THE RAW MATERIALS OP EVOLUTION 113 



Another suggestion, which has some interesting 

 experimental evidence behind it, is that important 

 changes in the environment — changes in chemical 

 composition, heat, light, and electrical conditions, 

 and so on — may saturate deeply through the 

 body and stimulate the germ-cell to change. We 

 shall return to this suggestion later on. 



GrERMiNAL, Selection. — In his theory of "ger- 

 minal selection" Weismann has elaborated an 

 interesting speculation in regard to the roots of 

 variation. With his characteristic way of following 

 an idea as far as it will lead him, he has extended 

 the concepts of struggle and selection to the 

 primary constituents (or determinants) within 

 the germ-cell. In consequence of unequal nutrition 

 these primary constituents are always varying. If 

 one of them, corresponding, let us say, to a sense- 

 cell, receives for a considerable time more abundant 

 food than before, it will grow in proportion, and 

 if the germ-ceU develops into an organism the 

 sensory cell may be twice as strong as it was in 

 the parent. 



But the strengthened determinant may begin 

 actively to nourish itself more abundantly — 

 attracting the food to itself, and in some measure 

 withdrawing it from its fellow-determinants. In 

 this way " it may get into a permanent upward 

 movement, and attain a degree of strength from 

 which there is no falling back." 



In a similar manner a downward variation of 

 a determinant may be started by diminished 

 nutrition, and the weakened determinant will 

 have less aflSnity for attracting nutriment because 

 of its diminished strength. " If a certain critical 

 stage of downward progress be passed, even 



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