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                THE GENETIC AND THE OPERATIVE EVIDENCE
                     RELATING TO SECONDARY SEXUAL
                              CHARACTERS

                            BY T. H. MORGAN

        [Illustration: CARNEGIE INSTITUTION OF WASHINGTON 1902]

          PUBLISHED BY THE CARNEGIE INSTITUTION OF WASHINGTON

                           WASHINGTON, 1919




                  CARNEGIE INSTITUTION OF WASHINGTON

                          PUBLICATION NO. 285

                    PRESS OF GIBSON BROTHERS, INC.
                           WASHINGTON, D. C.




CONTENTS.


PART 1.                                                             PAGE.

Introduction                                                           5

Castration of Sebrights                                                6

A male Sebright that did not become cock-feathered after castration   10

Transitional feathers                                                 10

Castration of F₁ hen-feathered males from Sebright by game            11

Castration of F₂ hen-feathered males                                  13

Hewitt’s Sebright hen that became cock-feathered in old age           14

Heredity of hen-feathering                                            14

Heredity of color in the cross between Sebright and Black-Breasted
     Game bantam                                                      17
    A. The F₁ birds                                                   18
    B. Description of F₂ birds                                        19
    C. Back-cross of F₁ to game                                       21
    D. The number of color factors involved                           22
    E. Back-cross of F₁ ♀ to Sebright ♂                               23
    F. Review of the heredity of the color of the plumage in poultry  23

Endocrine cells in ovary and testes of birds                          32

Luteal-cells in the testes of the male Sebright                       34

Endocrine cells in the testes of mammals                              35

Cyclical changes in the interstitial cells in hibernating mammals     36

Hermaphroditism in poultry and the secondary sexual characters        37


PART II.

Darwin’s theory of sexual selection                                   43

Other theories to account for secondary sexual characters             45

Display of the male                                                   50


PART III.

The genetic and the operative evidence                                62
    A. Evidence from mammals                                          64
    B. Evidence from birds                                            73
    C. Evidence from amphibia                                         86
    D. Evidence from crustaceans                                      89
    E. Evidence from insects                                          92


PART IV.

Summary and conclusions                                               95

Bibliography                                                         100

Description of plates                                                106




THE GENETIC AND THE OPERATIVE EVIDENCE RELATING TO SECONDARY SEXUAL
CHARACTERS.

BY T. H. MORGAN.




PART I.


There are a few races of poultry that have two kinds of males, one with
the feathering of the ordinary cock, the other with the feathering of
the hen. The Hamburgs and the Campines are perhaps the best known races
of this sort. Convention amongst breeders, in certain countries, has
determined that the cock-feathered bird shall be the standard, and at
other times and places that the hen-feathered males shall be the show
birds. In one breed, at least, viz., the Sebright bantams, the
hen-feathered cock is the only known type. Cock-feathered Sebrights have
never been seen, so far as I know. This breed is pure for
hen-feathering. As shown in plate 1, figure 3, the male Sebright lacks
the long, pointed saddle feathers at the base of the tail of the common
cock, also the peculiar back and neck feathers (hackles) of the cock
bird, as well as the male feathering on the bow of the wing. His
feathers in these parts are almost exactly like those of the hen (plate
4, fig. 4). The long sickle feathers covering the true tail are also
absent, although the two median ones sometimes occur in males of this
race.

The Sebrights seemed excellent material for studying the heredity of
this type of plumage in the male. In 1911 I began to study this problem,
and crossed Sebrights to Black-Breasted Game bantams. The latter race
was chosen not only because the males have the typical cock-feathering,
but also because the coloration of these birds resembles very closely
that of the jungle-fowl, from which many, perhaps all, of our
domesticated races have sprung.

In dissecting some of the F₂ birds from this cross I noticed that the
testis of the male was often more flattened than is the testis of the
typical male bird, that it was often somewhat pear-shaped, and that
frequently it was in part or entirely black. Recalling that male
Sebrights are said to be often partially sterile, the idea naturally
suggested itself that these birds are hen-feathered because the testes
have assumed some of the characteristics of the ovary. It had long been
supposed, and had been finally established by Goodale, that the presence
of the ovary in the female suppresses her potential development of
plumage, for when the ovaries of the hen are diseased or removed she
develops the plumage of the male. This reasoning led me to try the
experiment of castrating the hen-feathered males in order to see if
they would become cock-feathered. The outcome was immediately apparent;
the new feathers were those of the cock bird. While the “reasoning” that
led to the experiment is open to serious question, nevertheless the
“hint” furnished by the unusual condition of the testis led finally to
the discovery that luteal cells were present in abundance in the testes
of the male Sebright like those present only in the females of other
breeds. Whether or not the shape of the testis of the Sebright, that is
sometimes like that of the ovary, is connected with the unusual
abundance of luteal cells in the testis I do not know. If so, then the
hint that came from their shape was not so unreasonable as appears at
first sight.

The birds first operated upon were adult F₁ and F₂ hen-feathered birds.
The first one done by myself died, but a few, whose testes were removed
by Dr. H. D. Goodale at my request, lived and changed to cock-feathered
birds. Since then I have operated successfully on a number of F₁ and F₂
birds, as well as Sebright males. In these operations I have had
throughout the assistance of Dr. A. H. Sturtevant and for two years the
assistance of Dr. J. W. Gowen also. I wish to express my appreciation of
their help and advice, for without it I doubt whether I could have
carried out the work successfully. Since the main interest attaches to
the Sebright experiments, they will be described first, although they
were the last to be performed.




CASTRATION OF SEBRIGHTS.


Except for the similarities of the plumage, the male Sebright differs as
much from the female as do cocks of other races. The rose comb is very
large in the male, small in the female (plate 4, figs. 3, 4). The
wattles also are longer in the male. The cock carries himself erect, as
do the males of other breeds. His spurs are well developed and he shows
the aggressive behavior of his sex. On the other hand, the shortness of
the feathers on the back of the neck (the hackles), the absence of the
pointed feathers on the back and rump, and the usual absence of long
sickles and other tail-covert feathers make him hen-like. The detailed
account of the feathers in these critical regions will be given when
comparisons are made with the feathers of the castrated birds (plates 6
and 8).

Six males have been successfully operated upon and with one apparent
exception have all given the same results. The birds were of somewhat
different ages; they had been hatched about July, and were operated upon
about November of the same year, when they were either half grown or had
nearly reached maturity. At the time of the operation a few feathers
were removed from different regions of the body, and the new feathers
that regenerated in the course of 3 or 4 weeks showed all the
characteristics of those that came in later to replace the juvenile or
first adult coat. These regenerated feathers do not, therefore, call
for special notice. All of the new feathers were in shape, pattern, and
general coloration strikingly different from the original feathers, some
of which were at first still present, the old feathers of course showing
no change.

After completely molting, the appearance of the birds may be gathered
from the photographs (plate 5) and from the colored drawings (plates 1
and 3). The male now has in all points the plumage of a typical
cock-feathered male bird of other breeds. This is startlingly apparent
in the hackle, back, rump, sickle, and tail-covert feathers. Instead of
the laced feathers that are characteristic of both male and female, the
whole upper surface of the bird appears reddish or yellowish, the black
marginal edging of the feathers having disappeared. A detailed
comparison of the feathers of the different regions will show how great
a change has taken place. (See page 8.)

In plates 6 and 8 the feathers from characteristic regions of the normal
Sebright and of the castrated Sebright are shown in pairs.

One of the first Sebrights that was castrated was a lighter bird than
the others. Its lighter color was partly due to the narrower outer band
of the laced feathers, (plate 6, figure 1,) and partly to the lighter
color of the yellow-brown center of the feathers. The bird had a single
comb, but as this crops up occasionally in some stocks of Sebrights, it
need not be interpreted to mean that the bird was impure for color
factors. After being castrated the bird changed over completely to
cock-feathering and has remained in that condition for two or more
years. As shown in plate 5, figure 2, the plumage is even more fully
developed than in cock birds of some other breeds. The comb and wattles
are, however, shrunken and pale, as in a capon. The bird is timid and
scarcely or never crows. When killed (May 1919) no pieces of testes and
no trace of testicular tissue at the old situs were found.

The details of the feathers are shown in plate 6, figures 1 and 1a,
where, in each instance, one of the old and one of the new feathers from
the same region are placed side by side. The feathers on the head and
hackle are yellow, even to the base. At the base of the hackle--the
so-called cape--a few feathers have a small black tip. The feathers of
the back are entirely yellow, except that where the fluff begins there
is some dark pigment. The saddle feathers are for the most part all
yellow, but a few have at the base, near the fluff, black on each side.
The tail coverts are long, with a black margin at their tip. The tail
feathers are long, mossy, and have a black tip. The wing-bow feathers
are all yellow, except the black fluff at the base. The feathers on the
crop are mostly yellow with black margin around the end. Those on the
breast lower down are yellow with black tip and black fluff.

There was another Sebright operated upon at the same time that was a
darker bird (as the original feathers show, plate 8, figs. 1 to 4). It
had a rose comb. The feathers that were plucked at the time of
operation were replaced at once by new feathers of the cock-feathered
type. The new feathers that came in as the old ones were molted were
also cock-type, and the bird soon assumed the complete characteristic
cock-feathering. The comb was shrunken as in castrated birds (plate 5,
figure 5).


SEBRIGHT.

Plate 6 and 8.

     1. On the head (_a_) the feathers are small, dull black with
     lighter margin and reddish quill.

     2. On the hackle (_b_) the feathers are yellow bordered with black,
     especially at the base, and at the tip outside of this border there
     is a narrow yellow border (broader at base). The border is absent
     at tip.

     3. In the middle of the back (between the wings) the feathers are
     yellow with black margin at the tip. At the base there is some dark
     color.

     4. The saddle is made up of typical laced feathers with black where
     the fluff begins.

     5. The tail coverts are short; the upper ones, especially the short
     sickles, are slightly curved. The sickle feathers extend up only
     about half the length of the tail. They are yellow, laced, and have
     a black margin, tending to be lost at tip.

     6. The yellow tail is short and erect.

     7. The feathers on the wing bow are practically like those on the
     back, but shorter.

     8. Over the crop and lower breast the feathers are laced.


CASTRATE.

Plates 6 and 8.

     1. Feathers entirely yellow and more slender. Those on each side of
     the shrunken comb stand up from the head.

     2. Hackles on upper part of neck have a black base with red tip.
     The outer edge, without barbules, is narrow, then broader than at
     tip. Farther down the neck the edge with barbules is yellow with a
     narrow black margin.

     3. In the part of the feather with barbules there is a yellow
     center bordered by a broad black band, especially at base. In the
     part without barbules the feathers are yellow and more pointed.
     This region especially is deeper yellow than in the original
     Sebright.

     4. Saddle consists of long, slender laced feathers, except at tip,
     which is red. Barbules are absent along edge of outer third of
     feather.

     5. Tail coverts long, covering the tail as in cock birds of other
     breeds. The sickle feathers, especially the upper ones, are much
     curved, with black tips; the black margin is largely gone. The
     feathers are mossy, sometimes splotched (this is also sometimes
     noticeable in normal birds).

     6. The tail feathers themselves are almost twice as long as in
     normal bird; the upper feathers are more curved.

     7. The feathers of wing bow are like those of the back of the same
     bird, but shorter.

     8. The crop feathers are narrower, with a wider black margin, and a
     few may be also entirely black. Feathers on lower breast much like
     those in normal, but a little more pointed.

During the spring of 1917 it was noticed that the bird was going back
towards the hen-feathered type, and by the end of the summer he was in
the intermediate condition, as shown by the photograph and by the
individual feathers (plate 8, 1_b_, 2_b_, 3_b_, 4_b_). The comb had
begun to enlarge also. The bird was opened again (1918) and pieces of
testis about as big as peas were found on one side. Evidently a piece of
the old testis had been left behind and had regenerated. As it enlarged
the new feathers were affected so that the plumage returned towards the
normal type. The pieces of testis were removed and a few feathers
plucked out. The new feathers that came in were typically
cock-feathered, and, as the molting proceeded during the winter and
spring, the bird became cock-feathered for a second time as shown in
photograph (plate 5, figure 6) and by the feathers in plate 8, 1_c_,
2_c_, 3_c_, 4_c_. Here, then, is an excellent example of the connection
between the gonad and the condition of the plumage. On opening this bird
(May 1919) no pieces of testes were found. There was a very small
whitish lump at the situs of the old testes, which, when sectioned,
showed some glandular-like tissue, not in tubules, and no evidence of
testicular tissue.

Three other younger Sebrights of the same stock were successfully
castrated. They were hatched in June or July and castrated in November
of the same year. They remained quite small birds, despite their
elongation due to the long tail and tail coverts that they developed.
One of these birds in his cock-feathered plumage is shown in plate 3,
figure 1. One has died, the other two are alive and markedly
cock-feathered, as shown in plate 6, figure 2_a_. All three birds were
dark red-brown, much more so than the two preceding cases, especially
the first case. This color difference might be attributed to the earlier
age of the three birds when operated upon, or to a more complete (or
less complete) operation involving perhaps neighboring parts, or to the
birds having a somewhat different genetic composition (_i. e._,
modifying factors). There is no special reason why the operation if
performed early should have a different result on feathers that develop
after the bird is of adult age. Goodale has suggested that there may be
organs in the vicinity of the testis that have some influence on the
kind of plumage produced, and if there are such organs they might be
removed in one bird and accidentally left in another. It would not,
however, be probable that the bird operated on at first had received one
treatment and the later ones the other treatment. It seems to me more
probable that the birds have come from different genetic strains, and
that this genetic difference gives a more plausible explanation of the
darker cock-feathered plumage. Goodale observed, for the first time I
think, that the largest wing coverts of the castrated cock become
longer. I looked, therefore, with some interest at the condition of
these same feathers in the castrated Sebrights. As shown in plate 10,
figures 1, 1_a_, these feathers are also longer and narrower in the
castrated Sebright than in the normal bird.

The true tail feathers of the capon are said to be longer than those of
the cock. This holds also for the tail feathers of all of my castrated
Sebrights. Their true tail feathers are considerably longer than those
of the normal male, as seen by pulling them out and comparing the two.
Their length is concealed while on the bird by the excessively long
coverts that appear after castration.

In 1916 I operated on a Sebright male that lived for some months, but
died in the summer of 1917. At the time of his death he had assumed a
partial cock-feathering, as shown by the feathers in plate 9, figure 3,
3_a_. Dissection showed that some of the testes had been left, and as is
then to be expected, the change was incomplete.




A MALE SEBRIGHT THAT DID NOT BECOME COCK-FEATHERED AFTER CASTRATION.


One of the males that had been castrated with the others did not become
cock-feathered even after a year. Taking for granted that the castration
had been incomplete, the bird was opened, but as no pieces of the testes
were to be found in the normal position he was killed and carefully
dissected. There were no pieces of testes found in the normal situs. A
small whitish patch of material from this region was cut into sections,
but no testicular material was found in it. Then a large piece of the
back from the region of the attachment of the testes was prepared, but
as yet this piece has not been sectioned. Even were a small piece of
tissue to be found, it would seem unlikely that it would suffice to hold
back all indications of the cock-feathering, for after incomplete
removal of the testis there are nearly always at first some indications
of the lack of material. The most plausible view here is either that
some other gland may have assumed, provisionally, the function of the
missing testes, or else a detached piece has not yet been found.
Glandular cells like the luteal cells of the ovary have in fact been
described by some observers in other organs of the body. As yet I have
not found time to make a thorough histological study of the tissues of
this bird.




TRANSITIONAL FEATHERS.


In several birds new feathers had begun to develop at the time of the
operation under the influence of the testicular secretion. After the
removal of the testes, these feathers continued to grow and in the
absence of the original conditions changed over to the other type. The
outer end of these feathers shows the original or normal shape and
color, while the inner end shows the new characteristics. Such feathers
have been seen in nearly all of my castrated birds; a few from the
Sebright will suffice by way of illustration. In plate 10, figure 2_a_,
four such transitional feathers are shown. In _a_ and _b_ two feathers
from the hackle are photographed. The first (_a_) had begun as a normal
Sebright hackle feather, as seen in the condition of its tip; the rest
of the feather is the same as the feather of the castrated bird. For
comparison with this feather, two (2_b_) from the same bird are shown
that began to develop after the testes were removed, _i. e._, at the
same time as the change occurred in the former feather. At the time the
latter feather (_b_) had not yet completed its full growth. On the bow
of the wing a few intermediate feathers, like the one shown in 2_a_,
were present. (For comparison with normal and castrated feathers see
those on plates 6 and 8.)

An intermediate feather from the back is shown in 2_a_. For comparison
with the old feathers from the same region see plate 6, fig. 2. An
intermediate saddle feather is shown in 2_a_. For comparison with normal
feathers from the same region see figure 2. A still later feather from
the castrated bird is shown in 2_b_. The last was not yet complete when
removed from the bird.

It will be noticed that the change after castration involves the color,
the shape, and the presence and absence of barbules in those parts of
the bird that are peculiar in the last respect. The transition in these
characters is quite sharp--as sharp in fact as is compatible with the
passage from one structure to that of an entirely different kind without
any discontinuity of growth. Owing to the quickness of the response
shown by the feather, it will be possible to study more in detail the
length of time the secretion remains in the body of the bird after the
testes have been removed.




CASTRATION OF F₁ HEN-FEATHERED MALES FROM SEBRIGHT BY GAME.


Hen-feathering is dominant to cock-feathering. As shown in plate 2, the
F₁ male is almost as completely hen-feathered as is the male Sebright.
There is a somewhat greater color difference between the F₁ male and F₁
female than between the Sebright male and female. Two F₁ birds were
castrated for me by Goodale. At the time of operation, in the autumn of
1916, both birds were full grown, (plate 2, figure 1). After molting the
old feathers, both birds appeared as shown in plate 2, figure 4. Each is
completely cock-feathered. The plumage has also undergone a remarkable
change in color. In general, the color change is from yellow and black
to reddish yellow. The greatest change is over the upper surface. The
sickle, covert, and tail feathers are well formed and have now become
iridescent black. The breast has changed least of all. One bird died
February 12, 1919. When opened there was found on the left side a small
white lump; on the other side almost nothing. The lump was found to
consist of testicular tubules with loose glandular cells on its walls.

The extent to which the change has taken place is best shown by
comparison of individual feathers from identical regions--one before and
one after the new feathers (taken out two years later) have come in
(plate 7). The contrast between the old and new feathers of the hackle,
saddle, back, and wing-bow are the most striking. In all of these the
new feathers have become red on the exposed portion and the margin is
free from barbules, as in the cock bird. The increase in size and
change in shape of these feathers is remarkable. Equally great is the
change in the tail-coverts that grade into those of the saddle at the
base. The two median coverts or sickle feathers are longer than the tail
and much arched. They are jet black with a purplish iridescence and with
a yellow-red shaft. The tail itself has also changed; it has lost its
stippling, and has become black like the coverts. The increase in length
of the tail feathers is as remarkable as the increase in length of the
coverts. A detailed account of these changes follows:


F₁.

     1. The head feathers are yellow with black base that shows through
     on the head.

     2. The hackle feathers are yellow with black base (showing through
     on neck).

     3. The red feathers of the back are penciled. There is a black
     band, especially around tip just inside of the margin.

     4. The saddle feathers are much like those of the back, much
     stippled in center. The border is more distinct.

     5. The tail coverts are similar to those of the hen.

     6. The tail feathers are black, and with exception of the lower
     feathers they are partly stippled.

     7. The wing-bow feathers are penciled like those of the back.

     8. Feathers on crop yellow with black spot at tip; those lower down
     on breast have a bigger spot.


CASTRATE.

     1. The feathers are entirely red.

     2. The hackle feathers are entirely red.

     3. Feathers of back are red except for black at base. Barbules
     absent at end and side. Tip pointed.

     4. The saddle feathers are red with black base. They are long and
     pointed.

     5. The coverts are blue-black, with brown shaft. They are long,
     pointed, and curved.

     6. The tail feathers are black, not stippled, and have a black
     shaft.

     7. The wing-bow feathers are red with black base.

     8. Over the crop the feathers are orange-brown; on the breast they
     have the same color and a small black tip.

The F₁ bird from which the colored drawing (plate 2) was made and from
which the normal F₁ feathers were pulled was lent to Dr. Goodale in the
summer of 1917. The bird died in April 1918, and his skin was sent to
me. He also had begun to change over to cock-feathering (plate 9,
figures 2, 2_a_). Goodale recorded that the testis had dwindled to small
bodies only about 10 by 5 mm. This accounts for the change to
cock-feathering. For comparison I have added a third set of feathers to
the two former sets, showing the new hackle, back, saddle, wing, and bow
feathers of this bird. The feathers show that the change is in the same
direction as that shown by the castrated cock, but it has not gone so
far in the direction of cock-feathering. The tail is still short and the
feathers are black. The sickle feathers are not longer than the tail and
are stippled. It is probable that this is the old tail whose feathers
have not been molted since the testis dwindled. In fact, elsewhere the
old and the new feathers are both present, showing that a complete molt
had not taken place. The old feathers still present are practically like
those of the original F₁ bird, showing that the change was of recent
date, and due to the decrease in the testis which was probably caused by
disease.




CASTRATION OF F₂ HEN-FEATHERED MALES.


The F₂ hen-feathered males from this cross could not be utilized until
they had begun to assume the adult plumage, since before that time they
were like the cock-feathered F₂ males. Consequently, the operation is
more difficult and more dangerous to the bird. A good many birds have
died in consequence of the operation, but enough successful operations
(five) were made to show what the color of certain types of
hen-feathered bird would be when changed to cock-feathering.

A hen-feathered male (No. 292) that was darker than the F₁ male--in
fact, almost black, except for a yellow center in some of the dorsal
feathers that were mossy or penciled--was castrated. The details of
characteristic feathers may be gathered from the feathers in plate 7,
figure 2. A corresponding set of the new feathers after castration,
2_a_, are paired with the former. The castrated male in his new plumage
is shown in plate 2, figure 3. His dorsal surface is colored very much
as is the same region in the F₁ bird, but the breast is very much
darker, so that the bird as a whole presents a very different appearance
from the F₁ castrated male. A very small white mass was found when the
bird was killed in place of the old testis, composed, in sections, of a
reticulated mass of cells that look like old broken-down follicles of
testicular tubules with a few cell-layers lining the tubules.

An F₂ male (68) also had dark feathers (plate 3, figure 2, and plate 9,
figure 1). The castrated male in his new plumage is represented in plate
3, figure 3. Here again the upper surface is much like that of the last
castrate, and also like that of the F₁ castrate. The breast has changed
much less than the back; the centers of the feather are brown with a
black margin and a black band at the tip. The exposed portion of the
secondaries and the coverts are not so brown as in the last bird. The
spurs of this bird were bent back, looking like the horns of a ram. When
killed and examined, several small white pieces, that looked like pieces
of testes, were found in the abdominal cavity near the old attachment of
the testis. A histological study showed that these pieces contained
tubular tissue apparently testicular, but without germ-cells.

Another F₂ male (Band No. 221) was yellow in general color, the feathers
being irregularly penciled. After castration (plate 3, figure 4) the
bird became red above and deep brown below; the tail and coverts were
black.

A pale-yellow hen-feathered bird (No. 218) was also castrated. Here also
the change was most conspicuous over the upper surface, not only in a
greater depth of color than elsewhere, but in the shape, etc., of the
feathers. On the breast the original yellow color remains, but is
slightly deepened. When killed and opened (May 14, 1919), a few small,
whitish pieces were found. When these were sectioned it was seen that
they were made up, for the most part, of tubules looking like those of
the epididymus and also a few testicular tubules. At the old situs there
were some regenerated lumps, which in sections appeared to be loose
glandular tissue. No germ-cells were present and the tissue just
referred to may be old testicular tubules.




HEWITT’S SEBRIGHT HEN THAT BECAME COCK-FEATHERED IN OLD AGE.


Darwin records in Chapter XIII of Animals and Plants under Domestication
a change that took place in an old female Sebright:

     “Mr. Hewitt possessed an excellent Sebright gold-lace bantam hen,
     which, as she became old, grew diseased in her ovaria and assumed
     male characters. In this breed the males resemble the females in
     all respects except in their combs, wattles, spurs, and instincts;
     hence it might have been expected that the diseased hen would have
     assumed only those masculine characters which are proper to the
     breed, but she acquired, in addition, well-arched tail
     sickle-feathers quite a foot in length, saddle-feathers on the
     loins, and hackles on the neck--ornaments which, as Mr. Hewitt
     remarks, would be held to be abominable in this breed.”

This is the only record I know of showing the change that takes place in
the Sebright hen when the influence of her ovary is removed. There can
be no doubt from the above description that she changes in the same way
as does the castrated Sebright male.

Concerning the origin of the Sebright bantam Darwin states that the race
“originated about the year 1800 from a cross between a common bantam and
a Polish fowl, recrossed by a hen-tailed bantam, and carefully selected;
hence there can hardly be a doubt that the sickle feathers and hackles
which appeared in the old hen were derived from the Polish fowl or
common bantam; and we thus see that not only certain masculine
characters proper to the Sebright bantam, but other masculine characters
derived from the first progenitors of the breed, removed by a period of
about 60 years, were lying latent in this hen bird ready to be evolved
as soon as her ovaria became diseased.” To-day the problem appears to us
in a somewhat different light, since the secondary sexual characters
referred to by Darwin have simply been kept under for more than a
hundred years by the secretion produced in the ovary of the hen (as in
all breeds) and in the testis of the male Sebright.




HEREDITY OF HEN-FEATHERING.


In 1913 I found that hen-feathering as seen in the Sebright is a
dominant non-sex-linked character. A preliminary statement was given in
the first edition of my book on Heredity and Sex (1913), which treated
the character as a recessive one. This was a mistake due to a male
having been obtained that was like the game race, which subsequent work
showed must have been due to a sperm having been retained in the
oviduct of the female during her isolation period. In the second edition
published a few months later the mistake, having been found out, was
corrected.

If _one_ dominant suffices to produce hen-feathering, the F₂ ratio would
be 3 hen-feathered to 1 cock-feathered bird. The numbers found were 31
to 28. This realized ratio departs too far from a 3:1 ratio to make it
probable that the results are due to a single factor.

The F₂ expectation for _two_ dominants, both necessarily present to
produce hen-feathering, is 9 hen-feathered to 7 cock-feathered birds. If
the dominant factors are represented by H and H´ and their wild-type
(recessive) allelomorphs by h and h´, the expected F₂ recombinations are
given in the following table:

    +-----+-----+-----+-----+-----+
    |     | HH´ | Hh´ | hH´ | hh  |
    +-----+-----+-----+-----+-----+
    | HH´ |{HH´ | Hh´ | hH´ | hh  |
    |     |{HH´ | HH´ | HH´ | HH´ |
    |     +-----+-----+-----+-----+
    | Hh´ |{HH´ | Hh´ | hH´ | hh  |
    |     |{Hh´ | Hh´ | Hh´ | Hh´ |
    |     +-----+-----+-----+-----+
    | hH´ |{HH´ | Hh´ | hH´ | hh  |
    |     |{hH´ | hH´ | hH´ | hH´ |
    |     +-----+-----+-----+-----+
    | hh  |{HH´ | Hh´ | hH´ | hh  |
    |     |{hh  | hh  | hh  | hh  |
    +-----+-----+-----+-----+-----+

There are 9 classes containing both H and H´, 6 containing one or the
other, and one containing neither H nor H´. The realized numbers, 31 to
28, are in close approximation to 9:7.

In classifying the F₂ hen-feathered males, an attempt was made to divide
them into two classes, viz, type 1, hen-feathered to the same extent as
the Sebright, and type 2, intermediate between hen and cock feathering.
The line between intermediate and cock-feathering is sharp, all the
intermediates belonging distinctly to the hen-feathered group, but the
line between the two subdivisions of hen-feathered birds is not sharp,
and occasionally a bird is found that is difficult to place. These
statements hold also for the F₁ birds, whose skins I now have. Five of
these are classified as intermediates and one as completely
hen-feathered. The difference between these two classes, then, is
environmental or due to other modifying genetic factors, for which
either the Sebright or the game is not pure. Under these circumstances
it would not be profitable to attempt to find out (without additional
evidence) what genetic differences, if any, lie behind the hen-feathered
and intermediate-feathered birds in the F₂ classes.

Concerning the back-cross (F₁ by game) the expectation, for one dominant
factor-difference, is 1 hen-feathered to 1 cock-feathered male. There
were obtained 2 hen-feathered (intermediates) to 7 cock-feathered
birds. The numbers are too small to be significant, taken by themselves.
The expectation for 2 dominants, both essential to hen-feathering, is 1
to 3, and this is in agreement with 2 to 7 as found. It seems, then,
more probable from the evidence of the F₂ and of the back-cross combined
that there are two dominant factors present in the Sebright that make
the male hen-feathered, and since the race breeds true to
hen-feathering, both factors must be present in homozygous condition
unless an undetected lethal destroys some of the classes.[1]

+------------------------------------------+---------------------------------------------+
|                                          |                                             |
|_A_                                       |_C_                                          |
|     GOLD MALE      X    SILVER FEMALE    |                                             |
|                    |  (Product of English|                                             |
|  (Belgian imported |    type males for 7 |                                             |
|   from Brussels.)  |    generations in my|     SILVER MALE       X        GOLD FEMALE  |
|                    |    own yards)       |                       |                     |
|      +-------------+--------------+      |      +----------------+---------------+     |
|      |                            |      |      |                                |     |
|6 Silver Males             6 Gold Females |6 Silver Males              6 Silver Females |
|(Belgian type)                            |(English type)                               |
+------------------------------------------+---------------------------------------------+
|                                          |                                             |
|_B_                                       |_D_                                          |
|                                          |                                             |
|  SILVER CROSS-BRED  X GOLD CROSS-BRED    |   SILVER CROSS-BRED   X   SILVER CROSS-BRED |
|         MALE        |     FEMALE         |        MALE           |        FEMALE       |
|  (offspring of A)   | (offspring of A)   |   (Offspring of C)    |   (Offspring of C)  |
|                     |                    |                       |                     |
|    +--------+-------+-----+---------+    |       +---------------+---------------+     |
|    |        |             |         |    |       |               |               |     |
|4 Silver  2 Gold       2 Silver   5 Gold  | Silver Males    Silver Females  Gold Females|
| Males     Males        Females   Females | (English type)                              |
|(Belgian  (Belgian                        |                                             |
|  type)     type)                         |                                             |
+------------------------------------------+---------------------------------------------+
|                                                                                        |
|_E_                                                                                     |
|                                                                                        |
|       SILVER MALE (offspring of C)          X            GOLD FEMALE (pure)            |
|                                             |                                          |
|      +-------------+-------------+----------------+-------------+-------------+        |
|      |             |             |                |             |             |        |
|Silver Males  Silver Males    GOLD MALES       Gold Males  Silver Females Gold Female.  |
|(Eng. type)  (Belgian type) (ENG. TYPE 1912) (Belgian type)                             |
|                             ==============                                             |
|                             ==============                                             |
+----------------------------------------------------------------------------------------+
|                                                                                        |
|_F_                                                                                     |
|                                                                                        |
|      SILVER MALE (offspring of C)         X        SILVER FEMALE (pure)                |
|                                           |                                            |
|      +------------------------------------+------------------------+                   |
|      |                                    |                        |                   |
|Silver Males (English type)          Silver Females           Gold Females              |
+----------------------------------------------------------------------------------------+

Smith and Haig have reported the following curious case of
hen-feathering. Smith had a breed of White Leghorns with cocks of two
classes--those that assumed cock plumage at 6 months and those that are
like the hens for 8 months, after which they slowly assumed the
cock-feathering. The difference is hereditary and appears to segregate.
Possibly this breed had one factor at least for hen-feathering that is
effective for young birds, but not for older ones, or some of the birds
pass through a stage when they produce an internal secretion that
disappears later. But it is also possible, and perhaps more probable,
that the young birds, not cock-feathered, have remained longer in the
juvenile stage than the others, so that they might be said to be falsely
hen-feathered.

