PRINCIPLES OF SEXUAL SELECTION.
Secondary sexual characters—Sexual selection—Manner of action—Excess of
males—Polygamy—The male alone generally modified through sexual
selection—Eagerness of the male—Variability of the male—Choice exerted
by the female—Sexual compared with natural selection—Inheritance, at
corresponding periods of life, at corresponding seasons of the year,
and as limited by sex—Relations between the several forms of
inheritance—Causes why one sex and the young are not modified through
sexual selection—Supplement on the proportional numbers of the two
sexes throughout the animal kingdom—The proportion of the sexes in
relation to natural selection.
With animals which have their sexes separated, the males necessarily
differ from the females in their organs of reproduction; and these are
the primary sexual characters. But the sexes often differ in what
Hunter has called secondary sexual characters, which are not directly
connected with the act of reproduction; for instance, the male
possesses certain organs of sense or locomotion, of which the female is
quite destitute, or has them more highly-developed, in order that he
may readily find or reach her; or again the male has special organs of
prehension for holding her securely. These latter organs, of infinitely
diversified kinds, graduate into those which are commonly ranked as
primary, and in some cases can hardly be distinguished from them; we
see instances of this in the complex appendages at the apex of the
abdomen in male insects. Unless indeed we confine the term “primary” to
the reproductive glands, it is scarcely possible to decide which ought
to be called primary and which secondary.
The female often differs from the male in having organs for the
nourishment or protection of her young, such as the mammary glands of
mammals, and the abdominal sacks of the marsupials. In some few cases
also the male possesses similar organs, which are wanting in the
female, such as the receptacles for the ova in certain male fishes, and
those temporarily developed in certain male frogs. The females of most
bees are provided with a special apparatus for collecting and carrying
pollen, and their ovipositor is modified into a sting for the defence
of the larvae and the community. Many similar cases could be given, but
they do not here concern us. There are, however, other sexual
differences quite unconnected with the primary reproductive organs, and
it is with these that we are more especially concerned—such as the
greater size, strength, and pugnacity of the male, his weapons of
offence or means of defence against rivals, his gaudy colouring and
various ornaments, his power of song, and other such characters.
Besides the primary and secondary sexual differences, such as the
foregoing, the males and females of some animals differ in structures
related to different habits of life, and not at all, or only
indirectly, to the reproductive functions. Thus the females of certain
flies (Culicidae and Tabanidae) are blood-suckers, whilst the males,
living on flowers, have mouths destitute of mandibles. (1. Westwood,
‘Modern Classification of Insects,’ vol. ii. 1840, p. 541. For the
statement about Tanais, mentioned below, I am indebted to Fritz
Muller.) The males of certain moths and of some crustaceans (e.g.
Tanais) have imperfect, closed mouths, and cannot feed. The
complemental males of certain Cirripedes live like epiphytic plants
either on the female or the hermaphrodite form, and are destitute of a
mouth and of prehensile limbs. In these cases it is the male which has
been modified, and has lost certain important organs, which the females
possess. In other cases it is the female which has lost such parts; for
instance, the female glow-worm is destitute of wings, as also are many
female moths, some of which never leave their cocoons. Many female
parasitic crustaceans have lost their natatory legs. In some
weevil-beetles (Curculionidae) there is a great difference between the
male and female in the length of the rostrum or snout (2. Kirby and
Spence, ‘Introduction to Entomology,’ vol. iii. 1826, p. 309.); but the
meaning of this and of many analogous differences, is not at all
understood. Differences of structure between the two sexes in relation
to different habits of life are generally confined to the lower
animals; but with some few birds the beak of the male differs from that
of the female. In the Huia of New Zealand the difference is wonderfully
great, and we hear from Dr. Buller (3. ‘Birds of New Zealand,’ 1872, p.
66.) that the male uses his strong beak in chiselling the larvae of
insects out of decayed wood, whilst the female probes the softer parts
with her far longer, much curved and pliant beak: and thus they
mutually aid each other. In most cases, differences of structure
between the sexes are more or less directly connected with the
propagation of the species: thus a female, which has to nourish a
multitude of ova, requires more food than the male, and consequently
requires special means for procuring it. A male animal, which lives for
a very short time, might lose its organs for procuring food through
disuse, without detriment; but he would retain his locomotive organs in
a perfect state, so that he might reach the female. The female, on the
other hand, might safely lose her organs for flying, swimming, or
walking, if she gradually acquired habits which rendered such powers
useless.
We are, however, here concerned only with sexual selection. This
depends on the advantage which certain individuals have over others of
the same sex and species solely in respect of reproduction. When, as in
the cases above mentioned, the two sexes differ in structure in
relation to different habits of life, they have no doubt been modified
through natural selection, and by inheritance limited to one and the
same sex. So again the primary sexual organs, and those for nourishing
or protecting the young, come under the same influence; for those
individuals which generated or nourished their offspring best, would
leave, ceteris paribus, the greatest number to inherit their
superiority; whilst those which generated or nourished their offspring
badly, would leave but few to inherit their weaker powers. As the male
has to find the female, he requires organs of sense and locomotion, but
if these organs are necessary for the other purposes of life, as is
generally the case, they will have been developed through natural
selection. When the male has found the female, he sometimes absolutely
requires prehensile organs to hold her; thus Dr. Wallace informs me
that the males of certain moths cannot unite with the females if their
tarsi or feet are broken. The males of many oceanic crustaceans, when
adult, have their legs and antennae modified in an extraordinary manner
for the prehension of the female; hence we may suspect that it is
because these animals are washed about by the waves of the open sea,
that they require these organs in order to propagate their kind, and if
so, their development has been the result of ordinary or natural
selection. Some animals extremely low in the scale have been modified
for this same purpose; thus the males of certain parasitic worms, when
fully grown, have the lower surface of the terminal part of their
bodies roughened like a rasp, and with this they coil round and
permanently hold the females. (4. M. Perrier advances this case (‘Revue
Scientifique,’ Feb. 1, 1873, p. 865) as one fatal to the belief in
sexual election, inasmuch as he supposes that I attribute all the
differences between the sexes to sexual selection. This distinguished
naturalist, therefore, like so many other Frenchmen, has not taken the
trouble to understand even the first principles of sexual selection. An
English naturalist insists that the claspers of certain male animals
could not have been developed through the choice of the female! Had I
not met with this remark, I should not have thought it possible for any
one to have read this chapter and to have imagined that I maintain that
the choice of the female had anything to do with the development of the
prehensile organs in the male.)
