent on those to answer who conceive the
germinal substance to be composed of like units.

Applying, now, the explanation derived from the disappearance of organs to
the opposed transformation, namely, to the _enlargement_ of a part, the
presumption lies close at hand that the production of the long
tail-feathers of the Japanese cock does not repose solely on the
displacement directly effected by personal selection, of the zero-point of
variation upwards, but that _it is also fostered and strengthened by
germinal selection_. Were that not so, the phenomena of the transmutation
of species, in so far as fresh growth and the enlargement and complication
of organs already present are concerned, _would not be a whit more
intelligible than they were before_. We should know probably how it comes
to pass that the constitutional predisposition (group of determinants) of a
_single_ organ is intensified by selection, but the flood of objections
against the theory of selection touching its inability to modify _many_
parts at once would not be repressed by such knowledge. The initial impulse
conditioning the independent maintenance of the useful direction of
variation in the germ-plasm must rather be sought {45} in the utility of
the modification itself, and this also seems to me intelligible from the
side of the theory. For as soon as personal selection favors the more
powerful variations of a determinant, the moment that these come to
predominate in the germ-plasm of the species, at once the tendency must
arise for them to vary _still more strongly_ in the plus direction, not
solely because the zero-point has been pushed farther upwards, but because
they themselves now oppose a relatively more powerful front to their
neighbors, that is, actively absorb more nutriment, and upon the whole
increase in vigor and produce more robust descendants. From the relative
vigor or dynamic status of the particles of the germ-plasm, thus, will
issue spontaneously an ascending line of variation, precisely as the facts
of evolution require. For, as I have already said, it is not sufficient
that the augmentation of a character should be brought about by
uninterrupted personal selection, even supposing that the displacement of
the zero-point were possible without germinal selection.

Thus, I think, may be explained how personal selection imparts the initial
impulse to processes in the germ-plasm, which, when they are once set
agoing, persist of themselves in the same direction, and are, therefore, in
no need of the continued supplementary help of personal selection, _as
directed exclusively to a definite part_. If but from time to time, that
is, if upon the average the poorest individuals, the bearers of the weakest
determinants, are eliminated, the variational direction of the part in
question, now reposing on germinal selection, must persist, and it will
very slowly but very surely increase until further development is impeded
by its inutility and personal selection {46} arrests the process, that is,
ceases to eliminate the weaker individuals.

In this manner it becomes intelligible how a large number of modifications
varying in kind and far more so in degree can be guided _simultaneously_ by
personal selection; how in strict conformity with its adaptive wants every
part is modified, or preserved unmodified; how a given articulation can
undergo modifications, causing it to disappear on one side, to grow in
volume on another, and to continue unaltered on a third. For every part
that is perfectly adapted, although it can fluctuate slightly, yet can
never undergo a permanent alteration in the ascending or descending
direction because every plus and every minus variation which has attained
selective value would be eliminated by personal selection in the course of
time. Therefore, a definite direction of variation cannot arise in such
cases and we have also reached, as it seems to me, a satisfactory
explanation of the _constancy_ of well-adapted species and characters.

Hitherto I have spoken only of plus and minus variation. But there exist,
as we know, not only variations of size but also variations of _kind_; and
the coloration of the wings of butterflies, which we chose above as our
example, would fall, according to the ordinary usage of speech, under just
this head of variations of quality. The question arises, therefore, Have
the principles just developed any claim to validity in the explanation of
_qualitative_ modifications?

In considering this question it should be carefully borne in mind that by
far the largest part of the qualitative modifications falling under this
head rest on _quantitative_ changes. Of course, chemical transformations,
which usually also involve quantitative {47} alterations, cannot be reduced
to the processes of augmentation described, inasmuch as these, by their
very nature, can be effected only in living elements capable of increase by
propagation; but the interference of selection does not begin originally
with the constitutional predisposition (_Anlagen_) of the germ, i. e. with
the determinants, but with the ultimate units of life, the _biophores_.

A determinant must be composed of heterogeneous biophores, and on their
numerical proportion reposes, according to our hypothesis, their specific
nature. If that proportion is altered, so also is the character of the
determinant. But disturbances of this numerical proportion must result at
once on proof of their usefulness, or as soon as the modifications
determined thereby in the inward character of the determinant turn out to
be of utility. For fluctuations of nutriment and the struggle for
nutriment, with its sequent preference of the strongest, must take place
between the various species of the biophores as well as between the species
of the determinants. But changes in the quantitative ratios of the
biophores appear to us qualitative changes in the corresponding
determinants, somewhat as a simple augmentation of a determinant, for
example, that of a hair, may on its development appear to us as a
qualitative change, a spot on the skin where previously only isolated hairs
stood being now densely crowded with them, and assuming thus the character
of a downy piece of fur. The single hair need not have changed in this
process, and yet the spot has virtually undergone a qualitative
modification. The majority of the changes that appear to us qualitative
rest on invisible _quantitative_ changes, and such can be produced at all
times and _at all stages_ {48} _of the vital units_ by germinal selection.
In a similar manner are induced the most varied qualitative changes of the
corresponding determinants and of the characters conditioned thereby, just
as changes in the numerical proportions of atoms produce essential changes
in the properties of a chemical molecule.

In this way we acquire an approximate conception of the possible mechanical
_modus operandi_ of actual events--namely, of the manner in which the
useful variations required by the conditions of life _can_ always, that is,
very frequently, make their appearance. This possibility is the sole
condition of our being able to understand how different parts of the body,
absolutely undefined in extent, can appear as variational units and vary in
the same or in different directions, according to the special needs of the
case, or as the conditions of life prescribe. Thus, for example, in the
case of the butterfly's wings it rests entirely with utility to decide the
size and the shape of the spots that shall vary simultaneously in the same
direction. At one time the whole under surface of the wing appears as the
variational unit and has the same color; at another the inside half, which
is dark, is contrasted with the outside half which is bright; or the same
contrast will exist between the anterior and posterior halves; or, finally,
narrow stripes or line-shaped streaks will behave as variational units and
form contrasts with manifold kinds of spots or with the broader intervals
between them, with the result that the picture of a leaf or of another
protected species is produced.

I must refrain from entering into the details of such cases and shall
illustrate my views regarding the color-transformations of butterflies'
wings by the simplest {49} conceivable example--viz. that of the uniform
change of color on the entire under surface of the wing.

Suppose, for example, that the ancestral species of a certain
forest-butterfly habitually reposed on branches which hung near the ground
and were covered with dry or rotten leaves; such a species would assume on
its under surface a protective coloring which by its dark, brown, yellow,
or red tints would tend toward similarity with such leaves. If, however,
the descendants of this species should be subsequently compelled, no matter
from what cause, to adopt the habit of resting on the green-leafed branches
higher up, then from that period on the brown coloring would act less
protectively than the shades verging towards green. And a process of
selection will have set in which consisted first in giving preference only
to such persons whose brown and yellow tints showed a tendency to green.
Only on the assumption that such shades were possible by a displacement in
the quantitative proportions of the different kinds of biophores composing
the determinants of the scales affected, was a further development in the
direction of green possible. Such being the case, however, that development
_had to_ result; because fluctuations in the numerical proportions of the
biophores are always taking place, and consequently the material for
germinal selection is always at hand. At present it is impossible to
determine exactly the magnitude of the initial stages of the deviations
thus brought about and promoted by the sexual blending of characters; but
it may perhaps be ascertained in the future, with exceptionally favorable
material. Pending such special observations, however, it can only be said
_a priori_ that slight changes in the composition of a determinant do not
necessarily {50} condition similar slight deviations of the corresponding
character,--in this case the color,--just as slight changes in the atomic
composition of a molecule may result in bestowing upon the latter widely
different properties. As soon, however, as the beginning has been made and
a definite direction has been imparted to the variation, as the result of
this or that primary variation's being preferred, the selective process
must continue until the highest degree of faithfulness required by the
species in the imitation of fresh leaves has been attained.

That the foregoing process has actually taken place is evidenced not only
by the presence of the beginnings of such transformations, as found for
example in some greenish-tinted specimens of Kallima, but mainly by certain
species of the South American genus Catonephele, all of which are
forest-butterflies, and which, with many species having dark-brown under
surfaces, present some also with bright green under surfaces--a green that
is not like the fresh green of our beech and oak trees, but resembles the
bright under surface of the cherry-laurel leaf, and is the color of the
under surfaces of the thick, leathery leaves, colored dark-green above,
borne by many trees in the tropics.

