The Descent of Man

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 effect as to get the better of everything
else. But the male did not require this, the
female being an animal somewhat similar to
himself, was sufficient to rouse him." (33. 'Essays
and Observations,' by J. Hunter, edited by
Owen, 1861, vol. i. p. 194.)