Page:Popular Science Monthly Volume 84.djvu/19

Rh to treat biological problems in terms of mathematics he can make the same predictions from the data that can be treated without regard to the mechanism of the chromosomes. But since we find in the chromosomes all the machinery actually at hand for carrying out this procedure, it seems to me reasonable to base our conceptions on this mechanism until another is forthcoming. And if it should prove true that we have found the actual mechanism in the organism that accounts for segregation, assortment and linkage of hereditary factors we have made a distinct advance in our study of the constitution of the germ plasm.

It has been pointed out to me, more than once, that the views here presented concerning the "architecture" of the chromosome are similar to the views (assumed to be discredited) that Weismann advanced several years ago. But it should not be overlooked that Weismann's purpose in locating his determinants in the chromosomes was only that he might separate them again during development. He tried, in fact, to explain development in this way without, however, explaining what determines during development the orderly disintegration of the chromosomes. Nothing of the sort is postulated, or implied, on my view. Weismann's hypothesis was purely speculative. My own conception of the constitution of the chromosomes rests on numerical data obtained from hereditary characters. All of the chromosomes are supposed to go intact to every cell of the body as observation, so far as it goes, shows to be the case. How differentiation takes place is a question quite remote from the idea of the architecture of the chromosomes in their relation to hereditary characters.

There is but one fundamental similarity between my own view and that of Weismann. The chromosomes, looked upon as the vehicles of heredity, are assumed by both of us to have definite structures and not to be simply bags filled with a homogeneous fluid. The discrete parts (factors) of these structures are supposed to influence the course of differentiation, but there the resemblance ends. A factor, as I conceive it, is some minute particle of the chromosome whose presence in the cell influences the physiological processes that go on in the cell. Such a factor is supposed to be one element only in producing characters of the body. All the rest of the cell or much of it (including the inherited cytoplasm) may take part in producing the characters. So far as such things as unit characters exist I look upon them merely as the most conspicuous result of the activity of some part of the chromosome. A single factor may affect all parts of the body visibly, or a factor may preponderantly influence only a limited section of the body. As a matter of fact, if we look carefully, we can generally find farreaching effects of single factors. On the other hand, Weismann's idea of development emphasizes the intimate relation between his determinant and a specific character of the body. His writings often leave the