When the two sexes follow exactly the same habits of life, and the male
has the sensory or locomotive organs more highly developed than those
of the female, it may be that the perfection of these is indispensable
to the male for finding the female; but in the vast majority of cases,
they serve only to give one male an advantage over another, for with
sufficient time, the less well-endowed males would succeed in pairing
with the females; and judging from the structure of the female, they
would be in all other respects equally well adapted for their ordinary
habits of life. Since in such cases the males have acquired their
present structure, not from being better fitted to survive in the
struggle for existence, but from having gained an advantage over other
males, and from having transmitted this advantage to their male
offspring alone, sexual selection must here have come into action. It
was the importance of this distinction which led me to designate this
form of selection as Sexual Selection. So again, if the chief service
rendered to the male by his prehensile organs is to prevent the escape
of the female before the arrival of other males, or when assaulted by
them, these organs will have been perfected through sexual selection,
that is by the advantage acquired by certain individuals over their
rivals. But in most cases of this kind it is impossible to distinguish
between the effects of natural and sexual selection. Whole chapters
could be filled with details on the differences between the sexes in
their sensory, locomotive, and prehensile organs. As, however, these
structures are not more interesting than others adapted for the
ordinary purposes of life I shall pass them over almost entirely,
giving only a few instances under each class.
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 odours, 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.
Just as man can improve the breeds 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 shew 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.
From our ignorance on several points, the precise manner in which
sexual selection acts is somewhat uncertain. Nevertheless if those
naturalists who already believe in the mutability of species, will read
the following chapters, they will, I think, agree with me, that sexual
selection has played an important part in the history of the organic
world. It is certain that amongst almost all animals there is a
struggle between the males for the possession of the female. This fact
is so notorious that it would be superfluous to give instances. Hence
the females have the opportunity of selecting one out of several males,
on the supposition that their mental capacity suffices for the exertion
of a choice. In many cases special circumstances tend to make the
struggle between the males particularly severe. Thus the males of our
migratory birds generally arrive at their places of breeding before the
females, so that many males are ready to contend for each female. I am
informed by Mr. Jenner Weir, that the bird-catchers assert that this is
invariably the case with the nightingale and blackcap, and with respect
to the latter he can himself confirm the statement.
Mr. Swaysland of Brighton has been in the habit, during the last forty
years, of catching our migratory birds on their first arrival, and he
has never known the females of any species to arrive before their
males. During one spring he shot thirty-nine males of Ray’s wagtail
(Budytes Raii) before he saw a single female. Mr. Gould has ascertained
by the dissection of those snipes which arrive the first in this
country, that the males come before the females. And the like holds
good with most of the migratory birds of the United States. (5. J.A.
Allen, on the ‘Mammals and Winter Birds of Florida,’ Bulletin of
Comparative Zoology, Harvard College, p. 268.) The majority of the male
salmon in our rivers, on coming up from the sea, are ready to breed
before the females. So it appears to be with frogs and toads.
Throughout the great class of insects the males almost always are the
first to emerge from the pupal state, so that they generally abound for
a time before any females can be seen. (6. Even with those plants in
which the sexes are separate, the male flowers are generally mature
before the female. As first shewn by C.K. Sprengel, many hermaphrodite
plants are dichogamous; that is, their male and female organs are not
ready at the same time, so that they cannot be self-fertilised. Now in
such flowers, the pollen is in general matured before the stigma,
though there are exceptional cases in which the female organs are
beforehand.) The cause of this difference between the males and females
in their periods of arrival and maturity is sufficiently obvious. Those
males which annually first migrated into any country, or which in the
spring were first ready to breed, or were the most eager, would leave
the largest number of offspring; and these would tend to inherit
similar instincts and constitutions. It must be borne in mind that it
would have been impossible to change very materially the time of sexual
maturity in the females, without at the same time interfering with the
period of the production of the young—a period which must be determined
by the seasons of the year. On the whole there can be no doubt that
with almost all animals, in which the sexes are separate, there is a
constantly recurrent struggle between the males for the possession of
the females.
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. (7. Here is
excellent evidence on the character of the offspring from an
experienced ornithologist. Mr. J.A. Allen, in speaking (‘Mammals and
Winter Birds of E. Florida,’ p. 229) of the later broods, after the
accidental destruction of the first, says, that these “are found to be
smaller and paler-coloured than those hatched earlier in the season. In
cases where several broods are reared each year, as a general rule the
birds of the earlier broods seem in all respects the most perfect and
vigorous.”) 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. (8. Hermann Müller has come to this same conclusion
with respect to those female bees which are the first to emerge from
the pupa each year. See his remarkable essay, ‘Anwendung der
Darwin’schen Lehre auf Bienen,’ ‘Verh. d. V. Jahrg.’ xxix. p. 45.) 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.
But in very many cases the males which conquer their rivals, do not
obtain possession of the females, independently of the choice of the
latter. The courtship of animals is by no means so simple and short an
affair as might be thought. The females are most excited by, or prefer
pairing with, the more ornamented males, or those which are the best
songsters, or play the best antics; but it is obviously probable that
they would at the same time prefer the more vigorous and lively males,
and this has in some cases been confirmed by actual observation. (9.
With respect to poultry, I have received information, hereafter to be
given, to this effect. Even with birds, such as pigeons, which pair for
life, the female, as I hear from Mr. Jenner Weir, will desert her mate
if he is injured or grows weak.) Thus the more vigorous females, which
are the first to breed, will have the choice of many males; and though
they may not always select the strongest or best armed, they will
select those which are vigorous and well armed, and in other respects
the most attractive. Both sexes, therefore, of such early pairs would
as above explained, have an advantage over others in rearing offspring;
and this apparently has sufficed during a long course of generations to
add not only to the strength and fighting powers of the males, but
likewise to their various ornaments or other attractions.
In the converse and much rarer case of the males selecting particular
females, it is plain that those which were the most vigorous and had
conquered others, would have the freest choice; and it is almost
certain that they would select vigorous as well as attractive females.
Such pairs would have an advantage in rearing offspring, more
especially if the male had the power to defend the female during the
pairing-season as occurs with some of the higher animals, or aided her
in providing for the young. The same principles would apply if each sex
preferred and selected certain individuals of the opposite sex;
supposing that they selected not only the more attractive, but likewise
the more vigorous individuals.
NUMERICAL PROPORTION OF THE TWO SEXES.
