Page:Popular Science Monthly Volume 63.djvu/228

224 To compare heredity with memory explains nothing, of course, since we know as little of the physical basis of the one as of the other, but if the analogy be admitted it will prevent the too confident insistence upon the theory that heredity depends entirely upon positional or other mechanical relations of molecules, or is in some way embodied in particular granules or chromosomes.

But even the most fantastic theories often have some basis of suggestion in fact, and although we can not accept Professor Wilson's cytological explanation of Mendel's laws, nor even share his hope that cytology will elucidate evolution, it is by no means impossible that the normal individual diversity of organisms has a cytological as well as an evolutionary significance. That normal development or growth by cell division is advantaged by cross-fertilization may mean that the cells divide more readily and normally when they contain protoplasmic 'elements' of a proper degree of diversity than when they have only one kind of protoplasm, as would happen in narrow inbreeding, and also when cross-breeding is too wide for the intimate cooperation required for true fertilization. The Mendelian effect would then be explainable on the suggestion of a partial cooperation which has to be abandoned in the formation of new individuals, because, while the organism can follow either of two diverging parental roads with respect to any character, it is, as it were, a stranger to the path that an average would require.

The conjugation of cells may be viewed as a process quite distinct from reproduction, though it is a necessary preliminary to the long series of cell divisions required to build up the complex bodies of the higher animals and plants. As we descend in the organic scale the conjugating cells become more and more similar to each other and to the so-called vegetative or somatic cells of which the body of the organism is composed. Among simple organisms all the cells are alike, including those formed immediately before and after conjugation, and it is not strange that with the diversification of the cells which constitute the various tissues of the plant or animal body the germ cells should become specialized and unlike any of the others. The existence of special reproductive cells among the higher animals and plants is therefore to be looked upon as corresponding to the general complexity of the organism, rather than as an indication of a special mechanism of heredity resident in the germ cells. As founders of new cell-colonies or compound individuals they develop, it appears, on one or the other of divergent parental lines instead of striking out on an untraveled road between.

Notwithstanding their great significance Mendel's laws are negative rather than positive in their bearing upon descent, since we do