The difference between this and the old conception of the selection-process
consists not only in the fact that a large number of individuals with the
initial stages of the desired variation is present from the beginning, for
always innumerable plus and minus variations exist, but principally in the
circumstance that the constant uninterrupted progress of the process after
it is once begun is assured, that there can never be a lack of
progressively advantageous variations in a large number of individuals.
Selection, {51} therefore, is now not compelled to wait for accidental
variations but produces such itself, whenever the required elements for the
purpose are present. Now, where it is a question simply of the enlargement
or diminution of a part, or of a part of a part, these variations are
always present, and in modifications of quality they are at least present
in many cases.

This is the only way in which I can see a possibility of explaining
phenomena of _mimicry_--the imitation of one species by another. The useful
variations must be produced in the germ itself by internal
selection-processes if this class of facts is to be rendered intelligible.
I refer to the mimicry of an exempt species by two or three other species,
or, the aping of _different_ exempt patterns by _one_ species in need of
protection. It must be conceded to Darwin and Wallace that some degree of
similarity between the copy and the imitation was present from the start,
at least in very many cases;[17] but in no case would this have been
sufficient had not slight shades of coloring afforded some hold for
personal selection, and in this way furnished a basis for independent
germinal selection acting only in the direction indicated. It would have
been impossible for such a minute similarity in the design, and
particularly in the shades of the coloration, ever to have arisen, if the
process of adaptation rested entirely {52} on personal selection. Were this
so, a complete scale of the most varied shades of color must have been
continually presented as variations in every species, which certainly is
not the case. For example, when the exempt species _Acræa Egina_, whose
coloration is a brick-red, a color common only in the genus Acræa, is
mimicked by two other butterflies, a Papilio and a Pseudacræa, so
deceptively that not only the cut of the wings and the pattern of their
markings, but also that precise shade of brick-red, which is scarcely ever
met with in diurnal butterflies, are produced, assuredly such a result
cannot rest on accidental, but must be the outcome of a _definitely
directed_, variation, produced by utility. We cannot assume that such a
coloration has appeared as an _accidental_ variation in just and in only
these two species, which fly together with the _Acræa_ in the same
localities of the same country and same part of the world--the Gold Coast
of Africa. It is conceivable, indeed, that non-directed variation should
have accidentally produced this brick-red _in a single case_, but that it
should have done so three times and in three species, which live together
but are otherwise not related, is a far more violent and improbable
assumption than that of a causal connexion of this coincidence. Now
hundreds of cases of such mimicry exist in which the color-tints of the
copy are met with again in more or less precise and sometimes in
exceedingly exact imitations, and there are thousands of cases in which the
color-tint of a bark, of a definite leaf, of a definite blossom, is
repeated _exactly_ in the protectively colored insect. In such cases there
can be no question of accident, but _the variations presented to personal
selection must themselves have been produced by the principle of the
survival of the_ {53} _fit!_ And this is effected, as I am inclined to
believe, through such profound processes of selection in the interior of
the germ-plasm as I have endeavored to sketch to you to-day under the title
of germinal selection.

I am perfectly well aware how schematic my presentation of this process is,
and must be at present, owing mainly to our inability to gain exact
knowledge concerning the fundamental germinal constituents here assumed.
But I regard its existence as assured, although I by no means underrate the
fact that eminent thinkers, like Herbert Spencer, contest its validity and
believe they are warranted in assuming a germ which is composed of _similar
units_. I strongly doubt whether even so much as a _formal_ explanation of
the phenomena can be arrived at in this manner. So far as direct
observation is concerned, the two theories stand on an equal footing, for
neither my dissimilar, nor Spencer's similar, units of germinal substance
can be _seen_ directly.

The attempt has been recently made to discredit my _Anlagen_, or
constitutional germ-elements, on the ground that they are simply a
subtilised reproduction of Bonnet's old theory of preformation.[18] This
{54} impression is very likely based upon ignorance of the real character
of Bonnet's theory. I will not go into further details here, particularly
as Whitman, in several excellently written and finely conceived essays, has
recently afforded opportunity for every one to inform himself on the
subject. My determinants and groups of determinants have nothing to do with
the preformations of Bonnet; in a sense they are the exact opposites of
them; they are simply _those living parts of the germ whose presence
determines the appearance of a definite organ of a definite character in
{55} the course of normal evolution_. In this form they appear to me to be
an absolutely necessary and unavoidable inference from the facts. There
_must_ be contained in the germ parts that correspond to definite parts of
the complete organism, that is, parts that constitute the reason why such
other parts are formed.

It is conceded even by my opponents that the reason why one egg produces a
chicken and another a duck is not to be sought in external conditions, but
lies in a difference of the germinal substance. Nor can they deny that a
difference of germinal substance must also constitute the reason why a
slight _hereditary_ difference should exist between two filial organisms.
Should there now, in a possible instance, be present between them a second,
a third, a fourth, or a hundredth difference of hereditary character, each
of which could vary from the germ, then, certainly, some second, third,
fourth, or hundredth part of the germ must have been different; for whence,
otherwise, should the heredity of the differences be derived, seeing that
external influences affecting the organism in the course of evolution
induce only non-transmissible and transient deviations? But the fact that
every complex organism is actually composed of a very large number of parts
independently alterable from the germ, follows not only from the comparison
of allied species, but also and principally from the experiments long
conducted by man in artificial selection, and by the consequent and not
infrequent change of only a single part which happens to claim his
interest; for example, the tail-feathers of the cock, the fruit of the
gooseberry, the color of a single feather or group of feathers, and so on.
But a still more cogent proof is furnished by the degeneration of parts
grown {56} useless, for this process can be carried on to almost any extent
without the rest of the body necessarily becoming involved in sympathetic
alteration. Whole members may become rudimentary, like the hind limbs of
the whale, or it may be only single toes or parts of toes; the whole wing
may degenerate in the females of a butterfly species, or only a small
circular group of wing-scales, in the place of which a so-called "window"
arises. A single vein of the wing also may degenerate and disappear, or the
process may affect only a part of it, and this may happen in one sex only
of a species. In such cases the rest of the body may remain absolutely
unaltered; only a stone is taken out of the mosaic.

The assumption, thus, appears to me irresistible, that every such
hereditary and likewise independent and very slight change of the body
rests on some alteration of a _single_ definite particle of the germinal
substance, and not as Spencer and his followers would have it, on a change
of _all_ the units of the germ. If the germinal substance consisted wholly
of like units, then in every change, were it only of a single character,
_each_ of these units would have to undergo exactly the same modification.
Now I do not see how this is possible.

But it may be that Spencer's assumption is the _simpler_ one? Quite the
contrary, its simplicity is merely apparent. Whilst my theory needs for
each modification only a modification of _one_ constitutional element of
the germ, that is, of _one_ particle of the germinal substance, according
to Spencer _every_ particle of that substance must change, for they are all
supposed to be and to remain alike. But seeing that all hereditary
differences, be they of individuals, races, {57} or species, must be
contained in the germ, the obligation rests on these similar units, or
rather the capacity is required of them, to produce in themselves a truly
enormous number of differences. But this is possible only provided their
composition is an exceedingly complex one, or only on the condition that in
every one of them are contained as many alterable particles as according to
my view there are contained determinants in the whole germ. _The
differences that I put into the whole germ, Spencer and his followers are
obliged to put into every single unit of the germinal substance._ My
position on this point appears to me incontrovertible so long as it is
certain that the single characters can vary hereditarily; for, if a thing
can vary independently, that is, _of its own accord_, and _from the germ_,
then that thing must be represented in the germ by some particle of the
substance, _and be represented there in such wise that a change of the
representative particle produces no other change in the organism developing
from the germ than such as are connected with the part which depends on
it_. I conceive that even on the assumption of my constitutional elements
(_Anlagen_) the germ-plasm is complex enough, and that there is no need of
increasing its complexity to a fabulous extent. Be that as it may, the
person who fancies he can produce a complex organism from a _really_ simple
germinal substance is mistaken: he has not yet thoroughly pondered the
problem. The so-called "epigenetic" theory with its _similar_ germinal
units is therefore naught else than an evolution-theory where the primary
constitutional elements are reduced to the molecules and atoms--a view
which in my judgment is inadmissible. A _real_ {58} epigenesis from
absolutely _homogeneous_ and not merely _like_ units is not thinkable.

All value has been denied my doctrine of determinants[19] on the ground
that it only shifts the riddles of evolution to an invisible terrain where
it is impossible for research to gain a foothold.