I have remarked that sexual selection would be a simple affair if the
males were considerably more numerous than the females. Hence I was led
to investigate, as far as I could, the proportions between the two
sexes of as many animals as possible; but the materials are scanty. I
will here give only a brief abstract of the results, retaining the
details for a supplementary discussion, so as not to interfere with the
course of my argument. Domesticated animals alone afford the means of
ascertaining the proportional numbers at birth; but no records have
been specially kept for this purpose. By indirect means, however, I
have collected a considerable body of statistics, from which it appears
that with most of our domestic animals the sexes are nearly equal at
birth. Thus 25,560 births of race-horses have been recorded during
twenty-one years, and the male births were to the female births as 99.7
to 100. In greyhounds the inequality is greater than with any other
animal, for out of 6878 births during twelve years, the male births
were to the female as 110.1 to 100. It is, however, in some degree
doubtful whether it is safe to infer that the proportion would be the
same under natural conditions as under domestication; for slight and
unknown differences in the conditions affect the proportion of the
sexes. Thus with mankind, the male births in England are as 104.5, in
Russia as 108.9, and with the Jews of Livonia as 120, to 100 female
births. But I shall recur to this curious point of the excess of male
births in the supplement to this chapter. At the Cape of Good Hope,
however, male children of European extraction have been born during
several years in the proportion of between 90 and 99 to 100 female
children.
For our present purpose we are concerned with the proportions of the
sexes, not only at birth, but also at maturity, and this adds another
element of doubt; for it is a well-ascertained fact that with man the
number of males dying before or during birth, and during the first two
years of infancy, is considerably larger than that of females. So it
almost certainly is with male lambs, and probably with some other
animals. The males of some species kill one another by fighting; or
they drive one another about until they become greatly emaciated. They
must also be often exposed to various dangers, whilst wandering about
in eager search for the females. In many kinds of fish the males are
much smaller than the females, and they are believed often to be
devoured by the latter, or by other fishes. The females of some birds
appear to die earlier than the males; they are also liable to be
destroyed on their nests, or whilst in charge of their young. With
insects the female larvae are often larger than those of the males, and
would consequently be more likely to be devoured. In some cases the
mature females are less active and less rapid in their movements than
the males, and could not escape so well from danger. Hence, with
animals in a state of nature, we must rely on mere estimation, in order
to judge of the proportions of the sexes at maturity; and this is but
little trustworthy, except when the inequality is strongly marked.
Nevertheless, as far as a judgment can be formed, we may conclude from
the facts given in the supplement, that the males of some few mammals,
of many birds, of some fish and insects, are considerably more numerous
than the females.
The proportion between the sexes fluctuates slightly during successive
years: thus with race-horses, for every 100 mares born the stallions
varied from 107.1 in one year to 92.6 in another year, and with
greyhounds from 116.3 to 95.3. But had larger numbers been tabulated
throughout an area more extensive than England, these fluctuations
would probably have disappeared; and such as they are, would hardly
suffice to lead to effective sexual selection in a state of nature.
Nevertheless, in the cases of some few wild animals, as shewn in the
supplement, the proportions seem to fluctuate either during different
seasons or in different localities in a sufficient degree to lead to
such selection. For it should be observed that any advantage, gained
during certain years or in certain localities by those males which were
able to conquer their rivals, or were the most attractive to the
females, would probably be transmitted to the offspring, and would not
subsequently be eliminated. During the succeeding seasons, when, from
the equality of the sexes, every male was able to procure a female, the
stronger or more attractive males previously produced would still have
at least as good a chance of leaving offspring as the weaker or less
attractive.
POLYGAMY.
The practice of polygamy leads to the same results as would follow from
an actual inequality in the number of the sexes; for if each male
secures two or more females, many males cannot pair; and the latter
assuredly will be the weaker or less attractive individuals. Many
mammals and some few birds are polygamous, but with animals belonging
to the lower classes I have found no evidence of this habit. The
intellectual powers of such animals are, perhaps, not sufficient to
lead them to collect and guard a harem of females. That some relation
exists between polygamy and the development of secondary sexual
characters, appears nearly certain; and this supports the view that a
numerical preponderance of males would be eminently favourable to the
action of sexual selection. Nevertheless many animals, which are
strictly monogamous, especially birds, display strongly-marked
secondary sexual characters; whilst some few animals, which are
polygamous, do not have such characters.
We will first briefly run through the mammals, and then turn to birds.
The gorilla seems to be polygamous, and the male differs considerably
from the female; so it is with some baboons, which live in herds
containing twice as many adult females as males. In South America the
Mycetes caraya presents well-marked sexual differences, in colour,
beard, and vocal organs; and the male generally lives with two or three
wives: the male of the Cebus capucinus differs somewhat from the
female, and appears to be polygamous. (10. On the Gorilla, Savage and
Wyman, ‘Boston Journal of Natural History,’ vol. v. 1845-47, p. 423. On
Cynocephalus, Brehm, ‘Thierleben,’ B. i. 1864, s. 77. On Mycetes,
Rengger, ‘Naturgeschichte der Säugethiere von Paraguay,’ 1830, ss. 14,
20. On Cebus, Brehm, ibid. s. 108.) Little is known on this head with
respect to most other monkeys, but some species are strictly
monogamous. The ruminants are eminently polygamous, and they present
sexual differences more frequently than almost any other group of
mammals; this holds good, especially in their weapons, but also in
other characters. Most deer, cattle, and sheep are polygamous; as are
most antelopes, though some are monogamous. Sir Andrew Smith, in
speaking of the antelopes of South Africa, says that in herds of about
a dozen there was rarely more than one mature male. The Asiatic
Antilope saiga appears to be the most inordinate polygamist in the
world; for Pallas (11. Pallas, ‘Spicilegia Zoolog., Fasc.’ xii. 1777,
p. 29. Sir Andrew Smith, ‘Illustrations of the Zoology of S. Africa,’
1849, pl. 29, on the Kobus. Owen, in his ‘Anatomy of Vertebrates’ (vol.
iii. 1868, p. 633) gives a table shewing incidentally which species of
antelopes are gregarious.) states that the male drives away all rivals,
and collects a herd of about a hundred females and kids together; the
female is hornless and has softer hair, but does not otherwise differ
much from the male. The wild horse of the Falkland Islands and of the
Western States of N. America is polygamous, but, except in his greater
size and in the proportions of his body, differs but little from the
mare. The wild boar presents well-marked sexual characters, in his
great tusks and some other points. In Europe and in India he leads a
solitary life, except during the breeding-season; but as is believed by
Sir W. Elliot, who has had many opportunities in India of observing
this animal, he consorts at this season with several females. Whether
this holds good in Europe is doubtful, but it is supported by some
evidence. The adult male Indian elephant, like the boar, passes much of
his time in solitude; but as Dr. Campbell states, when with others, “It
is rare to find more than one male with a whole herd of females”; the
larger males expelling or killing the smaller and weaker ones. The male
differs from the female in his immense tusks, greater size, strength,
and endurance; so great is the difference in these respects that the
males when caught are valued at one-fifth more than the females. (12.
