otherwise inexplicable facts. These examples of incongruous thoughts I give to prepare the way; and will now go on to examine the chief propositions which the quoted passage contains.
The Duke of Argyll says that "heredity is the central idea of natural selection." Now it would, I think, be concluded that those who possess the central idea of a thing have some consciousness of the thing. Yet men have possessed the idea of heredity for any number of generations and have been quite unconscious of natural selection. Clearly the statement is misleading. It might just as truly be said that the occurrence of structural variations in organisms is the central idea of natural selection. And it might just as truly be said that the action of external agencies in killing some individuals and fostering others is the central idea of natural selection, No such assertions are correct. The process has three factors—heredity, variation and external action—any one of which being absent the process ceases. The conception contains three corresponding ideas, and if any one be struck out the conception cannot be framed. No one of them is the central idea, but they are coessential ideas.
From the erroneous belief that "heredity is the central idea of natural selection" the Duke of Argyle draws the conclusion, consequently erroneous, that "natural seclection includes and covers all the causes which can possibly operate through inheritance." Had he considered the cases which, in the Principles of Biology, I have cited to illustrate the inheritance of functionally produced modifications, he would have seen that his inference is far from correct. I have instanced the decrease of the jaw among civilised men as a change of structure which cannot have been produced by the inheritance of spontaneous, or fortuitous, variations. That changes of structure arising from such variations may be maintained and increased in successive generations, it is needful that the individuals in whom they occur shall derive from them advantages in the struggle for existence—advantages, too, sufficiently great to aid their survival and multiplication in considerable degrees. But a decrease of jaw, reducing its weight by even an ounce (which would be a large variation), cannot, by either smaller weight carried or smaller nutrition required, have appreciably advantaged any person in the battle of life. Even supposing such diminution of jaw to be beneficial (and in the resulting decay of teeth it entails great evils), the benefit can hardly have been such as to increase the relative multiplication of families in which it occurred generation after generation. Unless it has done this, however, decreased size of the jaw cannot have been produced by the natural selection of favorable variations. How can it then have
