224 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, the factors derived from the mother 

 separate from those derived from the father, each 

 pair by itself. The precise way in which this is 

 supposed to take place may differ shghtly on the 

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

 owe to Weismann more than to any other biologist 

 the conception of segregation at the reduction di- 

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

 mental importance in the appUcation of the chromo- 

 some theory to Mendehan heredity. The factorial 

 hypothesis postulates only three things about the 

 factors with which it works, viz.: (1) that they are 

 constant, (2) that they are usually in duplicate in 

 each cell of the body, and (3) that they usually segre- 

 gate in the maturing germ cells. 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 appUcation of the behavior 

 of the chromosomes to heredity, then, is the third 

 important contribution which modern genetics owes 