Dr. Campbell, in ‘Proc. Zoolog. Soc.’ 1869, p. 138. See also an
interesting paper by Lieut. Johnstone, in ‘Proceedings, Asiatic Society
of Bengal,’ May 1868.) The sexes of other pachydermatous animals differ
very little or not at all, and, as far as known, they are not
polygamists. Nor have I heard of any species in the Orders of
Cheiroptera, Edentata, Insectivora and Rodents being polygamous,
excepting that amongst the Rodents, the common rat, according to some
rat-catchers, lives with several females. Nevertheless the two sexes of
some sloths (Edentata) differ in the character and colour of certain
patches of hair on their shoulders. (13. Dr. Gray, in ‘Annals and
Magazine of Natural History,’ 1871, p. 302.) And many kinds of bats
(Cheiroptera) present well-marked sexual differences, chiefly in the
males possessing odoriferous glands and pouches, and by their being of
a lighter colour. (14. See Dr. Dobson’s excellent paper in ‘Proceedings
of the Zoological Society,’ 1873, p. 241.) In the great order of
Rodents, as far as I can learn, the sexes rarely differ, and when they
do so, it is but slightly in the tint of the fur.
As I hear from Sir Andrew Smith, the lion in South Africa sometimes
lives with a single female, but generally with more, and, in one case,
was found with as many as five females; so that he is polygamous. As
far as I can discover, he is the only polygamist amongst all the
terrestrial Carnivora, and he alone presents well-marked sexual
characters. If, however, we turn to the marine Carnivora, as we shall
hereafter see, the case is widely different; for many species of seals
offer extraordinary sexual differences, and they are eminently
polygamous. Thus, according to Peron, the male sea-elephant of the
Southern Ocean always possesses several females, and the sea-lion of
Forster is said to be surrounded by from twenty to thirty females. In
the North, the male sea-bear of Steller is accompanied by even a
greater number of females. It is an interesting fact, as Dr. Gill
remarks (15. ‘The Eared Seals,’ American Naturalist, vol. iv. Jan.
1871.), that in the monogamous species, “or those living in small
communities, there is little difference in size between the males and
females; in the social species, or rather those of which the males have
harems, the males are vastly larger than the females.”
Amongst birds, many species, the sexes of which differ greatly from
each other, are certainly monogamous. In Great Britain we see
well-marked sexual differences, for instance, in the wild-duck which
pairs with a single female, the common blackbird, and the bullfinch
which is said to pair for life. I am informed by Mr. Wallace that the
like is true of the Chatterers or Cotingidae of South America, and of
many other birds. In several groups I have not been able to discover
whether the species are polygamous or monogamous. Lesson says that
birds of paradise, so remarkable for their sexual differences, are
polygamous, but Mr. Wallace doubts whether he had sufficient evidence.
Mr. Salvin tells me he has been led to believe that humming-birds are
polygamous. The male widow-bird, remarkable for his caudal plumes,
certainly seems to be a polygamist. (16. ‘The Ibis,’ vol. iii. 1861, p.
133, on the Progne Widow-bird. See also on the Vidua axillaris, ibid.
vol. ii. 1860, p. 211. On the polygamy of the Capercailzie and Great
Bustard, see L. Lloyd, ‘Game Birds of Sweden,’ 1867, pp. 19, and 182.
Montagu and Selby speak of the Black Grouse as polygamous and of the
Red Grouse as monogamous.) I have been assured by Mr. Jenner Weir and
by others, that it is somewhat common for three starlings to frequent
the same nest; but whether this is a case of polygamy or polyandry has
not been ascertained.
The Gallinaceae exhibit almost as strongly marked sexual differences as
birds of paradise or humming-birds, and many of the species are, as is
well known, polygamous; others being strictly monogamous. What a
contrast is presented between the sexes of the polygamous peacock or
pheasant, and the monogamous guinea-fowl or partridge! Many similar
cases could be given, as in the grouse tribe, in which the males of the
polygamous capercailzie and black-cock differ greatly from the females;
whilst the sexes of the monogamous red grouse and ptarmigan differ very
little. In the Cursores, except amongst the bustards, few species offer
strongly-marked sexual differences, and the great bustard (Otis tarda)
is said to be polygamous. With the Grallatores, extremely few species
differ sexually, but the ruff (Machetes pugnax) affords a marked
exception, and this species is believed by Montagu to be a polygamist.
Hence it appears that amongst birds there often exists a close relation
between polygamy and the development of strongly-marked sexual
differences. I asked Mr. Bartlett, of the Zoological Gardens, who has
had very large experience with birds, whether the male tragopan (one of
the Gallinaceae) was polygamous, and I was struck by his answering, “I
do not know, but should think so from his splendid colours.”
It deserves notice that the instinct of pairing with a single female is
easily lost under domestication. The wild-duck is strictly monogamous,
the domestic-duck highly polygamous. The Rev. W.D. Fox informs me that
out of some half-tamed wild-ducks, on a large pond in his
neighbourhood, so many mallards were shot by the gamekeeper that only
one was left for every seven or eight females; yet unusually large
broods were reared. The guinea-fowl is strictly monogamous; but Mr. Fox
finds that his birds succeed best when he keeps one cock to two or
three hens. Canary-birds pair in a state of nature, but the breeders in
England successfully put one male to four or five females. I have
noticed these cases, as rendering it probable that wild monogamous
species might readily become either temporarily or permanently
polygamous.
Too little is known of the habits of reptiles and fishes to enable us
to speak of their marriage arrangements. The stickle-back
(Gasterosteus), however, is said to be a polygamist (17. Noel
Humphreys, ‘River Gardens,’ 1857.); and the male during the
breeding-season differs conspicuously from the female.
To sum up on the means through which, as far as we can judge, sexual
selection has led to the development of secondary sexual characters. It
has been shewn that the largest number of vigorous offspring will be
reared from the pairing of the strongest and best-armed males,
victorious in contests over other males, with the most vigorous and
best-nourished females, which are the first to breed in the spring. If
such females select the more attractive, and at the same time vigorous
males, they will rear a larger number of offspring than the retarded
females, which must pair with the less vigorous and less attractive
males. So it will be if the more vigorous males select the more
attractive and at the same time healthy and vigorous females; and this
will especially hold good if the male defends the female, and aids in
providing food for the young. The advantage thus gained by the more
vigorous pairs in rearing a larger number of offspring has apparently
sufficed to render sexual selection efficient. But a large numerical
preponderance of males over females will be still more efficient;
whether the preponderance is only occasional and local, or permanent;
whether it occurs at birth, or afterwards from the greater destruction
of the females; or whether it indirectly follows from the practice of
polygamy.
THE MALE GENERALLY MORE MODIFIED THAN THE FEMALE.