Now I have indeed to admit that no information can be gained concerning my
determinants, either with the aided or with the unaided eye. But
fortunately there exists in man another organ which may be of use in
fathoming the riddles of nature and this organ which is called the brain
has in times past often borne him out in the assumption of invisible
entities--entities that have not always proved unfruitful for science by
reason of that defect, in proof whereof we may instance the familiar
assumptions of atoms and molecules. Probably the biophores also will be
included under that head if the determinants should be adjudged utterly
unproductive. But so far I have always held that assumptions of this kind
_are_ really productive, if they are only capable of being used, so to
speak, as a _formula_, whereby to perform our computations, unconcerned for
the time being as to what shall be its subsequent fate. Now, as I take it,
the determinants have had fruitful results, as their application to various
biological problems shows. Is it no advance that we are able to reduce the
scission of a form of life into two or several forms subject to separately
continued but recurrent changes,--I refer to dimorphism and
polymorphism,--that we are able to reduce such phenomena to the formula of
male, female, and worker determinants? It has been, I think, {59} rendered
conceivable how these diverse and extremely minute adaptations could have
developed side by side in the same germ-plasm, under the guidance of
selection; how sterile forms could be _hereditarily_ established and
transformed in just that manner which best suits with their special duties;
and how they themselves under the right circumstances could subsequently
split up into two or even into three new forms. Surely at least the unclear
conception of an _adaptively_ transformative influence of food must be
discarded. It is true, we cannot penetrate by this hypothesis to the last
root of the phenomena. The hotspurs of biology, who clamor to know
forthwith how the molecules behave, will scarcely repress their
dissatisfaction[20] with such provisional knowledge--forgetful that _all
our knowledge is and remains throughout provisional_.

But I shall not enter more minutely into the question whether epigenesis or
evolution is the right foundation of the theory of development, but shall
content myself with having shown, first, that it is illusory to imagine
that epigenesis admits of a simpler structure of the germ, (the precise
opposite is true,) and secondly, that there are phenomena that can be
understood only by an evolution-theory. Such a phenomenon is {60} the
_guidance of variation by utility_, which we have considered to-day. For
without primary constituents of the germ, whether they are called as I call
them, determinants, or something else, _germinal selection_, or guidance of
variation by personal selection, is impossible; for where all units are
alike there can be no struggle, no preference of the best. And yet such a
guidance of variation exists and demands its explanation, and the early
assumptions of a "definitely directed variation" such as Nägeli and
Askenasy made are insufficient, for the reason that they posit only
_internal_ forces as the foundations thereof, and because, as I have
attempted to show, the harmony of the direction of variation with the
requirements of the conditions of life subsists and represents the riddle
to be solved. _The degree of adaptiveness which a part possesses itself
evokes the direction of variation of that part._

This proposition seems to me to round off the whole theory of selection and
to give to it that degree of inner perfection and completeness which is
necessary to protect it against the many doubts which have gathered around
it on all sides like so many lowering thunder-clouds. The moment variation
is determined substantially though not exclusively by the adaptiveness
itself, all these doubts fall to the ground, with _one_ exception, that of
the utility of the initial steps. But just this objection is the least
weighty. Without doubt the theory requires that the initial steps of a
variation should also have selective value; otherwise personal selection
and hence germinal selection could not set in. Since, however, as I have
before pointed out, _in no case can we pretend to a judgment regarding the
selective value of a modification, or have any_ {61} _experience thereof_,
therefore the assumption that in a given case where a character is
transformed the original initial steps of the variation did have selective
value, is not only as probable as the opposed assumption that they had
none, but is _infinitely more probable_, for with this we can give an
intelligible explanation of the mysterious fact of adaptation, while with
that we cannot. Consequently, unless we are resolved to give up all
attempts whatsoever at explanation, we are forced to the assumption that
the initial steps of all actually affected adaptations possessed selective
value.

The principal and fundamental objection that selection is unable to create
the variations with which it works, is removed by the apprehension that a
germinal selection exists. Natural selection is not compelled to wait until
"chance" presents the favorable variations, but supposing merely that the
groundwork for favorable variations is present in the transforming species,
that is, supposing merely that in the constitutional basis of the part to
be changed are contained components which render favorable variations
possible by a change of their numerical ratio, then those variations _must_
occur, for the reason that quantitative fluctuations are always happening,
and they must also be augmented as soon as personal selection intervenes
and permanently holds over them her protecting hand. Not only is the
marvelous _certainty and exactitude_ with which adaptation has operated in
so many individual cases, rendered intelligible in this manner, but what is
more difficult, we are able to understand the _simultaneity_ of numerous
and totally different modifications of the most diverse parts co-operant
towards some collective end, such as we see so frequently occur, {62} for
example, in the simultaneous rise of instincts and protective similarities,
or in the harmonious and simultaneous augmentation of two co-operant but
independent organs, as of the eye and of the centre of vision, or of the
nerve and its muscle, etc.

The "secret law," of which Wolff prophetically speaks in his criticism of
selection, is in all likelihood naught else than germinal selection. This
it is that brings it about that the necessary variations are always
present, that symmetrical parts, for example, the two eyes, usually vary
alike, but under circumstances may vary differently, for example, the two
visual halves of soles; that homodynamic parts, (for instance, the
member-pairs of Arthropoda,) have frequently varied alike, and not
infrequently and in conformity with the needs of the animal, have varied
differently. It brings it about also that conversely species of quite
different fundamental constitutions occasionally vary alike, as instances
of mimicry and numerous other cases of convergence show us. As soon as
utility itself is supposed to exercise a determinative influence on the
direction of variation, we get an insight into the entire process and into
much else besides that has hitherto been regarded as a stumbling-block to
the theory of selection, and which did indeed present difficulties that for
the moment were insuperable--as, for example, the like-directed variation
of a large number of already existing similar parts, seen in the origin of
feathers from the scales of reptiles. The utility in the last-mentioned
instance consisted, not in the transformation of one or two, but of _all_
the scales; consequently the line of variation of _all_ the scales must
have been started simultaneously in the same direction. A large part of the
objections to the theory of selection {63} that have been recently brought
forward by the acutest critics, as for example by Wigand, but particularly
by Wolff,[21] find, as I believe, their refutation in this doctrine of
germinal selection. The principle extends precisely as far as utility
extends, inasmuch as it creates, not only the direction of variation for
every increase or diminution demanded by the circumstances, but also every
qualitative direction of variation attainable by changes of quantity, so
far as that is at all possible for the organism in question.

Considering also the contrary process, the degeneration of useless parts by
the cessation of selection in regard to the normal size of that part, a
clear light is shed on that whole complex system of ascending and
descending modifications which makes up most of the transformations of a
living form, and we are led to understand how the fore extremity of a
mammal can change into a fin at the same time that the _hinder_ extremity
is growing rudimentary, or how one or two toes of a hoofed animal can
continue to develop more and more powerfully, whilst the others in the same
degree grow weaker and weaker until finally they have disappeared entirely
from the germ of most of the individuals of the species.

Possibly some of that large body of inquirers, mostly paleontologists, who
till now have considered the Lamarckian principle indispensable for the
explanation of these phenomena--perhaps some, I say, will not utterly close
their eyes to the insight that germinal selection performs the same
services for the understanding of observed transformations, particularly of
{64} the degeneration of superfluous parts, that a heredity of acquired
characters would perform, without rendering necessary so violent an
assumption. I have always conceded that many transformations actually do
run parallel to the use and disuse of the parts,[22] that therefore it does
really look as if functional acquisitions of the individual life were
hereditary. But if it be found that _passively functioning parts_, that is,
parts which are not alterable during the individual life by function, obey
the same laws and also degenerate when they become useless, then we shall
scarcely be able to refuse our assent to a view which explains both cases.
It certainly cannot be the physiological function which provokes
modifications in the individual, which are then subsequently transmitted to
the germ and in this way made hereditary, if _functionless parts also
change_ when they become useless. It is precisely this _uselessness_, then,
from which the initial impulse emanates, and the primary modification is
not in the soma but in the germ.

The Lamarckians were right when they maintained that the factor for which
hitherto the name of natural selection had been exclusively reserved, viz.,
_personal_ selection, was insufficient for the explanation of the
phenomena. They were also right when they declared that panmixia in the
form in which until recently I held the theory was also insufficient to
explain the degeneration of parts that had grown useless, but they {65}
erred when they ascribed hereditary effects to the selection-processes
which are enacted among the parts of the body (Wilhelm Roux) and which are
rightly regarded as the results of functioning. And they did this,
moreover, as they themselves admit, not because the facts of heredity
directly and unmistakably required it, but because they saw no other
possibility of explaining many phenomena of transformation. I am fain to
relinquish myself to the hope that now after another explanation has been
found, a reconciliation and unification of the hostile views is not so very
distant, and that then, we can continue our work together on the newly laid
foundations.

