The Descent of Man

Chapter VIII

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 Muller 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 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 Saugethiere
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, "verhalt 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. 'Vortrage 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 m