DARWIN’S THEORY OF SEXUAL SELECTION
Sexual Selection
The theory of sexual selection was formulated by Darwin, even in the
first edition of the “Origin of Species,” but was developed at much
greater length in “The Descent of Man.” “This form of selection depends,
not on a struggle for existence in relation to other organic beings or
to external conditions, but on a struggle between the individuals of one
sex, generally the males, for the possession of the other sex. The
result is not death to the unsuccessful competitor, but few or no
offspring. Sexual selection is, therefore, less rigorous than natural
selection. Generally the most vigorous males, those which are best
fitted for their place in nature, will leave most progeny. But in many
cases victory depends, not so much on general vigor, as on having
special weapons, confined to the male sex. A hornless stag or spurless
cock would have a poor chance of leaving numerous offspring. Sexual
selection, by always allowing the victor to breed, might surely give
indomitable courage, length to the spur, and strength to the wing to
strike in the spurred leg in nearly the same manner as the brutal
cock-fighter by the careful selection of his best cocks.” It is
important to notice that the theory of sexual selection is admittedly an
extension of the selection principle into a new field. Having accounted
for domesticated animals and plants by artificial selection, and for the
adaptations of wild species by natural selection, there remained only to
account for the secondary sexual differences between the sexes by the
principle of sexual selection.
There are two ways in which Darwin supposes sexual selection to act: (1)
through competition of the individuals of the same sex with each
other,—the strongest or best-equipped for fighting or for finding the
individuals of the other sex gaining an advantage; (2) through selection
by the individuals of one sex of certain preferred individuals of the
other sex.
The first category is natural selection applied to the members of one
sex in competition with each other, although the result does not lead to
the death of the unsuccessful individual, but excludes it from leaving
progeny. In the second category a new element is introduced, namely, the
_selective power_ of the individuals of one sex, usually the female. It
is this part that adds a distinctly new element to Darwin’s other two
theories of selection, and it is this part that we naturally think of as
the theory of sexual selection _par excellence_. Darwin makes, however,
no sharp distinction between these two sides of his theory, but includes
both under the heading of sexual selection.
In order to get the theory itself before us in as concrete form as
possible, let us examine some of the cases that Darwin has given to show
how he supposes the process to be carried out.
“There are many other structures and instincts which must have been
developed through sexual selection—such as the weapons of offence and
the means of defence of the males for fighting with and driving away
their rivals—their courage and pugnacity—their various ornaments—their
contrivances for producing vocal or instrumental music—and their glands
for emitting odors, most of these latter structures serving only to
allure or excite the female. It is clear that these characters are the
result of sexual and not of ordinary selection, since unarmed,
unornamented, or unattractive males would succeed equally well in the
battle for life and in leaving a numerous progeny, but for the presence
of better-endowed males. We may infer that this would be the case,
because the females, which are unarmed and unornamented, are able to
survive and procreate their kind. Secondary sexual characters of the
kind just referred to will be fully discussed in the following chapters,
as being in many respects interesting, but especially as depending on
the will, choice, and rivalry of the individuals of either sex. When we
behold two males fighting for the possession of the female, or several
male birds displaying their gorgeous plumage, and performing strange
antics before an assembled body of females, we cannot doubt that, though
led by instinct, they know what they are about, and consciously exert
their mental and bodily powers.”
This general statement gives an idea of the class of phenomena that
Darwin proposes to explain by the theory of sexual selection. The close
resemblance between this process and that of artificial selection may be
gathered from the following statement:—
“Just as man can improve the breed of his game-cocks by the selection of
those birds which are victorious in the cockpit, so it appears that the
strongest and most vigorous males, or those provided with the best
weapons, have prevailed under nature, and have led to the improvement of
the natural breed or species. A slight degree of variability leading to
some advantage, however slight, in reiterated deadly contests would
suffice for the work of sexual selection; and it is certain that
secondary sexual characters are eminently variable. Just as man can give
beauty, according to his standard of taste, to his male poultry, or more
strictly can modify the beauty originally acquired by the parent
species, can give to the Sebright bantam a new and elegant plumage, an
erect and peculiar carriage—so it appears that female birds in a state
of nature have, by a long selection of the more attractive males, added
to their beauty or other attractive qualities. No doubt this implies
powers of discrimination and taste on the part of the female which will
at first appear extremely improbable; but by the facts to be adduced
hereafter, I hope to be able to show that the females actually have
these powers. When, however, it is said that the lower animals have a
sense of beauty, it must not be supposed that such sense is comparable
with that of a cultivated man, with his multiform and complex associated
ideas. A more just comparison would be between the taste for the
beautiful in animals, and that in the lowest savages, who admire and
deck themselves with any brilliant, glittering, or curious object.”
Darwin did not close his eyes to the difficulties which the theory had
to contend against. One of the most formidable of these objections is
described in the following words: “Our difficulty in regard to sexual
selection lies in understanding how it is that the males which conquer
other males, or those which prove the most attractive to the females,
leave a greater number of offspring to inherit their superiority than
their beaten and less attractive rivals. Unless this result does follow,
the characters which give to certain males an advantage over others
could not be perfected and augmented through sexual selection. When the
sexes exist in exactly equal numbers, the worst-endowed males will
(except where polygamy prevails) ultimately find females, and leave as
many offspring, as well fitted for their general habits of life, as the
best-endowed males. From various facts and considerations, I formerly
inferred that with most animals, in which secondary sexual characters
are well developed, the males considerably exceeded the females in
number; but this is not by any means always true. If the males were to
the females as two to one, or as three to two, or even in a somewhat
lower ratio, the whole affair would be simple; for the better-armed or
more attractive males would leave the largest number of offspring. But
after investigating, as far as possible, the numerical proportion of the
sexes, I do not believe that any great inequality in number commonly
exists. In most cases sexual selection appears to have been effective in
the following manner.
“Let us take any species, a bird for instance, and divide the females
inhabiting a district into two equal bodies, the one consisting of the
more vigorous and better-nourished individuals, and the other of the
less vigorous and healthy. The former, there can be little doubt, would
be ready to breed in the spring before the others; and this is the
opinion of Mr. Jenner Weir, who has carefully attended to the habits of
birds during many years. There can also be no doubt that the most
vigorous, best-nourished and earliest breeders would on an average
succeed in rearing the largest number of fine offspring. The males, as
we have seen, are generally ready to breed before the females; the
strongest, and with some species the best-armed of the males, drive away
the weaker; and the former would then unite with the more vigorous and
better-nourished females, because they are the first to breed. Such
vigorous pairs would surely rear a larger number of offspring than the
retarded females, which would be compelled to unite with the conquered
and less powerful males, supposing the sexes to be numerically equal;
and this is all that is wanted to add, in the course of successive
generations, to the size, strength and courage of the males, or to
improve their weapons.”
I shall comment later on the points here raised, but we should not let
this opportunity pass without noticing, that even if the pairing were to
follow according to the method here imagined, still the argument breaks
down at the critical point, for there is no evidence that the more
precocious females would rear a larger number of offspring than the more
normal females, or even those that breed somewhat later.
The greater eagerness of the males which has been observed in so many
different classes of animals is accounted for as follows:—
“But it is difficult to understand why the males of species, of which
the progenitors were primordially free, should invariably have acquired
the habit of approaching the females, instead of being approached by
them. But in all cases, in order that the males should seek efficiently,
it would be necessary that they should be endowed with strong passions;
and the acquirement of such passions would naturally follow from the
more eager leaving a larger number of offspring than the less eager.”
Thus we are led to the rather complex conclusion, that the more eager
males will leave more descendants, and those that are better endowed
with ornaments will be the ones selected. But unless it can be shown
that there is some connection between greater eagerness and better
ornamentation, it might often occur that the less ornamented were the
more eager individuals, in which case there would be an apparent
conflict between the two acquirements.
After giving some cases of the greater variability of the males, in
respect to characters that are not connected with sexual selection, and
presumably not the result of any kind of selection, Darwin concludes:
“Through the action of sexual and natural selection male animals have
been rendered in very many instances widely different from their
females; but independently of selection the two sexes, from differing
constitutionally, tend to vary in a somewhat different manner. The
female has to expend much organic matter in the formation of her ova,
whereas the male expends much force in fierce contests with his rivals,
in wandering about in search of the female, in exerting his voice,
pouring out odoriferous secretions, etc.: and this expenditure is
generally concentrated within a short period. The great vigor of the
male during the season of love seems often to intensify his colors,
independently of any marked difference from the female. In mankind, and
even as low down in the organic scale as in the Lepidoptera, the
temperature of the body is higher in the male than in the female,
accompanied in the case of man by a slower pulse. On the whole, the
expenditure of matter and force by the two sexes is probably nearly
equal, though effected in very different ways and at different rates.”
Again: “From the causes just specified, the two sexes can hardly fail to
differ somewhat in constitution, at least during the breeding season;
and although they may be subjected to exactly the same conditions, they
will tend to vary in a different manner. If such variations are of no
service to either sex, they will not be accumulated and increased by
sexual or natural selection. Nevertheless, they may become permanent if
the exciting cause acts permanently; and in accordance with a frequent
form of inheritance they may be transmitted to that sex alone in which
they first appeared. In this case, the two sexes will come to present
permanent, yet unimportant, differences of character. For instance, Mr.
Allen shows that with a large number of birds inhabiting the northern
and southern United States, the specimens from the south are
darker-colored than those from the north; and this seems to be the
direct result of the difference in temperature, light, etc., between the
two regions. Now, in some few cases, the two sexes of the same species
appear to have been differently affected; in the _Agelæus phœniceus_ the
males have had their colors greatly intensified in the south; whereas
with _Cardinalis virginianus_ it is the females which have been thus
affected: with _Quiscalus major_ the females have been rendered
extremely variable in tint, whilst the males remain nearly uniform.”
The admissions contained in this statement would seem to jeopardize the
entire question, for, if it is admitted that, on account of the
difference in the constitution of the two sexes, the influence of the
surrounding conditions would produce a different effect on them, it
would seem that there is no need whatsoever for the theory of sexual
selection. What Darwin is probably attempting to show is that the
material for the further action of sexual selection is already given;
but the question may well be asked, if the external conditions have done
so much, why may they not have gone farther and produced the entire
result?
