In this chapter we have seen how evolution is possible under natural conditions. If the law of increase be true, if more are born than can survive to procreate their kind, natural selection is a logical necessity. We must not blame our forefathers for not seeing this. Until geology had extended our conception of time, no such conclusions could be drawn. If organisms have existed but six or seven thousand years, and if in the last thousand years little or no change in organic life has occurred, the supposition that they could have originated by any such process as natural selection is manifestly absurd. Lyell was the necessary precursor of Darwin. Given, then, increase and elimination throughout geological time, natural selection is a logical necessity. No one who adequately grasps the facts can now deny it. It is an unquestionable factor in organic evolution. Whether it is the sole factor, is quite another matter, and one we will consider in the chapter on "Organic Evolution." NOTES [M] Samuel Butler in England, and Ewald Hering in Prague, have ingeniously likened this hereditary persistence to "organic memory." What are ordinarily called memory, habit, instinct, and embryonic reconstruction, are all referable to the memory of organic matter. The analogy, if used with due caution, is a helpful one, what we call memory being the psychical aspect (under certain special organic and neural conditions) of what under the physical aspect we call persistence. [N] I have also to thank Mr. Edward Wilson for kindly giving me the measurements of three or four bats in the Bristol Museum. [O] A millimetre is about 1/25 of an inch, or more exactly .03937 inch. [P] In nearly all cases the measurements were checked by comparing the two wings. In one or two instances there were differences of as much as two or three millimetres between the bones of the two sides of the body, but in most cases they exactly corresponded. [Q] We are anxious to extend our observations and to compare series of bats from different localities. If any of my readers should feel disposed to help us, by sending specimens (_with the locality duly indicated_) to Mr. H. Charbonnier, 7, The Triangle South, Clifton, Bristol, we shall be grateful. [R] _Nature_, vol. xli. p. 393. The variation in molluscs is often considerable. In one of the bays in the basement hall of the Natural History Museum is a series showing the variation in size, form, and sculpturing of _Paludomus loricatus_, which is found in the streams of Ceylon. These varieties have in former times been named as ten distinct species!