Descent of Man [1871]
Charles
Darwin
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Part Two - Sexual Selection
Chapter VIII - Principles of Sexual
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 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 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.* 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) 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) 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.
* Westwood, Modern Classification of Insects, vol. ii., 1840, p.
541. For the statement about Tanais, mentioned below, I am indebted to
Fritz Muller.
*(2) Kirby and Spence, Introduction to Entomology, vol. iii.,
1826, p. 309.
*(3) 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, 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.*
* 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 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 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 selections 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.* 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.*(2) 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.
* J. A. Allen, on the "Mammals and Winter Birds of Florida,"
Bulletin of Comparative Zoology, Harvard College, p. 268.
*(2) 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 before-hand.
Our difficulty in regard to sexual selection lies in understanding
how it is that the males which conquer other males, or those which
prove the most attractive to the females, leave a greater number of
offspring to inherit their superiority than their beaten and less
attractive rivals. Unless this result does follow, the characters
which give to certain males an advantage over others, could not be
perfected and augmented through sexual selection. When the sexes exist
in exactly equal numbers, the worst-endowed males will (except where
polygamy prevails), ultimately find females, and leave as many
offspring, as well fitted for their general habits of life, as the
best-endowed males. From various facts and considerations, I
formerly inferred that with most animals, in which secondary sexual
characters are well developed, the males considerably exceeded the
females in number; but this is not by any means always true. If the
males were to the females as two to one, or as three to two, or even
in a somewhat lower ratio, the whole affair would be simple; for the
better-armed or more attractive males would leave the largest number
of offspring. But after investigating, as far as possible, the
numerical proportion of the sexes, I do not believe that any great
inequality in number commonly exists. In most cases sexual selection
appears to have been effective in the following manner.
Let us take any species, a bird for instance, and divide the females
inhabiting a district into two equal bodies, the one consisting of the
more vigorous and better-nourished individuals, and the other of the
less vigorous and healthy. The former, there can be little doubt,
would be ready to breed in the spring before the others; and this is
the opinion of Mr. Jenner Weir, who has carefully attended to the
habits of birds during many years. There can also be no doubt that the
most vigorous, best-nourished and earliest breeders would on an
average succeed in rearing the largest number of fine offspring.*
The males, as we have seen, are generally ready to breed before the
females; the strongest, and with some species the best armed of the
males, drive away the weaker; and the former would then unite with the
more vigorous and better-nourished females, because they are the first
to breed.*(2) 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.
* 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."
*(2) 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
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.* 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.
* 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.
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 statisties, 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 present 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.* 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*(2) 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.*(3) 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.*(4) 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.*(5) 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.
* On the Gorilla, Savage and Wyman, Boston Journal of Natural
History, vol. v., 1845-47, p. 423. On Cynocephalus, Brehm,
Illustriertes Thierleben, B. i., 1864, s. 77. On Mycetes, Rengger,
Naturgeschichte der Saugethiere von Paraguay, 1830, ss. 14, 20. On
Cebus, Brehm, ibid., s. 108.
*(2) 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.
*(3) 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.
*(4) Dr. Gray, in Annals and Magazine of Natural History, 1871, p.
302.
*(5) See Dr. Dobson's excellent paper in Proceedings of the
Zoological Society, 1873, p. 241.
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,* 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."
* "The Eared Seals," American Naturalist, vol. iv., Jan. 1871.
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 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.* 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 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.
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 know, 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 neighborhood, so many mallards were shot by the game-keeper
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;* and the male
during the breeding-season differs conspicuously from the female.
* 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 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."*
Two good authorities, Mr. Blackwell 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.*(2)
* Kirby and Spence, Introduction to Entomology, vol. iii., 1826,
p. 342.
*(2) 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* 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 shown 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.
* Essays and Observations, edited by Owen, vol. i., 1861, p. 194.
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, &c., 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.* 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.
* 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 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.* Good evidence also in favour of this conclusion can be
produced by a comparison of the two sexes in mankind. During the
Novara expedition*(2) 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,*(3) 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*(4) 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*(5) 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.*(6) Lastly the temperature is more variable in man
than in woman.*(7)
* Vortrage uber Viehzucht, 1872, p. 63.
*(2) 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.
*(3) Proceedings of the Royal Society, vol. xvi., July, 1868, pp.
519 and 524.
*(4) Proc. Royal Irish Academy, vol. x., 1868, p. 123.
*(5) Mass. Medical Society, ii., No. 3, 1868, p. 9.
*(6) Archiv fur Path. Anat. und Phys., 1871, p. 488.
*(7) The conclusions recently arrived at by Dr. J. Stockton Hough,
on the temperature of man, are given in the Pop. Sci. Review, Jan.
1, 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,
&c.: 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.* 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.*(2) 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.
* 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.
*(2) 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.
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, &c., 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.*
* 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 brightly 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.* 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.
* H. Muller, "Anwendung der Darwin'schen Lehre, &c.," Verh. d. n. V.
Jahrg., xxix. p. 42.
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,*
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.*(2) Hence in this breed
variations have occurred at, and been transmitted to, three
distinct periods of life. The pigeon offers a more remarkable case,
because the aboriginal parent species does not undergo any change of
plumage with advancing age, excepting that at maturity the breast
becomes more iridescent; yet there are breeds which do not acquire
their characteristic colours until they have moulted two, three, or
four times; and these modifications of plumage are regularly
transmitted.
* 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.
*(2) 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, &c., vol. i., pp. 160, 249; vol. ii., p. 77.
Inheritance at corresponding Seasons of the Year.- With animals in a
state of nature, innumerable instances occur of characters appearing
periodically at different seasons. We see this in the horns of the
stag, and in the fur of arctic animals which becomes thick and white
during the winter. Many birds acquire bright colours and other
decorations during the breeding-season alone. Pallas states,* that
in Siberia domestic cattle and horses become lighter-coloured during
the winter; and I have myself observed, and heard of similar
strongly marked changes of colour, that is, from brownish cream-colour
or reddish-brown to a perfect white, in several ponies in England.
Although I do not know that this tendency to change the colour of
the coat during different seasons is transmitted, yet it probably is
so, as all shades of colour are strongly inherited by the horse. Nor
is this form of inheritance, as limited by the seasons, more
remarkable than its limitation by age or sex.
* Novae species Quadrupedum e Glirium ordine, 1778, p. 7. On the
transmission of colour by the horse, see Variation of Animals and
Plants under Domestication, vol. i., p. 51. Also vol. ii., p. 71,
for a general discussion on "Inheritance as limited by Sex."
Inheritance as Limited by Sex.- The equal transmission of characters
to both sexes is the commonest form of inheritance, at least with
those animals which do not present strongly-marked sexual differences,
and indeed with many of these. But characters are somewhat commonly
transferred exclusively to that sex, in which they first appear. Ample
evidence on this head has been advanced in my work on Variation
under Domestication, but a few instances may here be given. There
are breeds of the sheep and goat, in which the horns of the male
differ greatly in shape from those of the female; and these
differences, acquired under domestication, are regularly transmitted
to the same sex. As a rule, it is the females alone in cats which
are tortoise-shell, the corresponding colour in the males being
rusty-red. With most breeds of the fowl, the characters proper to each
sex are transmitted to the same sex alone. So general is this form
of transmission that it is an anomaly when variations in certain
breeds are transmitted equally to both sexes. There are also certain
sub-breeds of the fowl in which the males can hardly be
distinguished from one another, whilst the females differ considerably
in colour. The sexes of the pigeon in the parent-species do not differ
in any external character; nevertheless, in certain domesticated
breeds the male is coloured differently from the female.* The wattle
in the English carrier pigeon, and the crop in the pouter, are more
highly developed in the male than in the female; and although these
characters have been gained through long-continued selection by man,
the slight differences between the sexes are wholly due to the form of
inheritance which has prevailed; for they have arisen, not from, but
rather in opposition to, the wish of the breeder.
* Dr. Chapuis, Le Pigeon Voyageur Belge, 1865, p. 87. Boitard et
Corbie, Les Pigeons de Voliere, &c., 1824, p. 173. See, also, on
similar differences in certain breeds at Modena, Le variazioni dei
Colombi domestici, del Paolo Bonizzi, 1873.
Most of our domestic races have been formed by the accumulation of
many slight variations; and as some of the successive steps have
been transmitted to one sex alone, and some to both sexes, we find
in the different breeds of the same species all gradations between
great sexual dissimilarity and complete similarity. Instances have
already been given with the breeds of the fowl and pigeon, and under
nature analogous cases are common. With animals under domestication,
but whether in nature I will not venture to say, one sex may lose
characters proper to it, and may thus come somewhat to resemble the
opposite sex; for instance, the males of some breeds of the fowl
have lost their masculine tail-plumes and hackles. On the other
hand, the differences between the sexes may be increased under
domestication, as with merino sheep, in which the ewes have lost their
horns. Again, characters proper to one sex may suddenly appear in
the other sex; as in those sub-breeds of the fowl in which the hens
acquire spurs whilst young; or, as in certain Polish sub-breeds, in
which the females, as there is reason to believe, originally
acquired a crest, and subsequently transferred it to the males. All
these cases are intelligible on the hypothesis of pangenesis; for they
depend on the gemmules of certain parts, although present in both
sexes, becoming, through the influence of domestication, either
dormant or developed in either sex.
There is one difficult question which it will be convenient to defer
to a future chapter; namely, whether a character at first developed in
both sexes, could through selection be limited in its development to
one sex alone. If, for instance, a breeder observed that some of his
pigeons (of which the characters are usually transferred in an equal
degree to both sexes) varied into pale blue, could he by
long-continued selection make a breed, in which the males alone should
be of this tint, whilst the females remained unchanged? I will here
only say, that this, though perhaps not impossible, would be extremely
difficult; for the natural result of breeding from the pale-blue males
would be to change the whole stock of both sexes to this tint. If,
however, variations of the desired tint appeared, which were from
the first limited in their development to the male sex, there would
not be the least difficulty in making a breed with the two sexes of
a different colour, as indeed has been effected with a Belgian
breed, in which the males alone are streaked with black. In a
similar manner, if any variation appeared in a female pigeon, which
was from the first sexually limited in its development to the females,
it would be easy to make a breed with the females alone thus
characterised; but if the variation was not thus originally limited,
the process would be extremely difficult, perhaps impossible.*
* Since the publication of the first edition of this work, it has
been highly satisfactory to me to find the following remarks (the
Field, Sept., 1872) from so experienced a breeder as Mr. Tegetmeier.