Darwin makes the following suggestion to account for those cases in
which the female is the more highly colored:—
“A few exceptional cases occur in various classes of animals, in which
the females instead of the males have acquired well-pronounced secondary
sexual characters, such as brighter colors, greater size, strength, or
pugnacity. With birds there has sometimes been a complete transposition
of the ordinary characters proper to each sex; the females having become
the more eager in courtship, the males remaining comparatively passive,
but apparently selecting the more attractive females, as we may infer
from the results. Certain hen birds have thus been rendered more highly
colored or otherwise ornamented, as well as more powerful and pugnacious
than the cocks; these characters being transmitted to the female
offspring alone.”
Then follows immediately the discussion as to whether a double process
of sexual selection may not be supposed to go on at the same time. “It
may be suggested that in some cases a double process of selection has
been carried on; that the males have selected the more attractive
females, and the latter the more attractive males. This process,
however, though it might lead to the modification of both sexes, would
not make the one sex different from the other, unless indeed their
tastes for the beautiful differed; but this is a supposition too
improbable to be worth considering in the case of any animal, excepting
man. There are, however, many animals in which the sexes resemble each
other, both being furnished with the same ornaments, which analogy would
lead us to attribute to the agency of sexual selection. In such cases it
may be suggested with more plausibility, that there has been a double or
mutual process of sexual selection; the more vigorous and precocious
females selecting the more attractive and vigorous males, the latter
rejecting all except the more attractive females. But from what we know
of the habits of animals, this view is hardly probable, for the male is
generally eager to pair with any female. It is more probable that the
ornaments common to both sexes were acquired by one sex, generally the
male, and then transmitted to the offspring of both sexes. If, indeed,
during a lengthened period the males of any species were greatly to
exceed the females in number, and then during another lengthened period,
but under different conditions, the reverse were to occur, a double but
not simultaneous process of sexual selection might easily be carried on,
by which the two sexes might be rendered widely different.”
The improbability of such a process is so manifest that the suggestion
can scarcely be looked upon as anything more than pure speculation. We
shall have occasion later to return to the same subject, and point out
its bearing more explicitly.
Nearly the whole animal kingdom is passed in review by Darwin from the
point of view of the sexual selection theory. There is brought together
a large number of extremely interesting facts, and if the theory did no
more than merely hold them together, it has served, in this respect, a
useful end. We may select some of the most instructive cases by way of
illustrating the theory.
In many of the lower animals in which the sexes are separated, and these
alone, of course, can be supposed to come within the range of the
theory, there are no striking differences between the sexes, in regard
to ornamentation, although in other respects differences may exist.
“Moreover it is almost certain that these animals have too imperfect
senses and much too low mental powers, to appreciate each other’s beauty
or other attractions, or to feel rivalry.
“Hence in these classes or subkingdoms, such as the Protozoa,
Cœlenterata, Echinodermata, Scolecida, secondary sexual characters, of
the kind which we have to consider, do not occur; and this fact agrees
with the belief that such characters in the higher classes have been
acquired through sexual selection, which depends on the will, desire,
and choice of either sex.”
There are some cases, however, in which animals low in the scale show a
difference in the ornamentation of the two sexes. A few cases have been
recorded in the roundworms, where different shades of the same tint
distinguish the sexes. In the annelids the sexes are sometimes so
different, that, as Darwin remarks, they have been placed in different
genera and even families, “yet the differences do not seem to be of the
kind which can be safely attributed to sexual selection.” In regard to
the nemertian worms, although they “vie in variety and beauty of
coloring with any other group in the invertebrate series,” yet McIntosh
states that he “cannot discover that these colors are of any service.”
In the copepods, belonging to the group of lower Crustacea, Darwin
excludes those cases in which the males alone “are furnished with
perfect swimming legs, antennæ, and sense-organs; the females being
destitute of these organs, with their bodies often consisting of a mere
distorted mass,” because these extraordinary differences between the two
sexes are no doubt related to their widely different habits of life.
Nevertheless, it is important to observe that such extreme differences
may exist in cases where sexual selection cannot come in, because of the
absence of eyes in the female.
In regard to another copepod, Saphirina, Darwin points out that the
males are furnished with minute scales, which exhibit beautiful changing
colors, and these are absent in the females; yet he states that it would
be extremely rash to conclude that these curious organs serve to attract
the females. Differences in the sexes are also found in one species of
Squilla, and a species of Gelasimus. In the latter case Darwin thinks
that the difference is probably due to sexual selection. In addition to
these cases, recorded by Darwin, there may be added the two remarkable
cases, shown in our Figure 2 A, B, of _Calocalanus pavo_, the female of
which has a gorgeous tail worthy of a peacock, and of _Calocalanus
plumulosus_, in which one of the setæ of the tail is drawn out into a
long featherlike structure. In the former, the male is much more
modestly adorned, as shown in Figure 2 C; in the latter species the male
is unknown.
[Illustration:
Fig. 2.—A male of the copepod, _Calocalanus plumulosus_.
B and C, a male and a female of _Calocalanus pavo_. (After
Giesbrecht.)]
In spiders, where as a rule the sexes do not differ much from each other
in color, the males are often of a darker shade than the females. “In
some species, however, the difference is conspicuous; thus the female of
_Sparassus smaragdulus_ is dullish green, whilst the adult male has the
abdomen of a fine yellow with three longitudinal stripes of rich red.”
Darwin believes that sexual selection must take place in this group,
because Canestrini has observed that the males fight for the possession
of the females. He has also stated that the males pay court to the
female, and that she rejects some of the males who court her, and
sometimes devours them, until finally one is chosen. Darwin believed, on
this evidence, that the difference in color of the sexes had been
acquired by sexual selection, “though we have here not the best kind of
evidence—the display by the male of his ornaments.” This evidence has,
however, now been supplied through the interesting observations of Mr.
and Mrs. Peckham. These accurate observers have studied the courtship of
the male, and observed that during the process, he twists and turns his
body in such a way as to show to best advantage his colors to the
female. From their account this certainly appears to be the result of
his movements, but whether this is really the case, and whether the
female makes any choice amongst her suitors, according to whether they
are more or less brilliantly marked, we are absolutely ignorant. The
following account given by Darwin should not pass unnoticed:—
“The male is generally much smaller than the female, sometimes to an
extraordinary degree, and he is forced to be extremely cautious in
making his advances, as the female often carries her coyness to a
dangerous pitch. De Geer saw a male that ‘in the midst of his
preparatory caresses was seized by the object of his attentions,
enveloped by her in a web and then devoured, a sight which, as he adds,
filled him with horror and indignation.’ The Rev. O. P. Cambridge
accounts in the following manner for the extreme smallness of the male
in the genus Nephila. ‘M. Vinson gives a graphic account of the agile
way in which the diminutive male escapes from the ferocity of the
female, by gliding about and playing hide and seek over her body and
along her gigantic limbs: in such a pursuit it is evident that the
chances of escape would be in favor of the smallest males, while the
larger ones would fall early victims; thus gradually a diminutive race
of males would be selected, until at last they would dwindle to the
smallest possible size compatible with the exercise of their generative
functions,—in fact probably to the size we now see them, _i.e._ so small
as to be a sort of parasite upon the female, and either beneath her
notice, or too agile and too small for her to catch without great
difficulty.’”
It is certainly surprising to find Darwin ascribing even this difference
in size between the sexes to the action of selection. Is it not a little
ludicrous to suppose that the females have reduced the males to a size
too small for them to catch?
There are many cases known in the animal kingdom where there is a
difference in size between the two sexes, especially in the group of
insects; but I doubt very much if they are to be accounted for as the
result of sexual selection. In some of these cases Darwin accounts for
the larger size of the female, on account of the large number of eggs
which she has to carry. In other insects where the male is larger, as in
the stag-beetle, the size is ascribed to the conflicts of the males,
leading to the survival of the larger individuals. In still other cases,
where the males are larger, but do not fight, an explanation is
admittedly wanting; but it is suggested that here there would be no
necessity for the males to be smaller than the females in order to
mature before them (as is supposed to happen in other species), for in
these cases the individuals are not short-lived, and there would be
ample time for pairing. Again, although the males of nearly all bees are
smaller than the females, yet the reverse is true in those forms in
which the females are fertilized during the marriage flight. The
explanation offered is that in these forms the male carries the female,
and this is assumed to require greater size on his part. This loose way
of guessing, as to a possible explanation, is characteristic of the
whole hypothesis of sexual selection. First one, and then another, guess
is made as to the causes of the differences between the sexes. It is not
shown in a single one of the instances that the postulated cause has
really had anything to do with the differences in question; and the
attempt to show that the theory is probable, by pointing out the large
number of cases which it appears to account for, is weakened to a very
great degree by the number of exceptional cases, for which an equally
ready explanation of a different kind is forthcoming. This way of giving
loose rein to the imagination has been the bane of the method that has
followed hard on the track of Darwin’s hypothesis, and for which his
example has been in no small measure responsible. Thus, in the case just
quoted, there are no less than four distinct conjectures made to account
for the differences in size between the sexes, and each guess involves
an entirely different set of processes. Considering the complicated
relation of the life of organisms, it may be doubted if any of the
imagined processes could bring about the result, and certainly not a
single one has been shown to be a real, or a sufficient, cause in the
evolutionary process. Neither the actuality of the postulated causes,
nor their application to a particular case, has been shown to exist.
In the Diptera, or flies, Wallace records one interesting case of sexual
difference in the genus Elaphomyia of New Guinea, in which the males are
furnished with horns, which the females lack. Darwin writes:—
“The horns spring from beneath the eyes, and curiously resemble those of
a stag, being either branched or palmated. In one of the species, they
equal the whole body in length. They might be thought to be adapted for
fighting, but as in one species they are of a beautiful pink color,
edged with black, with a pale central stripe, and as these insects have
altogether a very elegant appearance, it is perhaps more probable that
they serve as ornaments.”
Presumably, therefore, Darwin means these colored horns have been
acquired by sexual selection.
In the Hemiptera, or bugs, both sexes of some species are “beautifully
colored,” and as the members of the group are often unpalatable to other
animals, the color in this case is supposed to act as a warning signal.
