From what has been said of the nature of light, and the physical origin of colour, we see that to produce any distinct tint such as red, yellow, green, or blue, a definite physical structure must be formed, capable of reflecting certain rays of the same nature and absorbing others. Hence, whenever we see any distinct colour, we may be sure that a very considerable development in a certain direction has taken place. This is a most important conclusion, though not very obvious at first sight. Still, when we bear in mind the numbers of light waves of different lengths, and know that if these are reflected irregularly, we get only mixed tints such as indefinite browns; we can at once see how, in the case of such objects as tree trunks, and, still more, in inanimate things like rocks and soils, these, so-to-say, undifferentiated hues are just what we might expect to prevail, and that when definite colours are produced, it of necessity implies an effort of some sort. Now, if this be true of such tints as red and blue, how much more must it be the case with black and white, in which all the rays are absorbed or all reflected? These imply an even stronger effort, and _a priori_ reasoning would suggest that where they occur, they have been developed for important purposes by what may be termed a supreme effort. Consequently, we find them far less common than the others; and it is a most singular fact that in mimetic insects, these are the colours that are most frequently made use of. It would almost seem as if a double struggle had gone on: first, the efforts which resulted in the protective colouring of the mimicked species, and then a more severe, because necessarily more rapid, struggle on the part of the mimicker. Yet another point in this connection. If this idea be correct, it follows that a uniformly coloured flower or animal must be of extreme rarity, since it necessitates not merely the entire suppression of the tendency to emphasize important regions in colour, but also the adjustment of all the varying parts of the organism to one uniform molecular condition, which enables it to absorb all but a certain closely related series of light waves no matter how varied the functions of the parts. Now, such "self-coloured" species, as florists would call them, are not only rare, but, as all horticulturists know, are extremely difficult to produce. When a pansy grower, for instance, sets to work to produce a self-coloured flower--say a white pansy without a dark eye--his difficulties seem insurmountable. And, in truth, this result has never been quite obtained; for he has to fight against every natural tendency of the plant to mark out its corolla-tube in colour, and when this is overcome, to still restrain it, so as to keep it within those limits which alone allow it to reflect the proper waves of light. [Illustration: Plate I. KALLIMA INACHUS.]