After describing some curious cases in pigeons, of the transmission of
colour by one sex alone, and the formation of a sub-breed with this
character, he says: "It is a singular circumstance that Mr. Darwin
should have suggested the possibility of modifying the sexual
colours of birds by a course of artificial selection. When he did
so, he was in ignorance of these facts that I have related; but it
is remarkable how very closely he suggested the right method of
procedure."
On the Relation between the Period of Development of a Character and
its Transmission to one Sex or to both Sexes.- Why certain
characters should be inherited by both sexes, and other characters
by one sex alone, namely by that sex in which the character first
appeared, is in most cases quite unknown. We cannot even conjecture
why with certain sub-breeds of the pigeon, black striae, though
transmitted through the female, should be developed in the male alone,
whilst every other character is equally transferred to both sexes.
Why, again, with cats, the tortoise-shell colour should, with rare
exceptions, be developed in the female alone. The very same character,
such as deficient or super-numerary digits, colour-blindness, &c., may
with mankind be inherited by the males alone of one family, and in
another family by the females alone, though in both cases
transmitted through the opposite as well as through the same sex.*
Although we are thus ignorant, the two following rules seem often to
hold good- that variations which first appear in either sex at a
late period of life tend to be developed in the same sex alone; whilst
variations which first appear early in life in either sex tend to be
developed in both sexes. I am, however, far from supposing that this
is the sole determining cause. As I have not elsewhere discussed
this subject, and it has an important bearing on sexual selection, I
must here enter into lengthy and somewhat intricate details.
* References are given in my Variation of Animals and Plants under
Domestication, vol. ii., p. 72.
It is in itself probable that any character appearing at an early
age would tend to be inherited equally by both sexes, for the sexes do
not differ much in constitution before the power of reproduction is
gained. On the other hand, after this power has been gained and the
sexes have come to differ in constitution, the gemmules (if I may
again use the language of pangenesis) which are cast off from each
varying part in the one sex would be much more likely to possess the
proper affinities for uniting with the tissues of the same sex, and
thus becoming developed, than with those of the opposite sex.
I was first led to infer that a relation of this kind exists, from
the fact that whenever and in whatever manner the adult male differs
from the adult female, he differs in the same manner from the young of
both sexes. The generality of this fact is quite remarkable: it
holds good with almost all mammals, birds, amphibians, and fishes;
also with many crustaceans, spiders, and some few insects, such as
certain Orthoptera and Libellulae. In all these cases the
variations, through the accumulation of which the male acquired his
proper masculine characters, must have occurred at a somewhat late
period of life; otherwise the young males would have been similarly
characterised; and conformably with our rule, the variations are
transmitted to and developed in the adult males alone. When, on the
other hand, the adult male closely resembles the young of both sexes
(these, with rare exceptions, being alike), he generally resembles the
adult female; and in most of these cases the variations through
which the young and old acquired their present characters, probably
occurred, according to our rule, during youth. But there is here
room for doubt, for characters are sometimes transferred to the
offspring at an earlier age than that at which they first appeared
in the parents, so that the parents may have varied when adult, and
have transferred their characters to their offspring whilst young.
There are, moreover, many animals, in which the two sexes closely
resemble each other, and yet both differ from their young: and here
the characters of the adults must have been acquired late in life;
nevertheless, these characters, in apparent contradiction to our rule,
are transferred to both sexes. We must not however, overlook the
possibility or even probability of successive variations of the same
nature occurring, under exposure to similar conditions, simultaneously
in both sexes at a rather late period of life; and in this case the
variations would be transferred to the offspring of both sexes at a
corresponding late age; and there would then be no real
contradiction to the rule that variations occurring late in life are
transferred exclusively to the sex in which they first appeared.
This latter rule seems to hold true more generally than the second
one, namely, that variations which occur in either sex early in life
tend to be transferred to both sexes. As it was obviously impossible
even to estimate in how large a number of cases throughout the
animal kingdom these two propositions held good, it occurred to me
to investigate some striking or crucial instances, and to rely on
the result.
An excellent case for investigation is afforded by the deer
family. In all the species, but one, the horns are developed only in
the males, though certainly transmitted through the females, and
capable of abnormal development in them. In the reindeer, on the other
hand, the female is provided with horns; so that in this species,
the horns ought, according to our rule, to appear early in life,
long before the two sexes are mature and have come to differ much in
constitution. In all the other species the horns ought to appear later
in life, which would lead to their development in that sex alone, in
which they first appeared in the progenitor of the whole family. Now
in seven species, belonging to distinct sections of the family and
inhabiting different regions, in which the stags alone bear horns, I
find that the horns first appear at periods, varying from nine
months after birth in the roebuck to ten, twelve or even more months
in the stags of the six other and larger species.* But with the
reindeer the case is widely different; for, as I hear from Prof.
Nilsson, who kindly made enquiries for me in Lapland, the horns appear
in the young animals within four or five weeks after birth, and at the
same time in both sexes. So that here we have a structure, developed
at a most unusually early age in one species of the family, and
likewise common to both sexes in this one species alone.
* I am much obliged to Mr. Cupples for having made enquiries for
me in regard to the roebuck and red deer of Scotland from Mr.
Robertson, the experienced head-forester to the Marquis of
Breadalbane. In regard to fallow-deer, I have to thank Mr. Eyton and
others for information. For the Cervus alces of N. America, see Land
and Water, 1868, pp. 221 and 254; and for the C. Virginianus and C.
strongyloceros of the same continent, see J. D. Caton, in Ottawa Acad.
of Nat. Sc., 1868, p. 13. For Cervus eldi of Pegu, see Lieut.
Beaven, Proccedings of the Zoological Society, 1867, p. 762.
In several kinds of antelopes, only the males are provided with
horns, whilst in the greater number both sexes bear horns. With
respect to the period of development, Mr. Blyth informs me that
there was at one time in the Zoological Gardens a young koodoo (A.
strepsiceros), of which the males alone are horned, and also the young
of a closely-allied species, the eland (A. oreas), in which both sexes
are horned. Now it is in strict conformity with our rule, that in
the young male koodoo, although ten months old, the horns were
remarkably small, considering the size ultimately attained by them;
whilst in the young male eland, although only three months old, the
horns were already very much larger than in the koodoo. It is also a
noticeable fact that in the prong-horned antelope,* only a few of
the females, about one in five, have horns, and these are in a
rudimentary state, though sometimes above four inches long: so that as
far as concerns the possession of horns by the males alone, this
species is in an intermediate condition, and the horns do not appear
until about five or six months after birth. Therefore in comparison
with what little we know of the development of the horns in other
antelopes, and from what we do know with respect to the horns of deer,
cattle, &c., those of the prong-horned antelope appear at an
intermediate period of life,- that is, not very early, as in cattle
and sheep, nor very late, as in the larger deer and antelopes. The
horns of sheep, goats, and cattle, which are well developed in both
sexes, though not quite equal in size, can be felt, or even seen, at
birth or soon afterwards.*(2) Our rule, however, seems to fail in some
breeds of sheep, for instance merinos, in which the rams alone are
horned; for I cannot find on enquiry,*(3) that the horns are developed
later in life in this breed than in ordinary sheep in which both sexes
are horned. But with domesticated sheep the presence or absence of
horns is not a firmly fixed character; for a certain proportion of the
merino ewes bear small horns, and some of the rams are hornless; and
in most breeds hornless ewes are occasionally produced.
* Antilocapra Americana. I have to thank Dr. Canfield for
information with respect to the horns of the female: see also his
paper in Proceedings of the Zoological Society, 1866, p. 109. Also
Owen, Anatomy of Vertebrates, vol. iii., p. 627.
*(2) I have been assured that the horns of the sheep in north
Wales can always be felt, and are sometimes even an inch in length, at
birth. Youatt says (Cattle, 1834, p. 277), that the prominence of
the frontal bone in cattle penetrates the cutis at birth, and that the
horny matter is soon formed over it.
*(3) I am greatly indebted to Prof. Victor Carus for having made
enquiries for me, from the highest authorities, with respect to the
merino sheep of Saxony. On the Guinea coast of Africa there is,
however, a breed of sheep in which, as with merinos, the rams alone
bear horns; and Mr. Winwood Reade informs me that in one case observed
by him, a young ram, born on Feb. 10th, first shewed horns on March
6th, so that in this instance, in conformity with rule, the
development of the horns occurred at a later period of life than in
Welsh sheep, in which both sexes are horned.
Dr. W. Marshall has lately made a special study of the protuberances
so common on the heads of birds,* and he comes to the following
conclusion:- that with those species in which they are confined to the
males, they are developed late in life; whereas with those species
in which they are common to the two sexes, they are developed at a
very early period. This is certainly a striking confirmation of my two
laws of inheritance.
* "Uber die knochernen Schadelhocker der Vogel", in the
Niederland. Archiv fur Zoologie, B.i., Heft 2, 1872.
In most of the species of the splendid family of the pheasants,
the males differ conspicuously from the females, and they acquire
their ornaments at a rather late period of life. The eared pheasant
(Crossoptilon auritum), however, offers a remarkable exception, for
both sexes possess the fine caudal plumes, the large ear-tufts and the
crimson velvet about the head; I find that all these characters appear
very early in life in accordance with rule. The adult male can,
however, be distinguished from the adult female by the presence of
spurs; and conformably with our rule, these do not begin to be
developed before the age of six months, as I am assured by Mr.