The results published by the Rev. E. Lewis Jones in 1914, describing
crosses between two breeds of Campines, one called Belgian (which has
hen-feathered males), the other English (that has cock-feathered males),
are summarized in the table on page 16. They show the dominance of
hen-feathering with some probability. The table given there is the
original, to which the author has kindly added the numbers here prefixed
to some of the classes. The numbers are not large enough in all cases to
be satisfactory, but the dominance of the hen-feathering is, I think,
apparent, as well as its non-sex-linked transmission. The golden female
in C must have been English type, or at any rate heterozygous for
English-type feathering, for if Belgian her sons would have been Belgian
type.

Punnett and Bailey (1914) have published the result of a cross with
hen-feathered Silver Sebrights and Hamburgs. The dominance of
hen-feathering in the male is shown in the figures that illustrate their
paper, but as the paper deals solely with the inheritance of weight the
account of inheritance of hen-feathering was deferred to a later paper,
that has not yet appeared.




HEREDITY OF COLOR IN THE CROSS BETWEEN SEBRIGHT AND BLACK-BREASTED GAME
BANTAM.


The cross between the Sebright and the Black-Breasted Game bantam was
undertaken primarily to study the inheritance of hen-feathering. The
Sebright was chosen, on the one hand, because this race is pure for
hen-feathering, whereas in other races, such as the Campines, both kinds
of males are known. The hen-feathered birds of such races are, I
believe, frequently not pure for hen-feathering. The game race was
chosen because the cock has the typical plumage of the wild bird,
_Gallus bankiva_, and although his feathers are remarkably short, they
show the characteristic cock-feathered type.

Only secondarily was the experiment concerned with color inheritance.
The two breeds differ so markedly in coloration and pattern that the
very complex results that appeared in F₂ were to be expected. In
addition to the differences involving hen-feathering _versus_
cock-feathering, and Sebright plumage _versus_ game plumage, the game is
strongly dimorphic in the plumage, while in the Sebright the coloration
of the two sexes is closely similar. But the castration experiments have
shown that this difference is the result of hen-feathering in the
Sebright cock, and that the race carries the same potential dimorphism
as do other races of poultry.

The game cock is shown in plate 1, figure 1, and plate 4, figure 1. The
wattles and comb had been removed from the bird. The yellow-red back and
saddle are to be noted. The upper tail coverts and sickle feathers are
black, as is the tail. These parts are shorter in the game than in other
races, being one of the points selected for. The dorso-anterior edge of
the wing is black, this color meeting across the middle of the back.
Below this black area comes the red wing bow, followed by a double row
of blue-black feathers. The exposed portions of the secondaries are
brown, of the primaries black with green margin. The breast and entire
lower surface is black. The legs are greenish, the bill black and
yellow, the iris yellow.

The hen of the Black-Breasted game (plate 1, figure 2) is light
yellowish-brown. The back, saddle, and wing coverts are golden brown,
finely penciled with darker brown or black. The hackle is penciled; it
has a yellow border (without barbules); the back is more brown, the
forepart of the breast is salmon, the more posterior parts lighter
salmon. The sides of the body under and below the wings are stippled
gray.

The Sebright male is represented in plate 1, figure 3. Photographs of
the male and the female are given in plate 4, figures 3 and 4. Most of
the feathers have a yellow center and a black border. Such feathers are
said to be laced. The details of the different regions are shown in the
feather plates, 6 and 8.


A. THE F₁ BIRDS.

The F₁ birds were remarkably uniform. The sexual dimorphism is slight,
as a comparison of the male and female in plate 4, figures 5, 6, will
show. In the female the body feathers are penciled but very mossy, and
this holds for the male too, except that in the hackle, back, and
saddle, a change in color accompanies the change in shape, as seen in
the individual feathers in the feather chart (plate 7, figure 1). If
there are any sex-linked factors involved in the cross, we should expect
different types of F₁ hens in the direct cross and its reciprocal,
because in one case the F₁ hen gets her single X chromosome from one
father, and in the other case, the reciprocal cross, from the other.
Unfortunately no careful comparison can now be made, because the crosses
were carried out in different years and the changes due to age may have
affected the color sufficiently to obscure such slight difference that
may have existed. But the effects of such factors, if present, are very
small, since the birds seemed to be the same, regardless of the way in
which the cross was made. In the F₂ counts, although an attempt has been
made to keep apart the birds obtained in the two crosses (_i. e._, the
direct and the reciprocal crosses), it is very doubtful if the two
groups show any significant differences.


B. DESCRIPTION OF F₂ BIRDS.

All together there are 72 hens, 29 hen-feathered males, and 26
cock-feathered males, as shown in table 1:

                                         TABLE 1.
+-+-----------------------------------------------+-------------------------------------------+
| |       F₂ from Sebright ♀ by Game ♂.           |F₂ from Sebright ♂ by Game ♀.               |
| +---------+----------+--------------+-----------+---------+----------+----------+-----------+
| |         |Hen-feath.|Hen-feath.    |Cock-feath.|         |Hen-feath.|Hen-feath.|Cock-feath.|
| |Females. |♂ type 1. |  ♂ type 2.   |  ♂.       |Females. |♂ type 1. | ♂ type 2.|♂.         |
+-+---------+----------+--------------+-----------+---------+----------+----------+-----------+
|A|   15    |          |       3      |     2     |    1    |     1    |     4    |           |
|B|    1    |     1    |              |           |    5    |          |          |     1     |
|C|    3    |     1    |              |           |    4    |     1    |          |           |
|D|    3    |     2    |              |     2     |    2    |          |          |     1     |
|E|    6    |          |              |           |         |     1    |          |     1?    |
|F|    6    |     1    |       2      |     3     |         |     2    |     1    |     1     |
|G|   11    |     1    |       1      |     1     |    2    |     1    |          |     1     |
|H|    5    |          |              |           |         |          |          |           |
|I|    1    |          |              |     2     |    1    |          |          |     2     |
|J|    2    |     1    |              |     2     |    2    |     1    |     1    |     2     |
|K|         |          |       2      |     1     |         |          |          |     2     |
|L|         |          |              |           |    1    |          |          |           |
|M|         |          |              |           |    1    |          |          |           |
|N|         |          |              |           |         |          |          |           |
|O|         |          |              |           |         |          |          |     1     |
|P|         |          |              |           |         |          |     1    |           |
| +---------+----------+--------------+-----------+---------+----------+----------+-----------+
| |   53    |     7    |       8      |    13     |   19    |     7    |     7    |    13     |
+-+---------+----------+--------------+-----------+---------+----------+----------+-----------+


F₂ HENS.

     _A._ 8 females that are like F₁; 7 others resemble them below, but
     have stippled back and rump feathers (4 of these have yellow necks,
     and 3 black necks like those of F₁). Here, then, are two or three
     subdivisions, or perhaps main classes.

     _B._ 1 female is very close to game, having the characteristic
     stippling above and salmon breast below. She is darker colored than
     the game females, therefore more like the Leghorn female.

     _C._ 3 birds resemble the Sebright in plumage, but would not pass
     muster for real Sebrights.

     _D._ 3 others have spangled breast feathers like the Sebright, but
     a great amount of stippling on the back.

     _E._ 6 birds are yellow on breast, with stippled back.

     _F._ 6 others are yellow, stippled birds with a little yellow
     penciling on the breast.

     _G._ 11 black birds with some stippling on the back of the wing,
     and sometimes with traces of yellow in the hackle.

     _H._ 6 other birds are dark, but not as black as the last. There is
     some stippling, especially on breast. The hackle is always striped.
     (4 of these have yellow necks above and below; 2 have dark necks.)
     The series of feathers photographed in plate 10, figure 4, from a
     bird of class _H_, show all gradations between a spangled and a
     barred condition. It is practically certain that the barring seen
     here (as well as that under class _M_) is quite different from that
     of the Barred Plymouth Rock.

     _I._ 1 yellow hen with a little black (as a band) on wings and
     tail.

     _J._ 2 other yellow birds with a little black penciling on the
     back, the tail, and with long wing feathers.


F₂ MALES.

     _K._ 2 intermediate males with a black-splotched red breast and
     black tail. The saddle is coarsely stippled. The corresponding
     male-feathered bird is red above.

     _L._ A Sebright-like bird with black stippled feathers on back; the
     rest of the plumage heavily laced. Posterior part of breast and
     thigh black. The tail stippled.

     _M._ Yellow neck, the back, tail, and lower half of each secondary
     coarsely stippled. Posterior part of breast barred; cape and
     anterior breast penciled. The series of feathers photographed in
     plate 10, figure 5, from the breast of a bird in class _M_, shows
     all possible gradations from a penciled to a barred condition.

     _O._ 1 cock-feathered, Red-Breasted game with somewhat stippled
     feathers. The lower half of each secondary is penciled (Hamburg
     type).

     _P._ 1 intermediate male with yellow hackle that is black striped.
     He has a peculiar saddle, the general color of which is reddish
     brown. Each feather has a faint black edge, and is clear yellow
     along shaft; the rest of the feather is finely dusted on a yellow
     background (plate 10, fig. 3). Breast feathers (in front part) are
     laced with an outer black band edged with yellow. In the posterior
     region of the breast the feathers are broadly laced. Wing-bow and
     coverts red, laced with black. The exposed edges of primaries and
     secondaries are red-brown, the covered parts black. The tail is
     black.


BACK-CROSS HENS.

     _A._ Two dark F₁ types. The breast is between stippled and
     penciled, the head is black.

     _C._ (1) Sebright type. Very dark with much stippling. Some
     penciling on back. Breast dark; neck like that of Sebright.

     (2) Sebright type like (1), but not such clear yellow. Secondaries
     and tail feathers and coverts stippled (with black tips).

     (3) Yellow Sebright. Neck and breast yellow with black base and tip
     to feathers. Cape, breast, and wings (except bow) penciled to
     barred.

     _C._ to _D._ (1) Pale yellow, breast spangled, back lightly
     penciled, tail same. Secondaries yellow and little stippled. Upper
     web of primaries stippled.

     (2) Breast spangled, rest as in (1).

     (3) Same as (2).

     _L._ Dark Sebright. Back-feathers broadly laced and a little
     penciled. Neck black with yellow centers to feathers and yellow
     edges (reversed Sebright).

All of the preceding hens except A are in general Sebrights. The last
three are pale stippled Sebrights.


BACK-CROSS HEN-FEATHERED COCKS.

     _A._ One cock like F₁ male, but rather paler on back.

     _C._ Four cocks. Light Sebrights, but spangled, in general, instead
     of laced. Feathers clear, not stippled.

     _G._ One black.

     _L._ Dark Sebright. Back and rump black. Feathers with narrow
     center, not stippled.

     _Q._ Dark Sebright nearer to hen C (1). Thoroughly stippled with
     game tail. Neck and breast dark Sebright. Probably a new class
     nearer to (C).

     _R._ Two cocks. Pale yellow instead of reddish, and much less black
     than are other yellows. No class of hens to match.[2]

In regard to color inheritance the preceding 19 birds are too few to add
anything of significance to the other results, except that they serve to
emphasize the dominance of the factors making for Sebright coloration.
The hen-feathered cocks confirm the other results as to the dominance of
the factor or factors in question.

There can be little doubt that some of these classes are complex. They
almost merge into each other and in one part of the body individuals
may grade off into one class, in other parts into other classes. An
almost continuous series of types might be arranged from black to pale
yellow.

The difficulty of matching the hen-feathered males to their genetic
mates is almost insuperable. In table 1 an attempt was made to put these
males with their respective females. The difficulty is, of course,
greater for the cock-feathered birds, even with the castration evidence
(that is too meager at present for the purpose), but a few of the males
may be placed with certainty, and the rest guessed at.

One bird appears to be a _hen-feathered_ game male resembling in many
respects the female game, but darker and redder. There is more shafting
on cape and wing-bow. The breast is unusually dark-salmon. The hackle is
darker than is the game female. Upper wing-coverts broadly laced with
black. (Plate 10, fig. 3.)

The occurrence of this hen-feathered jungle-fowl is so unique and the
coloration of the bird so interesting that I have added to the plates
three feathers of such a bird, viz., a stippled saddle feather, a
feather from the back, a hackle feather, and a wing covert with stippled
center and a black border. The neck hackle departs somewhat from the
hackle of the jungle-fowl hen, but in the same direction as does the
neck hackle of the Sebright cock from his hen.

Looking over the F₂ group, the most noticeable thing is the large number
of blacks (E and G), all of which are stippled. Probably the factor came
from the game, because group E was present in the back-cross as well as
in F₂, and because these black birds are always stippled. The yellow
color (I and J) may have come from both, each breed having then a black
factor that, as a pattern, covers over most of the yellow. It is
difficult to distinguish penciling from stippling in the F₂ yellows.
Without figuring each of these types, their description in detail is not
of much value. The skins will be deposited for reference in the
Zoological Laboratory of Columbia University.


C. BACK-CROSS OF F₁ TO GAME.

As the back-cross of the F₁ to the game might appear more likely to
reveal the kinds of germ-cells present in the individual, the results
from such a cross may be given before discussing the genetic data. If it
were certain that the “game” contained all of the recessive factors that
are involved in the experiment, this method of testing the result would
be ideal, but there is no way of determining _a priori_ whether this is
the case. The question will be taken up later. The presence of two kinds
of males with corresponding but largely uncorrelated differences in
their plumage makes their classification as a group impossible. It is
simpler, therefore, to put the females into their classes first, after
which the hen-feathered males may be expected to fall into the same
groups (or nearly so), while the identity of the cock-feathered males,
_i. e._, their class relationship can only be determined for the classes
that resemble the F₁ and the P₁ birds. The F₂ hen-feathered males can in
part be further identified by means of the evidence that castration of
these types affords.

Two of the F₂ classes of hens can be identified in this back-cross, viz,
(_a_) 4 hens like the F₁ birds, (_b_) 3 hens like the game; (_c_) there
were 3 other hens with plain yellow, _i. e._, not stippled backs. The
upper surface was like that of the game female, but much lighter. The
first two classes (_a_), (_b_) might be again split into two types.
There were only two hen-feathered males, one nearly like the F₁ male,
the other blacker; they probably belong to different classes.

Of the 7 cock-feathered males, one was like the F₁ _castrated_ males;
another had a similar back, but a darker and differently marked breast;
2 were game-cock type; 3 were odd birds much like the game cock above
except for absence of black, with reddish heads without any black. The
males may be approximately classified as follows:

    +----------+-----------------------------------------------+
    |          |   Back-cross F₁ ♀ by game ♂.   |
    |          +---------------+--------------+----------------+
    |          | Hen-feathered | Intermediate | Cock-feathered |
    |          |    ♂.         |    ♂.        |     ♂.         |
    +----------+---------------+--------------+----------------+
    | F (or K) |               |      1       |        1       |
    | (C)      |               |      1       |        2       |
    | B        |               |              |        2       |
    | A        |               |              |        2       |
    |          |               |              +----------------+
    |          |               |              |        7       |
    +----------+---------------+--------------+----------------+

Four or five types may then be recognized in this rough grouping. None
of the groups seem uniform and probably might be split again.


D. THE NUMBER OF COLOR FACTORS INVOLVED.

The theoretical expectation for two pairs of factors calls for 4 classes
in the back-cross, but this assumes that the parent type used for
back-crossing contains all (here 2) recessives. But this simple
assumption can not be true in this case, for the F₁ bird would have been
like the Sebright. On the 3-factor assumption the expectation for the
back-cross is 8 classes, but this would apply only if the game were the
triple-recessive form, which, again, it is not, as shown by the F₁
cross. But if the dominance of one or more of the Sebright color factors
is incomplete, then either a 2 or 3 factor assumption might apply to the
back-cross.

If only 2 pairs of factors are present we should expect to recover the
game type once in 16 cases in F₂. But, as will be shown, only 1 game was
recovered out of the 49 F₂ females. This result fits better with a
3-factor assumption, for even with the small number in the back-cross
the indications are that more than 4 classes are present.

In the F₂ birds at least 11 classes may be distinguished, and some of
these appear composite. For 3 factors the maximum number of possible
classes (including heterozygotes) is 27. We can recognize at least 11 F₂
classes amongst the females alone, and a few others are doubtfully
present in the males.

In favor of the view that the heterozygous classes are here different
from the homozygous, the following evidence may be utilized:

(1) The F₁ birds are entirely different from either parent and they are
heterozygous for all the factor differences between the two types. The
only alternative explanation for the intermediate condition of F₁ would
be that each race carries one or more completely dominant factors. But
the latter view is improbable because more of each parent type would
then be expected in the F₂ generation.

(2) In the F₂ generation the F₁ type is not as frequent as would be
expected on the view that the heterozygotes could not be distinguished.


E. BACK-CROSS OF F₁ ♀ TO SEBRIGHT ♂.

It is possible to add, now, while this paper is passing through the
press (June 1919), the results of a back-cross of 4 F₁ females to a
Sebright male carried out during the summer of 1918. The birds being now
mature their permanent colors are evident. Making the back-cross in this
direction is much less advantageous than the reciprocal described above,
because the Sebright contains most of the dominant color factors. The
group of birds obtained appeared to be less variable in color than those
from the other back-cross, and one can see at a glance that more of them
approach the Sebright type; some quite closely.

All of the males are hen-feathered, as expected. No evidence was found
that two types of males exist, which would have been expected if the two
types noted in F₂ had any hereditary significance. If, then, as the F₂
results suggest, two factors for hen-feathering are present both are
dominant, and no genetic distinction is found between individuals in
which one or both of the dominant factors are duplex or simplex.

There were 9 adult hens and 10 hen-feathered cocks. An attempt is made
below to refer them to their corresponding F₂ classes.


F. REVIEW OF THE HEREDITY OF THE COLOR OF THE PLUMAGE OF POULTRY.

In poultry there are perhaps more different colors and color-patterns
than in any other species of domesticated animals. The genetic work has
advanced far enough to show that many of the differences depend on
Mendelian factors. It is probable that, in addition to the main factors,
there are many contributory, minor, or modifying factors that give the
finer details to “show birds.”

It is generally supposed that the wild bird from which some at least of
the domesticated races have come is _Gallus bankiva_ of India and
Indo-China, or else one or another of its subspecies. In any case, the
wild type of coloration is approximately known, since the known wild
races are colored alike in all essential respects. Even were the color
of the wild type not known, the original plumage could be deduced with
some degree of probability from the atavism that appears when some of
the races are hybridized. It is interesting to find that many of the new
plumage characters are dominant to the wild type. The same relation also
holds rather generally for other characters of poultry, such as the
comb, etc.

Amongst the uniform or single-colored races, the whites, blacks, reds,
and buffs have been studied. Bateson and Punnett were the first to show
that the white of the White Leghorn is dominant. They also showed that
the white of the White Rose Comb bantams is recessive. Another white,
that of the White Silky, is also recessive, but due to a different
factor from the white factor of the Rose Comb bantams; for, when these
two whites are bred together they give colored birds in the first
generation. Hurst showed later that the white of the Leghorn is dominant
over the black of the Hamburg and the buff of the Cochin. The dominance
is often not complete, since tints of black or of buff or even patches
of these colors may occur. The latter may be confined to the head, neck,
and breast. The black plumage of the Hamburg is dominant over the buff
of the Cochins, but incompletely so, as the black background may be
marked and shaded with brown. Whether we are dealing here with one pair
of factors, or two pairs, could only be determined by an F₂ ratio;
whether it is 3:1 or 9:3:3:1.

The blue color of the Andalusian is known not to be a simple color, but
to be a fine mosaic of splashed white and black. The color is produced
in birds that are heterozygous for splashed white and black, or at least
for certain kinds of white and black. This relation was first
demonstrated by Bateson and Punnett (1902 and 1905) and later Saunders
(1906). It appears also from certain crosses made by Davenport that some
of the whites (such as that of the Leghorn) and black (such as that of
the Minorca) may at times also give some blue birds when crossed.
Whether there are also other races with dominant white color different
from that of the Andalusian white (and the same holds for black races
also) or whether a special (recessive) white was present in this cross
when the blue appeared, was not made out by Davenport.

Lippincott has recently studied the Andalusian cross and obtained
essentially the same results as his predecessors. He calls attention to
an interesting fact in the splashed whites, namely, that the color
splashes are blue when they are found in those parts of the body where
the color is blue in the Andalusian. Although the Andalusian is always
spoken of as a blue bird, the hen only is entirely blue, while the male
is black above and blue below. The splashes on a white male correspond
to the black and blue of the Andalusian male, and are black if above and
blue if below.

Lippincott found also that the blue birds differ from the black in two
characteristics, viz, in the blues the pigment is in larger masses, _i.
e._, it is more clumped, leaving more white between the clumps than in
the blacks, and in the blues the pigment is absent in the extremities of
the barbules. If the clumping and the condition of the barbules are
treated as separate entities, each gives a 3:1 ratio. Lippincott
concludes, therefore, that the Andalusian cross is a 2-factor case. If
each of these characteristics was independent of the other in the sense
that some birds had clumped pigment and others deficiencies in the
barbules, then one might conclude that he was dealing with a 2-factor
case; but if these two characters are only different aspects of the same
gene, and when one is present the other is also, the situation is not
different from those that are very common, viz, two or more effects
produced by the same genetic factor.

Davenport has recorded results of crossing several breeds of different
colors (1906 and 1909). The white of the Leghorn was found dominant to
the black of the Minorca breed, although the hybrids, “at least the
females,” had some black feathers. This white was also found to be
dominant to the mottled Houdan and to the “Red-backed game.” On the
other hand, a male Tosa with wild-type plumage by recessive White Cochin
female gave “barred” males in F₁; the barring coming in, no doubt, from
the Cochin and although not at the time recognized by Davenport as
sex-linked inheritance, the statement that barring is “associated with
maleness” (as already pointed out by Darwin) indicated that the barring
that appeared within the cross was probably the sex-linked barring shown
by other breeds.

In Davenport’s cross of White Leghorn by Minorca two blues appeared (as
stated above), indicating that the same factors were here present that
in the Andalusian white and black strain gives the same result,[3] but
why only some of the F₁ appear as blue, while others are not blue, is
not yet made clear, unless two factors for white were present. White of
the Leghorn breed was found not to be as completely dominant over buff
as over black. Black was found dominant over the wild-type
(Black-Breasted game), but red is present in F₁ birds also to some
extent in those places where red is found in the game. Lacing, as shown
by the Dark Brahma, is dominant to the plumage of the Tosa. Penciling
also is said to be dominant, as shown in females of the cross between
the Dark Brahma and Tosa fowl.

In his later paper (1909) Davenport gives fuller information in regard
to some of the F₁ cases reported in his first paper, as well as the F₂
results. Thus, in the cross of Silky to Minorca, that gives black F₁
birds, the F₂ count gave 210 black, 57 game, and 95 white--approximately
the expectation for two pairs of factors, one of them giving white
(9:3:4). Silky by White Leghorn gave white F₁’s, but the males developed
red on the wing bow and saddle when they became mature, and the female a
faint blush of salmon (“red”) on the breast. In F₂ there were whites,
games, and blacks, approximating to expectation for three pairs of
factors, one being a dominant white (52:9:3). Silky by Buff Cochin gave
a washed-out buff, but with the jungle coloration partly developed in
the tail (black) and hackles and wing bow (redder buff). Davenport
represents the Buff Cochin as having lost the jungle patterns and
coloration, while the Silky retains it. The heterozygous condition of
the genes for the wild-type color in F₁ is made responsible for the part
development of color. The White Silky is represented as carrying the
factor for black (N), hence in F₂ both black and game-colored birds are
expected and they were obtained. When Black Cochin is crossed to Buff
Cochin, the F₁ males are in general like the game (black and red) while
the females are black (except for some red on the hackle). In this case
Davenport represents the Black Cochin as showing a factor for
jungle-fowl pattern, but lacking the color that is assumed in his other
formulæ to go with this pattern. What is meant by this change is not
quite clear to me, unless Davenport supposes there is an independent
factor for the jungle-fowl pattern which may be filled in by other
colors determined by other factors. But were there enough F₁ birds to
exclude the possibility that jungle-fowl birds would not appear in this
cross?

Davenport has reported a cross between a female White Cochin and a male
Tosa (wild type) from which the daughters were Tosa, except that the
shafting was broadened, and the saddle feathers and proximal secondaries
were obscurely barred (black and buff); the sons were also like the
Tosa, but every feather was repeatedly barred (see above). In F₂ there
were 15 white, 25 game, and 16 barred birds. Davenport concludes that
“barring is clearly heterozygous and confined to the male sex,” and in a
footnote he adds that the sex-linked barring factor of the Plymouth Rock
is different from that of this Cochin-Tosa cross, but Goodale informs me
that the barring that appeared in this cross is probably the same as
that in Barred Rocks.

As pointed out, an interesting feature of color inheritance in poultry
is the large number of cases of sex-linked inheritance. It might seem
probable here, as in the case of _Drosophila_, that this is due to a
well-recognized difference between sex-linked and autosomal characters,
namely, that a recessive mutation in one of the sex chromosomes of a
sperm-cell of the male bird will have a chance of showing its effect
immediately if that sperm-cell unites with an egg without a Z to form a
daughter, whereas it would not immediately show up in the offspring if
the mutation were autosomal.[4] In consequence the recessive mutant
would have a greater chance of being observed and selected if it
appeared in a sex chromosome. But dominant sex-linked characters,
however, have the same chance as dominant autosomal ones and the
question turns therefore on the kinds of characters shown in the cross.

The first indication of sex-linkage in fowls was furnished by evidence
that Spillman published in 1903 on information supplied by
poultry-men--information that has been proven subsequently to have been
accurate. Spillman pointed out clearly the similarity between the facts
he quoted and the then known cases of sex-linkage in the canary and in
the currant moth. The case referred to by Spillman was a cross between
Barred Plymouth Rock and Black Langshan. Goodale and I repeated the
cross, using both Plymouth Rock and American Dominques, publishing the
results in 1912. In addition to the F₁ results evidence was obtained for
the F₂ generation. The theory was also tested by back-crossing. The
results of such a cross that are typical for all cases of the sort are
briefly as follows: Plymouth Rock cock by Langshan hen gives F₁ barred
sons and barred daughters. These inbred give F₂ barred cocks and barred
and black hens (2:1:1).

In the following schemes the sex chromosomes are represented by Z and W,
while the exponents stand for the factors involved, viz, B for barred
and b for not-barred, which here means a black bird.

               Barred♂        Black♀
      P₁         ZᴮZᴮ           ZᵇbW
                      \     /
                       \   /
                  ZᴮZᵇ        ZᴮW

      F₁       Barred♂       Barred♀
                   \          /
                    \        /
               ZᴮZᴮ  ZᴮZᵇ     ZᴮW  ZᵇW

        Barred♂   Barred♂   Barred♀   Black♀

In the reciprocal cross, a black cock was mated to a barred hen. The
sons were barred, the daughters black (F₁). These inbred gave (F₂)
barred males and females, black males and females in the ratio of
1:1:1:1. The chromosome scheme of inheritance is as follows:

       Black♂             Barred♀
 P₁     ZᵇZᵇ                 ZᴮW
             \     /
              \   /
               \ /
          ZᴮZᵇ        ZᵇW
 F₁     Barred♂       Black♀
               \  /
                \/
    ZᴮZᵇ   ZᵇZᵇ        ZᴮW     ZᵇW
 Barred♂ Black♂   Barred♀  Black♀

One back-cross test consists in mating the F₁ barred males ZᴮZᵇ (from
both crosses) to a pure black female. The expectation is for equal
numbers of barred and black males and females, and the result was
realized. The F₁ barred hen of the first cross (ZᴮW) back-crossed to a
black cock is expected to give only barred males and black females, and
this result also was obtained. The explanation of the last cross, based
on the sex chromosomes, is as follows:

   Black♂       F₁ Barred ♀
     ZᵇZᵇ         ZᴮW
          \  /
           \/
      ZᴮZᵇ     ZᵇW
   Barred♂   Black♀

Before these experiments were finished Goodale had made other crosses
involving the barring factor, and had obtained results that showed the
sex-linked inheritance of this factor (1909). For example, he crossed
Buff Rock male (not barred) to white Plymouth Rock females. The sons
were barred and the daughters not barred. The reciprocal cross gave
barred sons and daughters. A White Rock male (carrying barring) mated to
a Brown Leghorn female gave barred sons and daughters. Reciprocally, the
chicks were of two kinds as to their down, viz, black chicks and chicks
with the down pattern of the barred rock. All these results with Barred
Plymouth Rocks show that they carry a sex-linked dominant factor for
barring. Its wild-type allelomorph would be game-color (jungle-fowl),
but since, when the dominant barring is absent in some of the
individuals in these crosses, they are black, it would seem to follow
that another dominant factor, one for black, that is not sex-linked, is
also present.