In other cases it is stated, however, that “a small pink and green
species” could hardly be distinguished from the buds on the trunks of
the lime trees which this insect frequents. In this case the color
appears “to be directly protective.” Thus without any means of forming a
correct judgment, the color of one animal is supposed to be the result
of natural selection, since it resembles its surroundings, but of sexual
selection if the color is present or more pronounced in one sex. Where
neither view can easily be applied, the color is ascribed in a general
way to the nature of the organism.
In respect to the group of Hymenoptera, or bees, Darwin records the
following cases:—
“In this order slight differences in color, according to sex, are
common, but conspicuous differences are rare except in the family of
bees; yet both sexes of certain groups are so brilliantly colored—for
instance in Chrysis, in which vermilion and metallic greens prevail—that
we are tempted to attribute the result to sexual selection. In the
Ichneumonidæ, according to Mr. Walsh, the males are almost universally
lighter-colored than the females. On the other hand, in the
Tenthredinidæ the males are generally darker than the females. In the
Siricidæ the sexes frequently differ; thus the male of _Sirex juvencus_
is banded with orange, whilst the female is dark purple; but it is
difficult to say which sex is the more ornamented.”
In other families of bees, differences in the color of the sexes have
been recorded, and since the males have been seen fighting for the
possession(?) of the females, and since bees are known to recognize
differences in color, Darwin believes that:—
“In some species the more beautiful males appear to have been selected
by the females; and in others the more beautiful females by the males.
Consequently in certain genera, the males of the several species differ
much in appearance, whilst the females are almost indistinguishable; in
other genera the reverse occurs. H. Müller believes that the colors
gained by one sex through sexual selection have often been transferred
in a variable degree to the other sex, just as the pollen-collecting
apparatus of the female has often been transferred to the male, to whom
it is absolutely useless.”
Although in beetles the sexes are generally colored alike, yet in some
of the longicorns there are exceptions to the rule. “Most of these
insects are large and splendidly colored. The males in the genus
Pyrodes, which I saw in Mr. Bates’s collection, are generally redder but
rather duller than the females, the latter being colored of a more or
less splendid golden-green. On the other hand, in one species the male
is golden-green, the female being richly tinted with red and purple. In
the genus Esmeralda the sexes differ so greatly in color that they have
been ranked as distinct species; in one species both are of a beautiful
shining green, but the male has a red thorax. On the whole, as far as I
could judge, the females of those Prionidæ, in which the sexes differ,
are colored more richly than the males, and this does not accord with
the common rule in regard to color, when acquired through sexual
selection.”
The great horns that rise from the heads of many male beetles are very
striking cases of sexual difference, and Darwin compares them to the
horns of stags and of the rhinoceros. They “are wonderful from their
size and shapes.” Darwin offers the following conjecture as to their
meaning: “The extraordinary size of the horns, and their widely
different structure in closely allied forms, indicate that they have
been formed for some purpose; but their excessive variability in the
males of the same species leads to the inference that this purpose
cannot be of a definite nature. The horns do not show marks of friction,
as if used for any ordinary work. Some authors suppose that as the males
wander about much more than the females, they require horns as a defence
against their enemies; but as the horns are often blunt, they do not
seem well adapted for defence. The most obvious conjecture is that they
are used by the males for fighting together; but the males have never
been observed to fight; nor could Mr. Bates, after a careful examination
of numerous species, find any sufficient evidence, in their mutilated or
broken condition, of their having been thus used. If the males had been
habitual fighters, the size of their bodies would probably have been
increased through sexual selection, so as to have exceeded that of the
females; but Mr. Bates, after comparing the two sexes in above a hundred
species of the Copridæ, did not find any marked difference in this
respect amongst well-developed individuals. In Lethrus, moreover, a
beetle belonging to the same great division of the lamellicorns, the
males are known to fight, but are not provided with horns, though their
mandibles are much larger than those of the female.”
“The conclusion that the horns have been acquired as ornaments is that
which best agrees with the fact of their having been so immensely, yet
not fixedly, developed,—as shown by their extreme variability in the
same species, and by their extreme diversity in closely allied species.
This view will at first appear extremely improbable; but we shall
hereafter find with many animals standing much higher in the scale,
namely fishes, amphibians, reptiles and birds, that various kinds of
crests, knobs, horns and combs have been developed apparently for this
sole purpose.”
It is asking a great deal to suppose that animals, so dull and sluggish
as these beetles, are endowed with a sufficient æsthetic discrimination
to select in each generation those males whose horns are a little longer
than the average. The resemblance of the horns to those of stags is, as
Darwin points out, obvious, but in the latter case also it remains to be
proven that they are the result of sexual selection, as Darwin believes
to be the case; but the evidence for this belief is not much better, as
we shall see in the case of the antlers of deer, than it is in these
beetles.
In regard to butterflies, the males and females are both often equally
brilliantly colored; in other species the differences in the sexes are
very striking. Darwin states:—
“Even within the same genus we often find species presenting
extraordinary differences between the sexes, whilst others have their
sexes closely alike.” The fine colors of the wings of many moths are
also supposed by Darwin to have arisen through sexual selection,
although the colors are usually on the lower wings, which are covered
during the day by the less ornamented upper wings. It is assumed that,
since the moths often begin to fly at dusk, their colors might at this
time be seen and appreciated by the other sex. It should not be
overlooked, however, that, in the case of some of the most highly
colored moths, it is known that the males find the females through the
sense of smell. Moreover, although moths are often finely colored,
Darwin points out that “it is a singular fact that no British moths
which are brilliantly colored, and, as far as I can discover, hardly any
foreign species, differ much in color according to sex; though this is
the case with many brilliant butterflies.”
Yet Darwin does not hesitate to conclude: “From the several foregoing
facts it is impossible to admit that the brilliant colors of
butterflies, and of some few moths, have commonly been acquired for the
sake of protection. We have seen that their colors and elegant patterns
are arranged and exhibited as if for display. Hence I am led to believe
that the females prefer or are most excited by the more brilliant males;
for on any other supposition the males would, as far as we can see, be
ornamented to no purpose. We know that ants and certain lamellicorn
beetles are capable of feeling an attachment for each other, and that
ants recognize their fellows after an interval of several months. Hence
there is no abstract improbability in the Lepidoptera, which probably
stand nearly or quite as high in the scale as these insects, having
sufficient mental capacity to admire bright colors. They certainly
discover flowers by color.”
So far as the evidence of ants having an attachment for each other is
concerned, we may eliminate this part of the argument, since the
evidence on which the statement is based is now regarded as only showing
that ants recognize each other by their sense of smell, which resides in
the antennæ. Hence the so-called fondling means only that the ants are
trying by smell to determine the odor of the other individual.
Darwin points out a number of cases in which the females are more
brightly colored than the males, and for such cases he reverses the
process of selection, supposing that the males have been discriminating,
and have not “gladly accepted any female.” No explanation is offered to
account for this reversal of instinct, in fact, no evidence to show that
such a reversal really exists. Darwin points out that in most cases the
male insect carries the female during the period of union, while in two
species of butterflies, _Colias edusa_ and _hyale_, the females carry
the males, and the females are here the more highly colored. He suggests
that since in this case “the females take the more active part in the
final marriage ceremony, so we may suppose that they likewise do so in
the wooing; and in this case we can understand how it is that they have
been rendered the more beautiful.”
A most significant fact in connection with the difference in sexual
coloration of butterflies did not escape Darwin’s attention.
“Whilst reflecting on the beauty of many butterflies, it occurred to me
that some caterpillars were splendidly colored; and as sexual selection
could not possibly have here acted, it appeared rash to attribute the
beauty of the mature insect to this agency, unless the bright colors of
their larvæ could be somehow explained. In the first place, it may be
observed that the colors of caterpillars do not stand in any close
correlation with those of the mature insect. Secondly, their bright
colors do not serve in any ordinary manner as a protection. Mr. Bates
informs me, as an instance of this, that the most conspicuous
caterpillar which he ever beheld (that of a Sphinx) lived on the large
green leaves of a tree on the open llanos of South America; it was about
four inches in length, transversely banded with black and yellow, and
with its head, legs, and tail of a bright red. Hence it caught the eye
of any one who passed by, even at the distance of many yards, and no
doubt that of every passing bird.”
Darwin applied to Wallace for a solution of this difficulty, and
received the reply that he “thought it probable that conspicuously
colored caterpillars were protected by having a nauseous taste; but as
their skin is extremely tender, and as their intestines readily protrude
from a wound, a slight peck from the beak of a bird would be as fatal to
them as if they had been devoured. Hence, as Mr. Wallace remarks,
‘distastefulness alone would be insufficient to protect a caterpillar
unless some outward sign indicated to its would-be destroyer that its
prey was a disgusting morsel.’ Under these circumstances it would be
highly advantageous to a caterpillar to be instantaneously and certainly
recognized as unpalatable by all birds and other animals. Thus the most
gaudy colors would be serviceable, and might have been gained by
variation and the survival of the most easily recognized individuals.”
It need scarcely be pointed out that an occasional peck can scarcely be
supposed to have led to the splendid development of color shown by some
caterpillars, and Darwin confesses that at first sight this hypothesis
appears bold, but nevertheless he believes that it will be found to be
true. He adds, “We cannot, however, at present thus explain the elegant
diversity in the colors of many caterpillars.”
A most important fact in this connection should not be overlooked,
namely, that in the caterpillar stage the sexual organs are so little
developed that it is generally impossible at this time to distinguish
between the sexes, unless a microscopic examination is made. This gives
us, perhaps, a clew as to the difference between the mature sexual
forms. These differences are connected with difference of sex itself.
This conclusion also fits in well with the fact that during the period
when the sexual organs are at the height of their development the
individuals are most brilliantly colored. The primary cause of the
brilliant color of many animals concerns us here only secondarily, for,
since it is known that many of the lower forms are no less brilliantly
and elaborately colored than are the sexes of the higher forms, it is
not surprising that the sexes themselves sometimes differ in this
respect.
Organs for producing sounds of different sorts are present in some
insects, and these organs Darwin includes under the head of secondary
sexual organs. In the group of Hemiptera, or bugs, the cicadas are the
most familiar species that produce sounds. The noise is made by the
males; the females are quite mute.
