278 THE FACTORIAL HYPOTHESIS 



alteration of either, and that these allelomorphs will 

 now perpetuate themselves unchanged although in 

 company with different factors. Today this as- 

 sumption is no longer an a priori deduction, but a 

 conclusion from experimental data. 



The second real and important point of agreement 

 between the factorial theory and Weismann's theory 

 is that both maintain that at one period in the history 

 of the germ cells, factors of diverse origin separate 

 from each other in an orderly manner, half of them 

 going to each pole. The precise way in which this 

 is supposed to take place differs greatly on the 

 two views, but the essential point is the same. We 

 owe to Weismann more than to any other biologist 

 the conception of segTegation at the reduction di- 

 vision of the egg and sperm — a conception of funda- 

 mental importance in the application of the chromo- 

 some theory to Mendelian heredity. 



The factorial theory as such deals with the be- 

 havior of its factors in an abstract way, quite apart 

 from any material basis of which they may happen to 

 be composed. In this way it may measure their con- 

 stancy, segregation, Unkage, etc. But the biologist 

 is not likely to stop here, for, to him the problem in- 

 volves cells about whose history and processes he has 

 come to know certain facts. Weismann, following 

 Roux, was the first to point out that these facts give a 

 mechanism showing how separation of factors might 

 take place. The specific appHcation of the behavior 

 of the chromosomes to heredity, then, is the third 

 important contribution which modern genetics owes 



