156 THE THEORIES OF EVOLUTION 



ories, for it accounts, at least hypothetically (the de- 

 terminants being hypothetical), for definite varia- 

 tions. It is a causal and mechanical interpretation of 

 orthogenesis. 



The j theory of germinal selection pretends to ex- 

 plain why useful variations are always observable; in 



reality it merely explains why they become more 

 accentuated after they once make their appearance. 

 It also explains, however, why any variation (unless 

 it is harmful enough to kill the organism) may be- 

 come more accentuated, and thus this theory defeats 

 its very purpose. 



External influences which were entirely ignored in 

 Weismann's original theory, are considered as an im- 

 portant factor in germinal selection. 



An increase or decrease of the food supply is bound 

 to play a part in the competition between determin- 

 ants; dearth, for instance, would kill the weakest 

 determinants and allow onlv the strongest to sur- 

 vive. Weismann himself admits that the initial dif- 

 ferences between determinants may depend upon an 

 increased or decreased food supply. The mode of 

 alimentation would also exert a deep influence upon 

 the development of the future organism and what is 

 more important, the modifications brought about by 

 alimentation would of necessity be hereditary, for, ac- 

 cording to the principle of the continuity of the germ 

 plasm, the weaker determinants, defeated and killed 



