THE MECHANISM OF HEREDITY 15 



fers 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 chro- 

 mosomes all the machinery actually at hand for carrying out this pro- 

 cedure, it seems to me reasonable to base our conceptions on this mech- 

 anism 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 sev- 

 eral years ago. But it should not be overlooked that Weismann's pur- 

 pose 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 chro- 

 mosomes. 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 T con- 

 ceive 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 charac- 

 ters 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 chromo- 

 some. 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 far- 

 reaching effects of single factors. On the other hand, Weismann's idea 

 of development emphasizes the intimate relation between his determi- 

 nant and a specific character of the body. His writings often leave the 