“With respect to the object of the music, Dr. Hartman, in speaking of
the _Cicada septemdecim_ of the United States, says, ‘the drums are now
(June 6th and 7th, 1851) heard in all directions. This I believe to be
the marital summons from the males. Standing in thick chestnut sprouts
about as high as my head, where hundreds were around me, I observed the
females coming around the drumming males.’ He adds, ‘this season
(August, 1868) a dwarf pear-tree in my garden produced about fifty larvæ
of _C. pruinosa_; and I several times noticed the females to alight near
a male while he was uttering his clanging notes.’ Fritz Müller writes to
me from S. Brazil that he has often listened to a musical contest
between two or three males of a species with a particularly loud voice,
seated at a considerable distance from each other: as soon as one had
finished his song, another immediately began, and then another. As there
is so much rivalry between the males, it is probable that the females
not only find them by their sounds, but that, like female birds, they
are excited or allured by the male with the most attractive voice.”
In the flies the following cases are given by Darwin:—
“That the males of some Diptera fight together is certain; for Professor
Westwood has several times seen this with the Tipulæ. The males of other
Diptera apparently try to win the females by their music: H. Müller
watched for some time two males of an Eristalis courting a female; they
hovered above her, and flew from side to side, making a high humming
noise at the same time. Gnats and mosquitoes (Culicidæ) also seem to
attract each other by humming; and Professor Mayer has recently
ascertained that the hairs on the antennæ of the male vibrate in unison
with the notes of a tuning-fork, within the range of the sounds emitted
by the female.”
In the crickets, grasshoppers, and locusts, the males “are remarkable
for their musical powers”; and it is generally assumed that the sounds
serve to call or to excite the female. In these forms the noise is made
by rubbing the wings over each other or the legs against the
wing-covers.
In some of these forms both sexes have stridulating organs, and in one
case they differ to a certain extent from each other. “Hence we cannot
suppose that they have been transferred from the male to the female, as
appears to have been the case with the secondary sexual characters of
many other animals. They must have been independently developed in the
two sexes, which no doubt mutually call to each other during the season
of love.”
Some beetles also possess rasping organs in different parts of the body,
but they cannot produce much noise by this means.
“We thus see that in the different coleopterous families the
stridulating organs are wonderfully diversified in position, but not
much in structure. Within the same family some species are provided with
these organs, and others are destitute of them. This diversity is
intelligible, if we suppose that originally various beetles made a
shuffling or hissing noise by the rubbing together of any hard and rough
parts of their bodies, which happened to be in contact; and that from
the noise thus produced being in some way useful, the rough surfaces
were gradually developed into regular stridulating organs. Some beetles
as they move, now produce, either intentionally or unintentionally, a
shuffling noise, without possessing any proper organs for the purpose.”
Darwin says that he expected from analogy to find in this group also
differences in the sexes, but none such were found, although in some
cases the males alone possess certain characters or have them more
highly developed.
It is important not to forget, when considering all questions connected
with sexual selection, that in order for the result to be successful it
is not only necessary that the female respond to the noises and music of
the other sex, but that she choose the suitor that makes the greatest,
or the most pleasing, noise. If the stridulating organs are only used by
the animals in finding each other, then the case might be considered as
coming under the head of natural selection. If this be granted, then it
may be claimed, and apparently Darwin is inclined to adopt this view,
that those males that make the most noise will be more likely to be
heard, and possibly approached. They will, therefore, be more likely to
leave descendants. We have already considered this question when dealing
with the theory of natural selection in the preceding chapter and need
not go over the ground again. This much may, however, be said again,
that even if it is probable that these organs are of use to the animals
in finding each other, and this seems not improbable, it does not follow
that the organs have been acquired through selection for this purpose.
Darwin finds his best examples of secondary sexual characters in the
group of vertebrates, and since in this group the intelligence is of a
higher order than in the other groups, the argument that the female
chooses the more pleasing suitor is made to appear more plausible.
The elongation of the lower jaw that occurs in a few fishes at the
breeding season is regarded as a secondary sexual character. On the
other hand, Darwin recognizes the following difficulty in regard to the
size of the males:—
“In regard to size, M. Carbonnier maintains that the female of almost
all fishes is larger than the male; and Dr. Günther does not know of a
single instance in which the male is actually larger than the female.
With some cyprinodonts the male is not even half as large. As in many
kinds of fishes the males habitually fight together, it is surprising
that they have not generally become larger and stronger than the females
through the effects of sexual selection. The males suffer from their
small size, for, according to M. Carbonnier, they are liable to be
devoured by the females of their own species when carnivorous, and no
doubt by other species. Increased size must be in some manner of more
importance to the females, than strength and size are to the males for
fighting with other males; and this perhaps is to allow of the
production of a vast number of ova.”
The last sentence implies that this particular case is to be explained
by the females becoming larger on account of the number of eggs that
they are to produce. But why was not the same explanation offered in the
case of the spiders? It is this uncertain way of applying any
explanation that suggests itself, that puts the whole method in an
unfortunate light.
In many species of fish the males are brighter in color than the
females. In the case of _Callionymus lyra_, Darwin states:—
“When fresh caught from the sea the body is yellow of various shades,
striped and spotted with vivid blue on the head; the dorsal fins are
pale brown with dark longitudinal bands, the ventral, caudal, and anal
fins being bluish black. The female, or sordid dragonet, was considered
by Linnæus, and by many subsequent naturalists, as a distinct species;
it is of a dingy reddish brown, with the dorsal fin brown and the other
fins white. The sexes differ also in the proportional size of the head
and mouth, and in the position of the eyes; but the most striking
difference is the extraordinary elongation in the male of the dorsal
fin. Mr. W. Saville Kent remarks that this ‘singular appendage appears
from my observations of the species in confinement, to be subservient to
the same end as the wattles, crests, and other abnormal adjuncts of the
male in gallinaceous birds, for the purpose of fascinating their
mates.’”
In the case of another fish, _Cottus scorpius_, there is also a great
difference between the sexes, and here the males become very brilliant
only at the breeding season. In other fishes, in which the sexes are
colored alike, the males may become more brilliant during the breeding
season. This, too, is explained by Darwin on the assumption that those
males that have varied at the breeding season, so as to become more
brightly colored, have been chosen in preference to the other males.
A few cases are cited in which it has been observed that the males
appear to exhibit themselves before the females, as in the following
case of the Chinese Macropus:—
“The males are most beautifully colored, more so than the females.
During the breeding season they contend for the possession of the
females; and, in the act of courtship, expand their fins, which are
spotted and ornamented with brightly colored rays, in the same manner,
according to M. Carbonnier, as the peacock. They then also bound about
the females with much vivacity, and appear by ‘l’étalage de leurs vives
couleurs chercher à attirer l’attention des femelles, lesquelles ne
paraissaient indifférentes à ce manége, elles nageaient avec une molle
lenteur vers les mâles et semblaient se complaire dans leur voisinage.’”
In this connection Darwin makes the following general statement:—
“The males sedulously court the females, and in one case, as we have
seen, take pains in displaying their beauty before them. Can it be
believed that they would thus act to no purpose during their courtship?
And this would be the case, unless the females exert some choice and
select those males which please or excite them most. If the female
exerts such choice, all the above facts on the ornamentation of the
males become at once intelligible by the aid of sexual selection.”
While it may readily be granted that display of the male may have for
its purpose the excitement of the female, it is another question as to
whether she will be more excited by the more beautiful suitor. The
attentions of the male may be supposed to have a purpose, even if the
female does not choose the more beautiful of her suitors. It is this
last proposition, so necessary for the theory of sexual selection, that
seems improbable. But even if it were probable, there are, as we shall
see, other difficulties to be overcome before we should be justified in
accepting Darwin’s statement quoted above, concerning the results of
sexual selection.
In regard to those species of fish in which both sexes are equally
ornamented, Darwin returns once more to his hypothesis that the color of
the male, acquired through sexual selection, may be transmitted to the
other sex, and then, as if in doubt on this point, he adds, that it may
be the result of the “nature of the tissues and of the surrounding
conditions.” He even makes the suggestion, somewhat further on, that the
colors may be warning, although it is confessedly unknown that these
fish are distasteful to fish-devouring animals.
In amphibians the crest on the back of the male triton, which becomes
colored along its edge, is described as a secondary sexual character.
The vocal sacs, present in some species of frogs, are found sometimes in
both sexes, but more highly developed in the males. In other species, as
in the toad, it is the male alone that sings. In the reptiles we find
that the two sexes of the turtles are colored alike, and this holds also
for the crocodiles. Some male turtles make sounds at the breeding
season, and the same is true for the crocodiles, the males of which are
said to make a “prodigous display.” In snakes the males are smaller, as
a rule, than the females, and the colors are more strongly pronounced,
and although some snakes are very brilliantly colored, Darwin puts this
down either to protective coloration, or to mimicry of other kinds of
snakes. But surely the extremely brilliant colors of many snakes cannot
be accounted for in any of these ways. The cause of the color of the
venomous kinds, that are supposed to be imitated by the others, “remains
to be explained and this may perhaps be sexual selection.”
“It does not, however, follow because snakes have some reasoning power,
strong passions and mutual affection, that they should likewise be
endowed with sufficient taste to admire brilliant colors in their
partners, so as to lead to the adornment of the species through sexual
selection. Nevertheless, it is difficult to account in any other manner
for the extreme beauty of certain species; for instance, of the
coral-snakes of South America, which are of a rich red with black and
yellow transverse bands.”
In lizards the erectile crests of the male _Anolis_, the brilliant
throat patches of _Sitaria minor_, which is colored blue, black, and
red, the skinny appendages present on the throat of the little lizards
of the genus Draco, which in the beauty of their colors baffle
description, are given as cases of sexual adornment. In the last case
cited the ornaments are present, however, in both sexes. The remarkable
horns in the males of different species of chameleons are imagined to
have been acquired through the battle of the males with each other.
In the group of birds we find some of the most striking cases of
secondary sexual differences. The spurs, combs, wattles, horns,
air-filled sacs, topknots, feathers with naked shafts, plumes, and
greatly elongated feathers are all secondary sexual characters. The
songs of the males, the rattling together of the quills of the peacock,
the drumming of the grouse, and the booming sounds made by the night
jars while on the wing, are further examples of secondary sexual
differences. The odor of the male of the Australian musk duck is also
put in the same category.
