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The Descent of Man and Selection in Relation to Sex

C >> Charles Darwin >> The Descent of Man and Selection in Relation to Sex




In a census of all the islands in 1850 (98. This is given in the Rev. H.T.
Cheever's 'Life in the Sandwich Islands,' 1851, p. 277.), 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. (99. 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 in 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 deer-
hounds. 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.


CHAPTER IX.

SECONDARY SEXUAL CHARACTERS IN THE LOWER CLASSES OF THE ANIMAL KINGDOM.

These characters absent in the lowest classes--Brilliant colours--Mollusca
--Annelids--Crustacea, secondary sexual characters strongly developed;
dimorphism; colour; characters not acquired before maturity--Spiders,
sexual colours of; stridulation by the males--Myriapoda.

With animals belonging to the lower classes, the two sexes are not rarely
united in the same individual, and therefore secondary sexual characters
cannot be developed. In many cases where the sexes are separate, both are
permanently attached to some support, and the one cannot search or struggle
for the other. Moreover it is almost certain that these animals have too
imperfect senses and much too low mental powers to appreciate each other's
beauty or other attractions, or to feel rivalry.

Hence in these classes or sub-kingdoms, such as the Protozoa, Coelenterata,
Echinodermata, Scolecida, secondary sexual characters, of the kind which we
have to consider, do not occur: and this fact agrees with the belief that
such characters in the higher classes have been acquired through sexual
selection, which depends on the will, desire, and choice of either sex.
Nevertheless some few apparent exceptions occur; thus, as I hear from Dr.
Baird, the males of certain Entozoa, or internal parasitic worms, differ
slightly in colour from the females; but we have no reason to suppose that
such differences have been augmented through sexual selection.
Contrivances by which the male holds the female, and which are
indispensable for the propagation of the species, are independent of sexual
selection, and have been acquired through ordinary selection.

Many of the lower animals, whether hermaphrodites or with separate sexes,
are ornamented with the most brilliant tints, or are shaded and striped in
an elegant manner; for instance, many corals and sea-anemones (Actiniae),
some jelly-fish (Medusae, Porpita, etc.), some Planariae, many star-fishes,
Echini, Ascidians, etc.; but we may conclude from the reasons already
indicated, namely, the union of the two sexes in some of these animals, the
permanently affixed condition of others, and the low mental powers of all,
that such colours do not serve as a sexual attraction, and have not been
acquired through sexual selection. It should be borne in mind that in no
case have we sufficient evidence that colours have been thus acquired,
except where one sex is much more brilliantly or conspicuously coloured
than the other, and where there is no difference in habits between the
sexes sufficient to account for their different colours. But the evidence
is rendered as complete as it can ever be, only when the more ornamented
individuals, almost always the males, voluntarily display their attractions
before the other sex; for we cannot believe that such display is useless,
and if it be advantageous, sexual selection will almost inevitably follow.
We may, however, extend this conclusion to both sexes, when coloured alike,
if their colours are plainly analogous to those of one sex alone in certain
other species of the same group.

How, then, are we to account for the beautiful or even gorgeous colours of
many animals in the lowest classes? It appears doubtful whether such
colours often serve as a protection; but that we may easily err on this
head, will be admitted by every one who reads Mr. Wallace's excellent essay
on this subject. It would not, for instance, at first occur to any one
that the transparency of the Medusae, or jelly-fish, is of the highest
service to them as a protection; but when we are reminded by Haeckel that
not only the Medusae, but many floating Mollusca, crustaceans, and even
small oceanic fishes partake of this same glass-like appearance, often
accompanied by prismatic colours, we can hardly doubt that they thus escape
the notice of pelagic birds and other enemies. M. Giard is also convinced
(1. 'Archives de Zoolog. Exper.' Oct. 1872, p. 563.) that the bright tints
of certain sponges and ascidians serve as a protection. Conspicuous
colours are likewise beneficial to many animals as a warning to their
would-be devourers that they are distasteful, or that they possess some
special means of defence; but this subject will be discussed more
conveniently hereafter.