That the application of the Malthusian principle was thoroughly justified
is now clear. _The entire process of the development of living forms is
guided by this principle._ The struggle for existence, _videlicet_, for
food and propagation, takes place at all the stages of life between all
orders of living units from the biophores recently disclosed upwards to the
elements that are accessible to direct observation, to the cells, and still
higher up, to individuals and colonies. Consequently, in all the divers
orders of biological units lying between the two extremes of biophores and
colonies, the modifications must be controlled by selective processes;
therefore, these govern every change of living forms no matter what its
significance, and bring it about that the latter fit their conditions of
life as wax does the mould; and the various stages of these processes, as
enacted between the divers orders of biological units, in all organisms not
absolutely simple, are involved in incessant and mutual interaction. The
three principal stages of selection, that of {66} _personal_ selection[23]
as it was enunciated by Darwin and Wallace, that of _histonal_ selection as
it was established by Wilhelm Roux in the form of a "struggle of the
parts," and finally that of _germinal selection_ whose existence and
efficacy I have endeavored to substantiate in this article--these are the
factors that have co-operated to maintain the forms of life in a constant
state of viability and to adapt them to their conditions of life, now
modifying them _pari passu_ with their environment, and now maintaining
them on the stage attained, when that environment is not altered.

Everything is adapted in animate nature[24] and has been from the first
beginnings of life; for adaptiveness of organisation is here equivalent to
the power to exist, and they alone have had the power to exist who have
permanently existed. _We know of only one natural principle of explanation
for this fact--that of selection {67} of the picking out of those having
the power to exist from those having the power to originate._ If there is
any solution possible to the riddle of adaptiveness to ends,--a riddle held
by former generations to be insoluble,--it can be obtained only through the
assistance of this principle of the self-regulation of the originating
organisms, and we should not turn our faces and flee at the sight of the
first difficulties that meet its application, but should look to it whether
the apparent effects of this single principle of explanation are not
founded in the imperfect application that is made of it.

If I am not mistaken the situation is as follows: We had remained standing
half way. We had applied the principle, but only to a portion of the
natural units engaged in struggle. If we apply the principle throughout we
reach a satisfactory explanation. Selection of _persons_ alone is _not
sufficient_ to explain the phenomena; _germinal_ selection must be added.
Germinal selection is the last consequence of the application of the
principle of Malthus to living nature. It is true it leads us into a
terrain which cannot be submitted directly to observation by means of our
organs of touch and by our eyes, but it shares this disadvantage in common
with all other ultimate inferences in natural science, even in the domain
of inorganic {68} nature: in the end all of them lead us into hypothetical
regions. If we are not disposed to follow here, nothing remains but to
abandon utterly the hope of explaining the adaptive character of life--a
renunciation which is not likely to gain our approval when we reflect that
by the other method is actually offered at least in principle, not only a
broad insight into the adaptation of the single forms of life to their
conditions, but also into the mode of formation of the living world as a
whole. The variety of the organised world, its transformation by adaptation
to new, and by reversed adaptation to old conditions, the inequality of the
systematic groups, the attainment of the same ends by different means, that
is, by different organisations, and a thousand and one other things assume
on this hypothesis in a certain measure an intelligible form, whilst
without it they remain lifeless facts.

And so in this case, I may say, that again doubt is the parent of all
progress. For the idea of germinal selection has its roots in the necessity
of putting something else in the place of the Lamarckian principle, after
that had been recognised as inadequate. That principle did, indeed, seem to
offer an easy explanation of many phenomena, but others stood in open
contradiction to it, and consequently that was the point at which the lever
had to be applied if we were to penetrate deeper into the phenomena in
question. For it is at the places where previous views are at variance with
facts that the divining rod of the well-seekers must thrice nod. There lie
the hidden waters of knowledge, and they will leap forth as from an
artesian well if he who bores will only drive undaunted his drill into
their depths.

       *       *       *       *       *


{69}

APPENDIX.

       *       *       *       *       *

I. THE REJECTION OF SELECTION.

Many years ago Semper[25] denied the power of selection to create an organ,
declaring that the organ must have previously existed before selection
could have increased and developed it. More recently Wolff[26] has
distinguished himself by the vigor with which he has attacked the "task" of
"setting aside the dogma of selection." Henry B. Orr[27] is also of opinion
that selection is not the real cause of improved organic states; he regards
it as a factor checking growth in certain directions, but not as a cause
producing growth. Likewise Yves Delâge,[28] in his recent voluminous but in
many respects excellent work, regards natural selection solely as a
subordinate principle which is devoid of all power to create species (p.
391), although he grants to it certain functions, and even characterises it
{70} as "an admirable and perfectly legitimate principle" (p. 371). A more
pronounced opponent of selection, of any kind, as a principle creating
species, is the Rev. Mr. Henslow,[29] whose views we shall discuss later,
in Division VII. of this Appendix.

Finally, must be mentioned the name of Th. Eimer, as that of a pronounced
and bitter enemy of the theory of selection. I shall leave it to others to
decide whether he can properly be called an "opponent" of the principle, in
the scientific acceptance of the word. I can see in the blind railings of
the Tübingen Professor nothing but a reiteration of the same unproved
assertions, mingled with loud praises of his own doughty performances and
captious onslaughts on every one who does not value them as highly as their
originator.[30]

The lack of confidence latterly placed in the theory of selection even by
professed adherents of the doctrine, is well shown by such remarks as the
following {71} from Emery,[31] who says: "Some pupils of Darwin have gone
beyond their master and discovered in natural selection the sole and
universal factor controlling variations. Thus there has arisen in the
natural course of things a reaction, especially on the part of those who,
while they accept evolution, will have naught to do with natural selection
or Darwinism as they call it." Emery then professes himself a Darwinian,
although not in the sense of Wallace and "other co-workers and pupils of
Darwin." For him "natural selection is a very important factor in
evolution, and in determining the direction of variation plays the highest
part; but it is far from being the only factor and is probably also not the
most efficient factor." Not the most efficient factor but plays the highest
part!

       *       *       *       *       *

II. CHEMICAL SELECTION.

If we refer adaptation to selection, we have also to trace back to this
source the origin of the organic combinations which make up the various
tissues of the body and which go by the collective name of muscular,
nervous, glandular substance, etc. Lloyd Morgan has prettily likened the
vital processes to the periodic formation and discharge of explosive
substances.[32] Unstable combinations are upon the application of a {72}
stimulus suddenly disintegrated into simpler and more stable compounds;
through this disintegration they evoke what is called the function of the
disintegrating part--for example, certain changes of form (muscular
contractions) or the excretion of the disintegrated products, etc.

Now how is it possible that such unstable chemical combinations, answering
exactly to the needs of life, could have arisen in such marvellous
perfection if the _useful_ variations had not always been presented to the
ceaselessly working processes of selection? or, if the constantly
increasing adaptation to the constantly augmenting delicacy of operation of
physiological substances had depended in its last resort on _accidental_
variations? Hence, not only with regard to the "form" of organs, but also
with regard to the chemical and physiological composition of their
materials, we are referred to the constant presence of appropriate
variations.

       *       *       *       *       *

III. VARIATION AND MUTATION.

I have still to add a few remarks on the subject touched on in the footnote
at page 31. The view there referred to was discussed by Professor Scott
before in an article published in the _American Journal of Science_, Vol.
XLVIII., for November, 1894, entitled "On Variations and Mutations."
Following the precedent of Waagen and Neumayr, Scott sharply discriminates
between the inconstant vacillating variations which it is supposed [?]
produce simultaneously occurring "varieties," and "mutations," or the
successively evolved _time_-variations of a phylum, which constitute the
stages of phyletic development. The facts on which this view is based are
those already {73} adduced in the text--the _Zielstrebigkeit_ (to use K. E.
von Bär's phraseology) displayed in the visible paleontological
development, the directness of advance of the modifications to a final
"goal." "The direct, unswerving way in which development proceeds, however
slowly, is not suggestive of many trials and failures in all directions
save one." And again, "The march of transformation is the resultant of
forces both internal and external which operate in a _definite manner_ upon
a changeable organism and similarly affect _large numbers of individuals_."

The two points which I have here italicised are actually the facts which
separate phylogenetic from common individual variation: the definite
_manner_ of the change, repeated again and again without modification, and
its occurrence in a _large number of individuals_.

