SCOPE OF GENETICS 31 



enough. No practical dog-breeder or seeds- 

 man can see the results of Mendelian recom- 

 bination without perceiving that here is a bit 

 of knowledge he can immediately apply. No 

 sociologist can examine the pedigrees illus- 

 trating the simple descent of a deformity or 

 a congenital disease, and not see that the 

 new knowledge gives a solid basis for prac- 

 tical action by which the composition of a 

 race could be modified if society so chose. 

 More than this : we know for certain in one 

 case, from the work of Professor BifFen, that 

 the power to resist a disease caused by the 

 invasion of a pathogenic organism, wheat- 

 rust, is due to the absence of one of the 

 simple factors or ingredients of which I have 

 spoken, and what we know to be true in that 

 one case we are beginning to suspect to be 

 true of resistance to certain other diseases. 

 No pathologist can see such an experiment 

 as this of Professor Biffen's without realizing 