Throughout the animal kingdom, when the sexes differ in external
appearance, it is, with rare exceptions, the male which has been the
more modified; for, generally, the female retains a closer resemblance
to the young of her own species, and to other adult members of the same
group. The cause of this seems to lie in the males of almost all
animals having stronger passions than the females. Hence it is the
males that fight together and sedulously display their charms before
the females; and the victors transmit their superiority to their male
offspring. Why both sexes do not thus acquire the characters of their
fathers, will be considered hereafter. That the males of all mammals
eagerly pursue the females is notorious to every one. So it is with
birds; but many cock birds do not so much pursue the hen, as display
their plumage, perform strange antics, and pour forth their song in her
presence. The male in the few fish observed seems much more eager than
the female; and the same is true of alligators, and apparently of
Batrachians. Throughout the enormous class of insects, as Kirby
remarks, “the law is that the male shall seek the female.” (18. Kirby
and Spence, ‘Introduction to Entomology,’ vol. iii. 1826, p. 342.) Two
good authorities, Mr. Blackwall and Mr. C. Spence Bate, tell me that
the males of spiders and crustaceans are more active and more erratic
in their habits than the females. When the organs of sense or
locomotion are present in the one sex of insects and crustaceans and
absent in the other, or when, as is frequently the case, they are more
highly developed in the one than in the other, it is, as far as I can
discover, almost invariably the male which retains such organs, or has
them most developed; and this shews that the male is the more active
member in the courtship of the sexes. (19. One parasitic Hymenopterous
insect (Westwood, ‘Modern Class. of Insects,’ vol. ii. p. 160) forms an
exception to the rule, as the male has rudimentary wings, and never
quits the cell in which it is born, whilst the female has
well-developed wings. Audouin believes that the females of this species
are impregnated by the males which are born in the same cells with
them; but it is much more probable that the females visit other cells,
so that close inter-breeding is thus avoided. We shall hereafter meet
in various classes, with a few exceptional cases, in which the female,
instead of the male, is the seeker and wooer.)
The female, on the other hand, with the rarest exceptions, is less
eager than the male. As the illustrious Hunter (20. ‘Essays and
Observations,’ edited by Owen, vol. i. 1861, p. 194.) long ago
observed, she generally “requires to be courted;” she is coy, and may
often be seen endeavouring for a long time to escape from the male.
Every observer of the habits of animals will be able to call to mind
instances of this kind. It is shewn by various facts, given hereafter,
and by the results fairly attributable to sexual selection, that the
female, though comparatively passive, generally exerts some choice and
accepts one male in preference to others. Or she may accept, as
appearances would sometimes lead us to believe, not the male which is
the most attractive to her, but the one which is the least distasteful.
The exertion of some choice on the part of the female seems a law
almost as general as the eagerness of the male.
We are naturally led to enquire why the male, in so many and such
distinct classes, has become more eager than the female, so that he
searches for her, and plays the more active part in courtship. It would
be no advantage and some loss of power if each sex searched for the
other; but why should the male almost always be the seeker? The ovules
of plants after fertilisation have to be nourished for a time; hence
the pollen is necessarily brought to the female organs—being placed on
the stigma, by means of insects or the wind, or by the spontaneous
movements of the stamens; and in the Algae, etc., by the locomotive
power of the antherozooids. With lowly-organised aquatic animals,
permanently affixed to the same spot and having their sexes separate,
the male element is invariably brought to the female; and of this we
can see the reason, for even if the ova were detached before
fertilisation, and did not require subsequent nourishment or
protection, there would yet be greater difficulty in transporting them
than the male element, because, being larger than the latter, they are
produced in far smaller numbers. So that many of the lower animals are,
in this respect, analogous with plants. (21. Prof. Sachs (‘Lehrbuch der
Botanik,’ 1870, S. 633) in speaking of the male and female reproductive
cells, remarks, “verhält sich die eine bei der Vereinigung activ,...die
andere erscheint bei der Vereinigung passiv.”) The males of affixed and
aquatic animals having been led to emit their fertilising element in
this way, it is natural that any of their descendants, which rose in
the scale and became locomotive, should retain the same habit; and they
would approach the female as closely as possible, in order not to risk
the loss of the fertilising element in a long passage of it through the
water. With some few of the lower animals, the females alone are fixed,
and the males of these must be the seekers. 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.
The great eagerness of the males has thus indirectly led to their much
more frequently developing secondary sexual characters than the
females. But the development of such characters would be much aided, if
the males were more liable to vary than the females—as I concluded they
were—after a long study of domesticated animals. Von Nathusius, who has
had very wide experience, is strongly of the same opinion. (22.
‘Vorträge uber Viehzucht,’ 1872, p. 63.) Good evidence also in favour
of this conclusion can be produced by a comparison of the two sexes in
mankind. During the Novara Expedition (23. ‘Reise der Novara:
Anthropolog. Theil,’ 1867, ss. 216-269. The results were calculated by
Dr. Weisbach from measurements made by Drs. K. Scherzer and Schwarz. On
the greater variability of the males of domesticated animals, see my
‘Variation of Animals and Plants under Domestication,’ vol. ii. 1868,
p. 75.) a vast number of measurements was made of various parts of the
body in different races, and the men were found in almost every case to
present a greater range of variation than the women; but I shall have
to recur to this subject in a future chapter. Mr. J. Wood (24.
‘Proceedings of the Royal Society,’ vol. xvi. July 1868, pp. 519 and
524.), who has carefully attended to the variation of the muscles in
man, puts in italics the conclusion that “the greatest number of
abnormalities in each subject is found in the males.” He had previously
remarked that “altogether in 102 subjects, the varieties of redundancy
were found to be half as many again as in females, contrasting widely
with the greater frequency of deficiency in females before described.”
Professor Macalister likewise remarks (25. ‘Proc. Royal Irish Academy,’
vol. x. 1868, p. 123.) that variations in the muscles “are probably
more common in males than females.” Certain muscles which are not
normally present in mankind are also more frequently developed in the
male than in the female sex, although exceptions to this rule are said
to occur. Dr. Burt Wilder (26. ‘Massachusetts Medical Society,’ vol.
ii. No. 3, 1868, p. 9.) has tabulated the cases of 152 individuals with
supernumerary digits, of which 86 were males, and 39, or less than
half, females, the remaining 27 being of unknown sex. It should not,
however, be overlooked that women would more frequently endeavour to
conceal a deformity of this kind than men. Again, Dr. L. Meyer asserts
that the ears of man are more variable in form than those of a woman.