The pugnacity of many male birds is well known, and it is imagined that
one of the results of the competition of the individuals of the same sex
with each other has led to the development of the organs of defence and
offence. The males that have been successful in these battles are then
supposed to mate with the best females. In this way those secondary
sexual differences, connected with the encounters of the males, are
supposed to have been formed. Darwin states in this connection:—
“Even with the most pugnacious species it is probable that the pairing
does not depend exclusively on the mere strength and courage of the
male; for such males are generally decorated with various ornaments,
which often become more brilliant during the breeding season, and which
are sedulously displayed before the females. The males also endeavor to
charm or excite their mates by love-notes, songs, and antics; and the
courtship is, in many instances, a prolonged affair. Hence it is not
probable that the females are indifferent to the charms of the opposite
sex, or that they are invariably compelled to yield to the victorious
males.”
Thus a double process of selection is imagined to take place; one, the
outcome of a competition of the males with each other, and the other,
through a choice of the more successful males by the females, the more
beautiful being supposed to be chosen.
It may be well not to lose sight of the fact that unless the selection
is severe in each generation, its good effects will be lost, as has been
stated in connection with the theory of natural selection. Still more
important is the consideration that unless the same variations appear at
the same time, in many of the surviving males, the results will be lost
through crossing. These statements will show that the difficulties of
the theory are by no means small, and when we are asked to believe
further that another process still has been superimposed on this one,
namely, the selection of the more beautiful males by the females, we can
appreciate how great are the difficulties that must be overcome in order
that the process may be carried out.
The love-antics and dances of male birds at the breeding season furnish
many curious data. The phenomena are imagined by Darwin to be connected
with sexual selection, for in the dances the males are supposed to
exhibit their ornaments to the females who are imagined to choose the
suitor that is most to their taste.
Hudson, who has studied the habits of birds in the field, asks some very
pertinent questions in connection with their performances of different
kinds. “What relation that we can see or imagine to the passion of love
and the business of courtship have these dancing and vocal performances
in nine cases out of ten? In such cases, for instance, as that of the
scissortail tyrant-bird, and its pyrotechnic displays, when a number of
couples leave their nests containing eggs and young to join in a wild
aërial dance; the mad exhibitions of ypecahas and ibises and the
jacana’s beautiful exhibition of grouped wings; the triplet dances of
the spur-winged lapwing, to perform which two birds already mated are
compelled to call in a third bird to complete the set; the harmonious
duets of the oven-birds and the duets and choruses of nearly all the
wood-hewers, and the wing-slapping aërial displays of the whistling
widgeons,—will it be seriously contended that the female of this species
makes choice of the male able to administer the most vigorous and
artistic slaps?”
“The believer in the theory would put all these cases lightly aside to
cite the case of the male cow-bird practising antics before the female,
and drawing a wide circle of melody around her, etc.... And this was in
substance what Darwin did.” “How unfair the argument is based on these
carefully selected cases gathered from all regions of the globe and
often not properly reported is seen when we turn to the book of nature
and closely consider the habits and actions of all the species
inhabiting any _one_ district.” Hudson concludes that he is convinced
that any one who will note the actions of animals for himself will reach
the conviction, that “conscious sexual selection on the part of the
female is not the cause of music and dancing performances in birds, nor
of the brighter colors and ornaments that distinguish the male.”
The differences in color in the sexes of birds are classified by Darwin
as follows: (1) when the males are ornamented exclusively or in a much
higher degree than the females; (2) when both sexes are highly
ornamented; (3) when the female is more brightly colored. A few examples
of each sort may be chosen for illustration.
“In regard to color, hardly anything need here be said, for every one
knows how splendid are the tints of many birds, and how harmoniously
they are combined. The colors are often metallic and iridescent.
Circular spots are sometimes surrounded by one or more differently
shaded zones, and are thus converted into ocelli. Nor need much be said
on the wonderful difference between the sexes of many birds. The common
peacock offers a striking instance. Female birds of paradise are
obscurely colored and destitute of all ornaments, whilst the males are
probably the most highly decorated of all birds, and in so many
different ways, that they must be seen to be appreciated. The elongated
and golden-orange plumes which spring from beneath the wings of the
_Paradisea apoda_, when vertically erected and made to vibrate, are
described as forming a sort of halo, in the centre of which the head
‘looks like a little emerald sun, with its rays formed by the two
plumes.’”
Male humming-birds are almost as splendidly colored as are the birds of
paradise, some having the feathers modified in a truly extraordinary
way. “Almost every part of their plumage has been taken advantage of,
and modified; and the modifications have been carried, as Mr. Gould
showed me, to a wonderful extreme in some species belonging to nearly
every subgroup. Such cases are curiously like those which we see in our
fancy breeds, reared by man for the sake of ornament: certain
individuals originally varied in one character, and other individuals of
the same species in other characters; and these have been seized on by
man and much augmented—as shown by the tail of the fantail pigeon, the
hood of the jacobin, the beak and wattle of the carrier, and so forth.
The sole difference between these cases is that in the one the result is
due to man’s selection, whilst in the other, as with humming-birds,
birds of paradise, etc., it is due to the selection by the females of
the more beautiful males.”
A remarkable bird of South America, the bell-bird, has a peculiar note
that “can be distinguished at the distance of nearly three miles and
astonishes every one who hears it.... The male is pure white, whilst the
female is dusky-green; and white is a very rare color in terrestrial
species of moderate size and inoffensive habits. The male, also, as
described by Waterton, has a spiral tube, nearly three inches in length,
which rises from the base of the beak. It is jet-black, dotted over with
minute downy feathers. This tube can be inflated with air, through a
communication with the palate; and when not inflated hangs down on one
side. The genus consists of four species, the males of which are very
distinct, whilst the females, as described by Mr. Sclater in a very
interesting paper, closely resemble each other, thus offering an
excellent instance of the common rule that within the same group the
males differ much more from each other than do the females. In a second
species (_C. nudicollis_) the male is likewise snow-white, with the
exception of a large space of naked skin on the throat and round the
eyes, which during the breeding season is of a fine green color. In a
third species (_C. tricarunculatus_) the head and neck alone of the male
are white, the rest of the body being chestnut-brown, and the male of
this species is provided with three filamentous projections half as long
as the body—one rising from the base of the beak, and the two others
from the corners of the mouth.”
The most familiar case of sexual difference amongst North American birds
is that of the scarlet tanager, in which the male is scarlet with
jet-black wings, while the female is an inconspicuous yellow-green
color. Amongst domesticated animals the peafowl shows the most beautiful
case of sexual differences. The magnificent tail of the male can be
lifted up, so as to be seen to best advantage when the male faces the
observer. Moreover the wild form, living in the forests of India, has
the same gorgeous train.
The male Argus pheasant has a remarkable series of spots, or ocelli, on
the secondary wing-covers. They are concealed until the male displays
them before the female. Darwin states that, while it may seem incredible
that such elegant shading and exquisite patterns could have been the
outcome of the taste of the female, yet the extraordinary attitude
assumed by the male during courtship appears entirely purposeless,
unless it be supposed that he is attempting to charm the female by a
display of his ornamentation.
Let us pass to the second class of cases, in which both sexes are
similarly and brightly colored, and in which the young have a plumage
different from the adults. For example, the male and the female of the
splendid scarlet ibis are alike, whilst the young are brown. The males
and females of many finely colored herons are ornamented alike, and this
plumage, Darwin admits, has a nuptial character. He even tries to
explain this by the curious assumption, that while the color has been
acquired through the selection of the males by the females, the results
attained in this way have been transmitted to both sexes. We find here
another example of the method so often employed by Darwin. When he meets
with facts that are not in conformity with the theory, he proceeds to
make a new assumption without establishing its validity. Thus, to assume
that in all cases where the sexes are colored differently, the
characters acquired by the males have been transmitted only to the same
sex, and in those cases where the sexes are colored alike the
transmission has been to both sexes, is most arbitrary.
In other cases, which are commoner than the last, the male and female
have the same color, and the young in their first plumage resemble the
adults. Darwin admits that here the facts are so complex that his
conclusions are doubtful. The following account of the tree-sparrow
shows how vague are the principles involved in the entire discussion in
relation to transmission:—
“Now with the tree-sparrow (_P. montanus_) both sexes and the young
closely resemble the male of the house-sparrow; so that they have all
been modified in the same manner, and all depart from the typical
coloring of their early progenitor. This may have been effected by a
male ancestor of the tree-sparrow having varied, firstly, when nearly
mature; or secondly, whilst quite young, and by having in either case
transmitted his modified plumage to the females and the young; or,
thirdly, he may have varied when adult and transmitted his plumage to
both adult sexes, and, owing to the failure of the law of inheritance at
corresponding ages, at some subsequent period to his young.”
The further admissions made in the following quotation are also
significant:—
“The plumage of certain birds goes on increasing in beauty during many
years after they are fully mature; this is the case with the train of
the peacock, with some of the birds of paradise, and with the crest and
plumes of certain herons, for instance, the _Ardea ludovicana_. But it
is doubtful whether the continued development of such feathers is the
result of the selection of successive beneficial variations (though this
is the most probable view with birds of paradise) or merely of
continuous growth. Most fishes continue increasing in size, as long as
they are in good health and have plenty of food; and a somewhat similar
law may prevail with the plumes of birds.”
We need not follow Darwin through his discussion of those cases in which
the adults have a winter and a summer dress and the young resemble the
one or the other in plumage, or are different from either. The
discussion of these cases, confessedly very complex, adds nothing to our
understanding of the theory, and little but conjecture is offered to
account for the facts.
The extreme to which even conjecture can be carried may be gathered from
the following quotation, taken from the section dealing with cases in
which the young in their first plumage differ from each other according
to sex, the young males resembling more or less closely the adult males,
and the young females more or less closely the adult females:
“Two humming-birds belonging to the genus Eustephanus, both beautifully
colored, inhabit the small island of Juan Fernandez, and have always
been ranked as specifically distinct. But it has lately been ascertained
that the one which is of a rich chestnut-brown color with a golden-red
head, is the male, whilst the other, which is elegantly variegated with
green and white with a metallic-green head, is the female. Now the young
from the first somewhat resemble the adults of the corresponding sex,
the resemblance gradually becoming more and more complete.
