The presence of some colour, or even of many brilliant colours, in animals and plants would require no other explanation than does that of the sky or the ocean, of the ruby or the emerald--that is, it would require a purely physical explanation only.

It is the wonderful individuality of the colours of animals and plants that attracts our attention--the fact that the colours are localised in definite patterns, sometimes in accordance with structural characters, sometimes altogether independent of them; while often differing in the most striking and fantastic manner in allied species.

We are thus compelled to look upon colour not merely as a physical but also as a biological characteristic, which has been differentiated and specialised by natural selection, and must, therefore, find its explanation in the principle of adaptation or utility.
_The Constancy of Animal Colour indicates Utility._ That the colours and markings of animals have been acquired under the fundamental law of utility is indicated by a general fact which has received very little attention.

As a rule, colour and marking are constant in each species of wild animal, while, in almost every domesticated animal, there arises great variability.

We see this in our horses and cattle, our dogs and cats, our pigeons and poultry.