(27. ‘Archiv fur Path. Anat. und Phys.’ 1871, p. 488.) Lastly the
temperature is more variable in man than in woman. (28. The conclusions
recently arrived at by Dr. J. Stockton Hough, on the temperature of
man, are given in the ‘Pop. Sci. Review,’ Jan. 1st, 1874, p. 97.)
The cause of the greater general variability in the male sex, than in
the female is unknown, except in so far as secondary sexual characters
are extraordinarily variable, and are usually confined to the males;
and, as we shall presently see, this fact is, to a certain extent,
intelligible. 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 vigour of
the male during the season of love seems often to intensify his
colours, independently of any marked difference from the female. (29.
Prof. Mantegazza is inclined to believe (‘Lettera a Carlo Darwin,’
‘Archivio per l’Anthropologia,’ 1871, p. 306) that the bright colours,
common in so many male animals, are due to the presence and retention
by them of the spermatic fluid; but this can hardly be the case; for
many male birds, for instance young pheasants, become brightly coloured
in the autumn of their first year.) 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. (30. For mankind, see Dr. J. Stockton Hough, whose
conclusions are given in the ‘Popular Science Review,’ 1874, p. 97. See
Girard’s observations on the Lepidoptera, as given in the ‘Zoological
Record,’ 1869, p. 347.) 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.
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 shews that with a large number of birds inhabiting the northern
and southern United States, the specimens from the south are
darker-coloured 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 Agelaeus
phoeniceus the males have had their colours 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.
(31. ‘Mammals and Birds of E. Florida,’ pp. 234, 280, 295.)
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 colours, 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 coloured or otherwise ornamented, as well as
more powerful and pugnacious than the cocks; these characters being
transmitted to the female offspring alone.
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.
We shall hereafter see that many animals exist, of which neither sex is
brilliantly coloured or provided with special ornaments, and yet the
members of both sexes or of one alone have probably acquired simple
colours, such as white or black, through sexual selection. The absence
of bright tints or other ornaments may be the result of variations of
the right kind never having occurred, or of the animals themselves
having preferred plain black or white. Obscure tints have often been
developed through natural selection for the sake of protection, and the
acquirement through sexual selection of conspicuous colours, appears to
have been sometimes checked from the danger thus incurred. But in other
cases the males during long ages may have struggled together for the
possession of the females, and yet no effect will have been produced,
unless a larger number of offspring were left by the more successful
males to inherit their superiority, than by the less successful: and
this, as previously shewn, depends on many complex contingencies.
Sexual selection acts in a less rigorous manner than natural selection.
The latter produces its effects by the life or death at all ages of the
more or less successful individuals. Death, indeed, not rarely ensues
from the conflicts of rival males. But generally the less successful
male merely fails to obtain a female, or obtains a retarded and less
vigorous female later in the season, or, if polygamous, obtains fewer
females; so that they leave fewer, less vigorous, or no offspring. In
regard to structures acquired through ordinary or natural selection,
there is in most cases, as long as the conditions of life remain the
same, a limit to the amount of advantageous modification in relation to
certain special purposes; but in regard to structures adapted to make
one male victorious over another, either in fighting or in charming the
female, there is no definite limit to the amount of advantageous
modification; so that as long as the proper variations arise the work
of sexual selection will go on. This circumstance may partly account
for the frequent and extraordinary amount of variability presented by
secondary sexual characters. Nevertheless, natural selection will
determine that such characters shall not be acquired by the victorious
males, if they would be highly injurious, either by expending too much
of their vital powers, or by exposing them to any great danger. The
development, however, of certain structures—of the horns, for instance,
in certain stags—has been carried to a wonderful extreme; and in some
cases to an extreme which, as far as the general conditions of life are
concerned, must be slightly injurious to the male. From this fact we
learn that the advantages which favoured males derive from conquering
other males in battle or courtship, and thus leaving a numerous
progeny, are in the long run greater than those derived from rather
more perfect adaptation to their conditions of life. We shall further
see, and it could never have been anticipated, that the power to charm
the female has sometimes been more important than the power to conquer
other males in battle.
LAWS OF INHERITANCE.
In order to understand how sexual selection has acted on many animals
of many classes, and in the course of ages has produced a conspicuous
result, it is necessary to bear in mind the laws of inheritance, as far
as they are known. Two distinct elements are included under the term
“inheritance”—the transmission, and the development of characters; but
as these generally go together, the distinction is often overlooked. We
see this distinction in those characters which are transmitted through
the early years of life, but are developed only at maturity or during
old age. We see the same distinction more clearly with secondary sexual
characters, for these are transmitted through both sexes, though
developed in one alone. That they are present in both sexes, is
manifest when two species, having strongly-marked sexual characters,
are crossed, for each transmits the characters proper to its own male
and female sex to the hybrid offspring of either sex. The same fact is
likewise manifest, when characters proper to the male are occasionally
developed in the female when she grows old or becomes diseased, as, for
instance, when the common hen assumes the flowing tail-feathers,
hackles, comb, spurs, voice, and even pugnacity of the cock.
Conversely, the same thing is evident, more or less plainly, with
castrated males. Again, independently of old age or disease, characters
are occasionally transferred from the male to the female, as when, in
certain breeds of the fowl, spurs regularly appear in the young and
healthy females. But in truth they are simply developed in the female;
for in every breed each detail in the structure of the spur is
transmitted through the female to her male offspring. Many cases will
hereafter be given, where the female exhibits, more or less perfectly,
characters proper to the male, in whom they must have been first
developed, and then transferred to the female. The converse case of the
first development of characters in the female and of transference to
the male, is less frequent; it will therefore be well to give one
striking instance. With bees the pollen-collecting apparatus is used by
the female alone for gathering pollen for the larvae, yet in most of
the species it is partially developed in the males to whom it is quite
useless, and it is perfectly developed in the males of Bombus or the
humble-bee. (32. H. Muller, ‘Anwendung der Darwin’schen Lehre,’ etc.,
Verh. d. n. V. Jahrg., xxix. p. 42.) As not a single other
Hymenopterous insect, not even the wasp, which is closely allied to the
bee, is provided with a pollen-collecting apparatus, we have no grounds
for supposing that male bees primordially collected pollen as well as
the females; although we have some reason to suspect that male mammals
primordially suckled their young as well as the females. Lastly, in all
cases of reversion, characters are transmitted through two, three, or
many more generations, and are then developed under certain unknown
favourable conditions. This important distinction between transmission
and development will be best kept in mind by the aid of the hypothesis
of pangenesis. According to this hypothesis, every unit or cell of the
body throws off gemmules or undeveloped atoms, which are transmitted to
the offspring of both sexes, and are multiplied by self-division. They
may remain undeveloped during the early years of life or during
successive generations; and their development into units or cells, like
those from which they were derived, depends on their affinity for, and
union with other units or cells previously developed in the due order
of growth.
