Let us now see in what way this theory has been applied to colouration. The colours, or, more strictly, the arrangement of colours, in patterns is of several kinds, viz.:—
1. General Colouration, or such as appears to have no very special function as colour. We find this most frequently in the vegetable kingdom, as, for instance, the green hue of leaves, which, though it has a most valuable function chemically has no particular use as colour, so far as we can see.
2. Distinctive Colouration, or the arrangement of colours in different patterns or tints corresponding to each species. This is the most usual style of colouring, and the three following kinds are modifications of it. It is this which gives each species its own design, whether in animals or plants.
3. Protective Resemblance, or the system of colouring which conceals the animal from its prey, or hides the prey from its foe. Of this class are the green hues of many caterpillars, the brown tints of desert birds, and the more remarkable resemblances of insects to sticks and leaves.
4. Mimetic Colouration, or the resemblance of one animal to another. It is always the resemblance of a rare species, which is the favourite food of some creature, to a common species nauseous to the mimicker's foe. Of this character are many butterflies.
5. Warning Colours, or distinctive markings and tints rendering an animal conspicuous, and, as it were, proclaiming noli me tangere to its would-be attackers.
6. Sexual Colours, or particular modifications of colour in the two sexes, generally taking the form of brilliancy in the male, as in the peacock and birds of paradise.
Under one or other of these headings most schemes of colouration will be found to arrange themselves.
At the outset, and confining ourselves to the animal kingdom for the present, bearing in mind the fierce intensity of the struggle for life, it would seem that any scheme of colour that would enable its possessor to elude its foes or conceal itself from its prey, would be of vital importance. Hence we might infer that protective colouring would be a very usual phenomenon; and such we find to be the case. In the sea we have innumerable instances of protective colouring. Fishes that lie upon the sandy bottom are sand-coloured, like soles and plaice, in other orders we find the same hues in shrimps and crabs, and a common species on our shores (Carcinus mænas) has, just behind the eyes, a little light irregular patch, so like the shell fragments around that when it hides in the sand, with eyes and light spot alone showing, it is impossible to distinguish it.