The Descent of Man

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 tortoiseshell
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 limi-