INHERITANCE AT CORRESPONDING PERIODS OF LIFE.
This tendency is well established. A new character, appearing in a
young animal, whether it lasts throughout life or is only transient,
will, in general, reappear in the offspring at the same age and last
for the same time. If, on the other hand, a new character appears at
maturity, or even during old age, it tends to reappear in the offspring
at the same advanced age. When deviations from this rule occur, the
transmitted characters much oftener appear before, than after the
corresponding age. As I have dwelt on this subject sufficiently in
another work (33. The ‘Variation of Animals and Plants under
Domestication,’ vol. ii. 1868, p. 75. In the last chapter but one, the
provisional hypothesis of pangenesis, above alluded to, is fully
explained.), I will here merely give two or three instances, for the
sake of recalling the subject to the reader’s mind. In several breeds
of the Fowl, the down-covered chickens, the young birds in their first
true plumage, and the adults differ greatly from one another, as well
as from their common parent-form, the Gallus bankiva; and these
characters are faithfully transmitted by each breed to their offspring
at the corresponding periods of life. For instance, the chickens of
spangled Hamburgs, whilst covered with down, have a few dark spots on
the head and rump, but are not striped longitudinally, as in many other
breeds; in their first true plumage, “they are beautifully pencilled,”
that is each feather is transversely marked by numerous dark bars; but
in their second plumage the feathers all become spangled or tipped with
a dark round spot. (34. These facts are given on the high authority of
a great breeder, Mr. Teebay; see Tegetmeier’s ‘Poultry Book,’ 1868, p.
158. On the characters of chickens of different breeds, and on the
breeds of the pigeon, alluded to in the following paragraph, see
‘Variation of Animals,’ etc., vol. i. pp. 160, 249; vol. ii. p. 77.)
Hence in this breed variations have occurred at, and been transmitted
to, three distinct periods of life. The Pigeon offers a more remarkable
case, because the aboriginal parent species does not undergo any change
of plumage with advancing age, excepting that at maturity the breast
becomes more iridescent; yet there are breeds which do not acquire
their characteristic colours until they have moulted two, three, or
four times; and these modifications of plumage are regularly
transmitted.
INHERITANCE AT CORRESPONDING SEASONS OF THE YEAR.
With animals in a state of nature, innumerable instances occur of
characters appearing periodically at different seasons. We see this in
the horns of the stag, and in the fur of Arctic animals which becomes
thick and white during the winter. Many birds acquire bright colours
and other decorations during the breeding-season alone. Pallas states
(35. ‘Novae species Quadrupedum e Glirium ordine,’ 1778, p. 7. On the
transmission of colour by the horse, see ‘Variation of Animals and
Plants under Domestication,’ vol. i. p. 51. Also vol. ii. p. 71, for a
general discussion on ‘Inheritance as limited by Sex.’), that in
Siberia domestic cattle and horses become lighter-coloured during the
winter; and I have myself observed, and heard of similar strongly
marked changes of colour, that is, from brownish cream-colour or
reddish-brown to a perfect white, in several ponies in England.
Although I do not know that this tendency to change the colour of the
coat during different seasons is transmitted, yet it probably is so, as
all shades of colour are strongly inherited by the horse. Nor is this
form of inheritance, as limited by the seasons, more remarkable than
its limitation by age or sex.
INHERITANCE AS LIMITED BY SEX. — The equal transmission of characters
to both sexes is the commonest form of inheritance, at least with those
animals which do not present strongly-marked sexual differences, and
indeed with many of these. But characters are somewhat commonly
transferred exclusively to that sex, in which they first appear. Ample
evidence on this head has been advanced in my work on ‘Variation under
Domestication,’ but a few instances may here be given. There are breeds
of the sheep and goat, in which the horns of the male differ greatly in
shape from those of the female; and these differences, acquired under
domestication, are regularly transmitted to the same sex. As a rule, it
is the females alone in cats which are tortoise-shell, the
corresponding colour in the males being rusty-red. With most breeds of
the fowl, the characters proper to each sex are transmitted to the same
sex alone. So general is this form of transmission that it is an
anomaly when variations in certain breeds are transmitted equally to
both sexes. There are also certain sub-breeds of the fowl in which the
males can hardly be distinguished from one another, whilst the females
differ considerably in colour. The sexes of the pigeon in the
parent-species do not differ in any external character; nevertheless,
in certain domesticated breeds the male is coloured differently from
the female. (36. Dr. Chapuis, ‘Le Pigeon Voyageur Belge,’ 1865, p. 87.
Boitard et Corbie, ‘Les Pigeons de Volière,’ etc., 1824, p. 173. See,
also, on similar differences in certain breeds at Modena, ‘Le
variazioni dei Colombi domestici,’ del Paolo Bonizzi, 1873.) The wattle
in the English Carrier pigeon, and the crop in the Pouter, are more
highly developed in the male than in the female; and although these
characters have been gained through long-continued selection by man,
the slight differences between the sexes are wholly due to the form of
inheritance which has prevailed; for they have arisen, not from, but
rather in opposition to, the wish of the breeder.
Most of our domestic races have been formed by the accumulation of many
slight variations; and as some of the successive steps have been
transmitted to one sex alone, and some to both sexes, we find in the
different breeds of the same species all gradations between great
sexual dissimilarity and complete similarity. Instances have already
been given with the breeds of the fowl and pigeon, and under nature
analogous cases are common. With animals under domestication, but
whether in nature I will not venture to say, one sex may lose
characters proper to it, and may thus come somewhat to resemble the
opposite sex; for instance, the males of some breeds of the fowl have
lost their masculine tail-plumes and hackles. On the other hand, the
differences between the sexes may be increased under domestication, as
with merino sheep, in which the ewes have lost their horns. Again,
characters proper to one sex may suddenly appear in the other sex; as
in those sub-breeds of the fowl in which the hens acquire spurs whilst
young; or, as in certain Polish sub-breeds, in which the females, as
there is reason to believe, originally acquired a crest, and
subsequently transferred it to the males. All these cases are
intelligible on the hypothesis of pangenesis; for they depend on the
gemmules of certain parts, although present in both sexes, becoming,
through the influence of domestication, either dormant or developed in
either sex.