Bartlett, and even at this age, the two sexes can hardly be
distinguished.* The male and female peacock differ conspicuously
from each other in almost every part of their plumage, except in the
elegant head-crest, which is common to both sexes; and this is
developed very early in life, long before the other ornaments, which
are confined to the male. The wild-duck offers an analogous case,
for the beautiful green speculum on the wings is common to both sexes,
though duller and somewhat smaller in the female, and it is
developed early in life, whilst the curled tail-feathers and other
ornaments of the male are developed later.*(2) Between such extreme
cases of close sexual resemblance and wide dissimilarity, as those
of the Crossoptilon and peacock, many intermediate ones could be
given, in which the characters follow our two rules in their order
of development.
* In the common peacock (Pavo cristatus) the male alone possesses
spurs, whilst both sexes of the Java peacock (P. muticus) offer the
unusual case of being furnished with spurs. Hence I fully expected
that in the latter species they would have been developed earlier in
life than in the common peacock; but M. Hegt of Amsterdam informs
me, that with young birds of the previous year, of both species,
compared on April 23rd, 1869, there was no difference in the
development of the spurs. The spurs, however, were as yet
represented merely by slight knobs or elevations. I presume that I
should have been informed if any difference in the rate of development
had been observed subsequently.
*(2) In some other species of the duck family the speculum differs
in a greater degree in the two sexes; but I have not been able to
discover whether its full development occurs later in life in the
males of such species, than in the male of the common duck, as ought
to be the case according to our rule. With the allied Mergus
cucullatus we have, however, a case of this kind: the two sexes differ
conspicuously in general plumage, and to a considerable degree in
the speculum, which is pure white in the male and greyish-white in the
female. Now the young males at first entirely resemble the females,
and have a greyish-white speculum, which becomes pure white at an
earlier age than that at which the adult male acquires his other and
more strongly-marked sexual differences: see Audubon, Ornithological
Biography, vol. iii., 1835, pp. 249-250.
As most insects emerge from the pupal state in a mature condition,
it is doubtful whether the period of development can determine the
transference of their characters to one or to both sexes. But we do
not know that the coloured scales, for instance, in two species of
butterflies, in one of which the sexes differ in colour, whilst in the
other they are alike, are developed at the same relative age in the
cocoon. Nor do we know whether all the scales are simultaneously
developed on the wings of the same species of butterfly, in which
certain coloured marks are confined to one sex, whilst others are
common to both sexes. A difference of this kind in the period of
development is not so improbable as it may at first appear; for with
the Orthoptera, which assume their adult state, not by a single
metamorphosis, but by a succession of moults, the young males of
some species at first resemble the females, and acquire their
distinctive masculine characters only at a later moult. Strictly
analogous cases occur at the successive moults of certain male
crustaceans.
We have as yet considered the transference of characters, relatively
to their period of development, only in species in a natural state; we
will now turn to domesticated animals, and first touch on
monstrosities and diseases. The presence of supernumerary digits,
and the absence of certain phalanges, must be determined at an early
embryonic period- the tendency to profuse bleeding is at least
congenital, as is probably colour-blindness- yet these
peculiarities, and other similar ones, are often limited in their
transmission to one sex; so that the rule that characters, developed
at an early period, tend to be transmitted to both sexes, here
wholly fails. But this rule, as before remarked, does not appear to be
nearly so general as the converse one, namely, that characters which
appear late in life in one sex are transmitted exclusively to the same
sex. From the fact of the above abnormal peculiarities becoming
attached to one sex, long before the sexual functions are active, we
may infer that there must be some difference between the sexes at an
extremely early age. With respect to sexually-limited diseases, we
know too little of the period at which they originate, to draw any
safe conclusion. Gout, however, seems to fall under our rule, for it
is generally caused by intemperance during manhood, and is transmitted
from the father to his sons in a much more marked manner than to his
daughters.
In the various domestic breeds of sheeps, goats, and cattle, the
males differ from their respective females in the shape or development
of their horns, forehead, mane, dewlap, tail, and hump on the
shoulders; and these peculiarities, in accordance with our rule, are
not fully developed until a rather late period of life. The sexes of
dogs do not differ, except that in certain breeds, especially in the
Scotch deerhound, the male is much larger and heavier than the female;
and, as we shall see in a future chapter, the male goes on
increasing in size to an unusually late period of life, which,
according to rule, will account for his increased size being
transmitted to his male offspring alone. On the other hand, the
tortoise-shell colour, which is confined to female cats, is quite
distinct at birth, and this case violates the rule. There is a breed
of pigeons in which the males alone are streaked with black, and the
streaks can be detected even in the nestlings; but they become more
conspicuous at each successive moult, so that this case partly opposes
and partly supports the rule. With the English carrier and pouter
pigeons, the full development of the wattle and the crop occurs rather
late in life, and conformably with the rule, these characters are
transmitted in full perfection to the males alone. The following cases
perhaps come within the class previously alluded to, in which both
sexes have varied in the same manner at a rather late period of
life, and have consequently transferred their new characters to both
sexes at a corresponding late period; and if so, these cases are not
opposed to our rule:- there exist sub-breeds of the pigeon,
described by Neumeister,* in which both sexes change their colour
during two or three moults (as is likewise the case with the almond
tumbler); nevertheless, these changes, though occurring rather late in
life, are common to both sexes. One variety of the canary-bird, namely
the London Prize, offers a nearly analogous case.
* Das Ganze der Taubenzucht, 1837, ss. 21, 24. For the case of the
streaked pigeons, see Dr. Chapuis, Le Pigeon Voyageur Belge, 1855,
p. 87.
With the breeds of the fowl the inheritance of various characters by
one or both sexes, seems generally determined by the period at which
such characters are developed. Thus in all the many breeds in which
the adult male differs greatly in colour from the female, as well as
from the wild parent-species, he differs also from the young male,
so that the newly-acquired characters must have appeared at a rather
late period of life. On the other hand, in most of the breeds in which
the two sexes resemble each other, the young are coloured in nearly
the same manner as their parents, and this renders it probable that
their colours first appeared early in life. We have instances of
this fact in all black and white breeds, in which the young and old of
both sexes are alike; nor can it be maintained that there is something
peculiar in a black or white plumage, which leads to its
transference to both sexes; for the males alone of many natural
species are either black or white, the females being differently
coloured. With the so-called cuckoo sub-breeds of the fowl, in which
the feathers are transversely pencilled with dark stripes, both
sexes and the chickens are coloured in nearly the same manner. The
laced plumage of the Sebright bantam is the same in both sexes, and in
the young chickens the wing-feathers are distinctly, though
imperfectly laced. Spangled Hamburgs, however, offer a partial
exception; for the two sexes, though not quite alike, resemble each
other more closely than do the sexes of the aboriginal parent-species;
yet they acquire their characteristic plumage late in life, for the
chickens are distinctly pencilled. With respect to other characters
beside colour, in the wild-parent species and in most of the
domestic breeds, the males alone possess a well-developed comb; but in
the young of the Spanish fowl it is largely developed at a very
early age, and, in accordance with this early development in the male,
it is of unusual size in the adult female. In the game breeds
pugnacity is developed at a wonderfully early age, of which curious
proofs could be given; and this character is transmitted to both
sexes, so that the hens, from their extreme pugnacity, are now
generally exhibited in separate pens. With the Polish breeds the
bony protuberance of the skull which supports the crest is partially
developed even before the chickens are hatched, and the crest itself
soon begins to grow, though at first feebly;* and in this breed the
adults of both sexes are characterised by a great bony protuberance
and an immense crest.
* For full particulars and references on all these points respecting
the several breeds of the fowl, see Variation of Animals and Plants
under Domestication, vol. i., pp. 250, 256. In regard to the higher
animals, the sexual differences which have arisen under
domestication are described in the same work under the head of each
species.
Finally, from what we have now seen of the relation which exists
in many natural species and domesticated races, between the period
of the development of their characters and the manner of their
transmission- for example, the striking fact of the early growth of
the horns in the reindeer, in which both sexes bear horns, in
comparison with their much later growth in the other species in
which the male alone bears horns- we may conclude that one, though not
the sole cause of characters being exclusively inherited by one sex,
is their development at a late age. And secondly, that one, though
apparently a less efficient cause of characters being inherited by
both sexes, is their development at an early age, whilst the sexes
differ but little in constitution. It appears, however, that some
difference must exist between the sexes even during a very early
embryonic period, for characters developed at this age not rarely
become attached to one sex.
Summary and concluding remarks.- From the foregoing discussion on
the various laws of inheritance, we learn that the characters of the
parents often, or even generally, tend to become developed in the
offspring of the same sex, at the same age, and periodically at the
same season of the year, in which they first appeared in the
parents. But these rules, owing to unknown causes, are far from
being fired. Hence during the modification of a species, the
successive changes may readily be transmitted in different ways;
some to one sex, and some to both; some to the offspring at one age,
and some to the offspring at all ages. Not only are the laws of
inheritance extremely complex, but so are the causes which induce
and govern variability. The variations thus induced are preserved
and accumulated by sexual selection, which is in itself an extremely
complex affair, depending, as it does, on the ardour in love, the
courage, and the rivalry of the males, as well as on the powers of
perception, the taste, and will of the female. Sexual selection will
also be largely dominated by natural selection tending towards the
general welfare of the species. Hence the manner in which the
individuals of either or both sexes have been affected through
sexual selection cannot fail to be complex in the highest degree.
When variations occur late in life in one sex, and are transmitted
to the same sex at the same age, the other sex and the young are
left unmodified. When they occur late in life, but are transmitted
to both sexes at the same age, the young alone are left unmodified.
Variations, however, may occur at any period of life in one sex or
in both, and be transmitted to both sexes at all ages, and then all
the individuals of the species are similarly modified. In the
following chapters it will be seen that all these cases frequently
occur in nature.