Pearl and Surface have also carried out crosses with Plymouth Rocks on a
much larger scale. Their results conformed in every way to the
foregoing. They crossed Barred Plymouth Rocks and Cornish Indian games.
The plumage of the male of the latter race is black with dark red on the
back and wing-bows; the females are also black laced with mahogany
ground-color on back, breast, wing, and tail coverts. When the male game
is mated to the barred hen the sons are barred and the daughters are
black. In the reciprocal cross both sons and daughters are barred. The
back-cross tests conformed to expectation. The results were the same as
those already stated above for the Langshan-Rock cross.

Sturtevant crossed Columbian Wyandottes and Brown Leghorns. The F₁ sons
were alike, whichever way the cross was made. They were fairly typical
Wyandottes, which race carries therefore more of the dominant plumage
characters (two or three?). There were two types of daughters, depending
on the direction in which the cross was made. When the father is
Wyandotte, the daughters are like him (except for stippling of the
Leghorn type). When the father is Brown Leghorn the daughters are
somewhat stippled red birds. In the former case the daughters getting
their Z chromosome from their Wyandotte father resemble him; in the
latter case the daughters getting their Z chromosome from their Leghorn
father look more like him. Their failure to look exactly like him must
be due to autosomal factors derived from the Wyandotte mother that
dominate other autosomal factors from the father.

Hagedoorn crossed Black Breasted Game bantams (like those used in my
Sebright crosses) to Brown-Breasted bantams. In the latter the black
breast feathers of the male are bordered by lemon; the hens are nearly
black. Black-breasted male to “brown-red” female gave both
black-breasted sons and daughters. In the reciprocal cross all the sons
were black-breasted (like the mother) and all the daughters were brown
red like the father. Evidently the factor here for Brown Breasted game
is sex-linked and recessive. In this case the new mutant sex-linked
character is recessive to the wild type.

Davenport (1912) crossed Brown Leghorns to Dark Brahmas. In the cross
and its reciprocal all the sons are alike. Two dominant sex-linked
factors were found,[5] viz, the white background characteristic of the
Dark Brahmas and the red upper wing-coverts (and back) characteristic of
the Brown Leghorns. On the other hand, the _daughters_ differ in the two
crosses, in each case resembling their father in their hackle color.

When two sex-linked characters are involved in a cross it is possible to
determine by suitable matings whether an interchange between the
chromosomes that bear them has taken place. In the case of the sex
chromosomes only one sex, the male, has both like chromosomes, viz, ZZ,
and we expect from analogy with the _Drosophila_ work that crossing-over
would be found between the sex chromosomes only in the male. Goodale has
recently (1917) made the important discovery that in poultry
crossing-over takes place between the sex chromosomes (ZZ) in the male,
but not in the female (ZW or ZO). This relation, therefore, is the
reverse in birds and flies, for, in the one, crossing-over takes place
in the female and in the other in the male. Whether this difference
extends also to the other chromosomes in birds as it does in flies is as
yet not known.

Several years ago some crosses between gold and silver Campines were
reported by Rev. E. Lewis Jones. The results are consistent with the
view that a sex-linked factor pair is responsible for this difference in
color, although the author does not apply this view to his results. The
results may be seen in the table on page 16, to which Jones has
prefixed the number of individuals. The cross also involved
hen-feathering _versus_ cock-feathering, which appears here (as in other
cases) to be a non-sex-linked dominant factor. As stated above there are
in the results a few apparent inconsistencies with this interpretation,
due possibly to heterozygous females having been used in the crosses.

Lefevre crossed Silver Spangled Hamburgs and Brown Leghorns. The
spangling was found to be a sex-linked dominant factor. A spangled cock
bred to a Leghorn hen gives spangled sons and daughters; a spangled hen
by a Leghorn male gave spangled sons and not spangled daughters. The
daughters do not transmit spangling. Other factors may obscure the
results, especially factors for black, or the localization of the
pattern. Lefevre says “it would seem probable that multiple factors for
black, introduced by the Brown Leghorns, are present, and that these
factors may have a cumulative effect, with the result that pigmentation
is developed to varying degrees of extension.” Whether the factors for
black spoken of as coming from the Leghorns are dominant wild-type
factors that have mutant allelomorphs in the Silver Spangled Hamburg is
not entirely clear from the quotation.

Baur gives in his Introduction to the Study of Heredity (1914, pp.
202-203) some results (unpublished) that Hagedoorn had obtained by
crossing gold and silver races of Assendelver birds. The factor is
sex-linked and is no doubt the same factor reported by Jones for gold
and silver Campines and by Sturtevant for Columbian Wyandottes. Silver
dominates gold and the sex relations are the same as those already
reported by others for poultry, viz, the male is ZZ, the female ZW. Gold
hens by a heterozygous silver[6] gave 162 silver cocks, 163 silver hens,
168 gold cocks, 160 gold hens, expressed graphically (_g_ for gold, _s_
for silver):


      Zᵍ--W♀ × Z--Zᵍ♂
      -----------------
      ZˢZᵍ--ZᵍZᵍ--ZˢW--ZᵍW
    Silver Gold Silver Gold
     male  male female female

When a silver hen was united to a gold cock there were 246 silver cocks
and 243 gold hens--crisscross inheritance.


_Summary._

From the standpoint of the Brown Leghorn type representing the wild
type, the following colors and patterns represent dominant mutations
from that type:

        _Dominants._

    White of White Leghorn.
    Silver of Dark Brahma.
    Black of Minorca.
    Lacing of Brahma.
    Barring of Plymouth Rock.
    Black (?) of Plymouth Rock.
    Buff (or red).

Each of these (in heterozygous condition of course) is dominant; in some
cases completely so, in others incompletely dominant. At three different
loci in the sex chromosome a dominant mutation has occurred; at three
loci in other chromosomes dominant mutant changes have also occurred.

        _Recessives._

    White of Rose Comb bantam.
    White of Silky.
    White of White Rock.
    Brown of Brown-breasted game.
    Penciling.

Whether the recessive white that is sometimes found in dominant White
Rock stock is different from both of the other recessive whites is not
known. There are, then, 5 or 6 recessive characters that are not
sex-linked and 1 recessive sex-linked character.

Owing to the relatively large number of color dominants in poultry, some
unnecessary confusion has arisen concerning the relation of the
dominants to the wild type, and especially to other mutant characters to
which they are said to be dominant, in the sense, however, of being
epistatic. An imaginary example will illustrate this. For example, if at
some locus in the wild type a mutation occurred that gave a dominant
black (_i. e._, a black that shows up when one gene for it is present)
and at the same time this black also showed up even when other recessive
mutant characters were present in homozygous form, then F₁ birds would
be black when black is crossed to such pure recessive stocks. Such cases
have indeed been described as dominant, but a knowledge of F₂ would have
shown at once the error of such a system. For, if black had been a real
dominant, the F₂ would have given 3 blacks to 1 of the other type (such
as the wild type), but if the case were one of epistasis, then there
would have been 9:3:3:1 classes in F₂ (or some modification of that
ratio). In this sense, then, epistasis may be defined as a result that
appears when one member of the pair of genes produces its effect
regardless of the constitution of the individual with respect to another
gene (or other pairs of genes). It is curious at least to note that in
the case of dominant white the term epistatic has been much less often
used than in the case of black. Theoretically the two situations are
exactly alike, but because black could so obviously conceal things
beneath it, while white is not thought of as doing so, it seemed
“natural” to make such a distinction. In reality it is not a question of
covering up at all, but a case of a dominant character (white or black)
preventing other colors from appearing.

In the case of recessive white the situation is somewhat different and
no one, so far as I know, has gone so far as to speak of such a white as
epistatic, although when the animal is white it certainly hides, when
completely effective, all the other effects of color-producing factors,
but allows them to “show through” in some of the cases. This means not
that they do “show through,” but that they only develop to a “lower”
degree. The difference between dominant and recessive whites rests on
the fact that in one case one member of a pair of factors gives white
and in the other both members are necessary. But obviously such a
distinction is not important, and if it were worth while the case might
be argued for recessive whites being also epistatic. The whole tangle
goes back to a false interpretation of presence and absence of
characters and presence and absence of factors. As I have gone over this
ground recently in my paper on the Theory of the Gene, I need not repeat
here what I tried to make clear there.




ENDOCRINE CELLS IN OVARY AND TESTES OF BIRDS.


The occurrence of gland-like cells with an internal secretion in the
ovary and testes of fowls has been described by a number of writers and
denied, at least for the testes, by others. The work of Boring and Pearl
has done much to bring this question to a satisfactory solution, for
they have tested out and made use of the best reagents that their
predecessors had discovered and have used a much greater amount of
material. As they have reviewed very fully the literature of the
subject, it will not be necessary to go over the ground again in detail.

In the follicles of the ovary there are present, according to Boring and
Pearl, groups or nests of cells lying among the connective tissue of the
inner theca. The cells are about three times as large as the ordinary
connective-tissue cells of the ovary. The cytoplasm is clear and
vacuolated, “only occasionally containing a few acidophile granules
which stain with the fuchsin in Mallory’s stain or the eosin of Mann’s
stain, while the real interstitial cells are crowded with granules.”

When the egg is set free from its follicle, the latter collapses and the
rupture becomes closed. A mass of cells collects in the center of the
collapsed structure which develop yellow pigment. The cells, lying in
the puckered edge of the follicle, may also develop such yellow color.
The cells that produce the yellow pigment come from the nests of cells
that lay originally mainly in the theca interna. Either by migration or
by division they come to fill up the central cavity. The yellow
substance in the cells is not fat, since it does not dissolve in the
clearing oils, nor can it be protein, for it does not take acid stains
as normal secretion granules of protein. It does not dissolve in HCl,
HNO₃, or H₂SO₄, nor in strong KOH, although the latter turns the pigment
a bright red color. Many other substances were also tried by Boring and
Pearl, but none of them dissolved the yellow pigment, which reacts in
this respect in the same way as does the yellow pigment in the luteal
cells of the mammal. The similarity in the nature of the pigments in the
two cases is an argument in favor of the view that the cells that
produce the pigment are the same in both groups. In the mammal the
yellow corpus luteum is a large, gland-like organ that develops after
the ovum is discharged; in the bird there is also a yellow spot on the
ovary, due to the pigment in the collapsed follicle, but it is smaller
and much less conspicuous than in the mammal. The evidence concerning
luteal cells in the testes of the bird is conflicting. One of the
difficulties in the situation is the identification of the cells, which
are sometimes regarded merely as the general connective-tissue stroma of
the testis that is undoubtedly present; at other times special secretory
cells are discerned embedded in the connective tissue, as individual
cells or in islands. Boring states (1912) that in newly hatched chicks
about half of the tissue of the testes is interstitial connective
tissue; the other half consists of tubes or cords whose principal
function is the development of the germ-cells. In the paper of 1912
Boring reached the conclusion that there are no “interstitial cells in
the testes of the domesticated chicken in the sense that this term has
been previously used,” and states that no evidence has been found that
an internal secretion of any kind is formed by any cells of the
interstitial tissue.

It is not necessary to discuss whether or not connective-tissue cells
are present in the testes of birds, for is it generally conceded that
they are found at least in certain stages, but it is important to look
into the question as to whether among these interstitial cells there are
others that have an endocrine function. Mazzetti gives pictures of such
gland-cells between the seminal tubules of the cock bird, but says that
they are rare, “even though this bird has very marked secondary sexual
characters” (Boring and Pearl). It may be remarked parenthetically that
if they had been more abundant the bird might have had no secondary
sexual plumage since it will be pointed out below that such glandular
cells may have as their special function the suppression of these
characters.

According to Des Cilleuls, interstitial cells are first found in males
about 30 days old and at this time the secondary sexual characters put
in their appearance. If, as will be shown in the sequel, he means by
interstitial cells the endocrine cells that suppress the development of
the male plumage in the female, the appearance of these cells at this
time would be significant; but if he implies that their occurrence in
the male incites the development of the secondary sexual characters, his
interpretation is open to serious doubt. Reeves found interstitial cells
in testes of cocks 3, 5½, 9, and 18 months--more in the earlier stages.

In a later communication by Boring and Pearl the whole question is taken
up again with improved methods, etc. Previously 21 male birds had been
studied, just hatched to 12 months old. More sections of this same
material were made which were stained according to Mann’s and Mallory’s
methods. In addition, a whole new series of preparations was made. A few
interstitial cells, i. e., granule containing-cells were found in newly
hatched chicks, but not in any of the 60 mature birds examined.




LUTEAL-CELLS IN THE TESTES OF THE MALE SEBRIGHT.


Finding that the testes of F₂ hen-feathered birds were often flat and
pear-shaped instead of rounded and cylindrical, as in ordinary cocks,
and that they were often black in color, suggested, as already stated,
that the testes of the Sebright might be hermaphrodite in some element.
It seemed not impossible that egg-cells might be found. I made a
considerable number of sections of the testes of these birds and
examined them under the microscope; not finding any egg or egg-like
bodies, the slides were laid aside, but the idea that in some other way
the Sebright’s testes might correspond to the ovary of the female next
recurred to my mind. Consequently, when in the summer of 1918 I had some
new material derived from a castrated Sebright male that had partly
regenerated its testes and was again going back to hen-feathering, and
pieces from one of the old testes of a castrated bird, I asked Miss
Boring, who was then in Woods Hole, to make some preparations and
examine them to see if she could detect any such elements in them as she
had found in the female. Miss Boring reported the occurrence of luteal
cells in the testes from hen-feathered males, and the results have been
published in a brief preliminary paper (1918). The abundance of these
clear cells, supposedly gland-cells with endocrine influences, in the
testes of hen-feathered birds is in sharp contrast to their absence in
the normal adult cock birds. It seems to follow, therefore, that the
hen-feathering in the Sebrights is due to the presence of these cells,
whose function is the same as of the similar cells in the female, _i.
e._, the suppression in both of cock-feathering. Castrating the Sebright
produces its effect by the removal of these cells that are responsible
for the suppression of cock-feathering.

The occurrence of luteal cells in young stages of other races of poultry
raises the question as to whether in these races the first or juvenile
plumage, that resembles that of the hen rather than that of the cock,
may not also be due to an internal secretion from these cells, or
whether this juvenile plumage is only the plumage of a characteristic
stage in development. Castration of young chicks ought to settle this
point. Such castration experiments have been made by Goodale. The
absence of any reference to any effect on the juvenile plumage in these
early castrated birds probably meant that they did not develop
precociously cock-feathering, and he writes me that he examined them
carefully and that their plumage is like that of the normal chicks.
Geoffrey Smith has reported the occurrence of two kinds of males in a
race of Leghorns, the males of one of which become cock-feathered before
the other. May not this difference depend on the length of time
endocrine cells remain or begin to develop? A histological study of the
two types would be of the greatest interest.




ENDOCRINE CELLS IN THE TESTES OF MAMMALS.


In man and other mammals it has long been recognized that in addition to
the germinal cells of the testis there are also present other cells,
sometimes called interstitial cells, that, so far as known, have no
immediate function in connection with the germ-cells, or at least that
have other important functions outside the relation to the reproductive
organ. That some internal secretion from these cells has an important
influence on the secondary sexual characters rather than anything done
by or produced by the germinal cells has been very clearly shown by
evidence derived from three separate sources, namely, from the operation
known as vasectomy, from an exceptional condition known as
cryptorchidism, and more indirectly from X-ray treatment. Vasectomy
involves either cutting the vasa deferentia in such a way that the cut
ends do not reunite. In consequence of the closure of the outlet of the
testis the germinal cells slowly degenerate, and finally completely
disappear. How such an effect is produced we do not know. That this
result does take place is borne out by the unanimous testimony of all
those who have successfully performed the operation. Ancel and Bouin
showed (1903) that breaking the continuity of the vas deferens
suppressed spermatogenesis in 8 to 12 months. Both the Sertoli cells
(the nourishing cells of the germinal epithelium) and the interstitial
cells persist. Such animals remain sexually active and their secondary
sexual characters are not affected. Marshall states that in the hedgehog
the remarkable periodic enlargement of the testis takes place even after
vasectomy, although the germ-cells have disappeared.

In mammals the testes fail at times to pass through the inguinal canal,
and, in consequence of their retention in the body-cavity, the
germ-cells fail to develop. On the other hand, the interstitial cells of
the testis develop normally. Cryptorchid individuals show the normal
secondary sexual characters of their species. How retention of the sperm
should give rise to the same result as cutting the duct, viz, absorption
of the germinal cells, is not known. A possible solution may be found in
the pressure exerted on the testes, both when retained in the abdomen
and when their outlets are stopped by tying or cutting the ducts.

Finally, it has been long known that continued or repeated exposure to
X-rays or to radium causes the destruction of the germ-cells, but leaves
the interstitial cells intact and presumably functional. Destruction of
the germ-cell by X-rays has no effect on the secondary sexual
characters.

This threefold evidence demonstrates that in the male of the mammalia
most, perhaps all, of the secondary sexual characters that are affected
by castration are not affected by the destruction of the germ-cells.
This conclusion supports very strongly the view that the interstitial
cells are the cellular element in the testes that influence through
internal secretion the development of the secondary sexual characters of
the male.

Equally important are the results that relate to the accessory organs of
reproduction, such as the glands that open into the vas deferens
(prostate, Cowper’s gland, etc.) and the copulatory organs also. In the
castrated mammals these organs diminish in size. On the other hand,
after destruction of the germ-cells in the testes (or even when they
fail to develop as in cryptorchid individuals) these accessory parts are
unaffected. In birds, as will be shown, the situation is entirely
different.




CYCLICAL CHANGES IN THE INTERSTITIAL CELLS IN HIBERNATING MAMMALS.


The changes that take place in the interstitial cells in mammals that
hibernate and in which there is a definite rutting season following
hibernation have been examined by several workers. The mole has been
studied by Regaud (1904), Lécaillon (1909), Tandler and Grosz (1911);
the marmot by Hauseman (1895) and Gaugini (1903); the hedgehog by
Marshall (1911); and the woodchuck by Rasmussan (1917). In the mole the
interstitial cells are most abundant when the tubules in which the
spermatogenesis is taking place are least developed, and _vice versa_.
In the hedgehog the increase in both tissues takes place at the same
time. In the woodchuck both tissues increase rapidly after hibernation
(during March and April), after which the spermatogenesis continues
actively for the two following months (May and June), while the
interstitial cells retrograde rapidly during April and remain at a low
level for the rest of the year. Retrogression in the germinal epithelium
begins in July, after the rutting season is past. It appears from this
evidence that the activity of the two tissues does not always run the
same course. Since the secondary sexual characters of the male, which
are not well developed in these animals, are not so far as known
affected by the condition of the testes, the evidence does not have any
very direct bearing on our present topic. How far the sexual behavior of
these mammals is determined by the quantity or by the activity of the
interstitial cells is not very clear from the evidence, although there
is a very noticeable increase in the amount of this tissue just before
and during the rutting season. In the mole also the interstitial cells
begin to increase just before the mating season, and the increase
continues for several months after mating has taken place. It is
difficult to judge how great or how little the change amounts to unless
the whole organ is considered, for the relative volumes of the seminal
tubes and the interstitial tissues does not give a measure of the total
volume of these tissues, since the testes may decrease greatly in size
when the seminal tubes retrograde, and the apparent increase of the
interstitial cells at the time may not increase the total amount of that
tissue present.

Probably more important than the ratio of interstitial tissue to tubules
is the activity of the former. Rasmussan states that in the woodchuck
the interstitial cells not only increase in number immediately after
hibernation, but the increase in amount of this tissue is largely due to
increase in the cytoplasm, in which there appears an accumulation of
fatty globules in the more peripheral parts of the cells. In the central
cytoplasm an abundance of fine lipoid granules develops.

Marshall has made some interesting experiments on the hedgehog at
different seasons. Castration in March prior to the breeding-season has
an influence on the accessory generative organs (vesiculæ seminales,
prostates, and Cowper’s glands). They remain in the same undeveloped
stage in which they were at the time of operation. If castration is
carried out very early in the breeding-season, when the accessory
reproductive organs are about half developed, their further enlargement
is prevented. In so far as the accessory organs rank as secondary sexual
organs, their complete development is thus shown to depend on the
testes. Transection of the vasa deferentia before the beginning of the
breeding-season affects somewhat the enlargement of the testes, but
produces no effect on the accessory organs.




HERMAPHRODITISM IN POULTRY AND THE SECONDARY SEXUAL CHARACTERS.


Several hermaphrodite birds have been described (Brandt, 1889; Shattock
and Seligman, 1906; Pearl and Curtis, 1909; Smith and Thomas, 1913;
Bond, 1914; etc.). The most recent and complete account of such birds is
that by Boring and Pearl. They examined in all 8 hermaphrodites, or at
least 8 birds that showed in their plumage, or other secondary sexual
characters, peculiarities of both sexes. Five of the birds came from
Herr Houwink in Meppel, Holland, who had a stock in which there
appeared, in 1911, two hermaphrodites out of 80 birds, and in 1912,
three out of 80 birds. These were the birds studied by Boring and Pearl.
In addition, when Pearl saw Herr Houwink’s birds in 1910, “there were
then on hand a considerable number of these supposed hermaphrodite
birds.” An anatomical study of the Holland birds showed that one of them
was nearly a normal female; three, the authors say, were “evidently
undeveloped females. They have infantile oviducts and embryonic
ovaries.” It should be added that there was a tumor more than twice the
size of the ovary attached to or part of the ovary. If the ovary itself
was affected by the tumor, or the tumor was a part of the ovary, the
slightly unusual condition of the birds might be accounted for. Of the
other 3 birds, 2 are also suspected to have ovarian tumors, while in the
third bird streaks of a secretion which resembles the substance of the
tumor of the other two were found. The change towards male plumage in
these 5 birds is probably due either to the incomplete development of
ovary or to the effect of the tumor on the ovary. Although luteal cells
are described as present, it seems probable that their total number
might be less than in a normal bird, and hence their insufficient
secretion would fail to suppress the development of male plumage. From
this point of view these birds are no more hermaphrodites than is a hen
with her ovary taken out.

The remaining Holland birds were entirely different. On the left side
there was an ovary in an inactive condition; on the right side there was
a testis, producing spermatozoa. Sections of the testis show that it is
normal, consisting of a mass of tubules with very little connective
tissue between them. In both ovary and testis there are “a few nests of
luteal cells near the surface. The ovary contains eggs, but is abnormal
to some extent.” The authors state:

     “In external appearance it is more like a male than the others,
     which fact correlates well with the active condition of the testis
     and inactive diseased ovary, with only one corpus luteum scar. The
     interstitial cells can scarcely be held accountable for the male
     secondary sex characters, as the only ones in an active secreting
     condition are a few in the ovary.”

It is not quite clear what is meant in this quotation by the statement
that the interstitial cells can scarcely be held accountable for the
male secondary characters unless to suggest that they cause the
development of these characters in the male, as they are supposed to do
in mammals--a view that the authors do not seem at other times to hold.

Another hermaphrodite (Atwood’s black) had an infantile oviduct and an
ovotestis. A second bird, too, had an ovotestis--mostly testis--as well
as a rather large oviduct. Collections of luteal cells are described
between the tubules of the testicular portion. If, as suggested by the
Sebright cases, these cells tend to suppress the female plumage, their
presence here in excess might at least be made to account for the female
part of the plumage of this bird. Comparing the last two birds (that
showed active sex-behavior as males) with the best of the Holland birds,
Boring and Pearl point out that the active sex behavior of the two
former can not be due to “interstitial cells that are absent in these
but present to a slight extent in the former.” They then add”
...though the differences can not be laid to the lutear cells, as they
are present in all three.” That the relative amounts of the latter or
their activity might still be accountable for the difference would not
seem entirely excluded from the evidence so far as it is given.

A fourth hermaphrodite (Dexter’s) laid 12 eggs and had a large coiled
oviduct. There was present “a large, lobulated reproductive organ on the
left,” which proved to be an ovotestis. Several ovarian tumors were
present and there was testicular tissue.

It is fairly evident, then, that four of these birds described by Boring
and Pearl were females with abnormal ovaries. The incomplete development
of the latter, or their abnormal condition due to tumors, may
sufficiently explain the occurrence of male secondary sexual characters.
That these tumors affect, to different degrees, such characters is
expected from what is shown by imperfectly spayed females of normal
breeds.

There are a few statements in the summary of this paper that call for
comment. The statement that the “development of comb, spurs, and wattles
does not stand in direct quantitative relation to the sex of the gonad,”
appears to be only intended as a statement of fact based on the author’s
observation. But in what sense is there an expectation that they should
stand in such relation beyond the obvious fact that in the cock the comb
and wattles are larger than in the hen, and that spurs are generally
present only on the cock. But if the expression “sex of the gonad”
implies the germ-cells it is not at all certain that there is any
expectation of a quantitative relation, and there is some probability at
least that other cells than the sex-cells are involved in the
development of combs, wattles, and possibly spurs. A castrated cock has
a small comb resembling that of the female bird. On the other hand,
removal of the ovary sometimes leads to an increase in the comb and
wattles. Here we have, to say the least, a paradoxical situation, for
the result looks superficially as though something in the ovary keeps
down the hen’s comb, while something in the testes keeps up the cock’s
comb, yet when the ovary is removed the hen develops a cock’s comb; when
the testes are removed the cock develops a hen’s comb. The real meaning
is, I think, that the genetic complex for femaleness (one Z or else ZW)
stands in itself for a full-sized comb, while the genetic complex for
maleness (two Z’s) stands in itself for small comb.

Boring and Pearl state that “body-shape and carriage have a genetic
relation to the sex of the gonad.” This statement means, I think, that
the amount of testicular matter present stands in some direct relation
to the shape of the body and carriage of the male. Castration, both of
the normal cock and the Sebright, seems to change the carriage somewhat
and perhaps the shape. Both lose something of the peculiar attitude of
the male, but I have not been able to my own satisfaction to analyze
what this means. As has been pointed out, and as the pictures show, the
castrated Sebright changes his attitude, but whether this is a change
due to his new contour, or to a new balance resulting from a large tail,
or to a let-down resulting principally from effects on the nervous
system, is difficult to determine. The same statements apply in part to
the castrated cock of ordinary breeds, but not to the same degree, since
the change after castration, in feathering and in carriage at least, is
slight.

The conclusions that the “amount of lutear cells or pigment (?) is in
precise correlation with the degree of external somatic femaleness
exhibited by the individual” is of especial interest in connection with
the Sebright evidence. It is difficult, however, to gather from the body
of the paper what the absolute amount of luteal cells is that is
present, for even in some of the more male-like birds with an ovotestis
the description leads one to suppose that there may be as much luteal
material present as in some of the more female birds with infantile
ovaries or cystic tumors.

Pearl and Curtis (1909) described “a case of incomplete hermaphroditism”
in a Barred Plymouth Rock fowl. Externally the bird looked like a hen,
but “the head and neck resembled these parts in the cockerel,”
especially the comb and wattles. The bird was never seen to tread a hen,
nor did it ever crow normally. An ovary and oviduct were found on the
left side, the former no larger than that of a laying hen after removal
of the large yolks. No eggs were visible on its surface. On the right
side a testis (9 mm. by 6 mm.) and vas deferens were present. No eggs
were found in the ovary, and it gave every indication of being in a
degenerating condition, with no eggs or egg follicles in it. The testis
had no “normal seminiferous tubules”, but indications of cellular rods
were present. The organ is in all probability a degenerating testis.

A Leghorn 2 years old has been described by Shattuck and Seligmann
(1906) that had the full-developed comb and wattles of the cock, but the
former drooped slightly to one side as in the hen. Well-developed spurs
were present. The plumage was mainly female, with neck-hackles
moderately developed, and with “saddle-hackles” practically absent. The
tail, though not typically female, lacks sickle feathers. The bird
excited no notice from other birds of either sex. A large left oviduct
and the distal end of a right oviduct were present. Two vasa deferentia
were also present. In the left side a flattened sex-gland (3 cm. high)
was found, made up of testicular tubules. Two small ova were found in
its posterior end. The right gonad was also tubular (testis).

The occurrence of real testicular tissue in one of the Holland birds and
in three others described by Boring and Pearl, as well as in one
described by Pearl and Curtis, and in another by Shattuck and Seligmann
calls for special comment, since the presence of both testicular and
ovarian tissue in the same bird is the essence of hermaphroditism. In
general there are two ways of looking at such a result. Either the
sex-determining factors have been changed so that in one part of the
body, where the reproductive organs are laid down, one condition can
prevail, in other parts other conditions; or a mixup of the sex
chromosomes has taken place. Until we get some more evidence concerning
such cases it is useless to speculate, although the former view might
seem the most probable of the two if the Holland birds of Herr Houwink’s
flock were in a high degree true hermaphrodites.

But in fact three of the four described by Boring and Pearl were due to
tumors of the ovary, which, if they suppress the normal development of
this organ, would be expected to call forth the appearance of the
secondary sexual characters of the cock. If the likelihood of developing
a tumor were inherited, the frequent occurrence of hen-feathered birds
in this flock would be explained. However, one true hermaphrodite in 4
birds is surprisingly high for a chance result, since hermaphrodite
birds are very rare.

The second interpretation suggested above is one that has been advanced
and established by genetic evidence in _Drosophila_, viz., dislocation
of the sex chromosomes. In the case of birds the male is supposed to be
duplex for the sex factors (ZZ), the female simplex (ZW), and
consequently the chromosome-dislocation hypothesis must be worked out
contrawise in birds and insects. We should have to suppose that such
birds start as males (ZZ), and that at some division of the cells of the
embryo one of the Z’s became lost (left at the cell-wall for example).
All the cells that got ZZ would be male; all that got Z would be female.
If the reproductive region included cells of these two kinds, an
ovotestis would result. The rest of the body should be the same, or
nearly so, since the soma of male and female birds is alike whether ZZ
or Z, except in so far as it is affected by the secretions from the
ovaries (in most races of poultry), or from the testes if the race be
Sebright, Campines, or Hamburgs. Birds with ovotestis might,
nevertheless, be expected, on this view, to show at times an
intermediate condition of the secondary sexual characters, according to
how much internal secretion is produced in the ovotestis. In other
words, the chromosome loss might involve much more extensive regions
than the reproduction organs, but show its effects first in that organ
and then indirectly other parts of the body be affected by the luteal
cells of the testis. There is one rather good piece of evidence that
seems opposed to this interpretation. In the hermaphrodites the oviduct
is present in all cases. Its conspicuous presence in the four
hermaphrodites would seem, therefore, to indicate that the birds
started as females (ZW), which is inconsistent with the dislocation
hypothesis. The alternate would be that in all these cases the Z part
always included the region of the oviduct, which seems improbable.