Still the two are not solely a result of observation, deduced from
paleontological data; they are also _a consequence of the theory of
selection_, as was shown in the text. If the theory in its previous form
was unable to fulfil this requirement, it is certainly now able to do so
after germinal selection has been added, and it is not in any sense
necessary to assume a difference of _character_ between phylogenetic and
ontogenetic variations. Bateson and Scott are wrong in imagining that I ask
them "to abrogate reason" in pronouncing the "omnipotence of natural
selection." On the contrary, the theory seems to me to accord so perfectly
with the facts that we might, by reversing the process, actually construct
the facts from the theory. What other than the actual conditions could be
expected, if it is a fact that selection favors only the useful variations
and singles them out from the rest by producing them in {74} increasing
distinctness and volume with every generation, and also in an increasing
number of individuals? The mere displacement of the zero-point of useful
variations alone must produce this effect, especially when it is supported
by germinal selection. It is impossible, indeed, to see how considerable,
that is perceptible, deviations could arise at all on the path of phyletic
development if in each generation a large number of individuals always
possessed the useful, that is, the phyletic variations? In fact, by the
assumption itself, the difference between useful and less useful variations
is merely one of degree, and that a slight one.

Hence, as I before remarked at page 31, I see no reason for assuming two
kinds of hereditary variations, _distinct as to their origin_, such as
Scott and the other palæontologists mentioned have been led to adopt,
although with the utmost caution. I believe there is only one kind of
variation proceeding from the germ, and that these germinal variations play
quite different rôles according as they lie or do not lie on the path of
adaptive transformation of the species, and consequently are or are not
favored by germinal selection. To repeat what I have said in the footnote
to page 31 only a relatively small portion of the numberless individual
variations lie on the path of phyletic advancement and so mark out under
the _guidance_ of germinal selection the way of further development; and
hence it would be quite possible to distinguish continuous, _definitely
directed_ variations from such as fluctuate hither and thither with no
uniformity in the course of generations. The origin of the two is the same;
they bear in them nothing that distinguishes the one from the other, and
their success alone, that {75} is, the actual resultant phyletic
modification, permits their being known as phyletic or as vacillating
variations. Uncertain fluctuations along the path of evolution are what the
geologists would be naturally led to expect from the theory of selection,
but which they were unable to discover in the facts; it is evident,
however, that these fluctuations are not a logical consequence of the
theory of selection as that is perfected by germinal selection, and there
seems to me to be no reason now for attributing "variations" to the union
of changing hereditary tendencies, while "mutations" are ascribed to the
effect "of dynamical agencies acting long in a uniform way, and the results
controlled by natural selection."

The idea which the Grecian philosophers evolved of the thousands of
non-adaptive formations that nature brings forth by the side of adaptive
ones, and which must subsequently all perish as being unfit to live, is
certainly correct in its ultimate foundations. But it is in need of far
more radical refinement than it underwent in the hands of Empedocles, or
than it seems likely to undergo at the hands of many contemporary
inquirers. We know now that nature did not produce isolated eyes, ears,
arms, legs, and trunks, and afterwards permit them to be joined together
just as the play of the fundamental forces of love and hatred directed,
leaving the monsters to perish and granting permanent existence only to
harmonious products. Yet there is a weak echo of this conception, although
infinitely far removed from its prototype, in the question as to where all
the non-adaptive individuals are preserved that have perished in the
struggle for existence and been eliminated from development by selection?
Where, for example, are the fossil remains {76} of the rejected individuals
in the line of the Horses? Certainly they should be forthcoming in far
larger numbers than the individuals lying directly in the path of
development, for by our very assumption the latter were greatly in the
minority in every generation. Doubtless the question would be a proper one
if our eyes were sufficiently keen-sighted to assign the life-value of the
various minute differences that distinguish the "better" from the "worse"
individuals of every generation. But this is a task which we can accomplish
at best only with selective processes which are artificially directed by
ourselves, as in the case of doves and chickens, and even there only with
the utmost difficulty and only with reference to a single characteristic
and not with any species which to-day exists in the state of nature.
Picture, then, the difficulties attending such a task as applied to the
meagre fossilic bones of prehistoric species, touching which the richest
discoveries never so much as remotely approach to the actual number of
individuals that have lived together for a _single_ generation in the same
habitat. If the differences between good and bad in a single generation
were striking enough to be immediately remarked _as such_ in fossil bones,
the development of species would take place so rapidly that we could
directly witness it in living species.

       *       *       *       *       *

IV. REMARKS ON THE HISTORY OF DEFINITELY DIRECTED VARIATIONS.

As to the attempt here made to apply the selective process to the elements
of the germinal substance (the idioplasm) and thus to acquire a foothold
for definitely directed variation not blind in its tendency but {77}
proceeding in the direction of adaptive growth, it is remarkable that the
same was not made long ago by some one or other of the many who have
thought and written on selection and evolution.

Allusions to a connexion between the direction of variation and the
selective processes are to be found, but they remained unnoticed or
undeveloped. I have been able to find at least two such observations, but
would not wish to assert that there are not more of them hidden somewhere
in the literature of the subject. One of them is old and comes from Fritz
Müller. It was appended by his brother Hermann as a "Supplementary Remark"
to his book _Die Befruchtung der Blumen durch Insecten_ (1873) and is dated
November 24, 1872. We read there: "My brother Fritz Müller communicates to
me in a letter which reached my hands only after the bulk of the present
work had passed through the press, the following law discovered by him,
which materially facilitates the explanation by natural selection of the
pronounced characters of sharply distinguished species: 'The moment a
choice in a definite direction is made in a variable species, progressive
modification from generation to generation in the same direction will set
in as the result of this choice, wholly apart from the influence of
external conditions. Transformation into new forms is thus greatly
facilitated and accelerated.'"

The facts on which F. Müller based the enunciation of his law, are the
results of several experiments with plants, the numbers of whose grains
(maize), or styles, or flowering leaves, were, by the exercise of choice in
the cultivation, made to change in definite directions. Accurately viewed
their significance is the same as that of numerous other cases of
artificial selection, for {78} example, that of the long-tailed Japanese
cock which was laid at the foundation of the theory in the text, although
the numerical form of the observation gives more precision and distinctness
to the reasoning based on them, than is to be observed in cases where we
speak of characters as being simply "longer" or "shorter."

F. Müller's opinion regarding the increase of characters by selection is
expressed as follows: "The simplest explanation of these facts appears to
be that every species possesses the faculty of varying within certain
limits; the crossing of different individuals, so long as no choice is
effected in a definite direction, maintains the mean round which the
oscillations take place at the same points, and consequently the extremes
also remain unaltered. If, however, one side is preferred by natural or
artificial selection, the mean is shifted in the direction of this side and
accordingly the extreme forms are also displaced towards that side, going
now beyond the original limit. However, this explanation does not satisfy
me in all cases."

It is not known to me that F. Müller ever returned to this conception
subsequently to the year 1872 or gave further developments of the same, nor
have I been able to discover that it has been mentioned by other writers or
incorporated in previous notions regarding selection.

The second naturalist who has approached the fundamental idea of my
doctrine of germinal selection, is a more recent writer. I refer to the
English botanist Thiselton-Dyer, a scientist whose occasional utterances on
the general questions of biology have more than once evoked my sympathetic
approval. In an article, "Variation and Specific Stability," which appeared
in {79} _Nature_ for March 14, 1895, this author enunciates twenty theses
touching this subject, many of which appear to me apposite and correct,
particularly the following: In every species there is a mean specific form
round which the variations are symmetrically grouped like shots around the
bull's eye of a target. As soon as natural selection comes into play and
favors one of these variations it must shift the centre of density.
Variations arise by a change in the outward conditions of life and can be
useful or indifferent; only in the first case will natural selection obtain
control of them and "the new variation will get the upper hand and the
centre of density will be shifted."

This is not germinal selection, but it is the same as what I have referred
to in this and in the preceding essay as displacement of the zero-point of
variation. Thiselton-Dyer did not draw the conclusion that a definitely
directed variation answering to utility resulted from this process, which
variation alone must cause the disappearance of useless parts, for the
reason that he never attempted to penetrate to the causes of the shifting
of the zero-point of variation. Neither Fritz Müller, whose utterances
Thiselton-Dyer was obviously ignorant of, nor Thiselton-Dyer himself pushed
his inquiries beyond the thought that the shifting in question resulted
entirely in consequence of personal selection. There is no gainsaying that
the degeneration of useless organs cannot be explained by personal
selection alone, seeing that though the minus variations may possibly have
a selective value at the beginning of a degenerative process, they
certainly cannot have such in the subsequent course of the same, when the
organ has dwindled down to a really minimal mass of substance as compared
with the whole {80} body. Of what advantage would it be to the whale if his
hinder leg, now concealed in a mass of flesh and no longer protruding
beyond the skin, should still be reduced one or several centimetres in
size? (Spencer.) If the minus variations have no selective value, how can
the upper limit of the variational field be constantly displaced downwards,
as actually happens? It is unquestionable but something different from
personal selection must come here co-determinatively into play.

       *       *       *       *       *

V. HISTORICAL REMARKS CONCERNING THE ULTIMATE VITAL UNITS.

(For this Appendix which is marked "Appendix V." in the German edition of
_Germinal Selection_ see the footnote at page 40.)