Sexual selection can never act on any animal before the age for
reproduction arrives. From the great eagerness of the male it has
generally acted on this sex and not on the females. The males have
thus become provided with weapons for fighting with their rivals, with
organs for discovering and securely holding the female, and for
exciting or charming her. When the sexes differ in these respects,
it is also, as we have seen, an extremely general law that the adult
male differs more or less from the young male; and we may conclude
from this fact that the successive variations, by which the adult male
became modified, did not generally occur much before the age for
reproduction. Whenever some or many of the variations occurred early
in life, the young males would partake more or less of the
characters of the adult males; and differences of this kind between
the old and young males may be observed in many species of animals.
It is probable that young male animals have often tended to vary
in a manner which would not only have been of no use to them at an
early age, but would have been actually injurious- as by acquiring
bright colours, which would render them conspicuous to their
enemies, or by acquiring structures, such as great horns, which
would expend much vital force in their development. Variations of this
kind occurring in the young males would almost certainly be eliminated
through natural selection. With the adult and experienced males, on
the other hand, the advantages derived from the acquisition of such
characters, would more than counterbalance some exposure to danger,
and some loss of vital force.
As variations which give to the male a better chance of conquering
other males, or of finding, securing, or charming the opposite sex,
would, if they happened to arise in the female, be of no service to
her, they would not be preserved in her through sexual selection. We
have also good evidence with domesticated animals, that variations
of all kinds are, if not carefully selected, soon lost through
inter-crossing and accidental deaths. Consequently in a state of
nature, if variations of the above kind chanced to arise in the female
line, and be transmitted exclusively in this line, they would be
extremely liable to be lost. If, however, the females varied and
transmitted their newly acquired characters to their offspring of both
sexes, the characters which were advantageous to the males would be
preserved by them through sexual selection, and the two sexes would in
consequence be modified in the same manner, although such characters
were of no use to the females: but I shall hereafter have to recur
to these more intricate contingencies. Lastly, the females may
acquire, and apparently have often acquired by transference,
characters from the male sex.
As variations occurring later in life, and transmitted to one sex
alone, have incessantly been taken advantage of and accumulated
through sexual selection in relation to the reproduction of the
species; therefore it appears, at first sight, an unaccountable fact
that similar variations have not frequently been accumulated through
natural selection, in relation to the ordinary habits of life. If this
had occurred, the two sexes would often have been differently
modified, for the sake, for instance, of capturing prey or of escaping
from danger. Differences of this kind between the two sexes do
occasionally occur, especially in the lower classes. But this
implies that the two sexes follow different habits in their
struggles for existence, which is a rare circumstance with the
higher animals. The case, however, is widely different with the
reproductive functions, in which respect the sexes necessarily differ.
For variations in structure which are related to these functions, have
often proved of value to one sex, and from having arisen at a late
period of life, have been transmitted to one sex alone; and such
variations, thus preserved and transmitted, have given rise to
secondary sexual characters.
In the following chapters, I shall treat of the secondary sexual
characters in animals of all classes, and shall endeavour in each case
to apply the principles explained in the present chapter. The lowest
classes will detain us for a very short time, but the higher
animals, especially birds, must be treated at considerable length.
It should be borne in mind that for reasons already assigned, I intend
to give only a few illustrative instances of the innumerable
structures by the aid of which the male finds the female, or, when
found, holds her. On the other hand, all structures and instincts by
the aid of which the male conquers other males, and by which he
allures or excites the female, will be fully discussed, as these are
in many ways the most interesting.
Supplement on the proportional numbers of the two sexes in animals
belonging to various classes.
As no one, as far as I can discover, has paid attention to the
relative numbers of the two sexes throughout the animal kingdom, I
will here give such materials as I have been able to collect, although
they are extremely imperfect. They consist in only a few instances
of actual enumeration, and the numbers are not very large. As the
proportions are known with certainty only in mankind, I will first
give them as a standard of comparison.
Man.- In England during ten years (from 1857 to 1866) the average
number of children born alive yearly was 707,120, in the proportion of
104.5 males to 100 females. But in 1857 the male births throughout
England were as 105.2, and in 1865 as 104.0 to 100. Looking to
separate districts, in Buckinghamshire (where about 5000 children
are annually born) the mean proportion of male to female births,
during the whole period of the above ten years, was as 102.8 to 100;
whilst in N. Wales (where the average annual births are 12,873) it was
as high as 106.2 to 100. Taking a still smaller district, viz.,
Rutlandshire (where the annual births average only 739), in 1864 the
male births were as 114.6, and in 1862 as only 97.0 to 100; but even
in this small district the average of the 7385 births during the whole
ten years, was as 104.5 to 100: that is in the same ratio as
throughout England.* The proportions are sometimes slightly
disturbed by unknown causes; thus Prof. Faye states "that in some
districts of Norway there has been during a decennial period a
steady deficiency of boys, whilst in others the opposite condition has
existed." In France during forty-four years the male to the female
births have been as 106.2 to 100; but during this period it has
occurred five times in one department, and six times in another,
that the female births have exceeded the males. In Russia the
average proportion is as high as 108.9, and in Philadelphia in the
United States as 110.5 to 100.*(2) The average for Europe, deduced
by Bickes from about seventy million births, is 106 males to 100
females. On the other hand, with white children born at the Cape of
Good Hope, the proportion of males is so low as to fluctuate during
successive years between 90 and 99 males for every 100 females. It
is a singular fact that with Jews the proportion of male births is
decidedly larger than with Christians: thus in Prussia the
proportion is as 113, in Breslau as 114, and in Livonia as 120 to 100;
the Christian births in these countries being the same as usual, for
instance, in Livonia as 104 to 100.*(3)
* Twenty-ninth Annual Report of the Registrar-General for 1866. In
this report (p. xii.) a special decennial table is given.
*(2) For Norway and Russia, see abstract of Prof. Faye's researches,
in British and Foreign Medico-Chirurg. Review, April, 1867, pp. 343,
345. For France, the Annuaire pour l'An 1867, p. 213. For
Philadelphia, Dr. Stockton-Hough, Social Science Assoc., 1874. For the
Cape of Good Hope, Quetelet as quoted by Dr. H. H. Zouteveen, in the
Dutch translation of this work (vol. i., p. 417), where much
information is given on the proportion of the sexes.
*(3) In regard to the Jews, see M. Thury, La Loi de Production des
Sexes, 1863, p. 25.
Prof. Faye remarks that "a still greater preponderance of males
would be met with, if death struck both sexes in equal proportion in
the womb and during birth. But the fact is, that for every 100
still-born females, we have in several countries from 134.6 to 144.9
stillborn males. During the first four or five years of life, also,
more male children die than females, for example in England, during
the first year, 126 boys die for every 100 girls- a proportion which
in France is still more unfavourable."* Dr. Stockton-Hough accounts
for these facts in part by the more frequent defective development
of males than of females. We have before seen that the male sex is
more variable in structure than the female; and variations in
important organs would generally be injurious. But the size of the
body, and especially of the head, being greater in male than female
infants is another cause: for the males are thus more liable to be
injured during parturition. Consequently the still-born males are more
numerous; and, as a highly competent judge, Dr. Crichton Browne,*(2)
believes, male infants often suffer in health for some years after
birth. Owing to this excess in the death-rate of male children, both
at birth and for some time subsequently, and owing to the exposure
of grown men to various dangers, and to their tendency to emigrate,
the females in all old-settled countries, where statistical records
have been kept,*(3) are found to preponderate considerably over the
males.
* British and Foreign Medico-Chirurg. Review, April, 1867, p. 343.
Dr. Stark also remarks (Tenth Annual Reports of Births, Deaths, &c.,
in Scotland, 1867, p. xxviii.) that "These examples may suffice to
show that, at almost every stage of life, the males in Scotland have a
greater liability to death and a higher death-rate than the females.
The fact, however, of this peculiarity being most strongly developed
at that infantile period of life when the dress, food, and general
treatment of both sexes are alike, seems to prove that the higher male
death-rate is an impressed, natural, and constitutional peculiarity
due to sex alone."
*(2) West Riding Lunatic Asylum Reports, vol. i., 1871, p. 8. Sir J.
Simpson has proved that the head of the male infant exceeds that of
the female by 3/8ths of an inch in circumference, and by 1/8th in
transverse diameter. Quetelet has shown that woman is born smaller
than man; see Dr. Duncan, Fecundity, Fertility, and Sterility, 1871,
p. 382.
*(3) With the savage Guaranys of Paraguay, according to the accurate
Azara (Voyages dans l'Amerique merid., tom. ii., 1809, pp. 60, 179),
the women are to the men in the proportion of 14 to 13.
It seems at first sight a mysterious fact that in different nations,
under different conditions and climates, in Naples, Prussia,
Westphalia, Holland, France, England and the United States, the excess
of male over female births is less when they are illegitimate than
when legitimate.* This has been explained by different writers in many
different ways, as from the mothers being generally young, from the
large proportion of first pregnancies, &c. But we have seen that
male infants, from the large size of their heads, suffer more than
female infants during parturition; and as the mothers of
illegitimate children must be more liable than other women to
undergo bad labours, from various causes, such as attempts at
concealment by tight lacing, hard work, distress of mind, &c., their
male infants would proportionably suffer. And this probably is the
most efficient of all the causes of the proportion of males to females
born alive being less amongst illegitimate children than amongst the
legitimate. With most animals the greater size of the adult male
than of the female, is due to the stronger males having conquered
the weaker in their struggles for the possession of the females, and
no doubt it is owing to this fact that the two sexes of at least
some animals differ in size at birth. Thus we have the curious fact
that we may attribute the more frequent deaths of male than female
infants, especially amongst the illegitimate, at least in part to
sexual selection.
* Babbage, Edinburgh Journal of Science, 1829, vol. i., p. 88;
also p. 90, on still-born children. On illegitimate children in
England, see Report of Registrar-General for 1866, p. xv.
