I have now given all the cases known to me of a difference in colour
between the sexes of mammals. Some of these may be the result of
variations confined to one sex and transmitted to the same sex, without
any good being gained, and therefore without the aid of selection. We
have instances of this with our domesticated animals, as in the males
of certain cats being rusty-red, whilst the females are tortoise-shell
coloured. Analogous cases occur in nature: Mr. Bartlett has seen many
black varieties of the jaguar, leopard, vulpine phalanger, and wombat;
and he is certain that all, or nearly all these animals, were males. On
the other hand, with wolves, foxes, and apparently American squirrels,
both sexes are occasionally born black. Hence it is quite possible that
with some mammals a difference in colour between the sexes, especially
when this is congenital, may simply be the result, without the aid of
selection, of the occurrence of one or more variations, which from the
first were sexually limited in their transmission. Nevertheless it is
improbable that the diversified, vivid, and contrasted colours of
certain quadrupeds, for instance, of the above monkeys and antelopes,
can thus be accounted for. We should bear in mind that these colours do
not appear in the male at birth, but only at or near maturity; and that
unlike ordinary variations, they are lost if the male be emasculated.
It is on the whole probable that the strongly-marked colours and other
ornamental characters of male quadrupeds are beneficial to them in
their rivalry with other males, and have consequently been acquired
through sexual selection. This view is strengthened by the differences
in colour between the sexes occurring almost exclusively, as may be
collected from the previous details, in those groups and sub-groups of
mammals which present other and strongly-marked secondary sexual
characters; these being likewise due to sexual selection.
Quadrupeds manifestly take notice of colour. Sir S. Baker repeatedly
observed that the African elephant and rhinoceros attacked white or
grey horses with special fury. I have elsewhere shewn (32. The
Variation of Animals and Plants under Domestication, 1868, vol. ii.
pp. 102, 103.) that half-wild horses apparently prefer to pair with
those of the same colour, and that herds of fallow-deer of different
colours, though living together, have long kept distinct. It is a more
significant fact that a female zebra would not admit the addresses of a
male ass until he was painted so as to resemble a zebra, and then, as
John Hunter remarks, she received him very readily. In this curious
fact, we have instinct excited by mere colour, which had so strong an
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