CONTROL OF HEREDITY: EUGENICS 391 



This distinction between somatic and germi- 

 nal variations, between those which concern 

 only the individual and those which are in- 

 herited and furnish material for evolution, 

 marks the greatest advance in the study of 

 evolution since the work of Darwin. And just 

 as these germinal variations are the only ones 

 of importance in the process of evolution so 

 the question of their origin is the greatest evo- 

 lutionary problem of the present day. How 

 are such germinal variations produced? 



There is evidence, as was pointed out in the 

 last lecture, that environmental changes of the 

 right sort acting upon germ cells at the right 

 stage may lead to permanent modification of 

 the hereditary organization. The observa- 

 tions of Hensen on yeast and of Tower and 

 Morgan on insects strongly support this 

 conclusion. 



It is probable that certain changes in en- 

 vironment acting upon germ cells at the time 

 of their maturation divisions may lead to new 

 distributions of chromosomes or even to 

 changes in the composition of individual chro- 

 mosomes, thus producing new hereditary 



