152 THE THEORIES OF EVOLUTION 



weaker, will go without food, develop more slowly and 

 "produce less vigorous daughter-determinants. The 

 result is that, in the second generation, certain parts 

 of the organism, which were represented in the ovum 

 by stronger determinants, will be more developed. 



As, furthermore, the germ cell of the second gener- 

 ation receives the parent's germ plasm, with all its 

 heterogeneous elements, the struggle will be resumed 

 between the strongest determinants, and in the third 

 generation certain characters will be even more 

 strongly marked. This accounts for the accumula- 

 tion of characters which was merely presupposed in 

 the Darwinian system, and for the fact that modifica- 

 tions accumulate in certain directions to the exclusion 

 of any others. These variations are not "predestined" 

 in the sense Naegeli gave the word, but merely caused 

 and directed by external conditions. 



When certain organs or parts thereof are favoured 

 by natural selection, the corresponding determinants 

 are better nourished and their offspring is stronger. 

 The degree of utility determines the line of variation 

 and this explains why useful variations are always 

 found present: their increase is, so to speak, auto- 

 matic. The only obscure point, Weismann says, is 

 the usefulness of variations in their incipient stages. 

 According to Weismann, variations must be useful 

 even in their incipient stages, or else natural selection 

 would have nothing upon which to exercise itself, and 