       *       *       *       *       *

VI. THE INITIAL STAGES OF USEFUL MODIFICATIONS.

In characterising as "least" weighty the old objection that the variations
are too small at the start to be useful and to be selected, I find myself
diametrically opposed to many writers of the present day, who have taken up
with renewed vigor this old stumbling block to the principle of selection.
Bateson[33] regards the deficient proof of the utility of initial stages as
the most serious objection that can be made to natural selection. New
organs must in the necessity of the case have first been imperfect; how,
then, could they have been selected since imperfect organs cannot be
useful? Answers from various quarters have already been {81} made to this
and to similar objections, and Darwin himself has referred to the fact that
even the smallest variations may have selective value; Dohrn, too, has
urged his principle of change of functions, which with regard to this
question of the utility of initial stages has certainly a wide
significance. Still, every transformation and new structure in the narrow
sense of the word does not rest on change of function, and neither Darwin
nor Wallace, nor any other more recent champion of the principle of
selection, can ever succeed in demonstrating in _every_ case the selective
value of an initial stage. One reason why this cannot be done is because
_in no case of morphological variation do we really know what these initial
stages are_. To say that "new organs were at first necessarily imperfect"
appears obvious enough, but it is at bottom a meaningless assertion, for it
is not only possible but certain, that "imperfect" organs may still have
selective value, and in by far the most cases have had selective value. The
fact that we see to-day a long graduated line of forest-butterflies which
possess resemblance to leaves and by this means are able in a measure to
conceal themselves from prying eyes, yet that this resemblance in many
species is very imperfect, in others more perfect, and in a very small
number very perfect, simply proves that even "imperfect" formations may be
of utility. The word "imperfect" in this connexion is itself very
imperfect, for it is utterly anthropomorphic and estimates the biological
value of a structure by our own peculiar artistic notions of its
faithfulness to a leaf-copy, whilst we are really concerned here only with
its protective value for the species in question, which is by no means
dependent merely on the faithfulness of the copying, on the {82}
faithfulness of the imitation, but on numerous other factors, such as the
frequency and sharp-sightedness of the enemies of the species, the
fertility of the species, their frequency and persecution in earlier
developmental stages, and so forth, in brief, on their need of protection
on the one hand and on their other means of protection on the other.

Now all this cannot be exactly calculated in any given case, and it will be
better, instead of haggling about individual cases concerning which we can
never judge with certainty, to take the position adopted in the text and
say: Since the utility of the initial stages _must_ be assumed unless we
are to renounce forever the explanation of adaptation, let us then take it
for granted. No contradiction of facts is involved in this assumption; in
fact, even individual variations exist whose eventual utility can be
demonstrated, for example, the invisible differences enabling Europeans of
certain constitutions to resist the attacks of tropical malarial
fevers,--or the differences of structure, likewise not directly visible,
which enable palms from the summits of the Cordilleras to withstand our
winter climate better than palms of the same species from along the
base-line of the mountains; and so on.

       *       *       *       *       *

VII. THE ASSUMPTION OF INTERNAL EVOLUTIONARY FORCES

Definite variation was not only postulated in the last decade by Nägeli and
Askenasy, but has also been repeatedly set up in recent years by various
other authors. The Rev. George Henslow, in his book _The Origin of Species
Without the Aid of Natural Selection_, 1894, regards the variations
occurring in the state {83} of nature as always definite and not with
Darwin as indefinite, and meets the objection that modification but not
adaptation to outward conditions of life can be inferred from this fact, by
the bold assumption that it is precisely the outward conditions of life or
the environment which "induces the best fitted to arise." He further
concludes that natural selection has nothing to do with the origin of
species. At the basis of his conviction lies the naturally correct view
that the summation of _accidental_ variations is insufficient for
transforming the species, but that definitely directed variation is
necessary to this end. But concerning the way in which external conditions
are always able to produce the fit variations, he can give us no
information--if I am not mistaken, for the simple reason that such is not
the fact, that the outward conditions only apparently determine the
direction of variations whilst in truth it is the adaptive requirement
itself that produces the useful direction of variation by means of
selectional processes within the germ.

C. Lloyd Morgan also has recently expressed himself in favor of the
necessity of definite variation, though likewise without assigning a basis
for its action, and without being able to show how its efficacy is
compatible with the plain fact of adaptation to the conditions of life. He
seeks to find the origin of variation in "mechanical stresses and chemical
or physical influences," but this conception is too general to be of much
help. He has, in fact, not been able to abandon completely the heredity of
acquired characters.

Emery[34] likewise sees only the alternative of a {84} "definitely directed
variation" from internal causes and of a summation of "accidental"
variations. He says: "A summation of entirely accidental variations in a
given direction is extremely difficult," because "natural selection thus
always awaits its fortune at the hands of accident whereby it is possible
that the little good thereby produced will be swept away by other accidents
(disadvantages of position) or obliterated in the following generations by
unfortunate crossings." We can, therefore, continues Emery, well conceive
"how many scientists look upon the whole theory of selection as a fable, or
else throw themselves into the arms of Lamarckism." Unquestionably Emery
has here singled out the insufficient points in the assumption of a
selection of "accidental" variations; he has recognised the necessity of
operating, not with single variations, but with "directions of variation."
He has not, however, attempted the derivation of directed tendencies of
variation from known factors; he apparently thinks of them as of something
which has sprung from unknown constitutional factors and consequently
ascribes to them the capacity of shooting beyond their mark, so to speak,
that is, of acting beyond and ahead of utility, and so of producing
modifications which may lead to the destruction of the species.

       *       *       *       *       *


{85}

INDEX.

  Accidental variations, 3, 83.
  Acquired variations, 33.
  Acracids, 19.
  Acræa, 52.
  Active selection, 38.
  Adaptations, 3, 10, 22, 61, 82.
  Adaptiveness, 66 footnote, 67, 74 et seq.
  Ageronia, 19.
  Anæa, 22.
  _Anlagen_, 35, 47, 53.
  Arthropoda, 32, 62.
  Articulata, 30.
  Artificial selection, 33.
  Askenasy, 24, 60, 82.
  Atoms, 57, 58.

  Bär, K. E. von, 73.
  Bateson, 18, 73, 80.
  "Better" individuals, 76.
  Biology, character of research in, 7.
  Biophores, 40, 47, 58.
  Boltzmann, 4, 5.
  Bonnet, 53.
  Bourne, footnote, 54.
  Brücke, 40.
  Butterflies, 14 et seq., 18 et seq., 81.

  Catonephele, 50.
  Chance, 61.
  Chemical selection, 71.
  Chitons, 28.
  Coadaptation, 30.
  Colorings, protective, 14 et seq.
  Constancy of species, 46.
  Constructs, 8.
  Cormi, 66 footnote.
  Correlation, 21.

  Danaids, 19.
  Darwin, 11, 25, 29, 36, 38, 66, 81, 83.
  Definite variation, 3, 4, 60, 76-79, 82.
  Degeneration, 30 et seq., 39 et seq. 55, 63, 64, 79.
  Delâge, Yves, 40, 69.
  Determinants, 6 et seq., 10, 36 et seq. 42, 54, 58.
  Developmental mechanics, 8, 9.
  De Vries, 40.
  Dimorphism, 58.
  Directions of variations, 83.
  Directive forces, 23, 24.
  Dixey, 51 footnote.
  Dohrn, 81.
  Driesch, Hans, 12.
  Dyer, Thiselton, 78-79.

  Eimer, 16, 70.
  Emery, 71, 83-84.
  Empedocles, 75.
  Epigenesis, 53 footnote, 58, 59.
  Euploids, 19.
  Europeans, exempt from malarial fevers, 82.
  Eurypheme, 22.
  Evolution, 53 footnote, 59.

  Fireworks, determinants and ids compared to, 7.
  "Fits," 6 footnote.
  Fluctuations of development, 74-75.
  Formative laws, 17 et seq., 23.
  Frog, 14.
  Functional adaptation, 29.
  Functionless parts, 64.

  Galton, 36.
  Germs, 7 et seq., 40 et seq.
  {86}
  Germinal selection, 3, 39, 44, 50-53, 59, 63, 66-68.
  Germinal substance, 55 et seq.
  Germ-plasm, 9, 44, 57.

  Haase, Eric, 70.
  Heliconids, 19, 20, 51 footnote.
  Henslow, G., 70, 82.
  Heredity, 4 et seq.
  Hertwig, O., 54 footnote, 58, 59.
  Hertz, 5, 6.
  Histonal selection, 66.
  Huxley, Thomas, 12.
  Hypna, 22.
  Hypotheses, nature of, 5 et seq.

