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. |