Page:Philosophical Review Volume 25.djvu/228

216 presence-absence hypothesis are of value in dealing with the inheritance of Mendelian characters, but we must modify the terminology when dealing with the origin of these character-differences. In reference, now, to the evolutionism of Bergson, the author is inclined to accept the criticisms of Bergson's metaphysics and epistemology offered by Bertrand Russel. Bergson is correct in his insistence upon phylogenetic divergence and occasional developments. He is also correct in his view that there is no predetermined course of evolution, if this means that the particular directions of various phylogenies are narrowly limited by conditions of the earth's surface. Bergson asks how we can explain the development of the eye in mollusks and vertebrates from purely fortuitous circumstances. But he increases the difficulty by assuming that inherited variations arise independently and simultaneously in different parts of the organism. Our view implies that the variations are all expressions of an original change in the fertilized egg. He finds difficulty in such variations being considered complementary. But one organ may influence and even produce another organ, as in the case of the tadpole's skin, which, when grafted over the developing optic vescicle, becomes a lens. Bergson asks how the small variations could have been preserved and accumulated. This question assumes that the various stages in the perfecting of an organ are in themselves of no service to the organism. The important fact that apparently new organs are often a remoulding of old organs must not be forgotten. But the changes must be correlated and must be such as to make survival and evolution possible, to be inherited at all. Some changes are advantageous, some bizarre, and some harmful. Let us now consider parallel development, as in the case of the molluscan and vertebrate eye. There are several types of eyes among the invertebrates, of which only one type is parallel to the vertebrate eye. The mollusks have frequently very many eyes, and these of different types, in the same organism. Bergson selects this complex case, and declares that science cannot explain it; the scientist points to simpler cases as affording a clue for the explanation. Thus wings have been evolved many times independently. Bergson finds a difficulty for science in the case of complex instincts, such as the instinct of the beetle Sitaris. But every variation implies a basis in the fertilized egg and is effective throughout the whole ontogeny. Every ontogenetic stage is modified by this initial change. This is clearly the case with structural modifications, and also applies to instincts, as these latter must have a structural foundation. No doubt it is hard to understand the transmission of complex instincts on the basis of germ-plasm, but this is no harder to conceive than the hereditary transmission of intellectual differences in man.

That the negative family morality of the past, with its command, 'Thou shalt not,' has failed, is proved by the present decreasing birthrate among the educated classes, the prevalence of divorce, illegitimacy and kindred evils.