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 segregation 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, linkage, 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 application of the behavior 
of the chromosomes to heredity, then, is the third 
important contribution which modern genetics owes 