There is another possibility, viz, that in birds a sex-factor is carried
by the W chromosome, and ZW is a female not because of one Z, but due to
the presence of W. If so, then one Z or two Z’s might give the same
result, viz, female. If a bird started as female, (ZW) and chromosomal
dislocation occurred, then the Z parts would be female and the male part
W. Until we get evidence on this point it is not worth elaborating.
Without genetic evidence from hybrids, the interpretation of
hermaphrodites in birds can have at present only a speculative interest.
We may hope some day to get the same kind of evidence as in the case of
_Drosophila_. Hermaphrodite hybrid pheasants that have been often
described might seem to furnish a hopeful field, for they appear to be
quite common and to show characteristics of both races. As yet, however,
no one has, I think, succeeded in finding a simple interpretation of the
results. It is also not unlikely that many of the pheasant cases are not
true hermaphrodites, but due to failure of normal development of the
reproductive gland, which gives an intermediate or mixed type of
secondary sexual characters.




PART II.




DARWIN’S THEORY OF SEXUAL SELECTION.


Darwin seems to have felt the necessity of giving some other explanation
for the secondary sexual differences between the male and female than
that such differences were only a by-product or concomitant of sex
itself. His reason for searching further was probably a part of the
general point of view he had reached in regard to the utility of special
structures of animals, namely, that their presence finds its explanation
on the basis of utility. Believing as he did that most of the
adaptations of plants and animals have been built up by the accumulation
of small steps, it must have appeared to Darwin inconceivable that the
highly developed ornamentation exhibited in the secondary sexual
characters could have been simply the by-product of sex itself,
especially when the ornamentation may have been entirely absent in males
of closely related species. To-day we are not, I think, so oppressed
with the difficulties of the situation, for we have become familiar with
the fact that very slight genetic differences may cause very great
differences in the end-product. In a word, the problem seems less
formidable to us than it did to Darwin.

Darwin appealed to three processes to account for the facts: (1) to
natural selection between the members of the same sex; (2) to choice on
the part of the “other” sex; (3) to the “inheritance of use.” Since each
of these appeals to a different procedure, let us take them up
separately.

Competition of the males with each other for the female would, Darwin
said, lead to the survival of those males best endowed with organs of
offense and defense. The spurs of the cock are weapons dangerous for
other birds; the horns of the bull and those of deer are used for
offense and defense; the mane of the lion is a protection against the
teeth of other lions. It is true that these same weapons and shields
serve for attack and defense outside the species; but since the female
lacks them or has them less developed, they would not seem necessary for
survival of the individual against aggression from without. They have
developed, then, through competition within the species.

Several objections of greater or less weight have been urged against
Darwin’s interpretation. It has been pointed out that the combats within
the species are seldom fatal and that the defeated rival finds another
mate. If, as a rule, there are as many females as males within the
species and monogamy is the rule, all males will find partners sooner or
later, all may have offspring, and the offspring have equally good
chances of survival. Under these circumstances it is not to be expected
that the combat would be likely to lead to the production of males with
longer spurs or larger horns.

Darwin realized this difficulty and tried to meet it by another
assumption, viz, that the better endowed males would _also_ be more
likely to have more offspring. How could this be made probable? Darwin
suggested that the strongest males would be in position to mate with the
first females to reach maturity, and if these were more likely to have
offspring, either because of maternal endowments that made them also
more prolific or because the earlier broods would have a better chance
of getting food, etc., then the successful competitor would sooner or
later impress his advantages on the race.

At other times Darwin suggested that the exceptional vigor that led to
the greater development of the character in question would itself be of
value and through transmission to the offspring lead to advance in the
development of the other character in question. But here the argument
shifts to another field of inquiry and survival is ascribed to greater
vigor, while the secondary sexual character is carried along in its wake
as a sort of correlated effect.

It will be conceded, I think, that such pleading does not help the
argument, but exposes rather its inherent weaknesses. There is, however,
a line of defense that is permissible. If monogamy is not the rule, if
the male captures or attracts several females and keeps a harem, as do
the fur seals and walruses, or rules a herd as does the bull, or has a
flock as does the cock, or mates more frequently with random females
than do some other males, then the advantage of his more developed
weapon might lead to more offspring. If it could be shown that such
intraspecific weapons prevail more frequently within polygamous species,
a fair argument for natural selection might be made. I do not know
whether such a census has been taken as yet, but it is true, I think,
that in most polygamous groups we find weapons of offense very highly
developed. The fur seal has a harem and the male is greater in size, in
strength, and in the development of his tusks than is the female.
Similarly for the walrus. The bull drives away rival bulls from the herd
until through age or injury, or through the development of a better
fighter, he is replaced. If the better endowment is due to a genetic
factor, we should expect natural selection to keep the race at the
highest possible level that variation supplies material for. If, then,
we confine the application of natural selection to cases of this sort,
the explanation is as valid as is the theory in other fields. Such a
conclusion becomes weakened when an attempt is made to apply it to other
groups of animals in which it appears improbable that the secondary
sexual characters of the male have any obvious value as organs of
offense. There are families of beetles, for example, in which the
development of the horns of the male are as striking as are those of the
ram or the stag. The males of these beetles are not known to fight with
each other, nor are they polygamous. It may seem that we must look here
for some other explanation, which, if found, might suffice to cover
also the case of birds and mammals. In answer to this criticism it may
be argued that it is also possible that the other explanation when found
need not necessarily apply to the higher animals, where the laws of
combat may still give the true explanation. On the whole, I think that,
for our present purpose, it will suffice to state it is consistent with
the theory of natural selection to accept _provisionally_ this part of
Darwin’s theory for those species in the higher groups in which polygamy
holds, conceding, however, that even here it may have to be altered when
fuller knowledge is gained.

We are more concerned with that special feature of Darwin’s theory of
sexual selection that is applied to those cases where the characters are
supposed to owe their special development to selection by the
individuals of the opposite sex. It is assumed that the female _chooses_
the better endowed males, _because_ of the strong appeal he makes to her
sense-organs. Here we must employ perforce or for brevity’s sake the
terms used in human psychology, and run the risk at every turn of
imputing to other animals the emotions and acquired associations which
man himself utilizes. Even granting that other animals possess somewhat
similar emotions to ours, there still remains always the danger, in the
absence of real evidence, of imputing to them the particular emotion
that we call “feeling for beauty”; and the greater danger of imputing an
esthetic sense so highly developed that the choice falls in the long run
on the suitor better ornamented than his rivals.




OTHER THEORIES TO ACCOUNT FOR SECONDARY SEXUAL CHARACTERS.


Wallace has always been an opponent of Darwin’s theory of sexual
selection in so far as it is based on female choice. As already stated,
he believes that the difference between the plumage of the male and
female in birds is due to natural selection keeping down the
ornamentation and high coloration in the female, because these would be
expected to expose the female while sitting on the nest to the attacks
of enemies, more especially of hawks. In support of this view he points
to a long series of species which build exposed nests and in them the
female is plainly and inconspicuously colored, while he also points out
that in such birds as parrots, toucans, woodpeckers, hangnests, and
starlings, which nest in holes or have covered nests, the female is
often as highly colored as the male. It can not be denied that he makes
out rather a strong case in support of this view, despite the fact that
there are other birds, like the Baltimore oriole, that have covered
nests and in which the sexes are very markedly different.

Wallace tries to meet cases like the last one by assuming that the
covering keeps off the rain; but, if so, why are the sexes still so
different? In the case of other highly colored birds, such as jays,
magpies, hawks, and crows, Wallace believes that these birds are all
aggressive, hence can protect their nests if attacked. As a further
support of his view, Wallace points out that in the few cases where the
female is more highly colored than the male (as the dotterel, species of
phalarope, an Australian creeper) the male incubates the eggs.

Wallace’s suggestion still leaves unexplained the ornamentation of the
male, which he tries to account for as the direct result of the greater
vitality of the male. He tries to show that excessive ornaments and high
coloration develop especially in those parts of the body to which there
is an unusual supply of blood or where nerves and blood-vessels emerge
to go to the skin or to the muscles.

     “If we have found a _vera causa_ for the origin of ornamental
     appendages of birds and other animals in a surplus of vital energy,
     leading to abnormal growths in those parts of the integument where
     muscular and nervous action are greatest, the continuous
     development of these appendages will result from the ordinary
     action of natural selection in preserving the most healthy and
     vigorous individuals, and the still further selective agency of
     sexual struggle in giving to the very strongest and most energetic
     the parentage of the next generation. And, as all the evidence goes
     to show that, so far as female birds exercise any choice, it is for
     ‘the most vigorous, defiant, and mettlesome male,’ this form of
     sexual selection will act in the same direction, and help to carry
     on the process of plume development to its culmination. That
     culmination will be reached when the excessive length or abundance
     of the plumes begins to be injurious to the bearer of them; and it
     may be this check to the further lengthening of the peacock’s train
     that has led to the broadening of the feathers at the ends, and the
     consequent production of the magnificent eye-spots which now form
     its crowning ornament.

     “The display of these plumes will result from the same causes which
     led to their production. Just in proportion as the feathers
     themselves increased in length and abundance, the skin-muscles
     which serve to elevate them would increase also; and the nervous
     development as well as the supply of blood to these parts being at
     a maximum, the erection of the plumes would become a habit at all
     periods of nervous or sexual excitement. The display of the plumes,
     like the existence of the plumes themselves, would be the chief
     external indication of the maturity and vigor of the male, and
     would, therefore, be necessarily attractive to the female. We have,
     thus, no reason for imputing to her any of those esthetic emotions
     which are excited in us, by the beauty of form, color, and pattern
     of these plumes; or the still more improbable esthetic tastes,
     which would cause her to choose her mate on account of minute
     differences in their forms, colors, or patterns.”

Wallace says, referring to the immense tuft of golden plumage in the
best known birds of paradise (_Paradisea apoda_ and _P. minor_) that
springs from a very small area on the side of the breast, that Mr. Frank
E. Beddard, who has kindly examined a specimen, says that “this area
lies upon the pectoral muscles, and near to the point where the fibers
of the muscle converge towards their attachment to the humerus. The
plumes arise, therefore, close to the most powerful muscle of the body,
and near to where the activities of that muscle would be at a maximum.
Furthermore, the area of attachment of the plumes is just above the
point where the arteries and nerves for the supply of the pectoral
muscles, and neighboring regions, leave the interior of the body. The
area of attachment of the plume is, also, as you say in your letter,
just above the junction of the coracoid and sternum.” “Ornamental plumes
of considerable size rise from the same part in many other species of
paradise birds, sometimes extending laterally in front, so as to form
breast shields. They also occur in many hummingbirds, and in some sun
birds and honey-suckers; and in all these cases there is a wonderful
amount of activity and rapid movement, indicating a surplus of vitality,
which is able to manifest itself in the development of these accessory
plumes.”[7]

There are two serious defects in such an attempt to explain the facts.
In the first place, it has been shown in several cases that have been
studied that it is not the lessened “vitality” of the female but the
suppression caused by the ovary that keeps down the development of the
full plumage in that sex. In the second place, the anatomical influences
appealed to are imaginary rather than real, for it is by no means
apparent that the local exits of blood-vessels and nerves to muscles are
at all correlated with the location of the ornamental parts, in the
skin. Even when larger blood-vessels run to the region of excessive
development of feather ornaments it may well be that they go there
because the ornaments in question use them for their nourishment; in
other words, Wallace puts the cart before the horse. The top of the
head, where crests so often develop, the throat coloration and throat
shields of hummingbirds and birds of paradise, the two long tail
feathers of several species of hummingbirds, etc., do not arise, so far
as known, from regions which are conspicuous for a rich supply of blood
and nerves. Wallace’s appeal to underlying organs such as muscles that
supposedly influence the special development of the feathers in the skin
above does not strike one as a fortunate appeal to physiological
principles.

Hudson, in his interesting book, “The Naturalist in La Plata,” has also
criticized Darwin’s theory of sexual selection. He has brought together
a considerable number of interesting observations that go to show that
the displays--dancing, singing, and combats--of males and females have
no relation to mating. Many of them involve birds already mated,
sometimes several males participating, sometimes males and females
together. Some of the tourneys he describes are more elaborate than the
mating instincts themselves, yet are not concerned with mating. He
attempts to explain them as overflow phenomena, _i. e._, as expressions
of the high vitality of the males, especially at this time. If he is
right, then elaborate exhibitions of these kinds have evolved that have
no special connection with mating. Are we called upon for a different
explanation for other differences that distinguish the sexes? One
example will suffice to bring out a curious emotional (?) display that,
elaborate as it is, has no apparent connection with mating (p. 269):

     “The lapwing display, called by the natives its ‘dance’ or ‘serious
     dance’--by which they mean square dance--requires three birds for
     its performance, and is, so far as I know, unique in this respect.
     The birds are so fond of it that they indulge in it all the year
     round, and at frequent intervals during the day, also on moonlight
     nights. If a person watches any two birds for some time--for they
     live in pairs--he will see another lapwing, one of a neighboring
     couple, rise up and fly to them, leaving his own mate to guard
     their chosen ground; and instead of resenting this visit as an
     unwarranted intrusion on their domain, as they would certainly
     resent the approach of almost any other bird, they welcome it with
     notes and signs of pleasure. Advancing to the visitor, they place
     themselves behind it; then all three, keeping step, begin a rapid
     march, uttering resonant drumming notes in time with their
     movements; the notes of the pair behind being emitted in a stream,
     like a drumroll, while the leader utters loud single notes at
     regular intervals. The march ceases; the leader elevates his wings
     and stands erect and motionless, still uttering loud notes; while
     the other two, with puffed-out plumage and standing exactly
     abreast, stoop forward and downward until the tips of their beaks
     touch the ground, and sinking their rythmical voices to a murmur
     remain for some time in this posture. The performance is then over
     and the visitor goes back to his own ground and mate, to receive a
     visitor himself later on.”[8]

Cunningham, who has brought together many interesting cases of secondary
sexual differences in his book on “Sexual Dimorphism in the Animal
Kingdom,” attempts to show that the development of the secondary sexual
characters of the males are due directly to the use of certain parts of
the body during courtship--the use of the parts leading to the
enlargement and excessive growth of the parts. The effects are believed
by him to be inherited, and he tries, furthermore, to show the way in
which such acquired characters could be inherited. He makes use of the
modern idea of hormones--substances that are elaborated in many organs
of the body, whose effects are often most conspicuously produced in
other parts of the body. He imagines these hormones to be collected in
the germ-cells and transmitted to the next generation, where their
presence contributes to the further development of the special region
(when it develops) that corresponds to the region in its parent in which
the hormone was made. His speculation meets in the first place with the
general objections inherent in Lamarck’s theory--objections so well
recognized to-day that I need not go over them here. His special appeal
to the hormone theory makes use of that theory in a way to which it was
never intended to be put, by assuming that an internal secretion formed
in one organ can be stored up in another organ, eggs and sperm--an
assumption not only unsupported by any evidence, but, as I have stated,
one quite foreign to the hormone theory. In fact, Cunningham’s
suggestion is nothing more than Darwin’s old idea of pangens, which,
being imaginary, could be endowed with all desirable properties. But one
can not invoke a chemical substance, even a hormone, and then at the
critical moment endow it with special virtues.

A rather unique explanation of the origin of secondary sexual characters
is made by Stolzmann. His argument runs as follows: (1) There is a great
excess of males in birds; (2) the males left over after mating are
useless to the species, since they can not propagate and they consume
food needed by the reproducing part of the population; (3) the
conspicuous coloration of the male has been evolved in order that he
could be seen more readily by birds of prey and the objectionable excess
of males removed; the comb of the cock has developed in order that he
may be the more easily killed by other cocks.

Stolzmann’s account of the origin of the plumes of the birds of paradise
should be immortalized in the literature of the subject:

     “Nous comprendrons aussi facilement la présence de longues plumes
     chez les males de nombreuses espèces, comme p. e. chez les oiseaux
     de paradis, chez les veuves (_Vidua_) et chez l’engoulevent
     africain (_Cosmetornis_). Telles plumes ont probablement pour but
     de relantir le vol des males. J’ai constate chez la _Loddigesia
     mirabilis_ (oiseaumouche péruvien), que le vieux male posséde
     l’aile quelques millimetres plus courte que le jeune male ou la
     femelle. Cet avortement des remiges provient assurément a cause de
     développement extraordinaire de retrices externes chez le vieux
     male de cet oiseaumouche. Si donc d’une part les retrices allongees
     rendent le vol plus difficile et d’hautre les ailes plus petites
     diminuent sa vélocité, le vol du male doit ètre plus lent que celui
     de la femelle, le poids du corps restant la même. Le développement
     extraordinaire soit des remiges soit des rectrices, en
     _relantissant_ le vol des males, rend leur rôle plus difficile, en
     facilitant en même temps celui des femelles. Nous pouvons prendre
     comme exemple le _Cosmetornis_, qui, comme tous les engoulevents,
     se nourrit d’insectes, qu’il attrape au vol. Chez cet oiseau
     quelques plumes des ailes se developpent extraordinairement pendant
     l’époque de reproduction, en retardant visiblement son vol. Il est
     donc facile a remarquer, qu’alors le male, ayant les mouvements
     plus lourds, n’est pas en êtat de se procurer la même quantité
     d’insectes qu’auparavant; ainsi donc la femelle a plus de chances
     de trouver une nourriture plus abondante.”[1]

Equally worthy of perpetuation is Stolzmann’s explanation of dancing and
singing birds:

     “Toutes les réunions des males, leurs danses bizarres, leur chant,
     enfin, ne servent pas probablement a séduire les femelles, mais
     pour distraire les males, ce qui rend plus faciles les besognes
     maternelles des femelles et au surplus les protege contre
     l’assiduite nuisible des célibataires. Darwin lui-mème constate le
     fait, qu’ordinairement pendant les réunions des males, quand ces
     derniers sont trop occupes par le combat ou la danse, la femelle
     s’echappe avec un d’eux pour copuler. Ainsi donc dans ce cas c’est
     bien la selection naturelle et non la selection sexuelle, qui agit
     pour la conservation d’équilibre sexuel.”[9]




DISPLAY OF THE MALE.


The antics of male birds at the mating season, their courtship
so-called, has played an important rôle in Darwin’s theory of sexual
selection. The behavior of many birds at this time is of such a kind as
to suggest that the male is exhibiting his plumage before the female for
the “purpose” of influencing her choice. The whole paraphernalia of
human psychology is imported into the situation and both the
consciousness of the male, his intentions so to speak, and the supposed
esthetic response or choice of the female is invoked. Even though it be
granted that the words that we must make use of, borrowed from human
behavior, are such as to imply much more in the direction of
consciousness and purpose than is desirable, and that most of the
behavior of animals should be stated in a more roundabout and objective
way, yet the theory will only work out on the assumption that the female
_chooses_ in some sense the more brilliant or ornamental (or effective)
male, whether she is “conscious” or unconscious of intention. I doubt if
anyone to-day would care to defend seriously the theory on the grounds
of consciousness or esthetic value of the exhibition, despite the fact
that Darwin’s language often takes this turn and the less-guarded
statements of some of his disciples, such as Romanes, show little
hesitation in anthropo-morphologizing the entire situation. It is,
however, not necessary for the working out of the theory that this
complication be introduced into it, for if the female is more likely to
mate with a more brilliantly colored than a less brilliantly colored
male, the theory may be made to apply regardless of whether she is
“conscious” or not of the difference to which she responds.

But there are weighty arguments against such an interpretation of the
behavior of the male and female during courtship. In the first place,
there is almost no direct evidence to show that the female mates with
the more ornamental male. As this is the all-essential requirement of
the theory, the almost complete absence of facts in its support leaves
the theory resting on a theoretical assumption. It can scarcely pass
unnoticed that while there exists a large mass of data describing the
secondary sexual characters, there is practically nothing in this
accumulation to show that the female makes her selection on differences
in coloration or ornamentation. And on the other hand, there is some
evidence showing that the female is ready to succumb to the
aggressiveness of the male rather than that she “chooses” him.

The behavior of the male under sexual excitement is often described to
be of a kind to exhibit before the female his peculiar adornments. That
the “purpose” of his exhibition is to show himself off before the female
may be conceded, with reservations as to what is meant here by
“purpose.” That the male is conscious of the probable results of his
conduct is scarcely probable the first time he courted; but that he may
have found out the most probable result after the first attempt through
“associative memory” is in accord with what the study of “animal
behavior” has shown to be possible. In this sense purpose would mean a
line of conduct that experience had shown to lead to a certain end.
Anticipation or far-sightedness would henceforth characterize such a
reaction. Here, however, we venture on very dubious grounds. But the
display of the male may be purposeful in a much simpler sense. His
activity may be an inborn reflex to visual or other sensory stimuli that
is a part of his attack on the female, or possibly a series of reflexes
that we may register under the old unanalyzed terms of “desire and
fear.” The action calls forth a responsive reflex in the female, for the
sexual act is not entirely active on one side, passive on the other, but
consists of a series of interreactions on the part of each sex, which,
if they pursue a given course, leads to the final mating. The mutual
responses appear to follow an automatic course in many cases if the
individuals are sexually ready to mate and the environment is
propitious. Types of behavior of this kind must be familiar to anyone
who has observed domesticated and semi-domesticated animals. The purpose
of the display may mean no more than a reaction that leads to a result
propitious to the perpetuation of the species if the situation is ripe
for such an outcome.

This conclusion still leaves open the question as to whether the display
is more likely to be successful, if certain special characters possessed
by the species are exhibited. In the absence of any sufficient evidence
to show that this is so, and in the light of the very great danger of
projecting “our human standards” into the world of other animals, and in
view of the fact that related species without such marks are as
successful in maintaining themselves, I can not but think that at
present we have a good deal to lose in the way of scientific procedure
and nothing to gain of scientific value in accepting Darwin’s
interpretation of sexual selection based on the display of the male as
furnishing an opportunity to the female to make the “best” selection
amongst her suitors on the basis of his adornment.

An excellent opportunity to study the problem as to “choosing” by the
female is furnished by the mutant races of _Drosophila_, some of which,
differing in a single mutant gene, have wings as different in coloration
as black, yellow, or gray, and eyes as differently colored as white,
vermilion, or red. Sturtevant put a yellow female with a gray
(wild-type) male and a yellow male. The male that first mated was noted
and the trio discarded. The female “chose” the gray males 25 times and
the yellow only 8 times. In the control combination, where a gray female
“chose” between the same two kinds of males, she took the gray male 60
times and the yellow male 12 times. In both cases it “appears” that the
female “prefers” the gray male, but this deduction may give an entirely
wrong impression as to what is taking place, for the result would be
the same in kind if the gray male were more active and mated quicker.
This was tested by putting a gray and a yellow female with a gray male
and then for control a gray and a yellow female with a yellow male. The
result was as follows:

    Red ♂ {Gray ♀     25   Yellow ♂  {Gray ♀     12
          {Yellow ♀   31             {Yellow ♀   30

Here the gray male mated slightly oftener with the yellow female than
with the other, whereas the yellow male mated much oftener with the
yellow female than with the gray one. Both results are explicable on the
view that the yellow female, being less active, is more easily captured
by the yellow male than is the gray female. This view fits in also with
the former experiment, where the yellow male is much less successful
than the more active gray male. Such a conclusion gives a more
consistent explanation of all the facts than does the theory of female
choice, for on the latter we must suppose that the yellow females prefer
the gray males and the yellow male prefers the yellow females, etc.

The following results were obtained by Sturtevant when red and white
eyed flies were competing:

    Red ♂    {Red ♀     54   Red ♀    {Red ♂     53
             {White ♀   82            {White ♂   14

    White ♂  {Red ♀    40    White ♀  {Red ♂     62
             {White ♀  93             {White ♂   19

The outcome can be interpreted in the same way as the yellow-gray
competition. The red male wins by virtue of his greater activity, while
the white female is chosen more often, especially by the white male,
because of her passivity (or weaker resistance). It may be claimed that
these results do not show that the female does not choose, for such
choice, if made, would be swamped by another condition of the
experiment, viz, the greater aggressiveness of one kind of male and
greater passivity of the other kind of female. This, of course, is true,
but the experiment still shows that in these flies other influences are
so much greater than “choice” by the female, if it exists, that the
postulated effect of the latter practically disappears from the
situation.

Mayer’s experiments with the large moth _Callosamia promethea_ furnish
important information as to the factors involved in mating. The results
are all the more significant from our present point of view because the
colors of male and female are in this species markedly different. The
wings of the male are black, those of the female reddish brown; the
antennæ of the male are large and bushy, those of the female small and
slender. Mayer found that the males are attracted by the female from
some distance when the latter are put into a glass jar covered by only
coarse mosquito-netting, but if the same jars are turned upside down the
males are unable to find the female. Females concealed in loose cotton
attracted males. Females were put into a box with an open chimney at one
end, the other open end being covered by mosquito-netting. A current of
air blew into the open end and out of the chimney. The males flew to the
end of the chimney from which the air came and fluttered about in the
neighborhood. Males are attracted to places where a female has been kept
even several hours after her removal. The male finds the female through
the sense-organs in his antennæ, for a male whose abdomen has been cut
off and the sides of whose thorax are covered with shellac will still
fly to the female, but if his antennæ he coated with any substance he no
longer seeks the female. If the eyes of the males are blackened they
will mate with females “in the normal manner.”

Mayer cut off the wings of females and glued male wings in their places,
so that the female looked like a male. Males readily mated with these
females. The wings of males were cut off and female wings glued in their
place. Mating occurred “with normal frequency, and I was unable to
detect that the female displayed any unusual aversion” to such males.
Males with female wings pass unnoticed by other males.

In a later paper (1901) Mayer and Soule describe how, when the wings of
the male were painted scarlet or green, the males were accepted as
readily as normals in competition with them. Experiments were also made
by them with the gipsy moth. Wingless males met with more “resistance”
from the female than do normal males, but when the eyes were covered the
wingless males succeeded as often as the normal males, but the number of
observations on which this statement is were far too few to be of any
value, and there are several other observations that make any such
conclusion from the evidence highly uncertain.

That it is the odor of the females that attracts the male can not be
doubted. It might still be claimed that the female chooses amongst her
suitors the darkest males, but the evidence gives no grounds for
inferring such a choice, and since she will even accept males with
female wings when they attempt to mate with her, it does not appear
probable that the color of the male is a factor in the result, or at
least if it is, then it must be entirely subordinate to the sense of
smell in finding the female and of touch after he arrives. There is
little or nothing in the behavior of these moths, or in that of the
silkworm moth, according to Kellogg, to suggest that vision plays any
significant rôle in courtship.

Concerning the genetic situation in insects, there are only a few cases
that have been studied. The most instructive are those in which more
than a single kind of male exists (two or three), one of which may be
like the female, the other quite different. The best worked out cases
are _Papilio memnon_ and _P. polytes_. De Meijere and Punnett have
shown from the breeding data that it is possible to frame an explanation
of such a sort that the aberrant female differs from the female
resembling the male in only a single genetic factor--one not sex-linked
(_i. e._, not carried by an X chromosome), but autosomal. The gene would
be of such a sort that it affects the female only--producing no visible
effect on the male. Such a conclusion, if established, helps,
theoretically at least, toward simplifying the situation in other
species, for it shows that genetic factors occur whose influence is on
one sex alone; hence the difference between the male and one type of
female does in such cases result from a single gene present in both but
causing them to be differently colored. There would be no need, then, to
assume that the difference had been slowly built up by selection, but
rather that the difference arose at some time by a single mutant step.
The incorporation of the step in the species would then follow if the
effect of the gene were useful in mating or if it had some other primary
significance for the welfare of the species, the different effect
produced on the male and female being only an unimportant by-product of
its action. On the other hand, it should be emphasized that because a
single factor difference between the two kinds of females will explain
the genetic results, it does not necessarily follow that the difference
did arise as a single mutation. The foregoing argument does no more than
imply that the difference in question may have arisen in this way, and
if so, that the situation, as it exists, would be the more easily
comprehended.

In insects and spiders, where dimorphism is as marked as in birds, the
mating habits have been studied by a number of naturalists. Here also
there are numerous accounts of the display of the male during courtship.
The account given by Dr. and Mrs. Peckham are particularly detailed and
call for careful consideration on account of their well-recognized
accuracy in observational work. Moreover, as a result of their
observations, along with those of Montgomery, Petrunkewitsch, and
others, we have really fuller information concerning the courtship of
spiders than of birds and of mammals.

In the great majority of species where the sexes are different the male
is more brightly colored or more ornamental. For example, in a group
such as the Attidæ of France, where both sexes are known, the Peckhams
state that in 26 cases the male is more conspicuous than the female; in
55 cases the sexes are alike, or if they differ the male is more
conspicuous. It appears that in other genera there are cases where the
female is more conspicuous than the male. The Peckhams state that
possibly as many as 250 species are in this condition. Those females
with brighter colors than the males are usually well armed by strong
spines. When very young they are like the males and begin to assume the
adult form and color when they are a quarter to a third grown. Whether
the change depends on changes in the ovary is not known.

The mating behavior of _Saitis pulex_, a species in which the males and
females are much alike, is described by the Peckhams as follows:

     “On May 24th we found a mature female and placed her in one of the
     larger boxes, and the next day we put a male in with her. He saw
     her as she stood perfectly still, twelve inches away; the glance
     seemed to excite him and he at once moved toward her; when some
     four inches from her he stood still and then began the most
     remarkable performances that an amorous male could offer to an
     admiring female. She eyed him eagerly, changing her position from
     time to time so that he might be always in view. He, raising his
     whole body on one side by straightening out the legs, and lowering
     it on the other by folding the first two pairs of legs up and
     under, leaned so far over as to be in danger of losing his balance,
     which he only maintained by sidling rapidly toward the lowered
     side. The palpus, too, on this side was turned back to correspond
     to the direction of the legs nearest it. (Fig. 13.) He moved in a
     semi-circle for about two inches and then instantly reversed the
     position of the legs and circled in the opposite direction,
     gradually approaching nearer and nearer to the female. Now she
     dashes toward him, while he, raising his first pair of legs,
     extends them upward and forward as if to hold her off, but withal
     slowly retreats. Again and again he circles from side to side, she
     gazing toward him in a softer mood, evidently admiring the grace of
     his antics. This is repeated until we have counted 111 circles made
     by the ardent little male. Now he approaches nearer and nearer and
     when almost within reach whirls madly around and around her, she
     joining and whirling with him in a giddy maze. Again he falls back
     and resumes his semi-circular motions, with his body tilted over;
     she, all excitement, lowers her head and raises her body so that it
     is almost vertical; both draw nearer; she moves slowly under him,
     he crawling over her head, and the mating is accomplished.