  Ids, their theoretical character, 7.
  Imagination, its function in science, 4.
  "Imperfect" formations, 81.
  Individual variations, 73 et seq.
  Inertia, law of organic, 15.
  Internal forces of evolution, 16, 23, 24, 31, 60, 82-4.
  Intrabiontic selection, 29.
  Ishikawa, Professor, 34.

  Japanese cocks, long-tailed, 34, 44, 78.

  Kallima, 22, 23, 50.
  Katagramma, 22.
  Knowledge, its character, 5.

  Lamarckian principles, 24, 29 et seq., 31 et seq., 38, 63-64, 68, 84.
  Leaves, imitated by butterflies, 20 et seq.
  Locomotive, simile of, 11.

  Malthusian principle, 65, 67.
  Markings, butterflies', 16 et seq.
  Maxwell, 4, 5.
  Mean of variation, 78-79.
  Meristic, 18.
  Mimicry, 19, 51 et seq.
  Minot, S., 54 footnote.
  Models, mental, 4 et seq.
  Molecules, 58.
  Morgan, Prof. C. Lloyd, 32, 71, 83.
  Müller, Fritz, 77-79.
  Müller, Hermann, 77.
  Mussels, 28.
  Mutations, 31 footnote, 72-76.

  Nägeli, 4, 11, 24, 60, 82.
  Neumayr, 72.
  Newton, 5.
  Nutrition of determinants, 36, 37, 41, 47.
  Nymphalidæ, 21.

  Ontogenesis, 8.
  Orr, Henry B., 69.
  Osborn, Prof. H. F., 33.
  Owen, Richard, 11.

  Paleontology, 31, 73, 75, 76.
  Palms from Cordilleras, 82.
  Pangenes, 40.
  Panmixia, 15, 39, 42, 43, 64.
  Papilio, 16, 52.
  Parallecta, 23.
  Parts, struggling of the, 29, 39, 66-67.
  Passively functioning parts, 30 et seq., 64.
  Personal selection, 30, 41, 42, 45, 52, 64-86, 80.
  Phyletic variation, 31-32 footnote.
  Phylogenesis, 8.
  Phylogenetic variations, 31-32, 73.
  Plasomes, 40.
  Plus and minus variations, 35, 42, 46, 50, 79-80.
  Polymorphism, 58.
  Poulton, 64 footnote.
  Predestined variation, 4.
  Pre-established harmony, 25.
  Preformation, 53.
  Protective colorings, 14 et seq.
  Protogonius, 22.
  Pseudocræa, 52.

  Qualitative modifications, 46.
  Quantitative changes, 46-47.

  Retrogressive development, 38.
  Round-worms, eggs of, 28.
  Roux, Wilhelm, 29, 39, 65, 66.

  Salamis, 22.
  Scott, Prof. W. B., 31 footnote, 72-74.
  Segmentation, 10.
  {87}
  Selection, natural, 10, 25 et seq., 50, 51, 67, 69-73, 81, 82.
  Selective value of variations, 60.
  Semper, 69.
  Siderone, 22.
  Snails, 28.
  Spencer, 14, 28, 29, 40, 53, 56, 80.
  Struggle for existence, 65.
  Survival of the fit, 52.
  Symphædra, 22.

  _Tabula rasa_, 27, 24.
  Tegetmeier, W. B., 34.
  Teleological principles, 10, 16, 25.
  Theories, nature of, 5 et seq.
  Turbellaria, 28.

  Units, vital, biological, physiological, etc., 8, 40, 41, 53, 56, 65, 80.
  Useful modifications, value of initial stages of, 80-82.
  Utility, 11, 18, 33, 45, 48, 62, 63, 82.

  Variations, necessary, their constant presence, 26 et seq., 31 et seq.,
      61;
    generally, 3, 11-14, 61, 71 et seq.

  Waagen, 72.
  Wallace, 11, 25, 29, 51, 66, 81.
  Weldon, 36.
  Whale, hind leg of, 42, 56, 80.
  Whitman, C. O., 53.
  Wiesner, 40.
  Wigand, Albert, 11, 63.
  Wings of butterflies, 14 et seq., 47-52, 56.
  Wolff, K. F., 53, 62, 63, 69.
  "Worse" individuals, 76.

  Zero-point of variation, 36 et seq., 45, 74, 79.

       *       *       *       *       *


Notes

[1] _Neue Gedanken zur Vererbungsfrage, eine Antwort an Herbert Spencer._
Jena. 1895.

[2] See Boltzmann, _Methoden der theor. Physik_, Munich, 1892. (In the
Catalogue of the Mathematical Exhibit.)

[3] Of late this saying of Newton's is frequently quoted as if Newton were
a downright contemner of scientific hypotheses. But if we read the passage
in question in its original context, we shall discover that his
renunciation of hypotheses referred solely to a definite case, viz., to
that of universal gravitation, of whose character Newton could form no
conception and hence was unwilling to construct hypotheses concerning it.
Indeed, such a wholesale repudiation of hypotheses is antecedently
incredible on the part of the inventor of the emission-theory of light, in
which, to speak of only one daring conjecture, "fits" were ascribed to the
luminous particles. Compare Newton, _Philosophiae Naturalis Principia
Mathematica_, second edition, 1714, page 484.

[4] H. Hertz, _Die Principien der Mechanik_.

[5] Hans Driesch, _Die Biologie als selbstständige Grundwissenschaft_,
Leipsic, 1893, p. 31, footnote. The sentence reads: "An examination of the
pretensions of the refuted Darwinian theory, so called, would be an affront
to our readers."

[6] _Die Allmacht der Naturzüchtung._ A Reply to Herbert Spencer. Jena,
1893, p. 27 et seq. [Also in the _Contemporary Review_ for September,
1893.]

[7] That is, by the law of exceedingly slow retrogression of superfluous
characters, which may be designated the law of organic inertia.

[8] _Materials for the Study of Variation with Especial Regard to
Discontinuity in the Origin of Species._ London, 1895.

[9] _Studien zur Descendenztheorie_, Leipsic, 1876. Vol. II. pp. 295 and
322.

[10] Compare my essay, _Neue Gedanken zur Vererbungsfrage_, Jena, 1895, p.
10, second footnote.

[11] On the same day on which the present address was delivered at the
International Congress of Zoölogists in Leyden, and on the same occasion,
Dr. W. B. Scott, Professor of Geology in Princeton College, New Jersey,
read a very interesting paper on the tertiary mammalian fauna of North
America, in which, without a knowledge of my paper, he took his stand
precisely on this argument and arrived at the opinion that it could not
possibly be the ordinary individual variations which accomplished phyletic
evolution, but that it was necessary to assume in addition phyletic
variations. I believe our views are not as widely remote as might be
supposed. Of course, I see no reason for assuming two kinds of hereditary
variations, different _in origin_. Still it is likely that only a
relatively small portion of the numberless individual variations lie on the
path of phyletic advancement and so under the _guidance_ of germinal
selection mark out the way of further development; and hence it would be
quite possible in this sense to distinguish continuous, _definitely
directed_ individual variations from such as fluctuate hither and thither
with no uniformity in the course of generations. The root of the two is of
course the same, and they admit of being distinguished from each other only
by their success, phyletic modification, or by their failure.

[12] H. F. Osborn, "The Hereditary Mechanism and the Search for the Unknown
Factors of Evolution," in _Biological Lectures delivered at the Marine
Biolog. Lab. at Wood's Holl in the Summer Session of 1894_. Boston, 1895.

[13] In 1886. See my paper on "Retrogression in Nature," published in
English in Nos. 105, 107, 108, and 109 of _The Open Court_, and also in my
essays on _Heredity_, Jena, 1892.

[14] _Neue Gedanken zur Vererbungsfrage_, Jena, 1895.