It has often been supposed that the relative age of the two
parents determine the sex of the offspring; and Prof. Leuckart* has
advanced what he considers sufficient evidence, with respect to man
and certain domesticated animals, that this is one important though
not the sole factor in the result. So again the period of impregnation
relatively to the state of the female has been thought by some to be
the efficient cause; but recent observations discountenance this
belief. According to Dr. Stockton-Hough,*(2) the season of the year,
the poverty or wealth of the parents, residence in the country or in
cities, the crossing of foreign immigrants, &c., all influence the
proportion of the sexes. With mankind, polygamy has also been supposed
to lead to the birth of a greater proportion of female infants; but
Dr. J. Campbell*(3) carefully attended to this subject in the harems
of Siam, and concludes that the proportion of male to female births is
the same as from monogamous unions. Hardly any animal has been
rendered so highly polygamous as the English race-horse, and we
shall immediately see that his male and female offspring are almost
exactly equal in number. I will now give the facts which I have
collected with respect to the proportional numbers of the sexes of
various animals; and will then briefly discuss how far selection has
come into play in determining the result.
* Leuckart, in Wagner's Handworterbuch der Phys., B. iv., 1853, s.
774.
*(2) Social Science Association of Philadelphia, 1874.
*(3) Anthropological Review, April, 1870, p. cviii.
Horses.- Mr. Tegetmeier has been so kind as to tabulate for me
from the Racing Calendar the births of race-horses during a period
of twenty-one years, viz., from 1846 to 1867; 1849 being omitted, as
no returns were that year published. The total births were 25,560,*
consisting of 12,763 males and 12,797 females, or in the proportion of
99.7 males to 100 females. As these numbers are tolerably large, and
as they are drawn from all parts of England, during several years,
we may with much confidence conclude that with the domestic horse,
or at least with the race-horse, the two sexes are produced in
almost equal numbers. The fluctuations in the proportions during
successive years are closely like those which occur with mankind, when
a small and thinly-populated area is considered; thus in 1856 the male
horses were as 107.1, and in 1867 as only 92.6 to 100 females. In
the tabulated returns the proportions vary in cycles, for the males
exceeded the females during six successive years; and the females
exceeded the males during two periods each of four years; this,
however, may be accidental; at least I can detect nothing of the
kind with man in the decennial table in the Registrar's Report for
1866.
* During eleven years a record was kept of the number of mares which
proved barren or prematurely slipped their foals; and it deserves
notice, as shewing how infertile these highly-nurtured and rather
closely-interbred animals have become, that not far from one-third
of the mares failed to produce living foals. Thus during 1866, 809
male colts and 816 female colts were born, and 743 mares failed to
produce offspring. During 1867, 836 males and 902 females were born,
and 794 mares failed.
Dogs.- During a period of twelve years, from 1857 to 1868, the
births of a large number of greyhounds, throughout England, were
sent to the Field newspaper; and I am again indebted to Mr. Tegetmeier
for carefully tabulating the results. The recorded births were 6878,
consisting of 3605 males and 3273 females, that is, in the
proportion of 110.1 males to 100 females. The greatest fluctuations
occurred in 1864, when the proportion was as 95.3 males, and in
1867, as 116.3 males to 100 females. The above average proportion of
110.1 to 100 is probably nearly correct in the case of the
greyhound, but whether it would hold with other domesticated breeds is
in some degree doubtful. Mr. Cupples has enquired from several great
breeders of dogs, and finds that all without exception believe that
females are produced in excess; but he suggests that this belief may
have arisen from females being less valued, and from the consequent
disappointment producing a stronger impression on the mind.
Sheep.- The sexes of sheep are not ascertained by agriculturists
until several months after birth, at the period when the males are
castrated; so that the following returns do not give the proportions
at birth. Moreover, I find that several great breeders in Scotland,
who annually raise some thousand sheep, are firmly convinced that a
larger proportion of males than of females die during the first year
or two. Therefore the proportion of males would be somewhat larger
at birth than at the age of castration. This is a remarkable
coincidence with what, as we have seen, occurs with mankind, and
both cases probably depend on the same cause. I have received
returns from four gentlemen in England who have bred lowland sheep,
chiefly Leicesters, during the last ten to sixteen years; they
amount altogether to 8965 births, consisting of 4407 males and 4558
females; that is in the proportion of 96.7 males to 100 females.
With respect to Cheviot and black-faced sheep bred in Scotland, I have
received returns from six breeders, two of them on a large scale,
chiefly for the years 1867-1869, but some of the returns extend back
to 1862. The total number recorded amounts to 50,685, consisting of
25,071 males and 25,614 females or in the proportion of 97.9 males
to 100 females. If we take the English and Scotch returns together,
the total number amounts to 59,650, consisting of 29,478 males and
30,172 females, or as 97.7 to 100. So that with sheep at the age of
castration the females are certainly in excess of the males, but
probably this would not hold good at birth.*
*I am much indebted to Mr. Cupples for having procured for me the
above returns from Scotland, as well as some of the following
returns on cattle. Mr. R. Elliot, of Laighwood, first called my
attention to the premature deaths of the males,- a statement
subsequently confirmed by Mr. Aitchison and others. To this latter
gentleman, and to Mr. Payan, I owe my thanks for large returns as to
sheep.
Of Cattle I have received returns from nine gentlemen of 982 births,
too few to be trusted; these consisted of 477 bull-calves and 505
cow-calves; i.e., in the proportion of 94.4 males to 100 females.
The Rev. W. D. Fox informs me that in 1867 out of 34 calves born on
a farm in Derbyshire only one was a bull. Mr. Harrison Weir has
enquired from several breeders of Pigs, and most of them estimate
the male to the female births as about 7 to 6. This same gentleman has
bred rabbits for many years, and has noticed that a far greater number
of bucks are produced than does. But estimations are of little value.
Of Mammalia in a state of nature I have been able to learn very
little. In regard to the common rat, I have received conflicting
statements. Mr. R. Elliot, of Laighwood, informs me that a rat-catcher
assured him that he had always found the males in great excess, even
with the young in the nest. In consequence of this, Mr. Elliot himself
subsequently examined some hundred old ones, and found the statement
true. Mr. F. Buckland has bred a large number of white rats, and he
also believes that the males greatly exceed the females. In regard
to moles, it is said that "the males are much more numerous than the
females":* and as the catching of these animals is a special
occupation, the statement may perhaps be trusted. Sir A. Smith, in
describing an antelope of S. Africa*(2) (Kobus ellipsiprymnus),
remarks, that in the herds of this and other species, the males are
few in number compared with the females: the natives believe that they
are born in this proportion; others believe that the younger males are
expelled from the herds, and Sir A. Smith says, that though he has
himself never seen herds consisting of young males alone, others
affirm that this does occur. It appears probable that the young when
expelled from the herd, would often fall a prey to the many beasts
of prey of the country.
* Bell, History of British Quadrupeds, p. 100.
*(2) Illustrations of the Zoology of S. Africa, 1849, pl. 29.
BIRDS.
With respect to the Fowl, I have received only one account,
namely, that out of 1001 chickens of a highly-bred stock of Cochins,
reared during eight years by Mr. Stretch, 487 proved males and 514
females; i.e., as 94.7 to 100. In regard to domestic pigeons there
is good evidence either that the males are produced in excess, or that
they live longer; for these birds invariably pair, and single males,
as Mr. Tegetmeier informs me, can always be purchased cheaper than
females. Usually the two birds reared from the two eggs laid in the
same nest are a male and a female; but Mr. Harrison Weir, who has been
so large a breeder, says that he has often bred two cocks from the
same nest, and seldom two hens; moreover, the hen is generally the
weaker of the two, and more liable to perish.
With respect to birds in a state of nature, Mr. Gould and others*
are convinced that the males are generally the more numerous; and as
the young males of many species resemble the females, the latter would
naturally appear to be the more numerous. Large numbers of pheasants
are reared by Mr. Baker of Leadenhall from eggs laid by wild birds,
and he informs Mr. Jenner Weir that four or five males to one female
are generally produced. An experienced observer remarks,*(2) that in
Scandinavia the broods of the capercailzie and black-cock contain more
males than females; and that with the Dal-ripa (a kind of ptarmigan)
more males than females attend the leks or places of courtship; but
this latter circumstance is accounted for by some observers by a
greater number of hen birds being killed by vermin. From various facts
given by White of Selborne,*(3) it seems clear that the males of the
partridge must be in considerable excess in the south of England;
and I have been assured that this is the case in Scotland. Mr. Weir on
enquiring from the dealers, who receive at certain seasons large
numbers of ruffs (Machetes pugnax), was told that the males are much
the more numerous. This same naturalist has also enquired for me
from the birdcatchers, who annually catch an astonishing number of
various small species alive for the London market, and he was
unhesitatingly answered by an old and trustworthy man, that with the
chaffinch the males are in large excess: he thought as high as 2 males
to 1 female, or at least as high as 5 to 3.*(4) The males of the
blackbird, he likewise maintained, were by far the more numerous,
whether caught by traps or by netting at night. These statements may
apparently be trusted, because this same man said that the sexes are
about equal with the lark, the twite (Linaria montana), and goldfinch.
On the other hand, he is certain that with the common linnet, the
females preponderate greatly, but unequally during different years;
during some years he has found the females to the males as four to
one. It should, however, be borne in mind, that the chief season for
catching birds does not begin till September, so that with some
species partial migrations may have begun, and the flocks at this
period often consist of hens alone. Mr. Salvin paid particular
attention to the sexes of the humming-birds in Central America, and is
convinced that with most of the species the males are in excess;
thus one year he procured 204 specimens belonging to ten species,
and these consisted of 166 males and of only 38 females. With two
other species the females were in excess: but the proportions
apparently vary either during different seasons or in different
localities; for on one occasion the males of Campylopterus
hemileucurus were to the females as 5 to 2, and on another
occasion*(5) in exactly the reversed ratio. As bearing on this
latter point, I may add, that Mr. Powys found in Corfu and Epirus
the sexes of the chaffinch keeping apart, and "the females by far
the most numerous"; whilst in Palestine Mr. Tristram found "the male
flocks appearing greatly to exceed the female in number."*(6) So again
with the Quiscalus major, Mr. G. Taylor says, that in Florida there
were "very few females in proportion to the males,"*(7) whilst in
Honduras the proportion was the other way, the species there having
the character of a polygamist.
