History of the World/History of the World (Bureau Development, Inc.)(1992).BIN/dp/0107/01076.txt

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$Unique_ID{how01076}
$Pretitle{}
$Title{Descent Of Man, The
Chapter 8.1}
$Subtitle{}
$Author{Darwin, Charles}
$Affiliation{}
$Subject{males
females
male
female
sexual
selection
sexes
birds
animals
organs}
$Date{1874}
$Log{}
Title:       Descent Of Man, The
Book:        Part II: Sexual Selection
Author:      Darwin, Charles
Date:        1874

Chapter 8.1

     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 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 defense 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 offense or means of
defense against rivals, his gaudy coloring 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, while the males, living on flowers, have mouths
destitute of mandibles. ^433 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 weevilbeetles (Curculionidae) there is a great
difference between the male and the female in the length of the rostrum or
snout; ^434 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 ^435 that the male uses his strong beak in chiseling the larvae of
insects out of decayed wood, while 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.

[Footnote 433: Westwood, "Modern Class of Insects," vol ii, 1840, p. 541. For
the statement about Tanais, mentioned below, I am indebted to Fritz Muller.]

[Footnote 434: Kirby and Spence, "Introduction to Entomology," vol. iii, 1826,
p. 309.]

[Footnote 435: "Birds of New Zealand," 1872, p. 66.]

     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, coeteris paribus, the greatest number to inherit their
superiority; while 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. ^436

[Footnote 436: M. Perrier advances this case ("Revue Scientifique," Feb. 1,
1873, p. 865) as one fatal to the belief in sexual selection, 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 censory 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 offense and the
means of defense of the males for fighting with and driving away their rivals
- their courage and pugnacity - their various ornaments - their contrivances
for producing vocal or instrumental music - and their glands for emitting
odors, most of these latter structures serving only to allure or excite the
female.  It is clear that these characters are the result of sexual and not of
ordinary selection, since unarmed, unornamented, or unattractive males would
succeed equally well in the battle for life and in leaving a numerous progeny,
but for the presence of better endowed males.  We may infer that this would be
the case, because the females, which are unarmed and unornamented, are able to
survive and procreate their kind.  Secondary sexual characters of the kind
just referred to, will be fully discussed in the following chapters, as being
in many respects interesting, but especially as depending on the will, choice,
and rivalry of the individuals of either sex.  When we behold two males
fighting for the possession of the female, or several male birds displaying
their gorgeous plumage and performing strange antics before an assembled body
of females, we cannot doubt that, though led by instinct, they know what they
are about, and consciously exert their mental and bodily powers.

     Just as man can improve the breed of his game-cocks by the selection of
those birds which are victorious in the cock-pit, so it appears that the
strongest and most vigorous males, or those provided with the best weapons,
have prevailed under nature, and have led to the improvement of the natural
breed or species.  A slight degree of variability leading to some advantage,
however slight, in reiterated deadly contests would suffice for the work of
sexual selection; and it is certain that secondary sexual characters are
eminently variable.  Just as man can give beauty, according to his standard of
taste, to his male poultry, or more strictly can modify the beauty originally
acquired by the parent species, can give to the Sebright bantam a new and
elegant plumage, an erect and peculiar carriage - so it appears that female
birds in a state of nature have by a long selection of the more attractive
males added to their beauty or other attractive qualities.  No doubt this
implies powers of discrimination and taste on the part of the female, which
will at first appear extremely improbable; but by the facts to be adduced
hereafter, I hope to be able to show that the females actually have these
powers. When, however, it is said that the lower animals have a sense of
beauty, it must not be supposed that such sense is comparable with that of a
cultivated man, with his multiform and complex associated ideas.  A more just
comparison would be between the taste for the beautiful in animals, and that
in the lowest savages, who admire and deck themselves with any brilliant,
glittering, or curious object.

     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 sectual selection has played
an important part in the history of the organic world.  It is certain that
among 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.
^437 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. ^438 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 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.

[Footnote 437: J. A. Allen on the "Mammals and Winter Birds of Florida," Bull.
Comp. Zoology, Harvard College, p. 268.]

[Footnote 438: Even with those plants in which the sexes are separate, the
male flowers are generally mature before the female.  As first shown 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-fertilized.  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.]

     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 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. ^439 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.
^440 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.

[Footnote 439: 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-colored 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."]

[Footnote 440: 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 den Darwin'schen Lehre auf Bienen," "Verh. d.
V. Jahrg.," xxix, p. 45.]

     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 the actual observation. ^441 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.

[Footnote 441: 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.]

     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 6,878 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 proportion 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 few 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 while
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 while 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
shown 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 favorable to the action of
sexual selection.  Nevertheless many animals which are strictly monogamous,
especially birds, display strongly marked secondary sexual characters; while
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 color, 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. ^442 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 S.
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 ^443 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 North America is polygamous, but, except in his greater size and 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. ^444 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 among 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 color of certain patches of hair on their
shoulders. ^445 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 color. ^446 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.

[Footnote 442: On the Gorilla, Savage and Wyman, "Boston Journal of Nat.
Hist.," vol. v, 1845-47, p. 423.  On Cynocephalus, Brehm, "Illust.
Thierleben," B.i, 1864, s. 77.  On Mycetes, Rengger, "Naturgesch.: Saugethiere
von Paraguay," 1830, ss. 14, 20.  Cebus, Brehm, ibid, s. 108.]

[Footnote 443: 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 showing incidentally which species of antelopes are gregarious.]

[Footnote 444: Dr. Campbell, in "Proc. Zoolog. Soc.," 1869, p. 138.  See also
an interesting paper, by Lieut. Johnstone, in "Proc. Asiatic Soc. of Bengal,"
May, 1868.]

[Footnote 445: Dr. Gray, in "Annals and Mag. of Nat. Hist.," 1871, p. 302.]

[Footnote 446: See Dr. Dobson's excellent paper in "Proc. Zoolog. Soc.," 1873,
p. 241.]

     As I hear from Sir Andrew Smith, the lion in S. 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 among 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, ^447 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."

[Footnote 447: The Eared Seals, "American Naturalist," vol. iv, Jan., 1871.]

     Among 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. ^448 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.

[Footnote 448: "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, 182.  Montagu and Selby speak of the Black Grouse as
polygamous and of the Red Grouse as monogamous.]

     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; while the sexes of the monogamous
red grouse and ptarmigan differ very little.  In the Cursores, except among
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 among 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 colors."

     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 neighborhood, 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; ^449 and the male during the breeding
season differs conspicuously from the female.

[Footnote 449: Noel Humphreys, "River Gardens," 1857.]

     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 shown 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 afterward
from the greater destruction of the females; or whether it indirectly follows
from the practice of polygamy.