Now it can be shown that no two individuals are exactly alike, in other words, that variation is constantly taking place, and that no animal or plant preserves its characters unmodified. This we might have expected if we attentively consider how impossible it is for any two individuals to be subjected to exactly the same conditions of life and habit. But for the proofs of variability we have not to rely upon theoretical reasoning. No one can study, even superficially, any class or species without daily experiencing the conviction that no two individuals are alike, and that variation takes place in almost every conceivable direction. Granted then the existence of the struggle for existence and the variability of individuals, and granting also that if any variation gives its possessor a firmer hold upon life, it follows as a necessity that the most favoured individuals will have the best chance of surviving and leaving descendants, and by the law of heredity, we know these offspring will tend to inherit the characters of their parents. This action is often spoken of as the preservation of favoured races, and as the survival of the fittest. The gradual accumulation of beneficial characters will give rise in time to new varieties and species; and in this way primarily has arisen the wonderful diversity of life that now exists. Such, in barest outline, is the theory of descent with modification. 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.