* Brehm (Illustriertes Thierleben, B. iv., s. 990) comes to the same
conclusion.
*(2) On the authority of L. Lloyd, Game Birds of Sweden, 1867, pp.
12, 132.
*(3) Nat. Hist. of Selborne, letter xxix., ed. of 1825, vol. i.,
p. 139.
*(4) Mr. Jenner Weir received similar information, on making
enquiries during the following year. To shew the number of living
chaffinches caught, I may mention that in 1869 there was a match
between two experts, and one man caught in a day 62, and another 40,
male chaffinches. The greatest number ever caught by one man in a
single day was 70.
*(5) Ibis, vol. ii. p. 260, as quoted in Gould's Trochilidae,
1861, p. 52. For the foregoing proportions, I am indebted to Mr.
Salvin for a table of his results.
*(6) Ibis, 1860, p. 137; and 1867, p. 369.
*(7) Ibis, 1862, p. 187.
FISH.
With fish the proportional numbers of the sexes can be ascertained
only by catching them in the adult or nearly adult state; and there
are many difficulties in arriving at any just conclusion.* Infertile
females might readily be mistaken for males, as Dr. Gunther has
remarked to me in regard to trout. With some species the males are
believed to die soon after fertilising the ova. With many species
the males are of much smaller size than the females, so that a large
number of males would escape from the same net by which the females
were caught. M. Carbonnier,*(2) who has especially attended to the
natural history of the pike (Esox lucius), states that many males,
owing to their small size, are devoured by the larger females; and
he believes that the males of almost all fish are exposed from this
same cause to greater danger than the females. Nevertheless, in the
few cases in which the proportional numbers have been actually
observed, the males appear to be largely in excess. Thus Mr. R. Buist,
the superintendent of the Stormontfield experiments, says that in
1865, out of 70 salmon first landed for the purpose of obtaining the
ova, upwards of 60 were males. In 1867 he again "calls attention to
the vast disproportion of the males to the females. We had at the
outset at least ten males to one female." Afterwards females
sufficient for obtaining ova were procured. He adds, "from the great
proportion of the males, they are constantly fighting and tearing each
other on the spawning-beds."*(3) This disproportion, no doubt, can
be accounted for in part, but whether wholly is doubtful, by the males
ascending the rivers before the females. Mr. F. Buckland remarks in
regard to trout, that "it is a curious fact that the males
preponderate very largely in number over the females. It invariably
happens that when the first rush of fish is made to the net, there
will be at least seven or eight males to one female found captive. I
cannot quite account for this; either the males are more numerous than
the females, or the latter seek safety by concealment rather than
flight." He then adds, that by carefully searching the banks
sufficient females for obtaining ova can be found.*(4) Mr. H. Lee
informs me that out of 212 trout taken for this purpose in Lord
Portsmouth's park, 150 were males and 62 females.
* Leuckart quotes Bloch (Wagner, Handworterbuch der Phys., B. iv.,
1853, s. 775), that with fish there are twice as many males as
females.
*(2) Quoted in the Farmer, March 18, 1869, p. 369.
*(3) The Stormontfield Piscicultural Experiments, 1866, p. 23. The
Field newspaper, June 29, 1867.
*(4) Land and Water, 1868, p. 41.
The males of the Cyprinidae likewise seem to be in excess; but
several members of this family, viz., the carp, tench, bream and
minnow, appear regularly to follow the practice, rare in the animal
kingdom, of polyandry; for the female whilst spawning is always
attended by two males, one on each side, and in the case of the
bream by three or four males. This fact is so well known, that it is
always recommended to stock a pond with two male tenches to one
female, or at least with three males to two females. With the
minnow, an excellent observer states, that on the spawning-beds the
males are ten times as numerous as the females; when a female comes
amongst the males, "she is immediately pressed closely by a male on
each side; and when they have been in that situation for a time, are
superseded by other two males."*
* Yarrell, Hist. British Fishes, vol. i., 1826, p. 307; on the
Cyprinus carpio, p. 331; on the Tinca vulgaris, p. 331; on the Abramis
brama, p. 336. See, for the minnow (Leuciscus phoxinus), Loudon's
Magazine of Natural History, vol. v., 1832, p. 682.
INSECTS.
In this great class, the Lepidoptera almost alone affords means
for judging of the proportional numbers of the sexes; for they have
been collected with special care by many good observers, and have been
largely bred from the egg or caterpillar state. I had hoped that
some breeders of silk-moths might have kept an exact record, but after
writing to France and Italy, and consulting various treatises, I
cannot find that this has ever been done. The general opinion
appears to be that the sexes are nearly equal, but in Italy, as I hear
from Professor Canestrini, many breeders are convinced that the
females are produced in excess. This same naturalist, however, informs
me, that in the two yearly broods of the ailanthus silk-moth (Bombyx
cynthia), the males greatly preponderate in the first, whilst in the
second the two sexes are nearly equal, or the females rather in
excess.
In regard to butterflies in a state of nature, several observers
have been much struck by the apparently enormous preponderance of
the males.* Thus Mr. Bates,*(2) in speaking of several species,
about a hundred in number, which inhabit the upper Amazons, says
that the males are much more numerous than the females, even in the
proportion of a hundred to one. In North America, Edwards, who had
great experience, estimates in the genus Papilio the males to the
females as four to one; and Mr. Walsh, who informs me of this
statement, says that with P. turnus this is certainly the case. In
South Africa, Mr. R. Trimen found the males in excess in 19
species;*(3) and in one of these, which swarms in open places, he
estimated the number of males as fifty to one female. With another
species, in which the males are numerous in certain localities, he
collected only five females during seven years. In the island of
Bourbon, M. Maillard states that the males of one species of Papilio
are twenty times as numerous as the females.*(4) Mr. Trimen informs me
that as far as he has himself seen, or heard from others, it is rare
for the females of any butterfly to exceed the males in number; but
three South African species perhaps offer an exception. Mr.
Wallace*(5) states that the females of Ornithoptera croesus, in the
Malay Archipelago, are more common and more easily caught than the
males; but this is a rare butterfly. I may here add, that in
Hyperythra, a genus of moths, Guenee says, that from four to five
females are sent in collections from India for one male.
* Leuckart quotes Meinecke (Wagner, Handworterbuch der Phys., B.
iv., 1853, s. 775) that the males of butterflies are three or four
times as numerous as the females.
*(2) The Naturalist on the Amazons, vol. ii., 1863, pp. 228, 347.
*(3) Four of these cases are given by Mr. Trimen in his
Rhopalocera Africae Australis.
*(4) Quoted by Trimen, Transactions of the Ent. Society, vol. v.,
part iv., 1866, p. 330.
*(5) Transactions, Linnean Society, vol. xxv., p. 37.
When this subject of the proportional numbers of the sexes of
insects was brought before the Entomological Society,* it was
generally admitted that the males of most Lepidoptera, in the adult or
imago state, are caught in greater numbers than the females: but
this fact was attributed by various observers to the more retiring
habits of the females, and to the males emerging earlier from the
cocoon. This latter circumstance is well known to occur with most
Lepidoptera, as well as with other insects. So that, as M. Personnat
remarks, the males of the domesticated Bombyx yamamai., are useless at
the beginning of the season, and the females at the end, from the want
of mates.*(2) I cannot, however, persuade myself that these causes
suffice to explain the great excess of males, in the above cases of
certain butterflies which are extremely common in their native
countries. Mr. Stainton, who has paid very close attention during many
years to the smaller moths, informs me that when he collected them
in the imago state, he thought that the males were ten times as
numerous as the females, but that since he has reared them on a
large scale from the caterpillar state, he is convinced that the
females are the more numerous. Several entomologists concur in this
view. Mr. Doubleday, however, and some others, take an opposite
view, and are convinced that they have reared from the eggs and
caterpillars a larger proportion of males than of females.
* Proceedings, Entomological Society, Feb. 17, 1868.
*(2) Quoted by Dr. Wallace in Proceedings, Entomological Society,
3rd series, vol. v., 1867, p. 487.
Besides the more active habits of the males, their earlier emergence
from the cocoon, and in some cases their frequenting more open
stations, other causes may be assigned for an apparent or real
difference in the proportional numbers of the sexes of Lepidoptera,
when captured in the imago state, and when reared from the egg or
caterpillar state. I hear from Professor Canestrini, that it is
believed by many breeders in Italy, that the female caterpillar of the
silk-moth suffers more from the recent disease than the male; and
Dr. Staudinger informs me that in rearing Lepidoptera more females die
in the cocoon than males. With many species the female caterpillar
is larger than the male, and a collector would naturally choose the
finest specimens, and thus unintentionally collect a larger number
of females. Three collectors have told me that this was their
practice; but Dr. Wallace is sure that most collectors take all the
specimens which they can find of the rarer kinds, which alone are
worth the trouble of rearing. Birds when surrounded by caterpillars
would probably devour the largest; and Professor Canestrini informs me
that in Italy some breeders believe, though on insufficient
evidence, that in the first broods of the ailanthus silkmoth, the
wasps destroy a larger number of the female than of the male
caterpillars. Dr. Wallace further remarks that female caterpillars,
from being larger than the males, require more time for their
development, and consume more food and moisture: and thus they would
be exposed during a longer time to danger from ichneumons, birds, &c.,
and in times of scarcity would perish in greater numbers. Hence it
appears quite possible that in a state of nature, fewer female
Lepidoptera may reach maturity than males; and for our special
object we are concerned with their relative numbers at maturity,
when the sexes are ready to propagate their kind.
