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.