     “After they have paired once, the preliminary courtship is not so
     long. When this same pair mated a second time, there was no
     whirling movement, nor did the female lift her body, as at first.”
     (pp. 37-38).[10]

The courtship of another species, _Dendryphantes capitatus_, in which
the sexes are entirely different, is described as follows:

     “The males of _capitatus_ are very quarrelsome, sparring whenever
     they meet, chasing each other about, and sometimes clinching. It is
     a very abundant spider with us, so that we often put eight or ten
     males into a box to see them fight. It seemed cruel sport at first,
     but it was soon apparent that they were very prudent little
     fellows, and were fully conscious that ‘he who fights and runs away
     will live to fight another day.’ In fact, after two weeks of hard
     fighting we were unable to discover one wounded warrior. When the
     males are approaching each other, they hold the first legs up in a
     vertical direction. Sometimes they drop the body on to one side as
     they jump about each other. These movement are very quick, and they
     are always ready for a passage at arms. When courting the females
     they have another movement. They approach her rapidly until within
     two to five inches, when they stop and extend the first legs
     directly forward, close to the ground, the legs being slightly
     curved with the tips turned up. (Fig. 18). Whether it be
     intentional or not, this position serves admirably to expose the
     whole of the bronze and white face to the attentive female, who
     watches him closely from a little distance. (Fig. 19.) The males
     also give their palpi a circular movement, much as a person does
     when washing his hands. As he grows more excited, he lies down on
     one side with his legs still extended. These antics are repeated
     for a very long time, often for hours, when at last the female,
     either won by his beauty or worn out by his persistence, accepts
     his addresses.” (Pp. 45, 46.)

In another species, _Dendryphantes elegans_, both sexes are brilliantly
colored.

     “The male is covered with iridescent scales, his general color
     being green; in the female the coloring is dark, but iridescent,
     and in certain lights has lovely rosy tints. In the sunlight both
     shine with the metallic splendor of hummingbirds. The male alone
     has a superciliary fringe of hairs on either side of his head, his
     first legs being also longer and more adorned than those of his
     mate. The female is much larger, and her loveliness is accompanied
     by an extreme irritability of temper which the male seems to regard
     as a constant menace to his safety, but his eagerness being great,
     and his manners devoted and tender, he gradually overcomes her
     opposition. Her change of mood is only brought about after much
     patient courting on his part. While from three to five inches
     distant from her he begins to wave his plumy first legs in a way
     that reminds one of a wind-mill. She eyes him fiercely and he keeps
     at a proper distance for a long time. If he comes close she dashes
     at him and he quickly retreats. Sometimes he becomes bolder and
     when within an inch, pauses, with the first legs outstretched
     before him, not raised as is common in other species; the palpi
     also are held stiffly out in front with the points together. Again
     she drives him off, and so the play continues. Now the male grows
     excited as he approaches her, and while still several inches away
     whirls completely around and around; pausing, he runs closer and
     begins to make his abdomen quiver as he stands on tip-toe in front
     of her. Prancing from side to side, he grows bolder and bolder,
     while she seems less fierce, and yielding to the excitement lifts
     up her magnificently iridescent abdomen, holding it at one time
     vertically and at another sideways to him. She no longer rushes at
     him, but retreats a little as he approaches. At last he comes close
     to her, lying flat, with his first legs stretched out and
     quivering. With the tips of his front legs he gently pats her; this
     seems to arouse the old demon of resistance, and she drives him
     back. Again and again he pats her with a caressing movement,
     gradually creeping nearer and nearer, which she now permits without
     resistance until he crawls over her head to her abdomen, far enough
     to reach the epigynum with his palpus”. (Pp. 46-47.)

If we lay no emphasis on the implied emotional elements in the behavior
of the spiders in this description--terms of emotion borrowed direct
from human psychology--there still remain the several types of
apparently significant reactions associated with courtship. The
statements leave no room for doubt that vision plays an important rôle
in the complex reflexes that lead gradually to successful mating. The
Peckhams insist that the display of the male is always of a kind to
bring before the female the special adornments of the male in whatever
part of the body they may lie. The chance of subjective interpretation
here is so great that unless the results are carefully checked up by
studies of the attitudes assumed by males in species in which the males
are without ornament, their interpretation must be taken with the
greatest reserve. Assigning, as our authors do, so much by gratuitous
implication to the emotional side of the picture prejudices one,
perhaps too greatly, against accepting a special (even an implied
intentional) exhibition of the specially ornamented parts. On the other
hand, if it be conceded that the conspicuousness of the male is an
element in the reaction, the very special adornments visible from the
front might be supposed to enhance the effect produced in the female.
Similar displays of special ornamentation in the male have been
described both for birds and insects, but here, too, the question has
been raised as to whether such exhibitions are more than an accidental
accompaniment of the posturing of the male, for the same kind of
behavior is known to occur in other cases where the male is unornamented
and resembles the female. Had such a male special ornamentation it would
no doubt appear to us that his behavior was “calculated” to display his
ornaments.

Dr. and Mrs. Peckham point out that their observations are entirely
inconsistent with Wallace’s interpretation of the origin of secondary
sexual characters. They find no evidence in favor of his view that the
male possesses greater “vital activity.” On the contrary, the female is
the more active and pugnacious of the two. They also object to Wallace’s
statement of a total absence of any evidence that the female notices the
display of the male. In spiders the females “_observe_” the males with
close attention during their courtship. They point out also that, in
spiders at least, as the female gradually becomes adult, a male if
preferred will have a chance of mating with several females, “and as the
mating season lasts for two or three weeks the more brilliant males may
easily be selected again and again.” In regard to Wallace’s argument as
to the distribution of accessory plumes in humming birds, the Peckhams
point out that--

     “The pectoral muscles reach their highest development in the
     hummingbirds, the diurnal birds of prey, and the swallows, and we
     may, therefore, fairly use these groups to test Mr. Wallace’s
     explanation of breast plumes. In the swallows and birds of prey we
     find no such appendages, in spite of their further claim to them,
     on the ground of great vigor and activity. As to the humming-birds,
     we find in the genus _Aglæactis_ six species with more or less
     developed breast-plumes, which are also found in nine other
     species, scattered through different genera--in all, only fifteen
     species out of four hundred and twenty-six; while we find in
     fifty-six species the lengthened and modified tail-feathers, which,
     according to Mr. Wallace’s view, should be peculiar to the
     Gallinaceæ.

     “Again, there are elongated feathers from the throat or from the
     side of the neck in thirty-five species, while seventeen have
     crests from the top of the head, and seventeen, downy puffs from
     the tarsi.”[11]

From this brief survey of the family we see that, contrary to what we
should expect from Mr. Wallace’s theory, although the breast muscles are
the seat of the highest activity, breast plumes are the least frequent
of all the forms of ornamental plumage.


     “We may fairly say, then, that the humming-birds completely refute
     the proposition that there is any relation between the development
     of color and accessory plumes and ‘surfaces where muscular and
     nervous development is considerable.’”[12]

What is true for birds is even more obvious for spiders where the
special ornaments are not confined to parts of the body with high
muscular development, etc. The writers make the very pertinent criticism
that while Wallace objects to assuming the emotional states in females,
he is less careful in regard to the males’ emotions when he speaks of
the display “under the influence of jealousy or sexual excitement....
The males, in their rivalry with each other, _would see what plumes were
most effective; and each would endeavor to excel his enemy_ as far as
voluntary exertion would enable him.”[13]

     “If the males have so complex an emotion as jealousy, and further,
     if they are conscious of the value of the plumes, may it not be
     asked why the female is unable to ‘see what plumes are most
     effective?’ The mental state in the male is without meaning unless
     we suppose the female to be affected and pleased.” (Peckham, _loc.
     cit._, p. 144.)

In regard to another interpretation of the courtship, the Peckhams point
out:

     “Mr. Pocock has suggested that the attitude of observant interest
     on the part of the female spider might be taken to indicate that
     she was preparing to spring upon her mate and devour him; or that
     it might simply mean that she was warily guarding herself from his
     approach. Neither of these suppositions is admissible. In some
     species the male is not attacked by the female, and when she does
     wish, as frequently happens, either to avoid or to destroy him, her
     attitude is totally different. In the former case she turns about
     and runs rapidly away, or suspends herself by a thread of web. In
     the second, there is a contraction of all the muscles, the legs are
     drawn together, and in this crouching position she creeps slowly
     toward him, as she might if he were a fly, only with something more
     malignant in her aspect. When she takes this stand the male
     incontinently flees. When, on the contrary, the female is
     interested in the male display, she seems perfectly absorbed in
     watching him, the muscles are all relaxed, unconscious of herself
     she directs her glance now here, now there, as he moves about; as
     he continues his mad antics, her appearance gives every indication
     of pleasurable excitement, and as he comes closer and closer, she
     yields herself to the impulses which he has awakened in her, and,
     as in _pulex_, joins in his dance and whirls around and around as
     though intoxicated. We claim, then, to have completely answered Mr.
     Wallace’s first objection.” (Peckham, _loc. cit._, pp. 145, 146.)

Finally, in regard to the specific character of the display of the
males, the Peckhams make the following significant statement:

     “The spider has four pairs of legs, and all are equally available
     for display or locomotion, and since all the movements are slow and
     on the ground they are entirely open to observation and study, and
     we are thus in a position to decide by facts whether their activity
     is simply an outlet for superfluous energy, and therefore
     meaningless, or whether there is a purpose in it. If the purpose of
     the antics is only to let off energy, then we should expect one
     pair to be flourished around quite as often as another, and that
     the pair flourished should as frequently be one that was not
     ornamented as one that was; and, moreover, their movements ought
     not to be of such a nature as to display the color or ornament,
     more frequently than the law of chance would explain. If the spider
     almost always moves the ornamented legs, and in such a way, too, as
     to bring out their beauty, it would seem to us, to say the least,
     highly improbable that the dance of the spider was merely a
     meaningless overflow of surplus energy. Such an explanation leaves
     much that needs explanation. The facts are, that the best foot is
     put forward; and this is just what Darwin’s theory requires and
     explains. Under Mr. Wallace’s view the facts are inexplicable. The
     better to show that these movements are not simply meaningless
     outlets of high vigor, we illustrate the several positions by
     figures taken from nature (figs. 7-12). The figures would seem to
     prove that the legs that are ornamented or contrasted in color are
     also the legs that are usually flourished; that where none of the
     legs have special ornament, then all are used; or, as sometimes
     happens, when an unornamented leg is used the movements are of such
     a character as to display some ornament that would otherwise have
     been more or less hidden from the female.” (Peckham, _loc. cit._,
     p. 147.)

In the tarantula, Petrunkewitsch finds that sight plays no rôle in
mating--that it is due entirely to accidental contact between the male
and female. Here the sexes are closely alike, except for a pair of hooks
on the front legs of the male, by means of which he grasps the mandibles
of the female, holding them during the elaborate process of transference
to her genital opening the sperm that he has already collected in the
genital spoon on his palpi. The hooks serve to guard the male against
injury or death, while at the same time they aid him in the act of
coitus.

In a common spider, _Mœvia villata_, two kinds of males exist. Both have
been seen to mate with the same female. No preference is given to either
type. The difference between them, according to Painter, is connected
with or caused by an additional pair of chromosomes in the gray male.
The two types may therefore have no connection with sexual selection,
but be directly due to a difference in the chromosome group.

Montgomery, who made observations on the courting habits of several
species of spiders, states that his “general theoretical conclusions
were quite different from those of the Peckhams.” It turns out, however,
that his objection to their view is based entirely on their assumption
that the male is conscious of his display and that the female is guided
by an esthetic sense in selecting the more beautiful male. It should be
pointed out that even after the removal of these gratuitous assumptions
as to the cause of the evolution of the male and female, enough still
remains in Montgomery’s own observations to include his results on
courtship under Darwin’s theory of sexual selection. For example,
Montgomery says:

     “The adult male is excited simultaneously by fear of and desire for
     the female, and his courtship motions are for the most part
     exaggerations of ordinary motions of fear and timidity. By such
     motions he advertises himself to the female as a male, but there is
     no proof that he consciously seeks to arouse her eagerness by
     esthetic display--there seems to be no good reason to hold that the
     female is actuated in her choice by sensations of beauty.... Thus
     my opinion was opposed to Darwin’s theory.”

Now, it is obvious that if a more brightly colored male has a better
chance of “advertising himself” to the female all the essential
requirements of Darwin’s theory are fulfilled, regardless of whether the
male is conscious of his ornamentation or the female makes use of an
“esthetic sense.” In another passage (p. 173) Montgomery concedes all
that any modern critical advocate of Darwin’s theory is likely to ask:

     “We have previously seen that conscious aesthetic choice by the
     female probably does not account for such male characters
     [secondary sexual characters with their ‘conspicuous color
     markings’]; that they are accordingly, probably not due to sexual
     selection. These characters of the males may be most readily
     explained as being conceived by simple natural selection. Peculiar
     ornamentation would be selected because unusually greater sex
     recognition therefore prompted mating.”

It is evident that Montgomery has only shifted the situation, although
to advantage, I think, but is essentially in accord with Darwin’s theory
of sexual selection, despite his protest to the contrary. The difference
lies in Darwin’s and especially in the Peckhams’ use of the term
“choice,” “aesthetic sense,” etc., to stand for the fact that the female
more promptly mates (as Montgomery prefers to put it) with a male
peculiarly ornamental.

The most critical observations on sexual selection that have been made
in the group of insects are those by Sturtevant on the pomace fly. The
courtship is described as follows:

     “The first and most noticeable act in courtship occurs when the
     male, being near the female, extends one wing at about right angles
     to his body, and vibrates it for a few seconds. The wing is then
     returned to the normal position and the process is repeated,
     usually with the other wing. But between times there is a
     scissors-like movement of the wings repeated several times. This
     vibrating of the wings is often repeated many times, and may be
     done in any position relative to the female, though the male always
     faces her. Usually, in fact, he swings quickly around her in a
     semicircle once, or oftener, during the process. Soon the male
     begins to protrude his genitalia and, if the female remains quiet,
     to lick her posterior end. Some white matter now protrudes from her
     ovipositor, and other males in the same vial are usually observed
     to become excited now and begin courting, indicating odor as a
     cause of sexual excitement. If the female runs or flies away the
     male is excited, moves his wings jerkily, and walks around
     rapidly, but seems unable to follow the female accurately or to
     locate her quickly. The penis is directed forward by bending up the
     abdomen underneath, towards the thorax, and is jerked toward the
     female (the male always standing facing her at this stage), but not
     always toward her genitalia, as I have seen it strike her in the
     eye. (The male in this case, however, had white eyes, and so was
     perhaps blind. Normally the aim is accurate.) If it does strike the
     mark the male mounts on the female’s back, between her wings.
     Mounting never takes place until after the actual copulation has
     occurred, in which respect _Drosophila_ differs from some related
     flies (_e. g._, Muscidæ, Anthomyidæ, Sepsidæ, Borboridæ, and
     Ephydrichæ, so far as my observations go). In these forms the male
     flies and lights on the female, after which copulation may or may
     not take place, probably depending upon the way the female
     responds.”[14]

To test whether the wings have any significance in courtship, the wings
of a male were clipped off and he was put into competition with a normal
male of the same stock, age, and size. A virgin female sexually mature
was given to these two males. The normal male mated 72 times before the
other, the clipped male 53 times. It might appear that the female
selected the normal male in preference to the clipped one, or possibly
that the male with normal wings drove the other male away. That the
operation on the wings may have an influence on the male himself is
shown in McEwen’s results. He found that clipped males lost their
heliotropism. It was also possible that the courtship of the normal male
might make the female ready to copulate and then she would mate with
either male. Sturtevant tested the last supposition by placing single
pairs in vials, testing each day an equal number of normal and clipped
males. The length of time before copulation was noted. The clipped male
began to court as soon as the normal, but a larger number of normal
males mated in the first 12 minutes than clipped males (50 to 25). Had
the females discriminated against the clipped males to an equal extent
we would have expected a much greater excess than 72 to 53 when they
were in competition. It appears, then, that the wings are useful in
shortening the time between the meeting of the individuals and
copulation. The display acts, however, almost as favorably for the other
male as for the exhibitor himself. The results show, therefore, that
here an esthetic sense of the female need not be postulated, for she
actually shows little preference when she has been brought to the point
of mating between the male that aroused her and the other male that did
not. This critical test puts the problem in a different relation from
that which Darwin’s theory of female choice was meant to throw light
upon.

The reverse experiment--a clipped and a normal female of the same age,
size, etc.--showed that the mate did not discriminate between them, for
in 52 first trials the normal female was paired with 25 times, the
clipped 27 times.




PART III.

THE GENETIC AND THE OPERATIVE EVIDENCE.


The genetic and operative evidence shows that there has been included
under the general term “secondary sexual characters” a complex of cases
that are the outcome of diverse physiological processes. Sex-linked and
sex-limited characters have often been confused; some characters depend
on the gonad; some of these involve the ovary, others the testes. Still
other characters fall under none of these groups, but are the direct
product of the male or female genetic constitution. It is not
surprising, therefore, that theories proposed on the information derived
from certain of these data are controverted by information derived from
other data. The theory of sexual selection, in its attempt to bring all
the facts under one point of view, has not escaped these difficulties,
even although it may be said that neither natural selection nor sexual
selection is concerned with the origin or even the kind of variations
with which it works. Nevertheless, the latter theory, by ignoring the
origin or the physiological process concerned in the production of
secondary sexual characters, may make assumptions that are difficult to
harmonize with the facts in the case, and we shall find several
instances of this sort. For example, if the hen had selected the cock
for his fine plumage (which, as we have seen, depends in part on
autosomal genes producing their effect without the cooperation of the
testes), she would be expected to endow herself with the same adornments
(if her selection worked), unless her ovary were already producing some
substance inimical to those that she is “calling forth” by selection of
the male. The problem is evidently, then, more complex than appears on
the surface, and is not so simple as it seemed when these essential
facts were unknown or ignored.

In the case of other theories, such as those of Wallace and of
Cunningham (that appeal more directly to the causes that are producing
the variation out of which the secondary sexual characters are built
up), the absence of information, physiological or genetic, has only too
often given these writers the opportunity to speculate without the
restraints which a more recent knowledge of the facts has imposed on us.

It is obvious from what we have learned that we shall have to proceed
with more caution in disentangling the evidence before we can hope to
“explain” it. Despite the meagerness of our present information, enough
has been found out to indicate that we must be content for a while with
tentative and partial explanations even in the best-known cases, and we
must, I think, be prepared to admit that no one theory may be able to
account for all of the secondary sexual differences that exist between
the sexes.

The genetic evidence shows, in the case of cock-feathering versus
hen-feathering in birds, that only one or two Mendelian factor
differences are involved. The result may seem to mean that the
secondary sexual _characters_ themselves have been acquired historically
by a single evolutionary step, and that in consequence the opportunity
for selection to have accomplished such a result has been enormously
facilitated. Such an argument rests, however, as we know to-day, on a
false interpretation of Mendelian heredity. What the evidence really
shows is that one or two genes if present cause the testes to produce
some substance that prevents the cock-feathering from developing. The
genetic complex may require a hundred or a thousand or more special
factors that are directly and indirectly concerned with the development
of the cock-feathering, but one or two other factors may suffice to
block this machinery; or, to change the metaphor, these dominant factors
may be no more than so much sand poured into the clock. The clock may
have been slowly built up historically by many contributory “factors,”
but a little sand may spoil its activity. Similarly in the hen something
produced by the ovary prevents the fullest possible genetic action from
taking place. Here at present we do not know whether a single factor or
a hundred “special” factors are necessary to produce such an inhibition,
but if, as one would like to suppose, it is the same or partly the same
genes involved in the ovary, and in the testes of hen-feathered males,
then a relatively few, one or two, factors will suffice to bar
cock-feathering from the female.

In a case like the clover butterfly, where the genetic relations work
out on the theory of one pair of factors that produce two types of
females and one type of male, it seems more reasonable to infer that
such a difference has not been slowly acquired by many smaller
mutational changes, because the two types are not adapted to live under
two different environments for which their differences fit them
respectively, but to live in the same environment. It has never been
claimed, so far as I know, that these two types of females have arisen
through some males preferring one, some another kind of female, so that
even although it may seem probable that the genetic situation is simple,
the simplicity can not be turned to the advantage of the theory of
sexual selection. It is unnecessary to discuss further the origin of the
factor or factors suppressing the development of one type in the male or
the probability of the multiplicity of such factors. In the case of such
species as _Papilio memnon_ and _P. polytes_, with three types of
females, the situation is the same as above, with the addition of the
theory of mimicry, that “explains” some advantage accruing to each type
of female. Since the latter is only a form of natural selection, we are
not further concerned with the change here. Punnett’s excellent
treatment of the problems involved in his recent book on mimicry brings
the subject down to date.

Meager as is the genetic and surgical evidence at present, it is enough
to show that only by further work along these lines can we hope to lay
a firm foundation for a scientific study of the subject. It is equally
important that critical evidence be obtained in regard to the effect on
the female of males of different types in competition. The instinctive
reactions of animals in these respects, their first reaction, the
associations that may or may not result, are practically an open field
for investigation. The entire equipment of human psychology of the
introspective school, that has been appealed to for help in a situation
itself little understood, reads often more like fiction than like
science.

So far as one branch of the subject goes--the possible interpretation of
ornamentation in the male--there seem to be two ways at least in which
the subject calls for immediate investigation: First, if it can be shown
that, other things being equal, a more adorned male rouses the female to
prompter mating, it may be inferred with some probability that in the
long run such conduct would lead to the establishment of the more
effective individual, but this would not be true unless the males mate,
as a rule, more than once, for any advantage that might accrue to a more
ornamented male would not affect the course of evolution of the species
if every other male found a mate too. Second, if it could be shown that
the special ornamentation of the male is only one of several effects of
a gene whose main effect is in some other direction, then the advantage
gained through natural selection in this other direction would carry in
its wake the advance in ornamentation, and if the change affects one sex
more than the other, owing to the difference in the genetic complex of
the two sexes, it would be called a secondary sexual character.


A. EVIDENCE FROM MAMMALS.

Owing to the differences in the secondary sexual characters of different
breeds of sheep, we have more genetic information about such characters
in this group than in other groups of mammals. Fortunately, also, in
some of the breeds both castration and ovariotomy have been performed,
and consequently we are in position to utilize both sources of
information for interpreting the situation. In certain breeds both males
and females have horns (Dorsets), in which case the horns of the male
are larger than those of the female. In other breeds neither males nor
females have horns (Suffolks). In still other breeds the males have
horns and the females are hornless (Merinos and Herdwicks). The clearest
evidence that we have, both genetic and operative, is that obtained by
Woods, as reported by Bateson, in which horned (Dorsets) and hornless
(Suffolks) breeds were crossed. In the Dorsets, where both sexes have
horns, those of the male are larger than those in the female. When the
young male is castrated the horns develop, but only as far as in the
female. It appears, therefore, that the presence of the testis, probably
through some secretion from it, contributes to the development of the
horns. The other race, the Suffolks, have no horns in either sex.
Castration produces no change in their hornless condition.

When a Dorset ram is crossed to a Suffolk ewe the sons have horns, the
daughters lack them. The reciprocal cross gives the same results. The
factor or factors involved are therefore not sex-linked. When the F₁’s
from the cross or from its reciprocal are inbred, four classes of
offspring are produced, namely: Horned male, 3; hornless male, 1; horned
female, 1; hornless female, 3. The ratios, as above, are approximately
3:1:1:3.

A simple Mendelian explanation covers the results. If we assume that the
Dorsets, both male and female, are homozygous in a factor for horns, H,
that is not in the sex chromosome, and that the Suffolks “lack this
factor,” _i. e._, that they have an allelemorphic factor for
hornlessness, the germ-cells are H-H and h-h, respectively. Only one
kind of individual, Hh, results in F₁. Since the male with this formula
develops horns, we must conclude that the presence of the testis
(through its secretions) causes horns to develop, while in the female of
this same composition horns are not produced because of the absence of
the testes. The sex-cells in these F₁ individuals are H-h and H-h.
Chance meeting of these gametes will give 3 classes of individuals,
irrespective of sex, namely, (1) HH, (2) Hh, (1) hh. The expectation for
the males is that those of the composition (1) HH and (2) Hh will
develop horns, while those of the composition hh will not develop horns.
There should be 3 horned to 1 hornless male. In the females we expect
those with the composition (1) HH to develop horns, since they have the
same formula as the pure Dorset; those with the formula Hh are not
expected to develop horns, because the F₁ females of this composition do
not have horns; those with the formula hh are not expected to develop
horns, because they have the same composition as have the pure Suffolk.
There should be 3 hornless to 1 horned female. Combining both sexes, the
expectation for F₂ is 4 horned to 4 hornless. Arranged according to sex,
these give the classes realized: Horned male, 3; hornless male, 1;
horned female, 1; hornless female, 3. That this is the correct
explanation is borne out by back-crossing the hornless F₁ female to a
hornless Suffolk ram. The former has two kinds of gametes, H and h, the
latter only gametes that bear the h factor. Half the sons should be
horned, half hornless, because half of them are Hh and half hh. But none
of the daughters should have horns, because neither the Hh nor the hh
females produce horns. This is the result realized, viz, 3 hornless
offspring to 1 horned.

The preceding account of the inheritance of the factor for horns is
based on the combination of Dorsets and Suffolks used by Wood. That
other conditions may exist in other breeds and even in races of the
same breed is claimed by Arkell as a result of a large number of crosses
that he has carried out. He states, for instance, that in the great
Merino class, with its various sub-breeds, there are flocks in which the
males only are horned, but even here there may be individual males that
are hornless “and at times the females may also show some signs of horn
growth.” In America, Arkell states, there are three types of
Merinos--the American, the Delaine, and the Rambouillet. He quotes Plumb
(Types and Breeds of Farm Animals, Boston, 1906) as stating that “the
American Merino ram carries heavy, spirally twisted horns, but the ewes
are hornless; ... that the rams of the National Standard or Victor-Beald
Delaines may or may not have horns; that the Dickinson Delaines may have
small horns, but a polled head is preferred,” etc. These conditions
suggest that there may be more than a single factor for horns in sheep
or that there may be modifying factors in certain breeds. In fact,
Arkell and Davenport attempt to cover the results of Arkell’s
experiments by assuming that there is an inhibiting factor for horns
that is carried by the sex chromosome. Such an inhibitor (I) would be
double in the XX female and single in the X male. It is assumed to be
incapable of preventing the development of horns in the heterozygous Hh
male, the inhibitor being there simplex (_i.e._, one I), while the
double inhibitor is capable of preventing the horns in the heterozygous
(Hh) condition, but not of preventing the development of horns when the
homozygous (HH) condition occurs. There are several objections to this
scheme: first, that there is no evidence that a _sex-linked_ inhibitor
is present that affects the hornless breeds, for the evidence indicates
rather that there is no factor for horns present in them, at least in
the Suffolks; second, the peculiar balance between the factors for horns
and the inhibitor seems an extremely artificial statement. Arkell and
Davenport intimate that races with horned males and hornless females do
not exist in a pure state. That breeds impure in these respects may
exist need not be denied, but that pure races for such a dimorphic
condition do exist seems probable. Castle states, for instance, that he
knows at first hand of such races of Merinos. Castle also states that
castrated Merino rams in this race do not develop horns, and this result
is in accordance with statements made by Marshall for Herdwicks (a race
with horned males and hornless females). Under the circumstances it is
certain that the presence of the testes is one of the factors in
determining whether horns develop at all (as in Merinos), or in
determining the extent to which they develop (as in the Dorsets), rather
than that the difference between the sexes is due only to an inhibiting
genetic factor. Nevertheless, it may be well to keep open the
possibility that there may be different factors for horns in different
races (allelomorphs or others), or conversely, that the genetic
composition of the races is different, the factor for horns remaining
the same, but producing a different effect.

It may be pointed out in passing that if, as Arkell assumes, the
hornless races are due to the presence in them of an inhibitor for
horns, the results can be worked out without postulating that the
inhibitor is sex-linked. For example, if the hornless male and female be
HHII and the horned male and female HHii, the F₁ horned males and
hornless females will be HHIi. The germ-cells will be HI and Hi in each
sex, which, by chance meeting, as shown below, gives the results
obtained by Wood. Thus:

      HI Hi  female.
        ×
      HI Hi  male.
    ----------------------
    1HIHI+2, HIHi+1, HiHi.

These formulæ give 3 horned males, 1 hornless male, 1 horned female, 3
hornless females. This formulation, while appealing apparently to a
different set of factors from those used by Arkell, is in reality the
same in principle, since the heterozygous condition is here represented
by Ii (instead of Hh) and sex determines that the heterozygous male is
horned and the female hornless.

The genetic relations of the Merino with horned males and hornless
females to the Dorsets, in which both sexes are horned (but in the male
the horns are larger), must be different from the genetic relation in
the other cross. There are two theoretical possibilities, viz., that a
different factor for horns is present that is either an allelomorph or
another different factor; or second, that a modifier is present in the
Merino that keeps down the development of the horns in the female. An
answer could be obtained by breeding Merinos to horned and to hornless
and getting F₂ from both crosses. Arkell’s data is not sufficient to
settle the question, because his numbers are often too small, but
chiefly because it appears that there were two genetic types present in
his flock of Merinos, one of which is characterized by scurs (very short
horns) in the females, the other by hornlessness in the female. He found
in a cross between a hornless father and Merino mother (that had knobs
or scab-like growths) that the daughters had horns or scurs and carried
a determiner for horns (as subsequent generations showed). On the other
hand, in other cases where the Merino mother was without horns, her F₁
daughters had no horns. In both cases the F₁ sons had horns. Arkell
cites this cross as “proving” that the knobs of Merino ewes depend for
their development upon two horn determiners (H´H´). It is not at all
evident that the results lead to such a conclusion, as other
explanations will cover the case as well.