“In considering this last case, if as before we take the plumage of the
young as our guide, it would appear that both sexes have been rendered
beautiful independently; and not that one sex has partially transferred
its beauty to the other. The male apparently has acquired his bright
colors through sexual selection in the same manner as, for instance, the
peacock or pheasant in our first class of cases; and the female in the
same manner as the female Rhynchæa or Turnix in our second class of
cases. But there is much difficulty in understanding how this could have
been effected at the same time with the two sexes of the same species.
Mr. Salvin states, as we have seen in the eighth chapter, that with
certain humming-birds the males greatly exceed the females in number,
whilst with other species inhabiting the same country the females
greatly exceed the males. If, then, we might assume that during some
former lengthened period the males of the Juan Fernandez species had
greatly exceeded the females in number, but that during another
lengthened period the females had far exceeded the males, we could
understand how the males at one time, and the females at another, might
have been rendered beautiful by the selection of the brighter-colored
individuals of either sex; both sexes transmitting their characters to
their young at a rather earlier age than usual. Whether this is the true
explanation I will not pretend to say; but the case is too remarkable to
be passed over without notice.”
The third group of cases include those in which the females are more
brightly colored, or more ornamented, than the males. These cases are
rare, and the differences between the sexes are never so great as when
the male is the more highly colored. Wallace thinks that since in these
cases the male incubates the eggs his less conspicuous colors have been
acquired through natural selection. In the genus Turnix the female is
larger than the male, and lacks the black on the throat and neck, and
the plumage as a whole is lighter than that of the male. The natives
assert that the females after laying their eggs associate in flocks, and
leave the males to do the incubating; and from other evidence Darwin
thinks that this is true.
In three species of painted snipe the females “are not only larger but
much more richly colored than the males,” and the trachea is more
convoluted in some species. “There is also reason to believe that the
male undertakes the duty of incubation.” In the dotterel plover the
female is larger and somewhat more strongly colored. The males take at
least a share in the incubation. In the common cassowary the female is
larger and the skin of the head more brightly colored than in the male.
The female is pugnacious during the breeding season and the male sits on
the eggs. The female emu is large and has a crest. She is more
courageous and pugilistic and makes a deep, hollow, guttural boom. The
male is more docile and can only hiss or croak. He not only incubates
the eggs, but defends the young against their own mother. “So that with
this emu we have a complete reversal not only of the parental and
incubating instincts, but of the usual moral qualities of the two sexes;
the females being savage, quarrelsome, and noisy, the males gentle and
good. The case is very different with the African ostrich, for the male
is somewhat larger than the female and has finer plumes with more
strongly contrasted colors; nevertheless he undertakes the whole duty of
incubation.”
Darwin attempts to explain these reversals of instincts on the
assumption that the males have turned the tables on the females, and
have themselves done the selecting; and incidentally, it may be pointed
out in passing, they have had to pay the penalty by incubating the eggs.
In the group of mammals, Darwin thinks that the male wins the female by
conquering other males rather than by charming her through his display.
The males, even when unarmed, engage in desperate conflicts with each
other, and sometimes kill, but more often only wound, their fellows. The
secondary sexual characters of the males have been acquired, therefore,
by natural selection applied to one sex, and less frequently through the
choice of the female. Since we are here more especially concerned with
the latter class of phenomena, we may examine only a few cases under the
first head.
The horns of stags are used by them in their conflicts with each other;
the tusks of the elephant make this animal the most dangerous in the
world, when in must. The horns of bulls, the canine teeth of many
mammals, the tusks of the walrus, are further examples of organs which
have been, according to Darwin, acquired through the competitions of the
males with each other.
The voices of mammals are used for various purposes, “as a signal of
danger, as a call from one member of the troup to another, and from the
mother to her lost offspring, or from the latter for protection.”
“Almost all male animals use their voices much more during the rutting
season than at any other time; and some, as the giraffe and porcupine,
are said to be completely mute excepting at this season. As the throats
(_i.e._ the larynx and thyroid bodies) of stags periodically become
enlarged at the beginning of the breeding season, it might be thought
that their powerful voices must be somehow of high importance to them;
but this is very doubtful. From information given to me by two
experienced observers, Mr. McNeill and Sir P. Egerton, it seems that
young stags under three years old do not roar or bellow; and that the
old ones begin bellowing at the commencement of the breeding season, at
first only occasionally and moderately, whilst they restlessly wander
about in search of the females. Their battles are prefaced by loud and
prolonged bellowing, but during the actual conflict they are silent.
Animals of all kinds which habitually use their voices utter various
noises under any strong emotion, as when enraged and preparing to fight;
but this may merely be the result of nervous excitement, which leads to
the spasmodic contraction of almost all the muscles of the body, as when
a man grinds his teeth and clenches his fists in rage or agony. No doubt
stags challenge each other to mortal combat by bellowing; but those with
the more powerful voices, unless at the same time the stronger,
better-armed, and more courageous, would not gain any advantage over
their rivals.”
“Some writers suggest that the bellowing serves as a call to the female;
but the experienced observers above quoted inform me that female deer do
not search for the male, though the males search eagerly for the
females, as indeed might be expected from what we know of the habits of
other male quadrupeds. The voice of the female, on the other hand,
quickly brings to her one or more stags, as is well known to the hunters
who in wild countries imitate her cry.
“As the case stands, the loud voice of the stag during the breeding
season does not seem to be of any special service to him, either during
his courtship or battles, or in any other way. But may we not believe
that the frequent use of the voice, under the strong excitement of love,
jealousy, and rage, continued during many generations, may at last have
produced an inherited effect on the vocal organs of the stag, as well as
of other male animals? This appears to me, in our present state of
knowledge, the most probable view.”
Here once more we find that Darwin makes use, as a sort of last resort,
of the principle of the inheritance of acquired characters. As long as
the theory of selection, in any of its forms, appears to offer a
satisfactory solution, we find the facts used in support of this theory,
but as soon as a difficulty arises the Lamarckian theory is brought to
the front. It is this shifting, as we have already more than once
pointed out, that shows how little real basis there is for the theory of
sexual selection.
The male gorilla has a tremendous voice, and he has, as has also the
orang, a laryngeal sac. One species of gibbon has the power of producing
a correct octave of musical notes.
“The vocal organs of the American _Mycetes caraya_ are one-third larger
in the male than in the female, and are wonderfully powerful. These
monkeys in warm weather make the forests resound at morning and evening
with their overwhelming voices. The males begin the dreadful concert,
and often continue it during many hours, the females sometimes joining
in with their less powerful voices. An excellent observer, Rengger,
could not perceive that they were excited to begin by any special cause;
he thinks that, like many birds, they delight in their own music, and
try to excel each other. Whether most of the foregoing monkeys have
acquired their powerful voices in order to beat their rivals and charm
the females—or whether the vocal organs have been strengthened and
enlarged through the inherited effects of long-continued use without any
particular good being thus gained—I will not pretend to say; but the
former view, at least in the case of the _Hylobates agilis_, seems the
most probable.”
The odor of some mammals is confined to, or more developed, in the
males; but in some forms, as in the skunk, it is present in both sexes.
In the shrew mice, abdominal scent glands are present, but since these
mice are rejected by birds of prey, their glands probably serve to
protect them; “nevertheless the glands become enlarged in the males
during the breeding season.” In many other quadrupeds the scent glands
are of the same size in both sexes, and their function is unknown.
“In other species the glands are confined to the males, or are more
developed than in the females; and they almost always become more active
during the rutting season. At this period the glands on the sides of the
face of the male elephant enlarge, and emit a secretion having a strong
musky odor. The males, and rarely the females, of many kinds of bats
have glands and protrudable sacs situated in various parts; and it is
believed that these are odoriferous.
“The rank effluvium of the male goat is well known, and that of certain
male deer is wonderfully strong and persistent. Besides the general
odor, permeating the whole body of certain ruminants (for instance, _Bos
moschatus_) in the breeding season, many deer, antelopes, sheep, and
goats, possess odoriferous glands in various situations, more especially
on their faces. The so-called tear-sacs, or suborbital pits, come under
this head. These glands secrete a semifluid fetid matter which is
sometimes so copious as to stain the whole face, as I have myself seen
in an antelope. They are ‘usually larger in the male than in the female,
and their development is checked by castration.’ According to Desmarest
they are altogether absent in the female of _Antilope subgutturosa_.
Hence, there can be no doubt that they stand in close relation with the
reproductive functions. They are also sometimes present, and sometimes
absent, in nearly allied forms. In the adult male musk-deer (_Moschus
moschiferus_), a naked space round the tail is bedewed with an
odoriferous fluid, whilst in the adult female and in the male until two
years old, this space is covered with hair, and is not odoriferous.”
Darwin believes in these cases that the odor serves to attract the
females. He admits that here, “active and long-continued use cannot have
come into play as in the case of the vocal organs.” He concludes,
therefore, that “the odor emitted must be of considerable importance to
the male, inasmuch as large and complex glands, furnished with muscles
for everting the sac, and for closing or opening the orifice, have in
some cases been developed. The development of these organs is
intelligible through sexual selection, if the most odoriferous males are
the most successful in winning the females, and in leaving offspring to
inherit their gradually perfected glands and colors.”
There is sometimes a difference in the mammals in the hair of the two
sexes both in amount and in color. In some species of goats the males
have a beard, in others it is present in both sexes. The bull, but not
the cow, has curly hair on the forehead. In some monkeys the beard is
confined to the male, as in the orang; in other species it is only
larger in the males.
“The males of various members of the ox family (Bovidæ), and of certain
antelopes, are furnished with a dewlap, or great fold of skin on the
neck, which is much less developed in the female.
“Now, what must we conclude with respect to such sexual differences as
these? No one will pretend that the beards of certain male goats, or the
dewlap of the bull, or the crests of hair along the backs of certain
male antelopes, are of any use to them in their ordinary habits.