[15] Delâge, in _La structure du protoplasma et les théories sur
l'hérédité_, etc., Paris, 1895, is mistaken in attributing to Herbert
Spencer the merit of having first pointed out the necessity of the
assumption of biological units ranking between the molecule and the cell.
Brücke set forth this idea three years previously to Spencer and
established it exhaustively in a paper which in Germany at least is famous
("Elementarorganismen," _Wiener Sitzungsberichte_, October 10, 1861, Vol.
XLIV., II., p. 381). Spencer's _Principles of Biology_ appeared between
1864 and 1868; consequently there can be no dispute touching the priority
of the idea. Strangely enough Delâge cites Brücke's essay in the
Bibliographical Index at the end of his book correctly, although Brücke's
name and views are nowhere mentioned in the book itself. It is to be
observed, however, that the elementary organisms of Brücke are not merely
the precursors of Spencer's "physiological units," but repose on much
firmer foundations than the latter, which, as Delâge himself remarks, are
at bottom nothing more than magnified molecules and not combinations of
different molecules of such character as to produce necessarily phenomena
of life. He aptly remarks on this point: "the physiological units of
Spencer are only chemical molecules of greater complexity than the rest,
and as he defines them they would be regarded as such by every chemist. He
attributes to them no property _essentially_ different from those of
chemical molecules." Assimilation, growth, propagation, in short the
attributes of life, are not attributed by Spencer to his units, while
Brücke by his very designation "elementary organisms" expresses the idea of
"ultimate living units," to use Wiesner's phrase. Of course this particular
aspect of the vital units was not emphasised by Brücke with the same
distinctness and sharpness as by recent inquirers, who took up Brücke's
ideas thirty years after. I refer to the conception that the union of a
definite combination of heterogeneous molecules into an invisibly small
unit, forms the cradle or focus of the vital phenomena. This was first done
and apparently on independent considerations by De Vries, and soon after by
Wiesner, and subsequently by myself (De Vries, _Intracelluläre Pangenesis_,
Jena, 1889; Wiesner, _Die Elementarstructur and das Wachsthum der lebenden
Substanz_, Vienna, 1892; Weismann, _Das Keimplasma_, Jena, 1892). Let me
say at the close of this note that it is not my intention in thus defending
the rights of a great physiologist, to censure in the least the
distinguished author of _L'hérédité_ who has set himself a remarkably high
standard of exactitude in such matters. Certainly, when we consider the
enormous extent of the literature that had to be mastered to produce his
book, embracing as it did all the various theories of recent times, such an
oversight is quite excusable.

[16] I speak here of determinants, not of groups of determinants, which is
the more correct expression, merely for the sake of brevity. It is a matter
of course that a whole extremity, such as we have here chosen, cannot be
represented in the germ by a single determinant only, but requires a large
group of determinants.

[17] That this is not so in all cases has recently been shown by Dixey from
observations on certain white butterflies of South America which mimic the
Heliconids and in which a small, yellowish red streak on the under surface
of the hind wing has served as the point of departure and groundwork of the
development of a protective resemblance to quite differently colored
Heliconids. "On the Relation of Mimetic Characters to the Original Form,"
in the _Report of the British Association for 1894_.

[18] Oscar Hertwig, _Zeit-und Streitfragen der Biologie_, Jena, 1894. It is
customary now to look upon the preformation-theory of Bonnet as a discarded
monstrosity, and on the epigenesis of K. F. Wolff as the only legitimate
view, and to draw a parallel between these two and what might be called
to-day "evolution" [i. e. unfoldment] and epigenesis. The evolution, or
unfoldment, of Bonnet and Harvey, however, was something totally different
from modern doctrines of evolution, and Whitman is quite right when he says
that even my theory of determinants would have appeared to the inquirers of
the last century as "extravagant epigenesis." Biologists in that day were
concerned with quite different questions from what they are at present, and
although now we probably all share the conviction of Wolff that new
characters do arise in the course of evolution, yet the acceptance of this
view is far from settling the question _as to how these new characters are
established in the germ-substance_--for in this substance they certainly
must have their foundation. When, therefore, O. Hertwig laments over my
regarding evolution and not epigenesis as the correct foundation of the
theory of development, his sorrow is almost as naïve as is the statement of
Bourne that epigenesis is a fact and not a theory "a statement of
morphological fact," _Science Progress_, April, 1894, page 108), or, as is
the latter's unconsciousness that facts originally receive their scientific
significance from thought, i. e. from their interpretation and combination,
and that thought is theory. And when S. Minot, as the leader of the
embryologists, carries his zeal to the pitch of issuing a general
pronunciamento against me as a corruptor of youth, in which he declares it
to be a "scientific duty to protest in the most positive manner against
Weismann's theory," I wonder greatly that he does not suggest the casting
of a general ballot in the matter. (See the _Biologisches Centralblatt_ of
August 1, 1895.) We see how with these gentlemen the wisdom of the
recitation-room regarding the infallibility of epigenesis has grown into a
dogma, and whoever ventures to disturb its foundations must be burnt as a
heretic.

[19] Oscar Hertwig, _Zeit- und Streitfragen der Biologie_, Jena, 1894.

[20] Nor will those, who demand a demonstration of "how the biophores and
determinants are constituted in every case, and must be arranged in the
architecture of the germ-plasm." (O. Hertwig, _loc. cit._, p. 137). As if
any living being could have the temerity even so much as to guess at the
actual ultimate phenomena in evolution and heredity! The whole question is
a matter of symbols only, just as it is in the matter of "forces," "atoms,"
"ether undulations," etc., the only difference being that in biology we
stumble much earlier upon the unknown than in physics.

[21] "Beiträge zur Kritik der Darwin'schen Lehre," _Biologisches
Centralblatt_, Vol. X., p. 449. 1890.

[22] Poulton has adverted to the fact that this is nevertheless not always
the case; for example, it is not so with the teeth, whose shape it had also
been sought to reduce to the mechanical effects of pressure and friction.
See "The Theory of Selection" in _The Proceedings of the Boston Society of
Natural History_, Vol. XX., page 389. 1894.

[23] As the highest stage of selective processes must be regarded that
between the highest biological units, the colonies or cormi--a stage,
however, which is not essentially different from personal selection. In
this stage the persons enact the part that the organs play in personal
selection. Like their prototypes they also battle with one another for food
and in this way maintain harmony in the colony. But the result of the
struggle endures only during the life of the individual colony and can be
transmitted through the germ-cells to the following generation as little as
can histological changes provoked by use in the individual person. Only
that which issues from the germ has duration.

[24] This statement has often been declared extravagant, and it is so if it
is taken in its strict literalness. On the other hand, it would also seem,
by a more liberal interpretation, as if there existed non-adaptive
characters, for example, rudimentary organs. Adaptiveness, however, is
never absolute but always conditioned, that is, is never greater than
outward and inward circumstances permit. Moreover, an organ can only
disappear gradually and slowly when it has become superfluous; yet this
does not prevent our recognising every stage of its degeneration as adapted
when compared with its precursor. Further, it does not militate against the
correctness of the above proposition that there are also characters whose
fitness consists in their being the necessary accompaniments of other
directly adapted features, as, for instance, the red color of the blood.

[25] Semper, _Die natürlichen Existenzbedingungen der Thiere_, Leipsic,
1880, pp. 218-219.

[26] Wolff, "Beiträge zur Kritik der Darwin'schen Lehre," _Biolog.
Centralblatt_, Vol. X., Sept. 15, 1890, and "Bemerkungen zum Darwinismus
mit einem experimentellen Beitrag zur Physiologie der Entwicklung,"
_Biolog. Centralblatt_, Vol. XIV., Sept. 1, 1894.

[27] Henry B. Orr, _A Theory of Development and Heredity_, New York, 1893.

[28] Yves Delâge, _La structure du protoplasma et les théories sur
l'hérédité et les grands problèmes de la biologie générale_, Paris, 1895.

[29] Henslow, _The Origin of Species Without the Aid of Natural Selection,
A Reply to Wallace_. 1894.

[30] If any one should deem these words too severe, let him read the
sarcastic passages in which Eimer has dispatched the late unfortunate Eric
Haase who had been presumptuous enough to oppose the Tübingen Professor's
deliverances on certain points. Haase, as we all know, fell a victim to the
climate of the tropics, shortly after resigning the post of Director of the
natural science collections in Bangkok, in order to return to Germany and
to work out the fruits of his tropical sojourn. The unfortunate end of this
accomplished man who had rendered important services to science had no
effect in mollifying the resentment of Herr Eimer at the opposition which
his views had encountered; and in twenty printed pages he takes him to task
in the most personal and rancorous manner for this affront, remarking at
the close: "In the meantime Herr Haase has died. Nevertheless I owe it to
myself, in spite of this occurrence, to make public the foregoing facts, in
order," etc. Any one who is interested in knowing the motives of Herr
Eimer's excuse may find them in his book _Artbildung and Verwandtschaft bei
den Schmetterlingen_, Part II., p. 66.

[31] "Gedanken zur Descendenz- und Vererbungstheorie." _Biolog.
Centralblatt_, July 15, 1893.

[32] C. Lloyd Morgan, _Animal Life and Intelligence_, London, 1890-1891, p.
30-33.

[33] _Materials for the Study of Variation with Especial Regard to
Discontinuity in the Origin of Species_, London, 1895, p. 16.

[34] "Gedanken zur Descendenz- and Vererbungstheorie," _Biolog.
Centralblatt_, 1893, Vol. XIII., p. 397.