The manner in which the males of certain moths congregate in
extraordinary numbers round a single female, apparently indicates a
great excess of males, though this fact may perhaps be accounted for
by the earlier emergence of the males from their cocoons. Mr. Stainton
informs me that from twelve to twenty males, may often be seen
congregated round a female Elachista rufocinerea. It is well known
that if a virgin Lasiocampa quercus or Saturnia carpini be exposed
in a cage, vast numbers of males collect round her, and if confined in
a room will even come down the chimney to her. Mr. Doubleday
believes that he has seen from fifty to a hundred males of both
these species attracted in the course of a single day by a female in
confinement. In the Isle of Wight Mr. Trimen exposed a box in which
a female of the Lasiocampa had been confined on the previous day,
and five males soon endeavored to gain admittance. In Australia, Mr.
Verreaux, having placed the female of a small Bombyx in a box in his
pocket, was followed by a crowd of males, so that about 200 entered
the house with him.*
* Blanchard, Metamorphoses, Moeurs des Insectes, 1868, pp. 225-226.
Mr. Doubleday has called my attention to M. Staudinger's* list of
Lepidoptera, which gives the prices of the males and females of 300
species or well-marked varieties of butterflies (Rhopalocera). The
prices for both sexes of the very common species are of course the
same; but in 114 of the rarer species they differ; the males being
in all cases, excepting one, the cheaper. On an average of the
prices of the 113 species, the price of the male to that of the female
is as 100 to 149; and this apparently indicates that inversely the
males exceed the females in the same proportion. About 2000 species or
varieties of moths (Heterocera) are catalogued, those with wingless
females being here excluded on account of the difference in habits
between the two sexes: of these 2000 species, 141 differ in price
according to sex, the males of 130 being cheaper, and those of only 11
being dearer than the females. The average price of the males of the
130 species, to that of the females, is as 100 to 143. With respect to
the butterflies in this priced list, Mr. Doubleday thinks (and no
man in England has had more experience), that there is nothing in
the habits of the species which can account for the difference in
the prices of the two sexes, and that it can be accounted for only
by an excess in the number of the males. But I am bound to add that
Dr. Staudinger informs me, that he is himself of a different
opinion. He thinks that the less active habits of the females and
the earlier emergence of the males will account for his collectors
securing a larger number of males than of females, and consequently
for the lower prices of the former. With respect to specimens reared
from the caterpillar-state, Dr. Staudinger believes, as previously
stated, that a greater number of females than of males die whilst
confined to the cocoons. He adds that with certain species one sex
seems to preponderate over the other during certain years.
* Lepidopteren-Doubletten Liste, Berlin, No. x., 1866.
Of direct observations on the sexes of Lepidoptera, reared either
from eggs or caterpillars, I have received only the few following
cases:*
* See following table.
So that in these eight lots of cocoons and eggs, males were produced
in excess. Taken together the proportion of males is as 122.7 to 100
females. But the numbers are hardly large enough to be trustworthy.
On the whole, from these various sources of evidence, all pointing
in the same direction, I infer that with most species of
Lepidoptera, the mature males generally exceed the females in
number, whatever the proportions may be at their first emergence
from the egg.
Males Females
The Rev. J. Hellins* of Exeter reared, during
1868, imagos of 73 species, which
consisted of 153 137
Mr. Albert Jones of Eltham reared, during
1868, imagos of 9 species, which
consisted of 159 126
During 1869 he reared imagoes from 4 species
consisting of 114 112
Mr. Buckler of Emsworth, Hants, during 1869,
reared imagos from 74 species,
consisting of 180 169
Dr. Wallace of Colchester reared from one
brood of Bombyx cynthia 52 48
Dr. Wallace raised, from cocoons of Bombyx
pernyi sent from China, during 1869 224 123
Dr. Wallace raised, during 1868 and 1869, from
two lots of cocoons of Bombyx yamamai 52 46
Total 934 761
* This naturalist has been so kind as to send me some results from
former years, in which the females seemed to preponderate; but so many
of the figures were estimates, that I found it impossible to
tabulate them
With reference to the other orders of insects, I have been able to
collect very little reliable information. With the stag-beetle
(Lucanus cervus) "the males appear to be much more numerous than the
females"; but when, as Cornelius remarked during 1867, an unusal
number of these beetles appeared in one part of Germany, the females
appeared to exceed the males as six to one. With one of the
Elateridae, the males are said to be much more numerous than the
females, and "two or three are often found united with one female;* so
that here polyandry seems to prevail." With Siagonium (Staphylinidae),
in which the males are furnished with horns, "the females are far more
numerous than the opposite sex." Mr. Janson stated at the
Entomological Society that the females of the bark feeding Tomicus
villosus are so common as to be a plague, whilst the males are so rare
as to be hardly known.
* Gunther's Record of Zoological Literature, 1867, p. 260. On the
excess of female Lucanus, ibid, p. 250. On the males of Lucanus in
England, Westwood, Modern Classification of Insects, vol. i., p.
187. On the Siagonium, ibid., p. 172.
It is hardly worthwhile saying anything about the proportion of
the sexes in certain species and even groups of insects, for the males
are unknown or very rare, and the females are parthenogenetic, that
is, fertile without sexual union; examples of this are afforded by
several of the Cynipidae.* In all the gall-making Cynipidae known to
Mr. Walsh, the females are four or five times as numerous as the
males; and so it is, as he informs me, with the gall-making
Cecidomyiidae (Diptera). With some common species of saw-flies
(Tenthredinae) Mr. F. Smith has reared hundreds of specimens from
larvae of all sizes, but has never reared a single male; on the
other hand, Curtis says,*(2) that with certain species (Athalia), bred
by him, the males were to the females as six to one; whilst exactly
the reverse occurred with the mature insects of the same species
caught in the fields. In the family of bees, Hermann Muller,*(3)
collected a large number of specimens of many species, and reared
others from the cocoons, and counted the sexes. He found that the
males of some species greatly exceeded the females in number; in
others the reverse occurred; and in others the two sexes were nearly
equal. But as in most cases the males emerge from the cocoons before
the females, they are at the commencement of the breeding-season
practically in excess. Muller also observed that the relative number
of the two sexes in some species differed much in different
localities. But as H. Muller has himself remarked to me, these remarks
must be received with some caution, as one sex might more easily
escape observation than the other. Thus his brother Fritz Muller has
noticed in Brazil that the two sexes of the same species of bee
sometimes frequent different kinds of flowers. With respect to the
Orthoptera, I know hardly anything about the relative number of the
sexes: Korte,*(4) however, says that out of 500 locusts which he
examined, the males were to the females as five to six. With the
Neuroptera, Mr. Walsh states that in many, but by no means in all
the species of the odonatous group, there is a great overplus of
males: in the genus Hetaerina, also, the males are generally at
least four times as numerous as the females. In certain species in the
genus Gomphus the males are equally in excess, whilst in two other
species, the females the are twice or thrice as numerous as the males.
In some European species of Psocus thousands of females may be
collected without a single male, whilst with other species of the same
genus both sexes are common.*(5) In England, Mr. MacLachlan has
captured hundreds of the female Apatania muliebris, but has never seen
the male; and of Boreus hyemalis only four or five males have been
seen here.*(6) With most of these species (excepting the Tenthredinae)
there is at present no evidence that the females are subject to
parthenogenesis; and thus we see how ignorant we are of the causes
of the apparent discrepancy in the proportion of the two sexes.
* Walsh in the American Entomologist, vol. i., 1869, p. 103. F.
Smith, Record of Zoological Lit, 1867, p. 328.
*(2) Farm Insects, pp. 45-46.
*(3) "Anwendung der Darwin'schen Lehre," Verh. d. n. Jahrg., xxiv.
*(4) Die Strich. Zug oder Wanderheuschrecke, 1828, p. 20.
*(5) "Observations on N. American Neuroptera," by H. Hagen and B. D.
Walsh, Proceedings, Ent. Soc. Philadelphia, Oct., 1863, pp. 168,
223, 239.
*(6) Proceedings, Ent. Soc. London, Feb. 17, 1868.
In the other classes of the Articulata I have been able to collect
still less information. With spiders, Mr. Blackwall, who has carefully
attended to this class during many years, writes to me that the
males from their more erratic habits are more commonly seen, and
therefore appear more numerous. This is actually the case with a few
species; but he mentions several species in six genera, in which the
females appear to be much more numerous than the males.* The small
size of the males in comparison with the females (a peculiarity
which is sometimes carried to an extreme degree), and their widely
different appearance, may account in some instances for their rarity
in collections.*(2)
8888
* Another great authority with respect to this class, Prof.
Thorell of Upsala (On European Spiders, 1869-70, part i., p. 205),
speaks as if female spiders were generally commoner than the males.
*(2) See, on this subject, Mr. O. P. Cambridge, as quoted in
Quarterly Journal of Science, 1868, page 429.
Some of the lower crustaceans are able to propagate their kind
sexually, and this will account for the extreme rarity of the males;
thus von Siebold* carefully examined no less than 13,000 specimens
of Apus from twenty-one localities, and amongst these he found only
319 males. With some other forms (as Tanais and Cypris), as Fritz
Muller informs me, there is reason to believe that the males are
much shorter-lived than the females; and this would explain their
scarcity, supposing the two sexes to be at first equal in number. On
the other hand, Muller has invariably taken far more males than
females of the Diastylidae and of Cypridina on the shores of Brazil:
thus with a species in the latter genus, 63 specimens caught the
same day included 57 males; but he suggests that this preponderance
may be due to some unknown difference in the habits of the two
sexes. With one of the higher Brazilian crabs, namely a Gelasimus,
Fritz Muller found the males to be more numerous than the females.
According to the large experience of Mr. C. Spence Bate, the reverse
seems to be the case with six common British crabs, the names of which
he has given me.
* Beitrage zur Parthenogenesis, p. 174.
The proportion of the sexes in relation to natural selection.