“Must we attribute all these appendages of hair or skin to mere
purposeless variability in the male? It cannot be denied that this is
possible; for in many domesticated quadrupeds, certain characters,
apparently not derived through reversion from any wild parent form, are
confined to the males, or are more developed in them than in the
females—for instance, the hump on the male zebu cattle of India, the
tail of fat-tailed rams, the arched outline of the forehead in the males
of several breeds of sheep, and, lastly, the mane, the long hairs on the
hind-legs, and the dewlap of the male of the Berbura goat.”
In these cases and in others that Darwin cites, which seem clearly to
indicate that some of these secondary sexual characters are not the
result of sexual selection, he concludes, “that they must be due to
simple variability, together with sexually limited inheritance.
“Hence it appears reasonable to extend this same view to all analogous
cases with animals in a state of nature. Nevertheless I cannot persuade
myself that it generally holds good, as in the case of the extraordinary
development of hair on the throat and fore-legs of the male Ammotragus,
or in that of the immense beard of the male Pithecia. Such study as I
have been able to give to nature makes me believe that parts or organs
which are highly developed, were acquired at some period for a special
purpose. With those antelopes in which the adult male is more strongly
colored than the female, and with those monkeys in which the hair on the
face is elegantly arranged and colored in a diversified manner, it seems
probable that the crests and tufts of hair were gained as ornaments; and
this I know is the opinion of some naturalists. If this be correct,
there can be little doubt that they were gained, or at least modified
through sexual selection; but how far the same view may be extended to
other mammals is doubtful.”
The astonishing colors in some of the monkeys cannot be passed over
without comment.
“In the beautiful _Cercopithecus diana_, the head of the adult male is
of an intense black, whilst that of the female is dark gray; in the
former the fur between the thighs is of an elegant fawn-color, in the
latter it is paler.
“In the _Cercopithecus cynosurus_ and _griseoviridis_ one part of the
body, which is confined to the male sex, is of the most brilliant blue
or green, and contrasts strikingly with the naked skin on the hinder
part of the body, which is vivid red.
“Lastly, in the baboon family, the adult male of _Cynocephalus
hamadryas_ differs from the female not only by his immense mane, but
slightly in the color of the hair and of the naked callosities. In the
drill (_C. leucophæus_) the females and young are much paler-colored,
with less green, than the adult males. No other member in the whole
class of mammals is colored in so extraordinary a manner as the adult
male mandrill (_C. mormon_). The face at this age becomes of a fine
blue, with the ridge and tip of the nose of the most brilliant red.
According to some authors, the face is also marked with whitish stripes,
and is shaded in parts with black, but the colors appear to be variable.
On the forehead there is a crest of hair, and on the chin a yellow
beard. ‘Toutes les parties supérieures de leurs cuisses et le grand
espace nu de leurs fesses sont également colorés du rouge le plus vif,
avec un mélange de bleu qui ne manque réellement pas d’élégance.’ When
the animal is excited all the naked parts become much more vividly
tinted.”
Darwin sums up the evidence in regard to the differences in color
between the male and female in the following statement:—
“I have now given all the cases known to me of a difference in color
between the sexes of mammals. Some of these may be the result of
variations confined to one sex and transmitted to the same sex, without
any good being gained, and therefore without the aid of selection. We
have instances of this with our domesticated animals, as in the males of
certain cats being rusty-red, whilst the females are tortoise-shell
colored. Analogous cases occur in nature: Mr. Bartlett has seen many
black varieties of the jaguar, leopard, vulpine phalanger, and wombat;
and he is certain that all or nearly all these animals, were males. On
the other hand, with wolves, foxes, and apparently American squirrels,
both sexes are occasionally born black. Hence it is quite possible that
with some mammals a difference in color between the sexes, especially
when this is congenital, may simply be the result, without the aid of
selection, of the occurrence of one or more variations, which from the
first were sexually limited in their transmission. Nevertheless it is
improbable that the diversified, vivid, and contrasted colors of certain
quadrupeds, for instance, of the above monkeys and antelopes, can thus
be accounted for.”
Finally, the case of man must be considered from the point of view of
sexual selection, for Darwin claims that man has acquired a number of
his secondary sexual characters in this way. For instance, the beard is
an excellent case of a secondary sexual character. Darwin’s
interpretation is that the beard has been retained, or even developed,
through the selection by the females of those males that had this
outgrowth best developed. Conversely, the absence of hair on the face of
the female is supposed by Darwin to have been brought about by men
selecting those women having less hair on their faces. The greater
intellect, energy, courage, pugnacity, and size of man are the outcome
of the competition of the males with each other, since the individual
excelling in these qualities will be able to select the most desirable
wife, or wives, and it is assumed will, therefore, leave more
descendants. The standard of beauty has been kept up by men selecting
the most beautiful women in each generation (the fate of the other
married women is ignored), and this beauty is supposed to have been
transmitted primarily to their daughters, but also to their sons.
Although all these forms of selection are imagined to be acting in man,
either alternately or simultaneously, yet Darwin recognizes in man a
number of checks to the action of sexual selection: amongst savages, the
so-called communal marriages; second, infanticide, generally of the
young females, which appears in some races to be practised to an
astonishing degree; third, early betrothals; fourth, the holding of
women as slaves.
When we recall that selection to be effective can only be carried out
under very exacting conditions, we cannot but be appalled at the demands
made here on our credulity. The choice of the women has produced the
beard of man, the choice of man the absence of a beard in women; the
competition of the males with each other is leading at the same time to
the development of at least half a dozen qualities that are supposed to
be male specialities, and while all this is going on the results are
being checked sometimes by one means, sometimes by another. Moreover,
even this is not all that we are asked to accept, for there are several
other qualities of the male that are put down as secondary sexual
characters. For example, let us examine what Darwin has to say in regard
to the development of the voice, and of singing in man.
In man the vocal cords are about a third longer than in woman and his
voice deeper. Emasculation arrests the development of the vocal
apparatus, and the voice remains like that of a woman. This difference
between the sexes, Darwin thinks, is due probably to long-continued use
by the male “under the excitement of love, rage, and jealousy.” In other
words, an appeal is again made to the Lamarckian theory, and in this
case to explain the origin of an organ that conforms to all the
requirements of the secondary sexual characters.
“The capacity and love for singing, or music, though not a sexual
character in man,” in the sense of being confined to one sex, yet is
supposed to have arisen through sexual selection in the following way:
“Human song is generally admitted to be the basis or origin of
instrumental music. As neither the enjoyment nor the capacity of
producing musical notes are faculties of the least use to man in
reference to his daily habits of life, they must be ranked amongst the
most mysterious with which he is endowed.”
Man is supposed to have possessed this faculty of song from a very
remote time, and even the most savage races make musical sounds,
although we do not enjoy their music, or they ours.
“We see that the musical faculties, which are not wholly deficient in
any race, are capable of prompt and high development, for Hottentots and
Negroes have become excellent musicians, although in their native
countries they rarely practise anything that we should consider music.
Hence the capacity for high musical development, which the savage races
of man possess, may be due either to the practice by our semi-human
progenitors of some rude form of music, or simply to their having
acquired the proper vocal organs for a different purpose. But in this
latter case we must assume, as in the above instance of parrots, and as
seems to occur with many animals, that they already possessed some sense
of melody.”
Darwin sums up the evidence in the two following statements, the
insufficiency of which to explain the phenomena is I think only too
obvious: “All these facts in respect to music and impassioned speech
become intelligible to a certain extent, if we assume that musical tones
and rhythm were used by our half-human ancestors, during the season of
courtship, when animals of all kinds are excited not only by love, but
by the strong passions of jealousy, rivalry, and triumph. From the
deeply laid principle of inherited associations, musical tones in this
case would be likely to call up vaguely and indefinitely the strong
emotions of a long past age.” Thus the difficulty is shifted to the
shoulders of our long-lost savage ancestors; or even, in fact, to our
simian forefathers, as the following paragraph indicates:—
“As the males of several quadrumanous animals have their vocal organs
much more developed than in the females, and as a gibbon, one of the
anthropomorphous apes, pours forth a whole octave of musical notes and
may be said to sing, it appears probable that the progenitors of man,
either the males or females or both sexes, before acquiring the power of
expressing their mutual love in articulate language, endeavored to charm
each other with musical notes and rhythm. So little is known about the
use of the voice by the Quadrumana during the season of love, that we
have no means of judging whether the habit of singing was first acquired
by our male or female ancestors. Women are generally thought to possess
sweeter voices than men, and as far as this serves as any guide, we may
infer that they first acquired musical powers in order to attract the
other sex. But if so, this must have occurred long ago, before our
ancestors had become sufficiently human to treat and value their women
merely as useful slaves. The impassioned orator, bard, or musician, when
with his varied tones and cadences he excites the strongest emotions in
his hearers, little suspects that he uses the same means by which his
half-human ancestors long ago aroused each other’s ardent passions
during their courtship and rivalry.”
We have now examined in some detail the evidence that Darwin has brought
forward in support of his hypothesis of sexual selection. A running
comment has been made while considering the individual cases, but it may
be well to sum up the matter by briefly indicating the reasons why the
hypothesis seems incompetent to explain the facts.
General Criticism of the Theory of Sexual Selection
1. Some of the objections that apply to the theory of natural selection
apply also with equal force to the theory of sexual selection in so far
as the results in both cases are supposed to be the outcome of the
selection of individual, or fluctuating, variations. If these variations
appear in only a few individuals, their perpetuation is not possible,
since they will soon disappear through crossing. It would be, of course,
preposterous to suppose that at any one time only those few individuals
pair and leave descendants that have the secondary sexual characters
developed to the highest point, but if something of this sort does not
occur, the extreme of fluctuating variations cannot be maintained. Even
if half of the individuals are selected in each generation, the
accumulation of a variation in a given direction could not go very far.
The assumption, however, that only half of all the individuals that
reach maturity breed, and that all of these are chosen on account of the
special development of their secondary sexual characters, seems
preposterous. Furthermore, if it is assumed that the high development of
the new character appears in a large number of individuals, then it is
not improbable that its continued appearance might be accounted for
without bringing in, at all, the hypothesis of sexual selection.