There is one difficult question which it will be convenient to defer to
a future chapter; namely, whether a character at first developed in
both sexes, could through selection be limited in its development to
one sex alone. If, for instance, a breeder observed that some of his
pigeons (of which the characters are usually transferred in an equal
degree to both sexes) varied into pale blue, could he by long-continued
selection make a breed, in which the males alone should be of this
tint, whilst the females remained unchanged? I will here only say, that
this, though perhaps not impossible, would be extremely difficult; for
the natural result of breeding from the pale-blue males would be to
change the whole stock of both sexes to this tint. If, however,
variations of the desired tint appeared, which were from the first
limited in their development to the male sex, there would not be the
least difficulty in making a breed with the two sexes of a different
colour, as indeed has been effected with a Belgian breed, in which the
males alone are streaked with black. In a similar manner, if any
variation appeared in a female pigeon, which was from the first
sexually limited in its development to the females, it would be easy to
make a breed with the females alone thus characterised; but if the
variation was not thus originally limited, the process would be
extremely difficult, perhaps impossible. (37. Since the publication of
the first edition of this work, it has been highly satisfactory to me
to find the following remarks (the ‘Field,’ Sept. 1872) from so
experienced a breeder as Mr. Tegetmeier. After describing some curious
cases in pigeons, of the transmission of colour by one sex alone, and
the formation of a sub-breed with this character, he says: “It is a
singular circumstance that Mr. Darwin should have suggested the
possibility of modifying the sexual colours of birds by a course of
artificial selection. When he did so, he was in ignorance of these
facts that I have related; but it is remarkable how very closely he
suggested the right method of procedure.”)
ON THE RELATION BETWEEN THE PERIOD OF DEVELOPMENT OF A CHARACTER AND
ITS TRANSMISSION TO ONE SEX OR TO BOTH SEXES.
Why certain characters should be inherited by both sexes, and other
characters by one sex alone, namely by that sex in which the character
first appeared, is in most cases quite unknown. We cannot even
conjecture why with certain sub-breeds of the pigeon, black striae,
though transmitted through the female, should be developed in the male
alone, whilst every other character is equally transferred to both
sexes. Why, again, with cats, the tortoise-shell colour should, with
rare exceptions, be developed in the female alone. The very same
character, such as deficient or supernumerary digits, colour-blindness,
etc., may with mankind be inherited by the males alone of one family,
and in another family by the females alone, though in both cases
transmitted through the opposite as well as through the same sex. (38.
References are given in my ‘Variation of Animals and Plants under
Domestication,’ vol. ii. p. 72.) Although we are thus ignorant, the two
following rules seem often to hold good—that variations which first
appear in either sex at a late period of life, tend to be developed in
the same sex alone; whilst variations which first appear early in life
in either sex tend to be developed in both sexes. I am, however, far
from supposing that this is the sole determining cause. As I have not
elsewhere discussed this subject, and it has an important bearing on
sexual selection, I must here enter into lengthy and somewhat intricate
details.
It is in itself probable that any character appearing at an early age
would tend to be inherited equally by both sexes, for the sexes do not
differ much in constitution before the power of reproduction is gained.
On the other hand, after this power has been gained and the sexes have
come to differ in constitution, the gemmules (if I may again use the
language of pangenesis) which are cast off from each varying part in
the one sex would be much more likely to possess the proper affinities
for uniting with the tissues of the same sex, and thus becoming
developed, than with those of the opposite sex.
I was first led to infer that a relation of this kind exists, from the
fact that whenever and in whatever manner the adult male differs from
the adult female, he differs in the same manner from the young of both
sexes. The generality of this fact is quite remarkable: it holds good
with almost all mammals, birds, amphibians, and fishes; also with many
crustaceans, spiders, and some few insects, such as certain orthoptera
and libellulae. In all these cases the variations, through the
accumulation of which the male acquired his proper masculine
characters, must have occurred at a somewhat late period of life;
otherwise the young males would have been similarly characterised; and
conformably with our rule, the variations are transmitted to and
developed in the adult males alone. When, on the other hand, the adult
male closely resembles the young of both sexes (these, with rare
exceptions, being alike), he generally resembles the adult female; and
in most of these cases the variations through which the young and old
acquired their present characters, probably occurred, according to our
rule, during youth. But there is here room for doubt, for characters
are sometimes transferred to the offspring at an earlier age than that
at which they first appeared in the parents, so that the parents may
have varied when adult, and have transferred their characters to their
offspring whilst young. There are, moreover, many animals, in which the
two sexes closely resemble each other, and yet both differ from their
young: and here the characters of the adults must have been acquired
late in life; nevertheless, these characters, in apparent contradiction
to our rule, are transferred to both sexes. We must not however,
overlook the possibility or even probability of successive variations
of the same nature occurring, under exposure to similar conditions,
simultaneously in both sexes at a rather late period of life; and in
this case the variations would be transferred to the offspring of both
sexes at a corresponding late age; and there would then be no real
contradiction to the rule that variations occurring late in life are
transferred exclusively to the sex in which they first appeared. This
latter rule seems to hold true more generally than the second one,
namely, that variations which occur in either sex early in life tend to
be transferred to both sexes. As it was obviously impossible even to
estimate in how large a number of cases throughout the animal kingdom
these two propositions held good, it occurred to me to investigate some
striking or crucial instances, and to rely on the result.
An excellent case for investigation is afforded by the Deer family. In
all the species, but one, the horns are developed only in the males,
though certainly transmitted through the females, and capable of
abnormal development in them. In the reindeer, on the other hand, the
female is provided with horns; so that in this species, the horns
ought, according to our rule, to appear early in life, long before the
two sexes are mature and have come to differ much in constitution. In
all the other species the horns ought to appear later in life, which
would lead to their development in that sex alone, in which they first
appeared in the progenitor of the whole Family. Now in seven species,
belonging to distinct sections of the family and inhabiting different
regions, in which the stags alone bear horns, I find that the horns
first appear at periods, varying from nine months after birth in the
roebuck, to ten, twelve or even more months in the stags of the six
other and larger species. (39. I am much obliged to Mr. Cupples for
having made enquiries for me in regard to the Roebuck and Red Deer of
Scotland from Mr. Robertson, the experienced head-forester to the
Marquis of Breadalbane. In regard to Fallow-deer, I have to thank Mr.
Eyton and others for information. For the Cervus alces of N. America,
see ‘Land and Water,’ 1868, pp. 221 and 254; and for the C. Virginianus
and strongyloceros of the same continent, see J.D. Caton, in ‘Ottawa
Acad. of Nat. Sc.’ 1868, p. 13. For Cervus Eldi of Pegu, see Lieut.
Beaven, ‘Proccedings of the Zoological Society,’ 1867, p. 762.) But
with the reindeer the case is widely different; for, as I hear from
Prof. Nilsson, who kindly made special enquiries for me in Lapland, the
horns appear in the young animals within four or five weeks after
birth, and at the same time in both sexes. So that here we have a
structure, developed at a most unusually early age in one species of
the family, and likewise common to both sexes in this one species
alone.