The Mechanism of Adaptation 349 



even if mutations of genes occurred with equal frequency 

 in all organisms (which seems improbable), those in which 

 the genes were most numerous and most differentiated 

 would furnish the largest number of mutations. If this 

 should prove true, the rate of mutation should be greatest 

 in the most highly differentiated organisms. Incidentally, 

 the fact that the number of mutations is not proportional 

 to the number of germ-cells that undergo maturation and 

 fertilization is evidence that evolution and adaptation do 

 not depend largely upon Mendelian segregation and recom- 

 bination of genes, and it also indicates that gene mutations 

 are not limited to the period of maturation of the germ-cells. 



In view of the fact, therefore, that there is in general a 

 much greater overproduction and elimination of individuals 

 in lower than in higher animals, it is possible to maintain 

 the Darwinian theory only by assuming that there is a 

 greater production and elimination of mutants in higher 

 forms than in lower ones ; if this be true, it would explain on 

 mechanistic grounds the more numerous and more complex 

 adaptations of higher as compared with lower organisms. 



On the other hand, the more rapid evolution and adapta- 

 tion of higher animals would be easily explained by Lamarck- 

 ian principles. If desire and intelligence are factors in evo- 

 lution, then it should follow that with increasing intelligence 

 there should be an increasing rate of evolution and adapta- 

 tion. Certainly these two intelligence and rapidity of evo- 

 lutionseem to be associated, but whether as cause and 

 effect we cannot say. Evolution has undoubtedly led to intel- 

 ligence; has intelligence in turn affected evolution? 



Finally, whether the Darwinian theory, as thus expanded, 

 is capable of explaining all the fitnesses of organisms or not, 

 it does succeed as no other theory does in offering a casual 

 or mechanistic explanation of very many of these wonderful 



