Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/211

Rh arrived at by a comparison of many biological generalizations, we then tried to show was the necessary consequence following from the Struggle for Existence combined with the effects of Inheritance and Variation: the resultant of these three forces being what Mr. Darwin calls Natural Selection. It must be remembered that Natural Selection does not explain the facts of Inheritance and Variation, but follows from them and the Struggle for Existence. The facts of Inheritance and Variation seem to depend upon Generation and Nutrition, which are chemical and physical phenomena still involved in much obscurity. Before leaving the subject, it seems proper to mention, as it does not appear to be generally understood, that the theory of the Evolution of Life may be accepted as true, and yet Natural Selection not be considered as a sufficient explanation. Suppose, now that the attention of naturalists has been drawn to the theory of Evolution, that most careful observations are made in reference to this subject, and that all biologists become convinced in time that plants and animals gradually change, the flora and fauna of a remote future differing very considerably from those of the present day,—the theory of the Evolution of Life might be demonstrated, and yet it might be shown that Natural Selection did not entirely produce it, or indeed the cause might still remain unknown. Let us repeat, then, that whatever may be thought of the causes advanced, as sufficient to bring about a development of life, the theory of Evolution remains the only explanation of the most important generalization of the comparative anatomy of plants and animals, their Paleontology, Embryology, and Geographical Distribution.