On the other hand, white flowers are not likely to produce red varieties, and we believe we may positively assert that they never produce a blue sport. Similarly, white animals appear not to give rise to colour varieties. We are never surprised to find that an ordinary upright plant produces as a sport or mutation a pendulous, or fastigiate form. These aberrant varieties, be it noted, occur in species which belong to quite different orders. De Vries points out that laciniated leaves appear in such widely separated trees and shrubs as the walnut, the beech, the hazel-nut, and the turnip. Another example of the definiteness of variation is furnished by what Grant Allen calls the “Law of Progressive Colouration” of flowers. On pp. 20, 21 of _The Colours of Flowers_, he writes, “All flowers, as we know, easily sport a little in colour. But the question is, do their changes tend to follow any regular and definite order? Is there any reason to believe that the modification runs from any one colour toward any other? Apparently there is. . . . All flowers, it would seem, were in their earliest form yellow; then some of them became white; after that a few of them grew to be red or purple; and finally a comparatively small number acquired the various shades of lilac, mauve, violet, or blue.” Over-development So among animals there are many colour patterns and structures that appear in widely different genera, as, for example, the magpie colouring in birds. With this phenomenon we shall deal more fully when speaking of animal colouration. There is certainly no small amount of evidence which seems to indicate that, from some cause or other, an impetus has been given to certain organs to develop along definite lines. The reduction of the number of digits in several mammalian families which are not nearly related is a case in point. This phenomenon is, as Cope points out, observed in Marsupials, Rodents, Insectivores, Carnivores, and Ungulates. He, being a Lamarckian, ascribes this to the inherited effects of use. Wallaceians attribute it solely to the action of natural selection. The assumption of a growth-force or tendency for the development of one digit at the expense of the others, would explain the phenomenon equally well. And it is significant that many palæontologists are believers in some kind of a growth-force. In the case of certain extinct animals we seem to have examples of the over-development of organs. “Palæontology,” writes Kellog on p. 275 of his _Darwinism To-day_, “reveals to us the one-time existence of animals, of groups of animals, and of lines of descent, which have had characteristics which led to extinction. The unwieldiness of the giant Cretaceous reptiles, the fixed habit of life of the crinoids, the coiling of the ammonities and the nautili, the gigantic antlers of the Irish stag—all these are examples of development along