Arkell’s mating between Dorsets and Merinos (tables IX and XVI)
corroborates his view “that the knob of the Merino female is represented
in the germ-plasm by the double determiner.” The 5 F₁ sons had long
horns, 3 F₁ daughters had horns present, and 2 had them absent (table
XVI). If some of the Merino mothers used were homozygous for a factor
that inhibits the development of horns in the female we can account for
the hornless daughters, and if other mothers did not have this factor
(or were heterozygous for it) we can account for the horned daughters.
Evidently more evidence is needed. Arkell himself assigns a
corresponding difference to the mothers in these cases, based on the
observed fact that the mother that had knobs or scurs were the ones that
gave birth to the horned daughters. If the above suggestion proves true,
it shows that the Merino condition dominates the Dorset condition. The
result is in harmony with the view that both have a common factor for
horns, but that in addition the Merinos have a non-sex-linked modifier
that holds down the development of the horns in the ewe.

What bearing have these results on the theory of sexual selection?
Clearly the Merino male, as constituted at present, develops horns
because he is a male, but only in the sense that his testes secrete some
substance that makes his horns grow. That maleness does not in itself
necessarily produce horn is shown by the absence of horns in the Suffolk
breed. Is it the same factor, present in the Merino, that produces horns
in both sexes of Dorsets when homozygous and in the male only when
heterozygous? If originally the ancestral race had no horns, the
appearance of factors for horns would, even in a heterozygous condition,
have sufficed in the males for the development of horns. If this gave
them any advantage either over the enemies of the race or in the eyes of
the female, such factors might be perpetuated, and through transferrence
to the females ultimately become homozygous in both sexes. Both would
then have horns, whether horns were or were not of any advantage to the
female, which would have them because they have an advantage to the
other sex.

Because the genetic evidence shows that a single factor difference
between the breeds with and without horns accounts for the horned
condition in one of them, it by no means follows that horns as they
exist arose as a single mutant factor change. True, they may have arisen
as a new single factor difference, but the Mendelian evidence can not be
claimed as evidence for this view. The _a priori_ argument based on the
relation of horns in an adaptive sense to the rest of the body would
appear rather to indicate that they could not have arisen at a single
mutational step.

Concerning the still broader bearing of this evidence on the theory of
sexual selection, two distinct questions are involved: first, how has
the present racial difference in horns arisen in domesticated sheep, and
secondly, what was the original condition of sheep. Reversing the order
of these questions, we find that sheep were domesticated in Asia and
Europe before the dawn of history. “Whether our well-known and useful
animal is derived from any one of the existing wild species, or from the
crossing of several, or from some now extinct species, is quite a matter
of conjecture” (Flower and Lydekker’s “Mammals”). Most of the wild
species of the genus (of which about 12 are recognized) have horns in
both sexes, but larger in the male. There are 3 wild species in which
the horns are lacking in the female, according to Flower and Lydekker.
If these have been crossed into the domesticated breeds the condition
shown by the Merino may go back to the wild state. The third condition
found in domesticated races, viz, hornlessness, may have appeared under
domestication. Such a change might have arisen in either of the two
other types and would be comparable to well-known losses of characters
shown by domesticated animals and plants. These losses of characters are
usually ascribed to actual losses of genes; any lost gene in the complex
of factors necessary for the production of horns might cause such a
change. But there is no advantage, in fact, in ascribing the _loss_ in
the character to a _loss_ in one of the factors producing that
character, for any change of any kind in the factor complex might bring
about the same result and the evidence from multiple allelomorphs should
put us on our guard against the all too easy assumption that a loss in a
character involves necessarily loss of a factor in the real sense in
which loss is used in ordinary speech.

The operative and genetic evidence for sheep shows that if the horns in
the male were developed through natural or sexual selection we should
expect them to develop also in the female. The greater development in
the male seems to be due to secretions from the testes which probably
are due to special factors that call them forth, but whether such
factors were also acquired to reinforce the effects being produced
through selection or were already present (reinforcement for horns being
only a by-product of their activity) can not of course be known. We can
suppose that special factors that suppress the development of horns in
the female may have arisen in the wild or in the domesticated races and
have been perpetuated because of some imagined benefit conferred; or
that in certain races factors were already present that kept down the
development of horns in the female. In any case such factors do not
cause their effects through secretions from the ovary, because after
ovariotomy horns do not develop; nor are they sex-linked factors. Any
speculation as to how natural or sexual selection has brought about the
evolution of the horns in sheep must reckon with the conditions imposed
on such speculation by the preceding information. So far as I can see,
it leaves the situation in this respect neither better nor worse off
than before.

In deer the effects of castration are well known, but there is no
genetic evidence to show the kind of factors involved, since no crosses
have been made between species with differences in their horns. If the
young male deer is castrated before the antlers have appeared, no horns
develop. If castrated at the time when the antlers have begun to
develop, incomplete or imperfect development follows. The antlers
remain covered with the velvet, and are said not to be thrown off
periodically as in the normal male. If the adult stag with antlers is
castrated, the horns are precociously dropped, and, if replaced at all,
the new antlers are imperfect and are not renewed. I do not know of any
cases in which females have been spayed, but no doubt the ovaries must
sometimes become diseased. There are, however, a few records of horns
developing in this sex in old age, or presumably after disease of the
ovaries. Both male and female reindeer are horned. Castration produces
no effect on the development of the horns.

In the case of deer it is evident that the presence of the testes in the
male causes the horns to develop. The genetic factor, or factors, for
horns may be supposed to be carried by both sexes, but the effects of
the factor can be seen only when the testes are present. In the reindeer
and eland, on the other hand, the genetic factor for sex can produce
horns without the need of the environment produced by the testes.[15]
Whether we are dealing here with the same factor or whether the rest of
the hereditary complex makes the result different can not be known
without breeding experiments.

There is apparently a connection between the stage of development of the
horns and the age of the animal, as the following statement by
Yarrell[16] (1858) indicates:

     “The fallow-buck is at his best in his sixth, or at most in his
     seventh year; after which, though the carcass may increase, the
     horns become smaller, and irregularly going back annually through
     something like their former stages of increase, a very old buck has
     from the state of his horns been mistaken for a young one. In the
     osteological department of the Museum at Paris there was, and may
     be now, the skeleton of a female reindeer in which the horns were
     reduced to little more than a rudiment of the beam and the
     brow-antler; this animal was so old that the molar teeth were worn
     down to the edges of the alveolar cavities.”

At first sight these results in the fallow deer appear to be only an age
condition, but since in old age a reverse process sets in, it may appear
more probable that the amount of secretion by the testes or other glands
may be the conditioning agent. In the case of the reindeer one may
hesitate to ascribe the change to the ovary without further evidence.

In cattle the effects of castration as seen in oxen have been studied.
There is little here that is useful for our present purpose. The horns
are not inhibited and may even be larger than in the bull. The absence
of horns in certain races of cattle is apparently a dominant character,
but as the character is neither sex-limited nor sex-linked, the evidence
has no further bearing on the present topic.

The effect of removal of the ovary from female calves has been studied
by Tandler and Keller. The height of the ovariotomized female is less
than that of the cow. The same difference is found between bull and ox.
Tandler and Keller call attention to the similarity of the head in male
and female lacking the gonads. They conclude that the ovariotomized
female does not come to resemble the male, but that removal of the gonad
causes both sexes to converge to a common type.

Castration is frequently performed in horses, dogs, and cats, but as the
secondary sexual differences, aside from size and behavior, are not very
well marked in these animals, the results need not be here considered.

Steinach’s experiments with rats are important, because by grafting
ovarian tissue into the castrated male, the male was caused to assume
certain characteristics peculiar to the female. The mammary glands that
are rudimentary in the male became much enlarged--not only the glandular
tissue increased in amount, but the mammæ themselves were greatly
developed. The hair of the male is coarser than that of the female. In
the feminized male the hair was soft like that of the female. The size
was smaller than that of the male. The skeleton also was affected, and
Steinach thinks that it changed in the direction of a female skeleton.
Even more striking was the sexual behavior of the feminized rat. The
individual no longer reacted as male, but showed some of the reflexes
peculiar to the female. These results, that stand almost alone, appear
to show that several of the secondary sexual characters of the female
rat are due directly to the presence of the ovary.

One of the most striking and definite results shown by castrated rats
(Steinach), guinea-pigs (Pirsche, Steinach), rabbits (Pauncet), hedgehog
(Marshall), and man is to be seen in the effect on the accessory glands
connected with the male ducts as well as on the penis. These remain
small and infantile. Some substances produced by the testes are
essential for the development of these parts. Natural selection rather
than sexual selection would be the agency that here comes into play.

In man the effects of castration have been often described. Eunuchs have
had a commercial value in some countries, as in Turkey and China, and
castration has been deliberately practiced on young children. Certain
religious sects, such as the Skops of Russia, have advocated and carried
out the operation. Disease has also at times necessitated the removal of
the testis, more often in adults than in the young. The full effects are
shown only when the operation has been carried out before the secondary
sexual characters have developed. The more striking difference between
the sexes involve the beard, and the hair on other parts of the body,
the voice, the shape of the pelvis, and the mammary glands. For a
detailed account of the results, the publications of Tandler and Grosz
and Marshall’s book on the “Physiology of Reproduction” should be
consulted.

The two most obvious changes in the eunuch are the absence of the beard
and mustache and the small larynx, which produces a high-pitched voice.
In both these respects man differs from woman; in both, however, the
eunuch is like the boy as much as he is like the woman. It is not
evident, therefore, whether the eunuch has retained the juvenile
condition or has become more like the female. Moreover, there is the
possibility that there is no difference in the present case between
these two conditions. The distribution of hair on the pubis of the
eunuch is often said to be more like that in the woman than that in the
man, but there is apparently no sufficient evidence to show that this is
more than the juvenile condition or an undeveloped condition of the
male. As to the voice, there is no way of determining whether the voice
of the eunuch is feminine or juvenile. The development of the mammæ in
the eunuch would be a better test, but it does not appear from the
literature on the subject that the mammary glands and the nipples of the
eunuch are changed toward the female type. On the contrary, it appears
rather that there is no such change. It is true that the tendency toward
the accumulation of fat may give the eunuch a somewhat feminine
appearance (since one of the foci of fat accumulation is in the region
of the breasts), but this in itself can scarcely be claimed to be
feminization, but due rather to the more slothful habit of the eunuch
that tends to obesity.

A more suggestive resemblance is found in the narrowness of the shoulder
girdle and broadness of the hips in the eunuch, but even these
resemblances to the female should be regarded skeptically, since other
changes in the bones that result from castration are certainly not a
development toward the female type, but a peculiar specific effect of
the absence of testes on the growth of the bones. For instance, the
bones of the arms and legs are much longer in the eunuch than in either
the normal man or woman, in fact, more in the direction of the male, who
has longer legs than the female. The explanation usually given is that
the ossification at the ends of the bones and of the epiphyses does not
take place so soon as in normal men and women. The condition here is
that characteristic of the juvenile state that is carried over into the
adult, but whether the narrowness of the chest and shoulder girdle of
the eunuch is correlated in some way with the more prolonged growth of
the other bones has not, so far as I know, been determined. That there
is no _apparent_ connection between the shortness of the one and the
greater length of the other does not necessarily lead to the conclusion
that there is no such connection. For the present I think we must hold
this point in reserve.

Steinach’s evidence for the feminized rats, if it may be extended to
man, indicates that some of the female characteristics are due to the
presence of the ovary holding in check the genetic possibilities of the
female, as well as leading to the development of such characteristic
traits as the mammæ, etc. In the case of the pelvis the female departs
from the juvenile type of both sexes, and here one might look for a
better criterion. It is stated that the pelvis of the ox is more like
that of the female than it is like that of the male, and it has been
said that this is true for the castrated rat and guinea-pig, but whether
a simple enlargement of the juvenile pelvis would make it resemble the
female type more than that of the male has not, so far as I know, been
carefully examined. Should it prove here that this is the case, the
evidence on this point would be no stronger than that for other
character differences. As has been stated, Tandler and Grosz think that
the changes in the skeleton of the ox, as well as those in the castrated
cow (skull, pelvis, and limb bones), are due directly to loss of the
gonads and are much the same in both. But their resemblance may possibly
be due more to an enlarged juvenile condition rather than that either of
them changes toward the normal skeleton of the other sex.

The statements that have been published concerning the effects of
removal of the ovaries in woman are, on the whole, unsatisfactory and
often contradictory. That the uterus and oviducts become smaller is
expected from what is known to occur in other mammals, and is definitely
recorded in the human female. That the breasts become smaller is stated
to be the case, but whether because of an actual decrease in the
glandular portion has not, so far as I know, been shown. That hair is
likely to develop on the upper lip of woman without ovaries is also
claimed as likely to occur, and this, too, is sometimes seen in old
women, but if it is interpreted to mean an approach to the bearded
condition of man it should be admitted that the development is hardly
sufficient to invite such a comparison. Finally, it has been stated that
the voice becomes deeper, more, therefore, like the male, but this has
also been denied. If it could be established that the voice changes and
that it was brought about by an enlargement of the larynx, similar to
that which takes place when the larynx of the boy changes to that of the
man, it might seem not improbable that the change was toward that of the
opposite sex. This would mean that the ovary produces some substance
that prevents the enlargement of the larynx in the female. But since it
has been shown that the enlargement in the male is caused by the
development of the testes, and that this enlargement is prevented by
castration, a paradoxical situation would present itself, viz, that the
testes cause the larynx to enlarge in the male and the ovary prevents
the enlargement in the female. Until convincing evidence is forthcoming,
the question is better left undecided.


B. EVIDENCE FROM BIRDS.

Probably a greater difference in the secondary sexual characters is
shown in birds than in any other group. It is true that there are
species, such as the doves and pigeons, in which the plumage of the male
is much like that of the female, but this is the exception rather than
the rule. At the other extreme are species like birds of paradise,
hummingbirds, fowls, pheasants, ducks, and many passerines, in which the
plumage of the two sexes is entirely different. Our knowledge as to the
relation between the nuptial plumage of the male and the condition of
the sex-organs rests largely on information gained by castration in
poultry and ducks and on the assumption of the nuptial plumage in
several species only at the mating season.

John Hunter in 1780 described a pheasant with male plumage. His account
of a similar change in a pea fowl is so complete that I venture to quote
it in full:

     “Lady Tynte had a favorite pyed pea-hen, which had produced
     chickens eight several times; having moulted when she was about
     eleven years old, she astonished the lady and her family by showing
     the feathers peculiar to the other sex, and appearing like a pyed
     peacock. In this process the tail, which was similar to that of a
     cock, first appeared after moulting. In the following year she
     moulted again, and produced the same feathers. In the third year
     she did the same; at the same time she had spurs similar to those
     of a cock. She died in the following winter during the hard frost,
     namely, in the winter 1775-6. She never bred after this change in
     her plumage. This bird is now preserved in the Museum of Sir Ashton
     Lever.”[17]

     “From what has been related of these two birds, may it not
     reasonably be inferred that it seems probable that all those wild
     pheasants of the female sex, which are found with the feathers of
     the cock, had changed the nature of their feathers, particularly at
     a certain age?

     “If this idea be just, it shews that there is a disposition in the
     female to come nearer and nearer to the male, at least in the
     secondary properties; or it may rather be said that the female is
     later in producing this change than the male is; for it has already
     been observed that both sexes when young differ not from each other
     in these respects, but that the male appears to be the one that by
     degrees separates from the female in its secondary properties.”

Statements in regard to the effect of castration on poultry go back, it
appears, to Aristotle. Yarrel in 1811 and again in 1850 has given an
excellent account of many of the effects produced. His account of the
effects on the cock seem to be based partly on hearsay, and while they
contain much accurate information, yet the statement that the plumage of
the capon is intermediate between that of the cock and hen is incorrect.
The further statement that by cutting the oviduct the hen assumes the
plumage of the capon has been shown by Sellheim to be erroneous. The
operation referred to by Yarrel must have been one in which the ovary
was removed.

Yarrel described a female pheasant that had assumed some of the
characteristic colors of the male. On dissection he found that the ovary
was diseased as well as the oviduct. He correctly assigns the change in
plumage to the condition of the ovary. He states furthermore that most
of the female pheasants that he had examined that had male plumage had
not assumed the complete coloration of the male. In one case, however, a
complete change had taken place. The change in pheasants he thought was
due to old age accompanied by partial or complete loss of function of
the ovary. For poultry he states:

     “In the imperfect female the comb increases; a short spur or spurs
     appear; the plumage undergoes an alteration, getting what is
     usually called ‘foul-feathered;’ she ceases to produce any eggs,
     and makes an imperfect attempt to imitate the crow of the cock.
     Being profitless in this state, she is usually made away with. The
     proverb says:

    A whistling woman and a crowing hen
    Are neither good for gods nor men.

     Our neighbors and allies the French, who seem to take a wider range
     in their prejudice against habits which they consider irregular,
     have the following proverb, which says:

    Poule qui chante, Prêtre qui danse
      Et Femme qui parle latin,
      N’arrivent jamais à belle fin.

     “I have seen two instances in which females of the wild duck have
     assumed to a considerable extent the appearance of the plumage of
     the mallard, even to the curled feathers of the tail. One of these
     birds, in my own collection, was given me when alive by my kind
     friend the late John Morgan, esq. When this bird was examined after
     death, the sexual organs were found to be diseased, as in the case
     of the hen pheasants referred to, and figured in the 2d volume of
     the History of our British Birds. In the published illustrations to
     his Fauna of Scandinavia, M. Nilsson has given a colored figure of
     a duck in this state of plumage (plate 163), which is called a
     barren female, and in which the curled tail-feathers are made very
     conspicuous.

     “From the general similarity in these females to the appearance
     assumed for a time by healthy males in July, I am disposed to refer
     this seasonal change in males, in this and in other species of
     ducks, to a temporary exhausted state of the male generative
     organs, and their consequent diminished constitutional influence on
     the plumage.

     “A male shut up by himself from early spring to the end of July
     undergoes no change in his plumage; but if he is allowed to
     associate with females till their season of incubation commences,
     he then goes through the change, and this appears to indicate the
     cause of the partial summer moulting.

     “The appearance is somewhat different, but yet very interesting in
     insects and crustacea. In these classes the sexual organs are
     double and distinct, arranged one on each side of the elongated
     mesial line. It sometimes happens, that a species in which the
     sexes are of a different color, or markings, or form has one sexual
     organ of each sort, male and female, in which case each half of the
     same insect is developed under the exclusive influence of the
     sexual organ on its own side. Instances are preserved among our
     collections of butterflies, mothes and beetles; and I have seen it
     twice in the common lobster.

     “Nor is the human race exempt from the operation of the law which
     prevails in the Mammalia. In women, at an advanced age, hair
     appears on the chin and upper lip, and the voice alters, becoming
     deep in its tone. The beard in old men becomes thin and soft, and
     our own inimitable Shakespeare has told us,

          * * * his big manly voice
    Turning again toward childish treble, pipes
    And whistles in his sound.”



Gurney (1888) has recorded several cases in which female birds have
assumed male plumage. For instance, he describes a female merganser,
_Mergus serrator_, assuming male plumage that showed no signs of disease
in the ovary. Mr. Cecil Smith had a female widgeon (_Mareca penelope_)
on his ponds near Trenton, which assumed the male plumage some years
ago, and which, so far as he knew, had not had young nor laid eggs.

     “On May 16th, 1887, a chaffinch (_Fringilla cœelebs_) in full male
     plumage was shot at Chapel Town, near Leeds, in Yorkshire, by the
     son of Mr. W. L. Jackson, M. P.; it was skinned by G. R. Grassham,
     assistant to Mr. W. E. Clarke at the Museum, who, much to his
     surprise, found that it was a female, and contained an egg, ready
     for laying, of a pale blue, without markings, and another egg in a
     less forward state. This chaffinch is in every way in perfect male
     plumage, and I am indebted to Mr. Clarke for his kindness in
     sending these particulars with the specimen, which he received from
     Grassham a few hours after the latter had dissected the bird.

     “In the ‘Norwich Nat. Trans.,’ an enumeration was given of female
     Redstarts (_Ruticilla phoenicurus_) assuming male plumage (_l.c._)
     to which the following may be added: a hen _R. phoenicurus_
     assuming male plumage, and very like Mr. Millais’ described in the
     ‘Norwich Nat. Trans.’ iv., p. 182, was caught by Mr. W. E. Clarke
     sitting upon her eggs, at Wike, near Leeds, in June, 1886; at the
     same time Mr. Clarke saw the cock close by, which appeared to be in
     the ordinary male plumage. The late Mr. Henry Doubleday’s
     collection contained a hen Redstart (_R. phoenicurus_) in male
     plumage, which had the ovaries ‘quite perfect and full of eggs’
     (_cf._ B. of Norf., i, p. 370, note), probably one of those alluded
     to by Yarrell (Brit. B. 1st ed. i, p. 240) in the remarks made by
     him on the plumage of this species. I have some recollection of
     this Redstart at the dispersal of Mr. Doubleday’s collection, but
     do not know who was the purchaser of it. There can be no doubt that
     more would soon turn up if looked for; and now that attention has
     been drawn to the subject, and the practice of dissection is
     getting more general among bird stuffers, it is certain to be the
     case, not only in _Ruticilla_, but in other genera besides. Why it
     should happen in _Ruticilla phoenicurus_ oftener than in other
     Passerine birds is hard to explain, but such is evidently the
     case.”

     “The same is recorded to have happened five or six times with the
     female Red-backed Shrike (_Lanius colluria_); see ‘the Field,’ June
     17, 1871, and April 25, 1885; Mag. N. H., iv, p. 344; ‘B. of
     Suffolk,’ p. 45; ‘Ibis,’ 1863, p. 292; but the number of hen
     Redstarts which have donned masculine attire is greater.

     “The following is a list of the species in which one or more
     instances of females assuming male plumage are ascertained to have
     occurred:

  Falco aesalon, fide Scully. (Cf. Sharpe, ‘Cat. Birds Brit. Mus.,’
      i, p. 407).
  Tinnunculus alaudarius, fide Sharpe; col. fig. P. Z. S., 1874, p. 580.
  Lanius collurio, fide Hoy.
  Lanius vittatus, fide Blyth.
  Ruticilla phœnicurus, fide Millais, Clarke and others.
  Fringilla cœlebs, fide Clarke.
  Linota cannabina, fide Blyth.
  Linota rufescens, fide Blyth.
  Nectarinia asiatica, fide Blyth.
  Gallus (domestic fowl), fide Yarrell and others; col. fig. “B.
      of Sherwood,” p. 183.
  Pavo (peahen), fide Latham; fig. “Synopsis,” ii, pl. 60.
  Meleagris (Turkey), fide Bechstein.
  Phasianus colchicus, fide Edwards and others. Of common occurrence
      in a semi-domesticated state.
  Thaumalea picta, fide Edwards.
  Euplocamus nycthemerus, fide Yarrell.
  Pucrasia nipalensis, fide Blyth.
  Tetrao tetrix, fide Bond; col. fig. Dresser, “B. of Eur.,” vi, 205.
  Tetrao urogallus, fide Nilsson; col. fig. “Unser Auer-, Rackel- und
     Birkwild und seine Abarten,” by A. B. Meyer.
  Otis tarda, fide Tiedmann.
  Anas (domestic duck), fide Rowley; col. fig. “Orn. Misc.,” i, p. 118.
  Anas boschas, fide Hancock; fig. col. “Scandinavisk Fauna,” pl. 163.
  Fuligula marila, fide Blyth; see also P. Z. S., 1885, p. 246.
  Mergus serrator, fide Gurney.
  Mareca penelope, fide Cecil Smith.

     “Perhaps the Kestrel (_Tinnunculus alaudarius_) ought not to be
     included in this catalogue, for so many have been seen with the
     lower part of the back blue or bluish, as to leave little doubt
     that the female generally becomes so if she lives long enough.

     “It is said that the females in _Oriolus_ generally become as
     bright as males in time (‘Ibis,’ 1864, p. 412; ‘Field,’ June 24th
     and July 8th, 1871).”

     “P. S.--Mr. W. Tegetmeier tells me he has known a barnyard cock
     moult into hen’s plumage, which is the converse of the instances
     narrated in this paper, and rather resembles the annual change
     which takes place in _Anas boschas_ and others of that tribe.”

In a later notice Gurney makes the following statement:

     “The bearded tit (_Panurus biarmicus_) may be added to the list of
     female birds which are known to occasionally assume male plumage.
     In the summer of 1882 a bearded tit, two years old, in Mr. J. G.
     Keulemans’ aviary, hatched five eggs and moulted, during which
     operation she suffered much from cold and stiffness, and when she
     recovered her plumage it was partly that of the male (_cf._ ‘The
     Field,’ Sept. 14, 1872).”

Brandt, who has reviewed the literature very thoroughly, cites the
following cases:

     “Galeinacei: Gallus bankiva domest., Phasianus pictus, torquatus,
     colchicus, mongolicus and nycthemerus, Pavo cristatus domest.,
     Meleagris gallopave domest., Perdix einerea, Tetrao urogallus,
     tetrix und bonasia.

     “Passeres: Fringilla coelebs, Pyrrhula vulgaris, coccinea, Loxia
     chloris, Turdus merula, Ruticilla phoenicurus, ochrura,
     chrysogastra, Cyanecula Wolfii, Sturnus vulgaris, Ampelis cotinga.

     “Scansores: Cuculus canorus, Edolius glandarius.

     “Grallatores: Machetes pugnax.

     “Natatores: Anas boschas domest.

     “Es ware denkbar, dass die Hahnenfedrigkeit, wenn auch in
     verkapptem Grade, allen Vögeln, selbst denjenigen zukomme, deren
     Gefieder uns geschlechtlich uniform zu sein scheint. Wie dem auch
     sei, einzelne Genera und Species scheinen mehr, andere weniger zur
     Arrhenoidie prädisponirt. So bemerkt J. Geoffrey St. Hilaire (p.
     511), dass Fasanen häufiger selbst als die Hühner hahnenfedrig
     werden, während für den Pfau, den man doch stets eines natürlichen
     Todes sterben lässt, ihm nur ein einziger Fall (der von Hunter)
     bekannt geworden. Während Lorenz (vide Tichomirow) auf dem Moskauer
     Markt häufiger hahnenfedrige Weibchen von Phasianus colchicus and
     mongolicus aufgefunden, ist ihm dieses fur Ph. chrysomelas bisher
     kein einziges Mal gelungen, obgleich die Zahl der jährlich in
     Moskau feilgebotenen Exemplare dieser Art sich auf 8000 Stück
     belaufen möchte.”

The preceding cases relate to exceptional changes in the plumage as
observed in nature, or in birds kept under domestication. We may next
examine the cases where the ovary or the testis has been removed.

The earlier observations of Berthold, Wagner, Hanau, Samuel, Sellheim,
Pirsche, Foges, Shattock, and Seligman are sufficiently covered by later
work quoted below. Sellheim’s work, however, is especially to be noted,
since he gives some measurements covering the weight of the brain,
heart, and body of the cock and capon, as well as observations on the
skull and skeleton. The weight of the brain is slightly less in the
capon, but the body-weight is greater. He questions whether the ovary
has ever been successfully removed, and he shows that the operation of
resecting the oviduct does not, as was supposed, lead to the
degeneration of the ovary. On the contrary, he found that after the
effects of the operation had been removed the ovary began again its
functions.

From Goodale’s careful summing up of the effects of castration only the
following points need be recalled: The feathers are little changed; some
of them, the hackles especially, become longer. The lowermost tier of
wing coverts are elongated as compared with those of the cock. The spurs
are practically the same in the capon and cock. The capon is disinclined
to give voice, but at times he crows. The molting is not affected. The
size of the capon is larger. He pays little attention to the hens. He is
not pugnacious, and if attacked will not often fight. As a rule he does
not pursue the hens, but if a hen squats down as the capon approaches he
will mount and go through the characteristic mating reaction. The comb
is extremely small, much smaller than that of the female of the same
race; it is infantile rather than feminine.

Comparing these results with those that I have observed in the castrated
Sebright, we find that aside from the assumption of the full plumage of
the cock-feathered bird the Sebright shows all of the characteristic
features of the capon. The spurs develop, perhaps even more fully than
in the normal Sebright cock. He seldom crows, and then weakly. The birds
appear large, but the excessive development of the feathers produces the
effect. I have not weighed them to show whether an actual increase in
size takes place. Two of my birds are notably large for Sebrights, but
the others are smaller. Both large and small cocks occur in the strain
that I have used. My Sebright and other capons neglect the hens, but I
have seen them tread the hens on occasion. They will fight each other,
if two strangers meet, but the attacks are not violent or prolonged. A
normal male beats them easily, and afterwards they run away from such
birds. The combs and wattles are very small and pale. If a piece of the
testis is left in, the comb is a fair index of its size. In the birds
that changed back toward a Sebright the comb slowly enlarged. After the
second operation it decreased again as the plumage once more changed to
that of the cock.

Goodale’s results with ovariotomized females are especially noteworthy,
since here for the first time we have definite information as to the
effects of the operation. By using a well-established breed, the brown
Leghorn, in which the dimorphism of the sexes is very striking, the
results are made all the more convincing. Goodale found that it was
possible to completely remove the ovary of young birds, for at an early
age the ovary is sufficiently compact to make its entire removal
possible. Later the ovary becomes more diffuse, and complete removal is
almost impossible. In a few successful cases, in which the ovary had
been completely removed, the bird assumed the full plumage of the
Leghorn cock, with red back, black breast, and long, pointed hackle and
saddle feathers. Spurs developed in all the operated females, even when
the ovary was not entirely removed. There can be little doubt that the
ovary holds back the development of the spurs, but as some hens
sometimes develop spurs, especially in certain breeds, it is not
entirely certain that in these cases the loss of the ovary is the cause
of the appearance. The comb (and wattles) developed to different
degrees; in some birds it was as large as in the cocks, in others no
larger than in the normal hen, but in all cases it was larger than in
the capon. What to conclude is doubtful. Tentatively it may be suggested
that the genetic complex that gives the female (ZW) produces a comb as
large as that shown by the female independently of the ovary, but beyond
this point the ovary inhibits the further development of the comb,
presumably by means of the same internal secretion that holds down the
cock plumage in the hen. In the male, on the other hand, the genetic
complex (ZZ) produces a comb much smaller than that of the female (no
more than that of the capon), and the testes produce a substance that
causes this comb to grow to the size of that of the cock. Possibly,
however, other internal secretions are involved.