There is reason to suspect that in some cases man has by
selection6 indirectly influenced his own sex-producing powers. Certain
women tend to produce during their whole lives more children of one
sex than of the other: and the same holds good of many animals, for
instance, cows and ho6rses; thus Mr. Wright of Yeldersley House
informs me that one of his Arab mares, though put seven times to
different horses, produced seven fillies. Though I have very little
evidence on this head, analogy would lead to the belief, that the
tendency to produce either sex would be inherited like almost every
other peculiarity, for instance, that of producing twins; and
concerning the above tendency a good authority, Mr. J. Downing, has
communicated to me facts which seem to prove that this does occur in
certain families of short-horn cattle. Col. Marshall* has recently
found on careful examination that the Todas, a hill-tribe of India,
consist of 112 males and 84 females of all ages- that is in a ratio of
133.3 males to 100 females. The Todas, who are polyandrous in their
marriages, during former times invariably practised female
infanticide; but this practice has now been discontinued for a
considerable period. Of the children born within late years, the males
are more numerous than the females, in the proportion of 124 to 100.
Colonel Marshall accounts for this fact in the following ingenious
manner. "Let us for the purpose of illustration take three families as
representing an average of the entire tribe; say that one mother gives
birth to six daughters and no sons; a second mother has six sons only,
whilst the third mother has three sons and three daughters. The
first mother, following the tribal custom, destroys four daughters and
preserves two. The second retains her six sons. The third kills two
daughters and keeps one, as also her three sons. We have then from the
three families, nine sons and three daughters, with which to
continue the breed. But whilst the males belong to families in which
the tendency to produce sons is great, the females are of those of a
converse inclination. Thus the bias strengthens with each
generation, until, as we find, families grow to have habitually more
sons than daughters."
* The Todas, 1873, pp. 100, 111, 194, 196.
That this result would follow from the above form of infanticide
seems almost certain; that is if we assume that a sex-producing
tendency is inherited. But as the above numbers are so extremely
scanty, I have searched for additional evidence, but cannot decide
whether what I have found is trustworthy; nevertheless the facts
are, perhaps, worth giving. The Maories of New Zealand have long
practised infanticide; and Mr. Fenton* states that he "has met with
instances of women who have destroyed four, six, and even seven
children, mostly females. However, the universal testimony of those
best qualified to judge, is conclusive that this custom has for many
years been almost extinct. Probably the year 1835 may be named as
the period of its ceasing to exist." Now amongst the New Zealanders,
as with the Todas, male births are considerably in excess. Mr.
Fenton remarks (p. 30), "One fact is certain, although the exact
period of the commencement of this singular condition of the
disproportion of the sexes cannot be demonstratively fixed, it is
quite clear that this course of decrease was in full operation
during the years 1830 to 1844, when the non-adult population of 1844
was being produced, and has continued with great energy up to the
present time." The following statements are taken from Mr. Fenton
(p. 26), but as the numbers are not large, and as the census was not
accurate, uniform results cannot be expected. It should be borne in
mind in this and the following cases, that the normal state of every
population is an excess of women, at least in all civilised countries,
chiefly owing to the greater mortality of the male sex during youth,
and partly to accidents of all kinds later in life. In 1858, the
native population of New Zealand was estimated as consisting of 31,667
males and 24,303 females of all ages, that is in the ratio of 130.3
males to 100 females. But during this same year, and in certain
limited districts, the numbers were ascertained with much care, and
the males of all ages were here 753 and the females 616; that is in
the ratio of 122.2 males to 100 females. It is more important for us
that during this same year of 1858, the non-adult males within the
same district were found to be 178, and the non-adult females 142,
that is in the ratio of 125.3 to 100. It may be added that in 1844, at
which period female infanticide had only lately ceased, the
non-adult males in one district were 281, and the non-adult females
only 194, that is in the ratio of 144.8 males to 100 females.
* Aboriginal Inhabitants of New Zealand (Government Report), 1859,
p. 36.
In the Sandwich Islands, the males exceed the females in number.
Infanticide was formerly practised there to a frightful extent, but
was by no means confined to female infants, as is shown by Mr. Ellis,*
and as I have been informed by Bishop Staley and the Rev. Mr. Coan.
Nevertheless, another apparently trustworthy writer, Mr. Jarves,*(2)
whose observations apply to the whole archipelago, remarks:-
"Numbers of women are to be found, who confess to the murder of from
three to six or eight children," and he adds, "females from being
considered less useful than males were more often destroyed." From
what is known to occur in other parts of the world, this statement
is probable; but must be received with much caution. The practice of
infanticide ceased about the year 1819, when idolatry was abolished
and missionaries settled in the islands. A careful census in 1839 of
the adult and taxable men and women in the island of Kauai and in
one district of Oahu (Jarves, p. 404), gives 4723 males and 3776
females; that is in the ratio of 125.08 to 100. At the same time the
number of males under fourteen years in Kauai and under eighteen in
Oahu was 1797, and of females of the same ages 1429; and here we
have the ratio of 125.75 males to 100 females.
* Narrative of a Tour through Hawaii, 1826, p. 298.
*(2) History of the Sandwich Islands, 1843, p. 93.
In a census of all the islands in 1850,* the males of all ages
amount to 36,272, and the females to 33,128, or as 109.49 to 100.
The males under seventeen years amounted to 10,773, and the females
under the same age to 9593, or as 112.3 to 100. From the census of
1872, the proportion of males of all ages (including half-castes) to
females, is as 125.36 to 100. It must be borne in mind that all
these returns for the Sandwich Islands give the proportion of living
males to living females, and not of the births; and judging from all
civilised countries the proportion of males would have been
considerably higher if the numbers had referred to births.*(2)
* This is given in the Rev. H. T. Cheever's Life in the Sandwich
Islands, 1851, p. 277.
*(2) Dr. Coulter, in describing (Journal R. Geograph. Soc., vol. v.,
1835, p. 67) the state of California about the year 1830, says that
the natives reclaimed by the Spanish missionaries, have nearly all
perished, or are perishing, although well treated, not driven from
their native land, and kept from the use of spirits. He attributes
this, in great part, to the undoubted fact that the men greatly exceed
the women in number; but he does not know whether this is due to a
failure of female offspring, or to more females dying during early
youth. The latter alternative, according to all analogy, is very
improbable. He adds that "infanticide, properly so called, is not
common, though very frequent recourse is had to abortion." If Dr.
Coulter is correct about infanticide, this case cannot be advanced
in support of Colonel Marshall's view. From the rapid decrease of
the reclaimed natives, we may suspect that, as in the cases lately
given, their fertility has been diminished from changed habits of
life.
I had hoped to gain some light on this subject from the breeding
of dogs; inasmuch as most breeds, with the exception, perhaps, of
greyhounds, many more female puppies are destroyed than males, just as
with the Toda infants. Mr. Cupples assures me that this is usual
with Scotch deerhounds. Unfortunately, I know nothing of the
proportion of the sexes in any breed, excepting greyhounds, and
there the male births are to the females as 110.1 to 100. Now from
enquiries made from many breeders, it seems that the females are in
some respects more esteemed, though otherwise troublesome; and it does
not appear that the female puppies of the best-bred dogs are
systematically destroyed more than the males, though this does
sometimes take place to a limited extent. Therefore I am unable to
decide whether we can, on the above principles, account for the
preponderance of male births in greyhounds. On the other hand, we have
seen that with horses, cattle, and sheep, which are too valuable for
the young of either sex to be destroyed, if there is any difference,
the females are slightly in excess.
From the several foregoing cases we have some reason to believe that
infanticide practised in the manner above explained, tends to make a
male-producing race; but I am far from supposing that this practice in
the case of man, or some analogous process with other species, has
been the sole determining cause of an excess of males. There may be
some unknown law leading to this result in decreasing races, which
have already become somewhat infertile. Besides the several causes
previously alluded to, the greater facility of parturition amongst
savages, and the less consequent injury to their male infants, would
tend to increase the proportion of live-born males to females. There
does not, however, seem to be any necessary connection between
savage life and a marked excess of males; that is if we may judge by
the character of the scanty offspring of the lately existing
Tasmanians and of the crossed offspring of the Tahitians now
inhabiting Norfolk Island.
As the males and females of many animals differ somewhat in habits
and are exposed in different degrees to danger, it is probable that in
many cases, more of one sex than of the other are habitually
destroyed. But as far as I can trace out the complication of causes,
an indiscriminate though large destruction of either sex would not
tend to modify the sex-producing power of the species. With strictly
social animals, such as bees or ants, which produce a vast number of
sterile and fertile females in comparison with the males, and to
whom this preponderance is of paramount importance, we can see that
those communities would flourish best which contained females having a
strong inherited tendency to produce more and more females; and in
such cases an unequal sex-producing tendency would be ultimately
gained through natural selection. With animals living in herds or
troops, in which the males come to the front and defend the herd, as
with the bisons of North America and certain baboons, it is
conceivable that a male-producing tendency might be gained by
natural selection; for the individuals of the better defended herds
would leave more numerous descendants. In the case of mankind the
advantage arising from having a preponderance of men in the tribe is
supposed to be one chief cause of the practice of female infanticide.
In no case, as far as we can see, would an inherited tendency to
produce both sexes in equal numbers or to produce one sex in excess,
be a direct advantage or disadvantage to certain individuals more than
to others; for instance, an individual with a tendency to produce more
males than females would not succeed better in the battle for life
than an individual with an opposite tendency; and therefore a tendency
of this kind could not be gained through natural selection.
Nevertheless, there are certain animals (for instance, fishes and
cirripedes) in which two or more males appear to be necessary for
the fertilisation of the female; and the males accordingly largely
preponderate, but it is by no means obvious how this male-producing
tendency could have been acquired. I formerly thought that when a
tendency to produce the two sexes in equal numbers was advantageous to
the species, it would follow from natural selection, but I now see
that the whole problem is so intricate that it is safer to leave its
solution for the future.
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