We can, in our ignorance of most of the lowest animals, only say that their
bright tints result either from the chemical nature or the minute structure
of their tissues, independently of any benefit thus derived. Hardly any
colour is finer than that of arterial blood; but there is no reason to
suppose that the colour of the blood is in itself any advantage; and though
it adds to the beauty of the maiden's cheek, no one will pretend that it
has been acquired for this purpose. So again with many animals, especially
the lower ones, the bile is richly coloured; thus, as I am informed by Mr.
Hancock, the extreme beauty of the Eolidae (naked sea-slugs) is chiefly due
to the biliary glands being seen through the translucent integuments--this
beauty being probably of no service to these animals. The tints of the
decaying leaves in an American forest are described by every one as
gorgeous; yet no one supposes that these tints are of the least advantage
to the trees. Bearing in mind how many substances closely analogous to
natural organic compounds have been recently formed by chemists, and which
exhibit the most splendid colours, it would have been a strange fact if
substances similarly coloured had not often originated, independently of
any useful end thus gained, in the complex laboratory of living organisms.

THE SUB-KINGDOM OF THE MOLLUSCA.

Throughout this great division of the animal kingdom, as far as I can
discover, secondary sexual characters, such as we are here considering,
never occur. Nor could they be expected in the three lowest classes,
namely, in the Ascidians, Polyzoa, and Brachiopods (constituting the
Molluscoida of some authors), for most of these animals are permanently
affixed to a support or have their sexes united in the same individual. In
the Lamellibranchiata, or bivalve shells, hermaphroditism is not rare. In
the next higher class of the Gasteropoda, or univalve shells, the sexes are
either united or separate. But in the latter case the males never possess
special organs for finding, securing, or charming the females, or for
fighting with other males. As I am informed by Mr. Gwyn Jeffreys, the sole
external difference between the sexes consists in the shell sometimes
differing a little in form; for instance, the shell of the male periwinkle
(Littorina littorea) is narrower and has a more elongated spire than that
of the female. But differences of this nature, it may be presumed, are
directly connected with the act of reproduction, or with the development of
the ova.

The Gasteropoda, though capable of locomotion and furnished with imperfect
eyes, do not appear to be endowed with sufficient mental powers for the
members of the same sex to struggle together in rivalry, and thus to
acquire secondary sexual characters. Nevertheless with the pulmoniferous
gasteropods, or land-snails, the pairing is preceded by courtship; for
these animals, though hermaphrodites, are compelled by their structure to
pair together. Agassiz remarks, "Quiconque a eu l'occasion d'observer les
amours des limacons, ne saurait mettre en doute la seduction deployee dans
les mouvements et les allures qui preparent et accomplissent le double
embrassement de ces hermaphrodites." (2. 'De l'Espece et de la Class.'
etc., 1869, p. 106.) These animals appear also susceptible of some degree
of permanent attachment: an accurate observer, Mr. Lonsdale, informs me
that he placed a pair of land-snails, (Helix pomatia), one of which was
weakly, into a small and ill-provided garden. After a short time the
strong and healthy individual disappeared, and was traced by its track of
slime over a wall into an adjoining well-stocked garden. Mr. Lonsdale
concluded that it had deserted its sickly mate; but after an absence of
twenty-four hours it returned, and apparently communicated the result of
its successful exploration, for both then started along the same track and
disappeared over the wall.

Even in the highest class of the Mollusca, the Cephalopoda or cuttle-
fishes, in which the sexes are separate, secondary sexual characters of the
present kind do not, as far as I can discover, occur. This is a surprising
circumstance, as these animals possess highly-developed sense-organs and
have considerable mental powers, as will be admitted by every one who has
watched their artful endeavours to escape from an enemy. (3. See, for
instance, the account which I have given in my 'Journal of Researches,'
1845, p. 7.) Certain Cephalopoda, however, are characterised by one
extraordinary sexual character, namely that the male element collects
within one of the arms or tentacles, which is then cast off, and clinging
by its sucking-discs to the female, lives for a time an independent life.
So completely does the cast-off arm resemble a separate animal, that it was
described by Cuvier as a parasitic worm under the name of Hectocotyle. But
this marvellous structure may be classed as a primary rather than as a
secondary sexual character.