The operated hens are quiet and nearly voiceless. None of Goodale’s
birds were heard to crow, yet this seems to be a well-known peculiarity
of old hens that have become cock-feathered. The operated hens are not
larger than the normal hens of the same breed. Their legs remain short,
as in the normal hen; and in this respect and in size the ovariotomized
bird is externally a female. The poullards “never visit the nests, never
sing or cackle, show none of the normal female reactions, and few or
none of the male.”

The influence of the ovary in suppressing the cock plumage has been
convincingly shown in an experiment of Goodale’s, in which, after
removal of both testes from the young Leghorn cock, pieces of ovaries
were inserted into the body-cavity. As dissection showed later, several
of these implanted pieces grew onto the wall of the body-cavity. The
birds developed the plumage of a hen, although some traces of the male
plumage were at times present. The difference between the sexes is so
great in Brown Leghorns that the hen-feathering of the feminized
cockerels leaves no doubt that the presence of the ovary had produced
the female coloration.

Geoffrey Smith and Mrs. Haig Thomas (1913) have examined a number of
hybrid pheasants, some of which were sterile. They found that the ovary
(and oviduct) was often small and degenerate. There was a more or less
corresponding tendency for such female hybrids to show male feathering,
at least in a part of the plumage. The degeneration of the sex element,
however, does not take place until after the time of synapsis, so that
the younger germ-cells may be normal. The later degeneration of these
cells is not likely to influence the secondary sexual characters, but
may be an index of changes in other parts of the ovary.

Geoffrey Smith had a breed of White Leghorns with cocks of two
classes--those that assumed cock plumage at 6 months, and those that are
like the hens for 8 months, after which they slowly assume the
cock-feathering. The difference is hereditary and appears to segregate.
Possibly this breed had one factor at least for hen-feathering that is
more effective for young birds than for older ones.

Smith states that birds and crabs (see _infra_) appear to give opposite
results, since removal of the ovary in the former leads to development
of secondary male characters and removal of testes in the latter to
secondary female characters. But he adds that he thinks the results are
really the same, because in the crab it is not the suppression of the
testis but the feminization of the male by the Sacculina that causes the
change.

There are a number of observations on ducks. Several cases have been
recorded where in old age the female assumed the male plumage (Darwin,
Shattock, and Sellheim). Also a few cases in which the testes were
removed. Those of Goodale are the most complete and striking. The male
duck has two characteristic plumages, one called the nuptial, also
called the summer or breeding plumage that is assumed at the molt in the
autumn, and the other the eclipse plumage, which is not identical with
but much like that of the female. Here, then, we find a new situation,
and one that invites comparison with the condition in Sebrights, in so
far as the male becomes hen-feathered at certain seasons.

Throughout the greater part of the year the Rouen drake has the nuptial
plumage. The head is green and the breast is claret. Two median tail
feathers are strongly curved; the next two are also often curved. These
four are called the sex feathers. At the close of the breeding-season
(July) both sexes molt. The male now has the same coat as the female, or
nearly so. The green head becomes brown to buff; the sex feathers are
straight. The change back again to the nuptial plumage begins at the end
of summer and is completed early in October. Thus in the race of Rouens
the eclipse plumage lasts only a very short time. In the mallard it
lasts longer. The eclipse plumage develops, therefore, only when the
testes are active, or, as Goodale puts it, “the presence of the active
testis is necessary for the drake to assume this plumage.” Conversely,
the nuptial plumage comes on in the late summer, when mating is over,
and when the testes have shrunken and are not active, at least as far as
the sex-cells are concerned. In some respects the situation is like that
in the fowls, for in both the testes are not necessary for the
development of the full plumage, but in other respects the situation is
different, because at the time in the ducks when the testes are active
the eclipse plumage develops. Are we to suppose that at the time of
sexual activity a substance is produced analogous to that produced by
the ovary of the female? This seems the most plausible assumption, for
we know that if the testis is removed the eclipse plumage does not
appear. Such a situation suggests a comparison with the Sebright, where
it has been shown that the testis must actively produce some substance
which, like that in the ovary, keeps down cock-feathering. It is
plausible, even if it can not be established, that the substance in the
duck and the inhibitory substance in the male Sebright are the same as
that produced in the female.

Goodale’s results with females (ducks) are not so clear cut, because the
ovariotomized females turned out to be of two sorts. One sort is almost
identical with the male, the other is more intermediate. There are
sufficient reasons for thinking, he says, that these differences are not
due to defective operations. Goodale suggests a genetic difference in
the females used, but this is apparently even to Goodale himself not a
very satisfactory solution. For our present purpose the important fact
is that the ovariotomized female may assume the perfect male plumage.
Evidently the ovary produces some substance which, as in the hen,
suppresses the potential plumage of the male. One such female known to
have had all the ovary removed never assumed the summer (eclipse)
plumage of the drake. On the other hand, another female developed first
the nuptial plumage, but this was replaced by the summer coat “of the
male of this variety.” Again, in the summers of 1914 and 1915 the change
to the eclipse plumage was followed in the autumn by a return to the
nuptial plumage.

How can we explain the apparent discrepancy of Goodale’s results? In one
case, the nuptial plumage was molted to nuptial plumage; in the other
case an eclipse plumage appeared at the breeding-season. Goodale regards
the latter case as a more perfect approach to the male than the former,
but this view undoubtedly offers serious theoretical difficulties. It
seems to me possible to suppose that in those cases where the summer
plumage appeared there was in reality enough ovarian tissue (or related
tissue) left after the operation to produce an effect at the normal
season for such ovarian tissue to become most active. It might then
suffice to eclipse the male plumage sufficiently to make it very similar
to the eclipse of the normal male. At any rate, on this basis we have a
consistent explanation of the entire complex of phenomena.

What bearing have these results relating to castration and
transplantation on the theory of sexual selection? Granting, of course,
that selection takes the materials as it finds them, there may still be
restrictions imposed on the theory by the kind of material offered. For
instance, the development of the plumage of the cock is independent of
the condition of his testes. Hence, if the female selected the more
vigorous male, she would not necessarily obtain one more ornate than his
less vigorous rivals. If the taste of the hen has built up the plumage
of the cock, it has been carried out then independently of the vigor
resulting from the greater activity of the testis. In a word, the more
vigorous male is not necessarily the most highly colored one. Darwin
concedes that these two conditions, high color and vigor, must go
together to insure success, or at least that the most vigorous and
therefore the most highly colored male will have more offspring.
Wallace’s contention that the greater vigor of the male accounts for his
greater development of plumage gets scant support from the facts of
castration. One might rather contend that the female must be more
vigorous, since she is obliged to suppress plumage that is allowed to
run riot in the male.

Wallace’s argument in favor of natural selection holding down the
plumage in the female as a protection to her while nesting might appear
to fit the facts better were it not that the quest for an explanation of
the male’s plumage is thereby abandoned. It should not be forgotten in
this connection that the nest is generally only partly concealed, that
bright color at rest need not be conspicuous, and that the male, exposed
as he is through a considerable part of the year, still manages to
maintain himself in about equal numbers with the female. Suppose,
however, for the sake of argument, that natural selection has kept under
the full possibilities of the female. The _modus operandi_ would be
competition between the least adorned females, suppression being brought
about by the activity of the ovary; while the male is left therefore to
exhibit the full possibilities of the genetic complex of his race
without restraint. The facts in the case are that the plumage of the
male is the direct result of his genetic composition; the female has the
same genetic composition (the sex-linked characters are duplex), but the
ovary produces a substance that holds them in restraint. Put in this
way, there is nothing further to be explained, unless we insist on
finding an explanation as to how the species came to have its genetic
constitution. In other words, if we are not satisfied with the statement
as to the actual situation, we must explain it by a utilitarian appeal
to a relation between the plumage and the world outside of the
individual or the species. To those who feel unsatisfied to leave the
case as it stands on a physiological basis, there is another
hypothetical means of escape. It may be assumed that the genetic factors
that are instrumental in producing the secondary sexual characters have
also other but unknown influences in the economy of the species, color
and ornamentation being by-products of these factors whose utility in
other directions accounts for their presence. Such a philosophy has
perhaps one redeeming feature, since it suggests the possibility of
searching for other influences--influences that only incidentally give
the striking coloration and ornamentation of the males.

At first sight the absence of cock-feathering in the Sebright may seem
to furnish the occasion for such a quest. It might appear that since
only one or two genetic factor differences are responsible for the
“nuptial” plumage of the male, that this plumage may have originated in
one or two genetic changes. Such an argument is fallacious, however, for
very many genetic factors may historically have been necessary to build
up the nuptial plumage of the male. The breeding experiment shows no
more than that one or two other factors have appeared that counteract
the effect of all that the others are capable of producing; the
experiment throws no light upon how many or how few these other factors
may be. That the nuptial complex is still present in the Sebright is
evident after castration. Castration shows only that the testes in the
Sebright produce some material that keeps down the effects of all the
other factors combined. This conclusion, it is true, somewhat simplifies
the problem for those who appeal to natural selection as suppressing in
the female the feathering of the cock, because it shows that this could
have been accomplished by one or two Mendelian factors that appeared of
such a kind that they caused the ovary to produce a substance
antagonistic to the influences coming from the genetic complex of the
species.

With this by way of provisional exposition, let us return to the
question as to whether the Sebright-game cross throws any other light on
the possibly useful character of the genetic factor or factors that
produce cock-feathering. It is obvious that the evidence gives us no
clue at all, for with the exception of the normal allelomorphs of the
dominant factor for hen-feathering, all the other factors are still
present in the Sebright. The normal allelomorph in question need not
have had any relation to the other complex; in fact, it seems not to
have any, because the castrated Sebright (with both normal allelomorphs
replaced by genes for hen-feathering) still develops the characteristic
cock-feathering.

The outcome in the duck with its double male plumage is still more
puzzling when we attempt to analyze the situation in the light of the
selection theory. At the height of the breeding-season, when his testes
are enlarged and functioning actively, a substance is being produced
that leads to the eclipse of the nuptial plumage. If the male were
selected by his partner for his plumage, he would be chosen for a
plumage that develops in the absence of the functioning testes. If the
male is chosen because of his greater aggressiveness or “activity” or
“vitality” due to the development of his testes, the result would be to
select males that would probably develop a better eclipse plumage. The
case is interesting because it gives an opportunity to distinguish
between a plumage that develops under the influence of the sexual organs
and one that does not; and the latter is paradoxically the nuptial
plumage. It is true that the male might be selected for his nuptial
suit, and, theoretically at least, female choice might still be made
responsible for this plumage, but this merely shifts the problem, for it
leaves “unexplained” the appearance historically of the effect of the
activity of the testes in suppressing this plumage for a short time
after maturity. No doubt an attempt might be made to show that natural
selection comes in at this time of the year in giving a protective color
to the male, but so long as any evidence is lacking as to the need of
this protection the argument serves rather to further complicate an
already difficult situation.

Goodale has written to me that there is an account, in the Agricultural
Journal, Union of South Africa, IV, 1912, of the effects of the removal
of the ovary of the female ostrich. I have not been able to see the
account, but according to my informant such female individuals assume
the male secondary characters.

Of unusual interest in connection with the seasonal change of plumage in
males of dimorphic species are Beebe’s experiments with scarlet tanagers
and bobolinks. In both species the males in their nuptial plumage are
very different from the females. Full-plumaged males of both species, at
the height of their “vocal and physical condition,” were confined in
small cages. The supply of light was gradually cut off and a slight
increase of the amount of food was allowed them. The birds became less
active in consequence and increased in weight. “The time for the fall
molt came and passed and not a single feather was shed.” The birds had
skipped the autumn molt and remained in their nuptial plumage. The song
soon died away; “the birds seldom uttered even a chirp.” From time to
time a bird was gradually brought into the light for a week or two and
meal-worms were added to the diet. This invariably resulted in a full
resumption of song.

     “I found that a sudden alteration in temperature--either lower or
     higher--wrought a radical change in the physical metabolism of the
     birds. They would stop feeding almost altogether, and one tanager
     lost weight rapidly. A few feathers on the neck fell out, and in
     the course of some two weeks this bird moulted almost every feather
     and came strongly into his normal winter plumage of olive green.
     The metabolism set up by the change in temperature, in its intent
     and rapidity, seems comparable only to the growth of a deer’s
     antlers.

     “Early in the following spring individual tanagers and bobolinks
     were gradually brought under normal conditions and activities, with
     quick result; just as the wild birds in their winter haunts in
     South America were at that time shedding their winter garb and
     assuming the most brilliant hues of summer, so the birds under my
     observation also moulted into the colors appropriate to the season.
     The old scarlet and black feathers fell from the tanagers and were
     replaced by others of the same color; from buff, cream, and black,
     the bobolinks moulted into buff, cream, and black! There was no
     exception; the moult was from nuptial to nuptial, not from nuptial
     to winter plumage. The dull colors of the winter season had been
     skipped.”

How are these results to be interpreted? Obviously the environment
prevented the autumn molting; hence the birds necessarily retained their
nuptial plumage. But is this the whole story? Did they not also remain
sexually active with their testes producing sperm as in the mating
season? In other words, if feathers had been plucked from them, would
not the new feathers have been like those already present? Despite the
author’s statement that not a single feather was molted, is it not
likely that occasionally a feather must have been accidentally lost. If
even one had been lost and an eclipse feather had replaced it, the
effect would not have escaped so keen an observer as Dr. Beebe. It seems
to me not unlikely that an occasional feather may have been lost and
replaced by a nuptial one. If so, then the results are most probably
interpreted as due to the birds having remained sexually active. This
condition suppressed the autumn molt, and at the same time would cause
any single feather lost to be like those still present. In support of
such a conclusion I can appeal to Beebe’s statement that after a week in
the light a full resumption of the song took place. It is unlikely that
sexual maturity would be attained in so short a time unless the birds
were already in the condition of sexual vigor. Perhaps one can appeal
also to Beebe’s other statement, viz, that after a sudden change in
temperature, followed by a changed metabolism and loss of weight, the
birds molted and assumed the eclipse (winter) plumage. Here I should
interpret the facts cited possibly to mean that the males lost their
sexual activity and in consequence developed the eclipse plumage.

Until further information is obtained judgment must be suspended. If, as
Beebe’s statements strongly suggest, the external conditions, acting
directly on the “metabolism,” cause the changes observed, then the
experiments mean that environmental conditions affect directly the
development of the nuptial and the eclipse plumage; but if, as I suggest
here, the effects observed are due directly to the environmental action
through its effects on the testes, then the results fall more nearly
into line with those of Goodale on ducks, etc.


C. EVIDENCE FROM AMPHIBIA.

The thumbs of frogs enlarge at the breeding-season and shrink
afterwards. The enlarged thumb is used by the male in clasping the
female during copulation, and the rough papillæ that appear over its
surface at this time may also help to anchor the male in his precarious
position on the back of the female. Since the pads and their papillæ are
used in copulation, they belong rather in the class of accessory organs
of reproduction than in the class of secondary sexual characters. Smith
and Schuster state for _Rana fusca_ that the testes are at their
smallest size in March and April after the breeding-season. From that
time until August they steadily increase in size and reach their maximum
size in September. From September to March they are inactive and full
size, until the shedding of the sperm in March brings them soon
afterward to their lowest point again. It is to be noted that the
increase after March is associated with the increase in division rate of
the spermatogonia. The ripening of the sperm is finished in October.

The thumb-pads with their pigmented papilla are “cast off” immediately
after the breeding-season, the thumb remaining smooth from May to
September. The reduction of the pad is usually due to the reduction of
the glands and the disappearance of the papillæ. Smith and Schuster
state: “During the months when the most active growth of the testis is
taking place the thumb-pads remain inactive and smooth.” The
implication, apparently, is that one ought to expect the growth in the
thumb to take place when the germ-cells are most actively dividing, if
its growth is connected with their activity; but there are no grounds
for such expectations, because the influence of the gonad may have
nothing to do with the division rate of the germ-cells, but rather with
interstitial or other cells, and even here less with their division rate
than with their period of greater secretive activity.

     “In August and September the epidermal papillæ begin to be obvious,
     and from this time onwards until about February a continuous
     increase of the epidermal papillæ and pigmentation occurs. During
     the greater part of this time, when the thumb-pads are attaining
     their characteristic rough and pigmented appearance, the testes
     remain inactive and unchanged--a fact which has been too readily
     overlooked by writers on the correlation of the primary and
     secondary sexual characters.”

Nussbaum (1909) and later Meisenheimer (1911) found that after
castration the thumb-pads disappear. Smith confirms this report in all
essential respects, although in certain details concerning the papillæ
he does not agree with the two former observers. His results show that
castration at the breeding-season is rapidly followed by the loss of the
outer papillated layer of the thumb-pads, but castration at any other
season does not have “any marked effect,” the papillæ remaining for 5
months and more in the same condition as at the time of castration. The
essential point here, however, is that the excessive and even special
development at the breeding-season does not take place nor is again
assumed (apparently), if castration has taken place at some other time
of the year.

Smith and Schuster’s attempts to transplant the testes into other males
or females were unsuccessful, as the testes degenerate after a time.
Auto-transplantation of the testes were more successful.

Removal of the ovary had no effect on the thumbs of the female, and even
the injection of testes extracts into such females did not cause them to
develop pads. Nussbaum and Meisenheimer had found that transplantation
of pieces of the testes, and even injection of testes extract, into
castrated frogs caused an enlargement of the thumb-pads. Smith shows
that this conclusion rests on uncritical evidence. At any rate, his own
more carefully planned experiments extending over the year show that the
results obtained by Nussbaum and by Meisenheimer may be accounted for on
other grounds than the effect of the injection or implantation.

The following statement by Smith is not without interest, since it bears
directly on an important question as to how internal secretions may
produce their effects.

     “The deduction, therefore, which has been unduly based on
     Nussbaum’s experiments, that the testis of the frog contains an
     internal secretion, which, on being circulated in the blood, calls
     for the development of the secondary sexual characters, either with
     or without the mediation of the nervous system, is without
     experimental foundation.... The fact that the developmental cycle
     of the thumb depends for its normal course on the presence of
     normal living testicular tissue can be equally well explained on
     the theory that the testicular cells enter into a chain of
     metabolic processes in the body which do not pursue their normal
     course in the absence of the testicular cells. This disturbance of
     the normal metabolic processes of the body, resulting in the
     failure of the metabolic organs of the body to give rise to their
     normal products in normal quantities, may have the result of
     inhibiting the further development of the secondary sexual
     characters. The development of these latter characters may depend,
     therefore, not directly on the action of an internal secretion or
     hormone derived from the gonad, but on the elaboration of other
     products in other organs of the body in their due proportions.
     These substances may be tentatively called ‘sexual formative
     substances,’ but we have no reason for supposing that they are
     entirely devoted to sexual or reproductive purposes, and that they
     take no part in the ordinary metabolic processes of the body.”

The arbitrary distinctions that Smith here sets up do not seem to me to
contribute anything to the situation, and in fact in the end it amounts
to practically the same thing whether the hormone acts directly on some
specific part of the body or whether in doing so it acts on other parts
as well. While it is more or less customary to limit the term “hormone”
to substances that do produce specific effects in a particular organ, no
one would, I suppose, deny that a substance was acting as a hormone if
at the same time it acted on other parts of the body also, or even if
its immediate action were on some part and its ultimate action on
another part of the animal. Moreover, there is nothing in the evidence
appealed to by Smith that supports one rather than the other contention.
It is not apparent that the simpler idea of hormone action may not still
apply. Failure to implant the testes in castrated male or female, and
failure of injections to produce the results sought for, may mean no
more than that the experimenter failed to fulfill some one of the
conditions present in the normal frog at the breeding-season. Granting
that the results recorded by Nussbaum and Meisenheimer are open to the
serious objections, pointed out by Smith and Schuster, the facts
recorded by all three writers indicate that the maximum development of
the pad takes place when the testes are at their greatest development
and that the pad suddenly decreases if at this time the testes are
removed. It would seem to follow that since the swelling is connected
with the presence of a certain condition of the testes, its enlargement
is to be referred directly to the latter, and the case comes under the
general category of “secondary sexual differences,” depending on the
gonad.

The secondary sexual characters of _Triton cristatus_ can not, as can
those of the frog, be supposed to be mechanically useful in mating, but
seem to be comparable in every respect with the secondary sexual
ornaments of higher animals. The work of Bresca has shown that their
development is under the influence of the testes. The most important
secondary sexual characters of the male are the dorsal comb and the
white stripes of the tail. The comb extends along the dorsal surface of
the body and of the tail (with a slight dip in the pelvic region). It is
fully developed during the breeding-season, when it reaches a height of
1.5 cm. In winter it is only 0.66 mm. high, or even less. The white
stripes also are fully developed in the breeding-season. They extend on
each side from the cloaca to the end of the tail. In the female the
white stripe is sometimes faintly seen. The angles of the tail and of
the cloaca thickening are black-brown or black. The belly of the male is
bright orange or “Ziegel rot”; that of the female sulphur-yellow or
orange, but the difference is not constant. The upper surface of the
head of the male is marbled, especially during the breeding-season
almost disappearing during the rest of the year. Bresca found, when the
testes were removed from sexually mature males, that in the course of a
year all the important secondary sexual characters disappeared,
including the comb, the white tail stripes, and the marbling of the
upper surface. Removal of the ovaries did not affect the characters of
the female. The black lower corner of the tail in the male is not
changed by castration.

When the skin along the middle line of the back of the female is
transplanted upon the back of a normal male (in place of his own comb)
the transplanted tissue develops into a comb. In other words, under the
influence of the testis, the dorsal mid-line tissues of the female
change into those characteristic of the male. When pieces of skin of a
male with the white tail stripes are grafted on the side of the tail of
another male, the stripe remains, but when grafted similarly on a female
the stripe slowly disappears. The result shows that its presence depends
on the testis.

A remarkably clear case of hermaphroditism in amphibians was found by V.
la Vallette St. George. He found an individual of _Triton tæniatus_ that
was outwardly a male with well-formed dorsal comb. In the interior were
two large testes in normal position and just lateral to these on each
side a large ovary. Sections showed ripe sperm in the testes and typical
ova in the ovary. Sperm-ducts were present, but no oviducts. The
presence of the testes will, of course, account for the development of
the secondary sexual characters of the male.

Other cases amongst the Anura have been recorded by Loisel and by
Marshall, Spengel, and Knappe. In the early stages of the gonad in frogs
there appears to be an hermaphroditic stage in which egg mother-cells
and sperm mother-cells are both present, at least in those individuals
that will later become males (Kusakowitsch).

The normal hermaphroditism of certain fish (_Serranus_) and its rare
occurrence in other species (recorded by Shattuck and Seligmann) need
not be recorded here.[18]


D. EVIDENCE FROM CRUSTACEANS.

In the Crustacea the secondary sexual characters are not marked, except
in a few cases. In the amphipods, Holmes has shown direct contact plays
the chief rôle in mating, and in the crayfish it has been shown by
Dearborn, Andrews, and Pearse that sex recognition is largely tactile.
Chidester also has shown this in crayfish. Even in crabs, and especially
those living on land which have well-developed eyes and good vision,
secondary sexual differences are as a rule slight and the mating
instincts simple. On the other hand, the enormous chela of the male of
the fiddler is supposed to be a secondary sexual difference (mainly
because no other use for it has been found). Pearse suggests that the
waving of this claw by the male is used as a sex signal, although he is
disinclined to accept Alcock’s view that it has become “conspicuous and
beautiful in order to attract the female.”

The most remarkable case known of a change in the secondary sexual
characters of one sex into those of the other was discovered by Giard
in 1886. As a result of infection by parasitic crustacea (_e. g._,
_Sacculina_), the male crab develops the secondary sexual characters of
the female. It has been generally supposed, following Giard, that this
result is due to the destruction of the testes of the male by the roots
of the parasite that invades the spaces between the organs of the host,
and, in the case of the testis, ultimately brings about its partial or
complete destruction. Not unnaturally the results here were supposed to
be parallel to those of castration in vertebrates, and received in fact
the name of “parasitic castration.” More recently Geoffrey Smith has
studied this phenomenon in the crab _Inachus_, infected by the parasite
_Sacculina_, and has reached the conclusion that the change is not due
to injury or to destruction of the testes, but to a change in the
metabolism of the crab brought about by the parasite.

Taking Geoffrey Smith’s case of _Inachus-Sacculina_ as typical, the
changes brought about are as follows: The parasites attach themselves to
the young crabs before the external secondary sexual differences have
appeared. In the females, the effect is to cause them to develop
prematurely the distinctively female characters. In the male, on the
other hand, the narrow abdomen of the male changes after a molt into the
broad abdomen of the female, which also develops ovigerous appendages on
its ventral surface like those of the female in every detail. The larger
claw of the male changes into that of the female, which is different in
form as well as in size. Some years ago I ventured to raise the question
as to whether these effects on the male might not be interpreted as
retention of the juvenile characters rather than development of the
female characters in the male. This might appear more especially the
case in the somewhat more juvenile shape of the anterior abdominal
appendages and possibly also in the shape of the broader abdomen; but
Smith has later shown that the results can not be interpreted as
juvenile, for when the changed organs are examined in detail they are
found to differ from the same organs in the juvenile condition, and to
be identical with those of the adult female. I think, therefore, that we
must accept this interpretation of Giard and of Smith as correct. But
Smith goes further and believes that the effects may be carried so far
that eggs develop in the old testes; in other words, that the testis
changes to an ovary. It seems to me that the evidence to support this
last point should be much stronger than that advanced by Smith before we
can accept this interpretation, for we lack the essential control for
this evidence. In only a single case were eggs found--in the testis of a
male that had been infected, but from which the parasite had fallen off,
and which was presumably recovering from the effects of its presence.
Now, it is known that in the testes of some male animals a few eggs may
occasionally be found where there is no suspicion that the animal has
changed its sex. In some crustacea, in scorpions, and in insects,
isolated instances of this kind have been found. Abnormal division of a
spermatogonial cell, of such a kind that both sex chromosomes (in the
case of insects at least) got into the same cell might be expected to
cause such a cell to become, even in the male, an egg-cell rather than a
sperm-cell. The degenerative changes of the testes in the hermit crab
caused by the parasite might be imagined to favor such abnormal division
with its consequences. More significant, however, is the fact that the
parasite causes the absorption of the ovary when it infects a young
female, so that even all its eggs disappear. In other words, the
parasite is as injurious to the peculiarly female organ as it is to the
testis. Why then, one can not but ask, should an influence that causes
such effects on the ovary first change a male into a female so long as
it is present and then when the parasite has disappeared leave an
influence behind of a kind that causes the ovary to develop--an organ
which the parasite destroys when the parasite is present? Is it not more
probable that only the secondary sexual organs were changed, without
change in sex, the single case of eggs observed being caused in another
way? This point can only be settled by direct experimentation either by
removal of the testis, by injuring it, or by injection, grafting, or
feeding experiments. The extent of the testis and its position make it
impossible to remove it by an operation, as I have found after repeated
attempts. It seemed easier to destroy it by radium. This I have tried to
do, using very powerful tubes, treating the crab (fiddler crabs) for
several hours. The crabs had had one claw removed--the enormously large
one--and were kept until the next molt, that occurred from a week to six
weeks later. In none of the cases was any change produced. The large
claw of the male regenerated, of course, not full size after only one
molt, but after several nearly full size and always with the
peculiarities of the male crab. The abdomen and the appendages were not
changed. Whether the significant cells of the testes, if there are such
cells apart from the germ-cells, were destroyed, can not be told, for as
yet the histological examination of the material has not been made.
Until a successful operation has been done, I think we must hesitate to
accept Smith’s argument, although based as it is on a series of
interesting observations. His speculation is as follows:

     “The reason why _Sacculina_ causes the assumption of the adult
     female state in _Inachus_ is found in the facts: (1) that the roots
     of _Sacculina_ elaborate a yolk-substance from the blood of
     _Inachus_ of a similar nature to that which is elaborated in the
     ovaries of an adult _Inachus_; (2) that in order to elaborate this
     yolk-substance the roots take up from the blood of _Inachus_ the
     female sexual formation substance, which is the necessary material
     for forming the yolk; (3) that the female sexual formative
     substance being absorbed by the _Sacculina_ roots is regenerated in
     excess; (4) that the presence of the female formative substance
     continually circulating in large quantities in the body-fluids of
     the infected crabs causes the production of adult female secondary
     sexual characters, and, when the parasite dies, of yolk-containing
     eggs.”

In brief, the evidence consists in showing that in the parasite a
yolk-substance appears, which Smith says comes from the blood of the
crab that produces it under the influence of the parasite.
Incidentally, as it were, this is said to be the same yolk-substance
(but no sufficient evidence that it is the same is given) that the egg
stores up inside itself, and it is _assumed_ that it is a formative
substance that causes the cell that gets it (or contains it or secretes
it--details are wanting) to become an egg-cell. It is the excess of this
substance produced by the male crab, while still a male, under the
influence of the parasite, that affects the abdomen and its appendages
in such a way that they assume the female condition. There are too many
assumptions in the argument, some of which are scarcely of a kind that
our knowledge of development, incomplete as it is, can allow us to
accept without more direct evidence in their support, to make this view
very plausible. Until better evidence is forthcoming, I fail to be
convinced by Smith’s interpretation of his facts.

Into Smith’s and Robson’s interesting observations on the blood of
crabs, described in Smith’s later paper (part 7, 1911), it is not
necessary to enter here, since the evidence taken as a whole offers
little further in support of his view than had been already assumed. The
argument on page 263 should not, however, pass unchallenged. Smith says:

     “It is clear that the old and familiar idea of an internal
     secretion produced by the gonad being the stimulus for the
     development of the secondary sexual character could not be applied
     here, since at the time that the alterations in the secondary
     sexual characters take place no ovary is present to give rise to
     the required stimulus. It is suggested, therefore, that in some way
     the stimulus must reside in the roots of the _Sacculina_,” etc.