2. But even supposing that the females select the most beautiful males,
then, since in the vast majority of higher animals the males and the
females are in equal numbers, the others will also be able to unite with
each other in pairs after this first selection has taken place. Nothing
will therefore be gained in the next generation. It is interesting to
see how Darwin attempts to meet this argument. He tries to show in the
case of birds, that there are always unpaired individuals, but since the
few facts that he has been able to collect show that there are as many
additional females as males, the argument proves too much. A few species
are polygamous, one male having a number of female birds; but on this
basis we can only account, at best, for the development through
competition of the organs of offence and defence used to keep away the
weaker males. Yet it is just amongst these birds that we often find the
ornamental characters well developed. In fact, since all the females in
such cases are selected, and since they will transmit the characters of
all the males, it is evident that the secondary sexual characters could
not be formed in the way imagined.
3. If the female fails to select only the more ornamental males, no
result will follow. It has not been shown that she is capable of making
such a choice, and in the lower forms particularly, it does not seem
probable that this is done. The argument that Darwin often employs,
namely, that unless she does select, the display of the males before her
is meaningless, is not to the point. So far as we can detect the “cause”
of the display of the male, it appears to be due to his own excitement;
and even if we go so far as to admit that the “purpose” is to attract
the other sex, it still does not in the least follow that the most
ornamental male is selected, and unless this occurs the display has no
bearing on the hypothesis of sexual selection.
4. The two forms of sexual selection, namely, competition of the males
with one another (really one form of natural selection), and the
selection of the most ornamental or gifted individuals, are both used by
Darwin to explain secondary sexual characters, the one for organs of
offence and defence, and the other for ornamental characters. If we
fully appreciate the difficulties that any theory of selection meets
with, we shall realize how extraordinarily complex the action must be,
when two such processes are carried out at the same time, or even during
alternating periods.
5. It has been objected to Darwin’s theory of sexual selection, that he
suddenly reverses its mode of action to explain those cases in which the
female is the stronger and more ornamented sex; but if, as Darwin shows,
the instincts of the male have also changed, and have become more like
those of the female, I can see no inherent difficulty in this way of
applying the theory. A much more serious objection, it seems to me, is
that the male is supposed to select the female for one set of
characteristics, and the female to select the male for another set. It
sounds a little strange to suppose that women have caused the beard of
man to develop by selecting the best-bearded individuals, and the
compliment has been returned by the males selecting the females that
have the least amount of beard. It is also assumed that the results of
the selection are transmitted to one sex only. Unless, in fact, the
character in question were from the beginning peculiar to only one sex
as to its inheritance, the two sexes might go on forever selecting at
cross-purposes, and the result would be nothing.
6. The development, or the presence, of the æsthetic feeling in the
selecting sex is not accounted for on the theory. There is just as much
need to explain why the females are gifted with an appreciation of the
beautiful, as that the beautiful colors develop in the males. Shall we
assume that still another process of selection is going on, as a result
of which those females are selected by the males that appreciate their
unusual beauty, or that those females whose taste has soared a little
higher than that of the average (a variation of this sort having
appeared) select males to correspond, and thus the two continue heaping
up the ornaments on one side and the appreciation of these ornaments on
the other? No doubt an interesting fiction could be built up along these
lines, but would any one believe it, and, if he did, could he prove it?
Darwin assumes that the appreciation on the part of the female is always
present, and he thus simplifies, in appearance, the problem, but he
leaves half of it unexplained.
7. It has been pointed out, that it is important to distinguish between
the possible excitement of the female by the display or antics of the
male, and the selection of the more beautiful or agile performer. Darwin
himself records a few cases, which plainly show that the more beautiful
is not always the more successful. It has also been suggested that the
battles of the males are sometimes sham performances, and even when they
are real, if the less vigorous do not remain to be destroyed but run
away, they live to find mates of their own. In fact, the conduct of the
males at the breeding season appears to be much more the outcome of
their own excitement than an attempt to attract the females.
8. There is another side to the question, the importance of which is so
great, that it is surprising that Darwin has not taken any notice of it.
If, in order to bring about, or even maintain, the results of sexual
selection, such a tremendous elimination of individuals must take place,
it is surprising that natural selection would not counteract this by
destroying those species in which a process, so useless for the welfare
of the species, is going on. It is curious that this has not been
realized by those who believe in both of these two hypotheses.
9. What has just been said applies also with almost equal force to the
development of such structures as the horns of deer, bison, antelopes,
and the brilliant colors of many insects and birds. If in nature,
competition between species takes place on the scale that the Darwinian
theory of natural selection postulates, such forms, if they are much
exposed, would be needlessly reduced in numbers in the process of
acquiring these structures. So many individuals would have been at such
a disadvantage in breeding, that if competition is as severe as the
theory of natural selection postulates, these species could hardly be
expected to compete successfully with other species in which sexual
selection was not taking place.
10. Darwin admits that, in certain cases, external conditions may have
acted directly to produce the colors in certain forms, and if these were
not injurious he thinks they might have become constant. Such cases are
left unexplained in the sense that they are not supposed to be
adaptations to anything in particular. That colors produced in this way
might afterward be found useful, irrespective of how they arose, is
admitted as one of the ways in which sexual differences may have arisen.
11. It is baffling to find Darwin resorting to the Lamarckian
explanation in those cases in which the improbability of the hypothesis
of sexual selection is manifest. If either principle is true, we should
expect it to apply to all phenomena of the same sort; yet Darwin makes
use of the Lamarckian principle, in the hypothesis of sexual selection,
only when difficulties arise.
12. In attempting to explain the development of the musical sense in
man, it is clear that the hypothesis of sexual selection fails to give a
satisfactory explanation. To suppose that the genius of a Beethoven or
of a Mozart could have been the result of a process of sexual selection
is too absurd to discuss. Neither the power of appreciation nor of
expression in music could possibly have been the outcome of such a
process, and it does not materially help the problem to refer it back to
a troop of monkeys making the woods hideous with their cries.
We come now to some of the special cases to which Darwin’s hypothesis
has been applied.
13. In one case at least, it is stated that a bird living on the ground
might have acquired the color of the upper surface of the body through
natural selection, while the under surface of the males of the same
species might have become ornamented through the action of sexual
selection. Thus in one and the same individual the two processes are
supposed to have been at work, and it does not lessen the difficulty
very much by supposing the two processes to have been carried out at
different times, because it is evident that what had been gained at one
time by one process might become lost while the color of certain parts
was being acquired through the other process.
14. Darwin points out that “the plumage of certain birds goes on
increasing in beauty during many years after they are fully mature,” as
in the peacock, and in some of the birds of paradise, and with the
plumes and crests of some herons. This is explained as possibly merely
the result of “continued growth.” The improbability of selection is
manifest in these cases, but if “continued growth” can accomplish this
much, why may not the whole process be also the outcome of such growth?
At any rate, whatever the explanation is, it is important to find a case
of a secondary sexual character that the hypothesis obviously is
insufficient to explain.
15. It is admitted in a number of cases, as in the stag for instance,
that, although the larynx of the male is enlarged, this is not, in all
probability, the outcome of sexual selection, but in other forms this
same enlargement is ascribed to the selection process.
16. It is admitted that in none of the highly colored British moths is
there much difference according to sex, although when a difference of
color is found in butterflies this is put down to the action of sexual
selection. If such wonderful colors as those of moths can arise without
the action of selection, why make a special explanation for those cases
in which this difference is associated with sex?
17. It is well known that birds sing at other times of the year than at
the breeding season, and an attempt is made to account for this in that
birds take pleasure in practising those instincts that they make use of
at other times, as the cat plays with the captive mouse. Does not this
suggest that, if they had certain instincts, they would be more likely
to employ them at the times when their vitality or excitement is at its
highest without regard to the way in which they have come by them?
18. The color of the iris of the eyes of many species of hornbills is
said to be an intense crimson in the males, and white in the females. In
the male condor the eye is yellowish brown, and in the female a bright
red. Darwin admits that it is doubtful if this difference is the result
of sexual selection, since in the latter case the lining of the mouth is
black in the males, and flesh-colored in the females, which does not
affect the external beauty. Yet if these colors were more extensive and
on the exterior, there can be little doubt that they would have been
explained as due to sexual selection.
19. When the females in certain species of birds differ more from each
other than they do from their respective males, the case is compared to
“those inexplicable ones, which occur independently of man’s selection
in certain sub-breeds of the game-fowl, in which the females are very
different, whilst the males can hardly be distinguished.” Here then is a
case of difference in color associated with sex, but not the outcome of
sexual selection.
20. The long hairs on the throat of the stag are said possibly to be of
use to him when hunted, since the dogs generally seize him by the
throat, “but it is not probable that the hairs were specially developed
for this purpose; otherwise the young and the females would have been
equally protected.” Here also is a sexual difference that can scarcely
be ascribed to selection.
Some cases of differences in color between the sexes “may be the result
of variations confined to one sex, and transmitted to the same sex
without any good being gained, and, therefore, without the aid of
selection. We have instances of this with our domesticated animals, as
in the males of certain cats being rusty-red while the females are
tortoise-shell colored. Analogous cases occur in nature: Mr. Bartlett
has seen many black varieties of the jaguar, leopard, vulpine phalanger,
and wombat; and he is certain that all or nearly all of these animals
were males.” If changes of this sort occur, associated with one sex, why
is there any need of a special explanation in other cases of difference?
* * * * *
In the light of the many difficulties that the theory of sexual
selection meets with, I think we shall be justified in rejecting it as
an explanation of the secondary sexual differences amongst animals.
Other attempts to explain these differences have been equally
unsuccessful. Thus Wallace accounts for them as due to the excessive
vigor of the male, but Darwin’s reply to Wallace appears to show that
this is not the cause of the difference. He points out that, while the
hypothesis might appear plausible in the case of color, it is not so
evident in the case of other secondary sexual characters, such, for
instance, as the musical apparatus of the males of certain insects, and
the difference in the size of the larynx of certain birds and mammals.
Darwin’s theory served to draw attention to a large number of most
interesting differences between the sexes, and, even if it prove to be a
fiction, it has done much good in bringing before us an array of
important facts in regard to differences in secondary sexual characters.
More than this I do not believe it has done. The theory meets with fatal
objections at every turn.
In a later chapter the question will be more fully discussed as to the
sense in which these secondary sexual differences may be looked upon as
adaptations.
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