Although with the Mollusca sexual selection does not seem to have come into
play; yet many univalve and bivalve shells, such as volutes, cones,
scallops, etc., are beautifully coloured and shaped. The colours do not
appear in most cases to be of any use as a protection; they are probably
the direct result, as in the lowest classes, of the nature of the tissues;
the patterns and the sculpture of the shell depending on its manner of
growth. The amount of light seems to be influential to a certain extent;
for although, as repeatedly stated by Mr. Gwyn Jeffreys, the shells of some
species living at a profound depth are brightly coloured, yet we generally
see the lower surfaces, as well as the parts covered by the mantle, less
highly-coloured than the upper and exposed surfaces. (4. I have given
('Geological Observations on Volcanic Islands,' 1844, p. 53) a curious
instance of the influence of light on the colours of a frondescent
incrustation, deposited by the surf on the coast-rocks of Ascension and
formed by the solution of triturated sea-shells.) In some cases, as with
shells living amongst corals or brightly-tinted seaweeds, the bright
colours may serve as a protection. (5. Dr. Morse has lately discussed
this subject in his paper on the 'Adaptive Coloration of Mollusca,' 'Proc.
Boston Soc. of Nat. Hist.' vol. xiv. April 1871.) But that many of the
nudibranch Mollusca, or sea-slugs, are as beautifully coloured as any
shells, may be seen in Messrs. Alder and Hancock's magnificent work; and
from information kindly given me by Mr. Hancock, it seems extremely
doubtful whether these colours usually serve as a protection. With some
species this may be the case, as with one kind which lives on the green
leaves of algae, and is itself bright-green. But many brightly-coloured,
white, or otherwise conspicuous species, do not seek concealment; whilst
again some equally conspicuous species, as well as other dull-coloured
kinds live under stones and in dark recesses. So that with these
nudibranch molluscs, colour apparently does not stand in any close relation
to the nature of the places which they inhabit.

These naked sea-slugs are hermaphrodites, yet they pair together, as do
land-snails, many of which have extremely pretty shells. It is conceivable
that two hermaphrodites, attracted by each other's greater beauty, might
unite and leave offspring which would inherit their parents' greater
beauty. But with such lowly-organised creatures this is extremely
improbable. Nor is it at all obvious how the offspring from the more
beautiful pairs of hermaphrodites would have any advantage over the
offspring of the less beautiful, so as to increase in number, unless indeed
vigour and beauty generally coincided. We have not here the case of a
number of males becoming mature before the females, with the more beautiful
males selected by the more vigorous females. If, indeed, brilliant colours
were beneficial to a hermaphrodite animal in relation to its general habits
of life, the more brightly-tinted individuals would succeed best and would
increase in number; but this would be a case of natural and not of sexual
selection.

SUB-KINGDOM OF THE VERMES: CLASS, ANNELIDA (OR SEA-WORMS).

In this class, although the sexes, when separate, sometimes differ from
each other in characters of such importance that they have been placed
under distinct genera or even families, yet the differences do not seem of
the kind which can be safely attributed to sexual selection. These animals
are often beautifully coloured, but as the sexes do not differ in this
respect, we are but little concerned with them. Even the Nemertians,
though so lowly organised, "vie in beauty and variety of colouring with any
other group in the invertebrate series"; yet Dr. McIntosh (6. See his
beautiful monograph on 'British Annelids,' part i. 1873, p. 3.) cannot
discover that these colours are of any service. The sedentary annelids
become duller-coloured, according to M. Quatrefages (7. See M. Perrier:
'L'Origine de l'Homme d'apres Darwin,' 'Revue Scientifique', Feb. 1873, p.
866.), after the period of reproduction; and this I presume may be
attributed to their less vigorous condition at that time. All these worm-
like animals apparently stand too low in the scale for the individuals of
either sex to exert any choice in selecting a partner, or for the
individuals of the same sex to struggle together in rivalry.

SUB-KINGDOM OF THE ARTHROPODA: CLASS, CRUSTACEA.

In this great class we first meet with undoubted secondary sexual
characters, often developed in a remarkable manner. Unfortunately the
habits of crustaceans are very imperfectly known, and we cannot explain the
uses of many structures peculiar to one sex. With the lower parasitic
species the males are of small size, and they alone are furnished with
perfect swimming-legs, antennae and sense-organs; the females being
destitute of these organs, with their bodies often consisting of a mere
distorted mass. But these extraordinary differences between the two sexes
are no doubt related to their widely different habits of life, and
consequently do not concern us. In various crustaceans, belonging to
distinct families, the anterior antennae are furnished with peculiar
thread-like bodies, which are believed to act as smelling-organs, and these
are much more numerous in the males than in the females. As the males,
without any unusual development of their olfactory organs, would almost
certainly be able sooner or later to find the females, the increased number
of the smelling-threads has probably been acquired through sexual
selection, by the better provided males having been the more successful in
finding partners and in producing offspring. Fritz Muller has described a
remarkable dimorphic species of Tanais, in which the male is represented by
two distinct forms, which never graduate into each other. In the one form
the male is furnished with more numerous smelling-threads, and in the other
form with more powerful and more elongated chelae or pincers, which serve
to hold the female. Fritz Muller suggests that these differences between
the two male forms of the same species may have originated in certain
individuals having varied in the number of the smelling-threads, whilst
other individuals varied in the shape and size of their chelae; so that of
the former, those which were best able to find the female, and of the
latter, those which were best able to hold her, have left the greatest
number of progeny to inherit their respective advantages. (8. 'Facts and
Arguments for Darwin,' English translat., 1869, p. 20. See the previous
discussion on the olfactory threads. Sars has described a somewhat
analogous case (as quoted in 'Nature,' 1870, p. 455) in a Norwegian
crustacean, the Pontoporeia affinis.)