The argument seems to imply that, since the secondary sexual characters
of the female can not be produced by an ovary in the infected male,
therefore the _Sacculina_ must take the place of the ovary. But why make
such a supposition, for if the testes simply keep down the development
of the female characters, as Giard supposes, there is no need either for
an ovary or for a _Sacculina_ to develop them. One might as well argue
that since the cock does not develop the secondary sexual characters of
the hen that an ovary is essential for their development--which is true,
but not in the sense implied.

Stamati (1888) states that he attempted to remove the testes of adult
crayfish and apparently succeeded, but since no effects are expected
until after a molt occurs (that may not take place for two years or
more), no results were obtained. Injections of the gonads with an acid
failed, since the animals died.


E. EVIDENCE FROM INSECTS.

In 1899 Oudemans succeeded in finding a method of removing the testes
and ovaries from caterpillars, using a dimorphic species, _Ocneria
dispar_, the gipsy moth. The results were negative; none of the
secondary sexual characters of the male or female moths or the accessory
organs of copulation were in the least affected by the operation. The
castrated male copulated as readily with the female as did the normal
male, while the spayed females also behaved as normal individuals of
that sex behave. Kellogg, in 1904, repeated the same operation in the
silkworm moth on a small scale with the same results. Kopec and
Meisenheimer, in 1909, repeated in a more detailed way Oudemans’s work.
A further important addition was made by Kopec and by Meisenheimer. They
transplanted ovaries into a castrated male and testes into a spayed
female. Neither gonad produced any effect on the characters of the other
sex. It is interesting to note that the testes underwent their normal
development in the body of a spayed female, and even in one with the
ovaries present, and that the ovary also underwent normal development in
the body of the male. In other words, there is no intolerance of the
tissue of one sex to the gonad of the other. This result is all the more
unexpected, because other observations have shown that the color of the
blood, and its chemical properties, is quite different in the male and
female moths of certain species.

In the case of moths, therefore, if these cases be regarded as typical,
the situation from the point of view of sexual selection is much simpler
than in birds in the sense that the secondary sexual characters are
directly the product of the genetic constituents of all the cells, and
not influenced indirectly by the secretions from the testes or the
ovaries. Sexual selection, therefore, if it is an agent in the evolution
of the differences between males and females, has acted on the genetic
complex to produce these effects on either sex without the result being
involved in the condition of the ovary or the testes.

Regen castrated crickets, _Gryllus campestris_, in the larval stages and
found no effects on the adult structures. The castrated males chirped
like normal males and mated with the females. Spayed females were like
normal females; they bored holes in the ground, but laid no eggs in
them, of course, as the ovary had been completely removed.

The only genetic evidence in the group of insects, outside of the
vinegar fly, relating to the secondary sexual inheritance of the
secondary sexual characters is the following important experiments made
by Foot and Strobell:

The male of one of the bugs, _Euchistus variolarius_, has a black spot
on the end of the abdomen--a spot that is not present in the female.
Foot and Strobell crossed a female of this species to another bug, _E.
servus_, that lacks the spot in both sexes. The daughters had no spot,
the sons a faint spot less developed than in _variolarius_. These inbred
gave (in F₂) 249 females without a spot, 107 males with a spot, and 84
males without a spot. The results are explicable on the view that a
single dominant Mendelian factor, not-sex-linked, causes the spot in the
males, but the presence of the gene in the female produces no effect.
The effect, therefore, is sex-limited, _i. e._, its expression is
determined by the rest of the complex male or female.

The very important breeding experiments carried out by Goldschmidt on
varieties of the gipsy moth should be referred to in this connection,
but as I have recently reviewed these results in the paper on
gynandromorphs written in collaboration with C. B. Bridges,[19] I need
only refer to that account here.

     [Note added April 21, 1919.]

     Shortly after the preceding paper was finished a theses by A.
     Pézard on the secondary sexual characters of birds reached me. In
     it the author gives an account of a number of experiments that he
     has made with poultry and with pheasants. His description of the
     changes that take place after castration are more exact and more
     detailed than any other so far recorded; but in general the results
     obtained by Pézard, through castration, are the same as those that
     had been obtained by others. Castration of 4 male silver pheasants
     are reported. No change in the plumage results, although the
     changes that take place in the comb and wattles are the same in
     kind as those observed in fowls. The sexual instincts and
     peculiarities of the voice and their belligerency are also lost.
     Similarly 4 golden pheasants that were operated on gave the same
     results.

     Three pheasants with mixed plumage (_Phasianus colchicus_) were
     examined. Their testes proved, on histological examination, to be
     imperfectly developed. It is not evident what relation existed
     between the facts and the mixed plumage. The suggestions made by
     Pézard seem inadequate to cover the cases.

     Testicular tissue transplanted into castrated cocks whose comb,
     wattles, etc., had undergone retrogressive changes brought about a
     return to the normal conditions after an interval during which the
     implanted nodules had begun to regenerate.

     Testicular extract from the cryptorchid testes of swine was
     injected into castrated cocks. In one case this resulted in a rapid
     growth in size of the comb, which, after 2 months, had reached its
     full size. Cessation of the injections led immediately to a
     cessation of growth. Before injection the bird exhibited the
     pacifistic characteristics of the capon, but the injections brought
     out little by little the aggressive behavior of the normal male.
     The voice reappeared and “nous assistons á une véritable crise de
     puberte.”

     A histological study of the testes of the fowl and of pheasants
     showed that much connective tissue is characteristic of young
     birds. In the adult cock, and during the mating season of the
     pheasant, the connective tissue becomes largely crowded out by the
     enlargement of the tubules. Pézard concludes that the
     “interstitial” cells in birds have nothing to do with the secondary
     sexual characters, but that these come rather under the influence
     of the germinal cycle of cells of the testes. The submergence of
     the connective-tissue cells of pheasants during the breeding-season
     and their reappearance during the rest of the year might appear to
     have some relation to the facts that I have recently described in
     Sebrights, but as the nuptial plumage of the male remains the same
     throughout the year we can not ascribe any direct influence to this
     tissue. Nevertheless, the different tissues of the testes in birds
     that show seasonal dimorphism of plumage should be carefully
     examined.

     Pézard made a few observations on hens whose ovary had been
     removed. His results are in accord with those of Goodale, except
     that he thinks that the ovary has no influence on the erectile
     organs (comb, etc.) which acquire in the spayed bird the same
     _length_ as that of the normal female.

     Two hens showing male characteristics and a pheasant similarly
     affected are described. In all three cases an examination of the
     ovary was found to be undeveloped or abnormal.




PART IV.

SUMMARY AND CONCLUSIONS.


1. The two principal results obtained were: (_a_) that castration of
hen-feathered Sebright males causes them to develop the full plumage
characteristic of the cock-bird; (_b_) that complete hen-feathering is
due to two dominant Mendelian genes.

2. A striking change takes place when the Sebright male is castrated
(plate 1, figs. 3, 4; plate 3, fig. 1). The new feathers on the upper
surface of the head, neck, back, wings, rump, and tail-coverts assume a
different color and distribution of their pigment; they take on a new
shape, and in those regions where in the cock the barbules are absent
from a part of the margin of the feather, the same absence occurs in the
castrated birds. Such feathers are present on the neck, back, wing-bow,
and rump. The transition is shown in the figures in plate 6, where for
comparison one of the old and one of the new feathers lie side by side.
The tail-coverts in the hen-feathered bird are short, and like those in
the hen do not cover the true tail. After castration they become
excessively long--longer, in fact, than in many cocks--and cover the
true tail feathers. The tail feathers themselves, moreover, become
increased in length, as do the posterior row of feathers of the
wing-coverts. On the breast and sides the change is less marked. The
castrated Sebright loses his erect carriage, but how far this is due to
the changes in his plumage and how far is real (as a result of a new
balance due possibly to the lengthening tail and its coverts) I can not
decide.

3. While castration causes the hen-feathered male to make additions in
color, length, and size of many feathers, it causes at the same time the
other retrogressive changes characteristic of the capon (a castrated
cock-feathered bird); the comb and wattles shrink and become pale, the
birds almost cease crowing, and become timid. They do not make much
effort to mate with the hens, but when they do they show the usual
copulatory reactions.

4. If feathers are removed at the time of castration, the new feathers
show the full effect of the removal of the testes, although they must
have begun to develop immediately afterward. It is suggested that by
means of this delicate test the time relations of the internal secretion
can be profitably studied.

5. Feathers that may have started their development at the time of the
operation show the old influence at the tip of the feathers (plate 10)
and the new one in the rest of the feather. The change is abrupt,
although the transition is perfect.

6. Incomplete castration of the hen-feathered male leads to smaller
changes in the same direction than those following complete castration.

Where such small pieces of the testis were left that complete
cock-feathering followed, the bird slowly changed back to
hen-feathering as the testes began to regenerate. When the regenerated
pieces were removed the bird became cock-feathered again.

7. One Sebright male whose testes appear to have been completely removed
did not change the character of the plumage. No testes were found on
autopsy. It is suggested that some other endocrine organs have taken
over the function of the testes, but as yet none such can be indicated.

8. In one case an old hen-feathered (F₁) male began to change over to
cock-feathering. It was found that his testes had dwindled (probably
through disease) to very small size (10 by 5 mm.).

9. The F₁ male of the cross between the Sebright and game is also
hen-feathered (plate 2, fig. 1). After castration he becomes
cock-feathered (plate 2, fig. 4) and shows thereby the genetic type of
the heterozygous cock-feathered class in which his hen belongs. The
change in this male is even more striking than that in the Sebright. The
change in the individual feathers is shown in plate 7, figs. 1 and 1_a_.

10. Three types of F₂ hen-feathered castrated males are shown in plate
2, figure 3, and plate 3, figure 3 and figure 4. The first was a dark
bird that changed to a lighter red above. The third a gray bird that
became bright red; the second was a light yellow that became deep
yellow, etc. The class of hens to which such males belong, as
cock-feathered birds, can thus be found out by castration. In this way
the F₂, and back-cross, hen-feathered cocks can be classified with the
corresponding F₂ cock-feathered males.

11. In the F₂ generation, made up of birds from the direct and
reciprocal crosses taken together, there were 29 hen-feathered and 26
cock-feathered males. In the back-cross (F₁ hen by game male) the
classes were 2 and 7. The results seem in better accord with the
assumption that two factors are present in the Sebright that stand for
hen-feathering; that either alone will give hen-feathered birds
(intermediate type?), but that both together give the extreme type of
hen-feathering seen in the Sebright.

12. The difference in color in the two races (Sebright and Black
Breasted Game bantams) is very great. The former have almost uniformly
laced feathers, while the latter has the varied plumage of the
jungle-fowl. The game is strongly dimorphic in color and color-pattern;
the Sebright has the same type of coloration and pattern both in the
male and female, but this is deceptive, as castration shows, because the
castrated male is as strikingly different from the normal Sebright
female as is the cock of other birds from the hen. The resemblance of
male and female in this race is due to the suppression of the true male
plumage by something produced in the testes. Therefore the heredity of
dimorphism resolves itself here into the problem of the heredity of
hen-feathering. That the female Sebright has the same genetic factors as
the male is shown by the fact that she transmits hen-feathering in the
same way as does the male, and also by the fact, as Darwin pointed out,
that an old female Sebright whose ovaries had degenerated developed not
the hen-feathered plumage of her own cock, but cock-feathered plumage
like that of most male poultry.

13. The color of the F₁ birds is shown in plate 2, figs. 1 and 2. In
general, the feathers are stippled, black and light yellow being the two
most conspicuous ingredients. Since hen-feathering dominates, the
dimorphism is absent, or at least is so slight as to not attract
attention--little more, in fact, than in the Sebright race. The carriage
of the male is like that of the Sebright male. The F₁ male and female
are alike in the direct cross and the reciprocal, or at least no
conspicuous difference is found between the two classes of hens,
indicating that no important sex-linked factors are involved in the
cross.

14. The F₂ birds show a great variety of color and pattern, but those
obtained can be approximately grouped into 16 classes. The classes are,
however, admittedly not uniform, indicating minor factors not here
reckoned with. The classification of the hens is easiest; the F₂
hen-feathered males can then in many cases be referred to the proper
classes; the F₂ cock-feathered males can not be accurately classified
with their corresponding hens, except in the case of those that resemble
the two P₁ males, the F₁ male, and those that castration experiments of
the hen-feathered males have shown to belong to certain hen types.

15. Despite the admitted difficulties of classification, it is suggested
that three factor-pairs of differences will cover the main color classes
seen in the F₂ and in the back-cross. One or two of these seem to be
incompletely dominant, since the F₁ birds are not like either parent in
any single character, nor are they like the wild type in so far as this
is represented by the game.

16. A histological examination of the testis of the male Sebright by
Boring and Morgan has shown that it contains cells like those present in
the ovary of all breeds of poultry. These cells are called luteal cells
by Pearl and Boring, from their resemblance to the cells of that name
found in the corpora lutea of mammals. In the mammals similar cells are
supposed to produce internal secretions that act as hormones. Their
function in the female bird is unknown, but the fact that after the
removal of the ovary the female develops the secondary sexual plumage of
the male suggests that some secretion from these cells performs this
function. Their occurrence in the male Sebright and their complete
absence, or paucity, in the males of other races supports strongly the
view that these cells are concerned with the suppression of the
secondary sexual plumage.

17. While in mammals the interstitial cells have been supposed to
produce an internal secretion that causes the development of some of the
secondary sexual characters of the male, and the fuller elaboration of
others, in birds no such connection exists, if we except the case of
the Sebright. Castration of ordinary males does not affect deleteriously
the secondary sexual plumage (although it does the comb, behavior,
etc.), in fact may even enhance their effects. But, while in the mammal
a secretion is necessary for the full development of the secondary
sexual characters, in the Sebright a secretion inhibits certain of them.
What element in the ordinary bird and in the Sebright causes the full
development of the comb, wattles, sexual behavior, etc., is not known.
Possibly it is the sexual elements themselves, but possibly it is a
secondary influence of the luteal cells producing a contrary effect on
these parts from its effects on the feathers; but possibly more than one
kind of secretory cell is present in the testis of the cock.

18. The causes of the development of the secondary sexual characters are
seen to be of such diverse physiological kinds that one may well
hesitate to apply the same explanation as to their evolution. In fact,
it is pointed out that several of the theories that have been suggested
run counter to the conditions that bring about the development of the
secondary sexual characters.

19. An attempt is made to give a critical review of Darwin’s theory of
sexual selection in the light of the modern genetic and operative
results on the secondary sexual characters of the vertebrates. It is
pointed out that far from extending the general theory in its
applications, the modern work has shown in the first place that the
underlying conditions that call forth the development of the secondary
sexual differences are so diverse in the different groups of animals
that it is a priori very unlikely that this evolution can have been
directed by the same external agent, such as the choice of the female,
for such an assumption carries with it in several cases other
implications concerning the causes of the suppression of these same
characters in the female herself, etc. In the second place, it is
pointed out that the problem of the excessive development of certain
characters in the male whose genes are present in both sexes no longer
oppresses us as it did Darwin, for it has been shown both by the genetic
and by the operative work that a single factorial difference may be at
the root of exceedingly great differences in the individual. Such
results, while they admittedly do not _in most cases_ tell us that the
differences involved have arisen at a single progressive step, show us
nevertheless that such differences may depend on very simple initial
differences, and if so, the entire problem becomes enormously
simplified. To Darwin the excessive development of color and
ornamentation appeared due to a long, slow process of evolution
laboriously brought about by the female through selection of those males
a little more ornamented than their fellows. To-day we have found out
that in many cases the genetic composition of a male with such
ornamentation and of a female without it may be almost identical, except
that the genes in one chromosome are duplex in one sex and simplex in
the other. Owing to this initial difference, the female in birds
produces an internal secretion that suppresses in her the ornamentation
shown by the male, and in the mammal an internal secretion produced by
the testes causes the full development in the male of the secondary
sexual characters. If, as seems probable, these secretions are some
particular kind of substance, the condition that led to their appearance
historically need not have been very complex; and if not, the problem
appears simplified. It still remains to give some reasonable explanation
as to why such substances should continue to be produced if their
products--the secondary sexual characters--possess no “beauty” for the
female. Here more work is necessary, but the modern genetic point of
view may possibly give an important clue. We are coming to realize more
fully that the hereditary genes generally have more than a single effect
on the characters of the animal. The secondary sexual characters may,
then, be only by-products of genes whose important function lies in some
other direction. If, for example, the secretion produced by the cells of
the male have an important influence on his output of energy, or
strength, or activity, their secondary influence over certain parts of
the body would not call for any further explanation on the modern view
of natural selection. If the secretions of the ovary of the female bird
have some direct relation to her physiological processes that are
important in the development of the oviduct, for instance, it would be a
matter of no importance from an evolutionary point of view if that same
secretion suppresses in her the development of the high color shown by
the male.




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DESCRIPTION OF PLATES.


PLATE 1.

     FIG. 1. Black-Breasted Game bantam cock. He is typically
     cock-feathered, but, as in all games, his hackles and tail-coverts
     are shorter than in the cocks of other breeds. The comb was dubbed
     by the breeder.

     FIG. 2. Black-Breasted Game bantam hen. The great contrast in color
     between the cock and hen is practically the same as that in the
     Brown Leghorn, in most races of Tosa fowls, and in the wild type
     _Gallus bankiva_.

     FIG. 3. Sebright cock, “hen-feathered.” The short hackles, the
     rounded feathers of the back and saddle, and the shortness of the
     tail-coverts are characteristic features of these males. For
     details of individual feathers from different regions see plates 6
     and 8.

     FIG. 4. A castrated Sebright male. The drawing was made about a
     year after the operation. This particular bird developed a lighter
     color than did other castrated Sebrights (see plate 3, fig. 1). The
     entire dorsal region has changed its color, and the feathers have
     also changed in shape, length, etc. Note especially the very long
     hackle and saddle feathers (for details see plate 6, fig. 1_a_) and
     the change in the wing-bow. The tail-coverts have also grown long.


PLATE 2.

     FIG. 1. F₁ hen-feathered male out of Game by Sebright. The
     hen-feathering in this bird is as complete as in the Sebright.

     FIG. 2. F₁ female out of Game by Sebright.

     FIG. 3. Castrated male originally hen-feathered (292), nearly black
     in color, as shown by the individual feathers of plate 7, figure 2.
     After castration the bird has become red above, with black
     iridescent tail-coverts, and deeper yellow (or red) below.

     FIG. 4. Castrated F₁ male, originally like figure 1. Note
     especially the change in color of the whole upper surface that has
     become red, like that of the jungle-fowl. The tail-coverts have
     grown long and are now iridescent black. The breast has changed
     least, but is a richer yellow. The comb and wattles and ear lobes
     are shrunken, as in all capons.


PLATE 3.

     FIG. 1. A castrated Sebright male. The operation was performed on a
     juvenile bird; the drawing was made a year later. The bird is
     typical as to the change in color that takes place in the Sebright.
     He was darker red than the bird shown in plate 1, figure 4. The red
     was more mahogany than the picture shows. The original feathers
     were like those in plate 6, fig. 2 (there erroneously referred to
     as those of light-colored Sebright).

     FIG. 2. An F₂ hen-feathered very dark male. The condition of his
     plumage at the time of the operation is shown in this figure. The
     change that took place after castration is shown in the next
     figure.

     FIG. 3. The change that took place in the bird drawn in figure 2 is
     shown here. The whole upper surface has become red, except the
     tail-coverts, which are iridescent black. Note also the change in
     color on the wing-bow. For the details of the feathers see plate 9,
     figures 1, 1_a_.

     FIG. 4. A castrated F₂ bird that had been hen-feathered and had
     changed over to cock-feathering, as shown here. The color and the
     details of the original hen-feathering are shown in plate 9,
     figures 2 and 2_a_.


PLATE 4.

     FIG. 1. One of the original Black-Breasted Game males used in the
     breeding experiments. Compare with colored drawing, plate 1, figure
     1.

     FIG. 2. A Black-Breasted Game hen used in the breeding experiments.
     Compare with colored drawing, plate 1, figure 2.

     FIG. 3. A Sebright male. The bird was used in the later
     back-crosses and not in the original experiments. He is typical of
     his breed.

     FIG. 4. A Sebright female. One of the birds used in the original
     experiments.

     FIG. 5. An F₁ male. This bird had just reached maturity and was
     younger than the one drawn in plate 2, figure 1.

     FIG. 6. An F₁ hen of the same age as the last. The pattern changed
     a little as the bird became older.


PLATE 5.

     FIG. 1. An adult Sebright male for comparison with the next figure.

     FIG. 2. A castrated Sebright male. This photograph shows the same
     bird from which the drawing, plate 1, figure 4, was made. It is the
     lighter colored bird referred to in the text.

     FIG. 3. One of the two F₁ castrated birds. For comparison see the
     colored drawing in plate 2, figure 4.

     FIG. 4. A castrated Sebright. This bird is darker, and in this
     sense more typical than figure 2.

     FIG. 5. One of the castrated Sebright males which at one time after
     castration was as extremely cock-feathered as figure 2, but slowly
     “went back” towards hen-feathering, as the figure shows especially
     in the hackle and saddle. The details are much better shown in the
     feathers photographed in plate 8, figures 1, 2, 3, 4, 1_a_, 2_a_,
     3_a_, 4_a_, 1_b_, 2_b_, 3_b_, 4_b_.

     FIG. 6. The same bird was opened and the regenerated pieces of the
     testis removed. He returned later, as shown here, to full
     cock-feathering.


PLATE 6.

     FIGS. 1, 1A. Typical old (1) and new (1_a_) feathers (after
     castration) of the same bird. This is the “lighter” male drawn in
     plate 1, figure 4, and photographed in plate 5, figure 2.

     FIGS. 2, 2A. Typical old (2) and new (2_a_) (after castration)
     feathers of another Sebright. This bird developed after castration
     darker feathers than did the last bird. Its feathers were more like
     those that other castrated Sebrights developed. Legend on plate 6
     erroneous as far as 2 and 2a are concerned.


PLATE 7.

     FIGS. 1, 1A. Typical old (1) and new (1_a_) (after castration)
     feathers of an F₁ bird. (See plate 2, figures 1 and 4.)

     FIGS. 2, 2A. Typical old (2) and new (2A) (after castration)
     feathers of bird shown in plate 3, figures 2 and 3 (No. 292).


PLATE 8.

     Typical feathers of “dark” Sebright (1, 2, 3, 4) that after
     incomplete castration changed to cock-feathering (1_a_, 2_a_, 3_a_,
     4_a_), then later, as pieces of the testes that had been left
     behind in the old situs regenerated, began to go back towards
     hen-feathering (1_b_, 2_b_, 3_b_, 4_b_). The bird was then opened
     again, and the regenerated pieces removed, when it again became
     cock-feathered (1_c_, 2_c_, 3_c_, 4_c_), and has so remained for
     more than a year.


PLATE 9.

     FIGS. 1, 1A. Typical feathers of hackle and saddle from
     hen-feathered bird (No. 68) plate 3, figure 2, that changed over to
     the cock-feathered bird of plate 3, figure 3.

     FIGS. 2, 2A. Typical feathers of an F₁ male (2) that changed over
     partly as a result of degeneration of his testes, into a
     cock-feathered bird (2A). The change was not so great as it is
     after castration.

     FIGS. 3, 3A. Typical feathers of Sebright male that slightly
     changed towards cock-feathering (old hackle feather missing).


PLATE 10.

     FIGS. 1, 1A. Old (1) and new (1A) wing-coverts of normal Sebright
     (1) and castrated (1A).

     FIGS. 2A, 2B. Upper row, to right, “Transitional” hackle feathers
     (2A), and a slightly later changed-over feather from wing-bow (2A),
     and from back (2B). Second row, to left, old (2), transitional
     (2_a_), and changed-over feather (2_b_), from saddle of Sebright.

     FIG. 3. Three feathers (tail-covert, wing-bow, and saddle) of an F₂
     hen-feathered game-like male.

     FIG. 4. A series of breast feathers from an F₂ bird. At one end of
     the series (the left) the feather is spangled, at the other barred.

     FIG. 5. A series of breast feathers from another F₂ bird. At one
     end of the series (the left) the feathers are penciled, at the
     other end they are barred.

[Illustration:

T. H. MORGAN       PLATE 1

JULIUS BIEN, N.Y.

1. Black Breasted Game Bantam male.
2. Female.
3. Sebright male.
4. Castrated Sebright male.
]

[Illustration:

T. H. MORGAN        PLATE 2

JULIUS BIEN, N.Y.

1. Hen-feathered F₁ male.
2. F₁ female.
3. Castrated F₂ male.
4. Castrated F₁ male (Fig. 1).
]

[Illustration:

T. H. MORGAN        PLATE 3

JULIUS BIEN, N.Y.

1. Castrated Sebright male.
2. F₂ Hen-feathered male.
3. Same castrated.
4. F₂ Castrated male.
]

[Illustration:

MORGAN       PLATE 4

1. Black-Breasted Game male.
2. Black-Breasted Game female.
3. Sebright male.
4. Sebright female.
5. Hybrid male.
6. Hybrid female.
]

[Illustration:

MORGAN        PLATE 5

1. Adult Sebright male.
2. Castrated Sebright male.
3. Castrated F₁ male.
4. Another castrated Sebright male.
5. Castrated Sebright male with testes regenerating.
6. Same as 5 after second removal of testes.
]

[Illustration:

T. H. MORGAN       PLATE 6

JULIUS BIEN, N.Y.

Feathers of “light” colored Sebright (1, 2) that changed to
cock-feathers after castration (1ªa, 2ª).]

[Illustration:

T. H. MORGAN       PLATE 7

JULIUS BIEN, N.Y.

Feathers of F₁ hen-feathered male before (1), and after (1ª) castration.

Feathers of a darker hen-feathered male before (2), and after (2ª)
castration.]

[Illustration:

T. H. MORGAN        PLATE 8

JULIUS BIEN, N.Y.

Feathers from hen-feathered male Sebright (1, 2, 3, 4) that changed to
cock-feathered male (1ª, 2ª, 3ª, 4ª) after castration; and then began to
go back as the testes regenerated (1ᵇ, 2ᵇ, 3ᵇ, 4ᵇ); then changed again
to cock-feathering after castration (1ᶜ, 2ᶜ, 3ᶜ, 4ᶜ).]

[Illustration:

T. H. MORGAN       PLATE 9

JULIUS BIEN, N.Y.

Feathers showing complete (1) or incomplete (2 and 3) change from
hen-feathering to cock-feathering (1ª, 2ª, 3ª) after castration.]

[Illustration:

T. H. MORGAN      PLATE 10

JULIUS BIEN, N.Y.

Normal. 1, 2; transitional, 1ª, 2ª, and changed-over feathers, 2ᵇ, of
Sebright, 3, 4 and 5. Feathers from F₂ birds.]


FOOTNOTES:

[1] The expectation for 1 dominant and 1 recessive factor is so nearly
the same as for 1 dominant alone that for the numbers obtained no
difference between the two cases could be detected.

[2] There is one other bird, not given in the above list, that is
pure Sebright except that his legs are yellow. Until I find out by
further breeding of the Sebright stock whether yellow legs are present
in it, this case must remain doubtful. On the basis of a two factor
color-difference one Sebright (as to color) is expected in 16 birds,
and one in 64 on a three factor basis. Some Sebrights had been raised
along with the back cross, hence the possibility of contamination.

[3] Provided that the blue classification was based on the adult
plumage and not on down color.

[4] If the recessive mutation occurs first in the Z chromosome of an
egg of the female it will not appear in the next generation; then if
it has passed into a male, half his daughters will show it. The single
factor-pair involved is carried by the sex chromosomes ZZ.

[5] One may be either sex-linked or sex-limited so far as the evidence
goes.

[6] No mention is made by Baur that a heterozygous male instead of a
pure silver male was used, although the male is made heterozygous in
the formulæ.

[7] For activity and pugnacity in hummingbirds, see Tropical Nature,
pp. 130, 213.

[8] The Naturalist in La Plata, W. H. Hudson, London, 1892, pp. 269-270.

[9] Proceedings of the Zoological Society of London, 1885, p. 431,
Quelques remarques sur le dimorphisme sexuel. Jean Stolzmann.

[10] George W. and Elizabeth G. Peckham. Observations on Sexual
Selection in Spiders of the Family Attidæ. Nat. Hist. Soc. of
Wisconsin, Vol. I, 1889, pp, 46, 47.

[11] _Loddigesia mirabilis_ has the tail about three times as long
as the body. Similar modifications are found in the genera _Sappho_,
_Cynanthus_, _Lesbia_, _Stegnura_, _Discura_, _Gouldia_, _et al._

[12] Among the most remarkable of this wonderful family are the nine
species of coquettes (_Lophornis_), which have elongated feathers,
with metallic tips, springing from the sides of the neck; some have
also beautiful crests. (George W. and Elizabeth G. Peckham, Additional
Observations on Sexual Selection in Spiders of the Family Attidæ, Nat.
Hist. Soc. of Wisconsin, 1889, vol. I, pp. 141, 142.)

[13] Tropical Nature, p. 210. The italics are ours.

[14] A. H. Sturtevant, Experiments on Sex Recognition and the Problem
of Sexual Selection in _Drosophila_. Journ. Animal Behavior, Sept.-Oct.
1915, vol. 5, No. 5, pp. 352, 353.

[15] In the eland as well as in the reindeer, in which both sexes have
horns that begin in the latter at least to develop before the gonads
ripen, it is stated that castration does not prevent the development of
the horns in the male, but whether they are as large as in the normal
male is apparently not definitely stated.

[16] Yarrell also states that after the fallow buck has reached the
height of its maturity and has 6 prongs in its antler, removal of one
testis causes the next antler to have but 5 prongs.

[17] It might be supposed that this bird was really a cock which had
been changed for a hen; but the following facts put this matter beyond
a doubt: First, there was no other pyed pea-fowl in the country.
Secondly, the hen had knobs on her toes, which were the same after
her change. Thirdly, she was as small after the change as before,
therefore too small for a cock. Fourthly, she was a favorite bird, and
was generally fed by the lady, and used to come for her meat, which she
still continued to do after the change in the feathers.

[18] See the latter also for references to _Lacertilia_ and _Chelonia_.

[19] Carnegie Inst. Wash. Pub. No. 278, 1918.