Thus the occurrence not only of conspicuous, but of complicated, coloration is explained.
Darwin has shown that, in the equipment developed by the males in their competition for the possession of the females, it is by no means only those characters which may be considered 'beautiful' in themselves that have to be considered; it is rather the striking characteristics which mark their possessor and distinguish it from others that are primarily important. In fact, it is the principle of 'mode' or 'fashion' which is operative; something new is demanded, and as far as possible something quite different from that which was previously considered beautiful. Thus the starting-point for such processes of selection may have been afforded by white spots on a black ground, or, indeed, by any light spots on a dark ground, which may have been the primitive colour in most cases. If in the course of a long series of generations these spots became the common property of all the males, a possibility of further change was opened up as soon as a new contrast cropped up as a chance variation, which would then, under favourable conditions, be the starting-point of a new process of selection. Darwin has cited some cases in which, from a comparison of the dress of the young bird with that of the adult, we may conclude that a transformation of the colouring of the whole plumage must have taken place in the course of the phylogenetic history.
In other cases the course of the process of selection has been such that, though the general colouring has not been changed, variations have appeared in particular regions of the body--spots or stripes which accumulated through the ages and co-operated to form an increasingly diverse and complex colour-scheme, such a 'marking' of the animal as we may observe to-day, especially in butterflies, but also in birds.
It is a fine corroboration of the origin of bright colours through sexual selection that, even in those groups of the animal kingdom which are in general sexually monomorphic, there always occur some species in which male and female are quite different, and a host of species in which both sexes are alike in the main, yet with differences in certain minor points. Among the parrots similarity of colouring prevails as a general rule, but in New Guinea there lives a parrot the female of which is a gorgeous blood-red and the male a beautiful light-green; minor differences occur in many species, for instance, the female of the horned parrot (_Cyanorhamphus cornutus_ Gm.) lacks the two long black and red feathers on the head, that of the grass-parakeet (_Melopsittacus undulatus_) is a slightly paler green and has not the beautiful blue spots on the cheeks which the male possesses. Innumerable similar instances might be cited, serving to show that all these distinguishing characters of the males have been acquired step by step and piece by piece, and are slowly and independently transferred to the females--if, indeed, at all.