[Fig.4. Labidocera Darwinii (from Lubbock). Labelled are:
a. Part of right anterior antenna of male, forming a prehensile organ.
b. Posterior pair of thoracic legs of male.
c. Ditto of female.]

In some of the lower crustaceans, the right anterior antenna of the male
differs greatly in structure from the left, the latter resembling in its
simple tapering joints the antennae of the female. In the male the
modified antenna is either swollen in the middle or angularly bent, or
converted (Fig. 4) into an elegant, and sometimes wonderfully complex,
prehensile organ. (9. See Sir J. Lubbock in 'Annals and Mag. of Nat.
Hist.' vol. xi. 1853, pl. i. and x.; and vol. xii. (1853), pl. vii. See
also Lubbock in 'Transactions, Entomological Society,' vol. iv. new series,
1856-1858, p. 8. With respect to the zigzagged antennae mentioned below,
see Fritz Muller, 'Facts and Arguments for Darwin,' 1869, p. 40, foot-
note.) It serves, as I hear from Sir J. Lubbock, to hold the female, and
for this same purpose one of the two posterior legs (b) on the same side of
the body is converted into a forceps. In another family the inferior or
posterior antennae are "curiously zigzagged" in the males alone.

[Fig. 5. Anterior part of body of Callianassa (from Milne-Edwards),
showing the unequal and differently-constructed right and left-hand chelae
of the male. N.B.--The artist by mistake has reversed the drawing, and
made the left-hand chela the largest.

Fig. 6. Second leg of male Orchestia Tucuratinga (from Fritz Muller).

Fig. 7. Ditto of female.]

In the higher crustaceans the anterior legs are developed into chelae or
pincers; and these are generally larger in the male than in the female,--so
much so that the market value of the male edible crab (Cancer pagurus),
according to Mr. C. Spence Bate, is five times as great as that of the
female. In many species the chelae are of unequal size on the opposite
side of the body, the right-hand one being, as I am informed by Mr. Bate,
generally, though not invariably, the largest. This inequality is also
often much greater in the male than in the female. The two chelae of the
male often differ in structure (Figs. 5, 6, and 7), the smaller one
resembling that of the female. What advantage is gained by their
inequality in size on the opposite sides of the body, and by the inequality
being much greater in the male than in the female; and why, when they are
of equal size, both are often much larger in the male than in the female,
is not known. As I hear from Mr. Bate, the chelae are sometimes of such
length and size that they cannot possibly be used for carrying food to the
mouth. In the males of certain fresh-water prawns (Palaemon) the right leg
is actually longer than the whole body. (10. See a paper by Mr. C. Spence
Bate, with figures, in 'Proceedings, Zoological Society,' 1868, p. 363; and
on the nomenclature of the genus, ibid. p. 585. I am greatly indebted to
Mr. Spence Bate for nearly all the above statements with respect to the
chelae of the higher crustaceans.) The great size of the one leg with its
chelae may aid the male in fighting with his rivals; but this will not
account for their inequality in the female on the opposite sides of the
body. In Gelasimus, according to a statement quoted by Milne Edwards (11.
'Hist. Nat. des Crust.' tom. ii. 1837, p. 50.), the male and the female
live in the same burrow, and this shews that they pair; the male closes the
mouth of the burrow with one of its chelae, which is enormously developed;
so that here it indirectly serves as a means of defence. Their main use,
however, is probably to seize and to secure the female, and this in some
instances, as with Gammarus, is known to be the case. The male of the
hermit or soldier crab (Pagurus) for weeks together, carries about the
shell inhabited by the female. (12. Mr. C. Spence Bate, 'British
Association, Fourth Report on the Fauna of S. Devon.') The sexes, however,
of the common shore-crab (Carcinus maenas), as Mr. Bate informs me, unite
directly after the female has moulted her hard shell, when she is so soft
that she would be injured if seized by the strong pincers of the male; but
as she is caught and carried about by the male before moulting, she could
then be seized with impunity.

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