Divergent Theories 399 



plant or animal undergoes, in the course of his development, 

 the kind of alterations that fit him to further life in the 

 given circumstances, so the species adapts itself to changing 

 conditions from generation to generation. Is it true that 

 the effects of the individual's response to environmental 

 stimuli and disturbances are passed on to the offspring? If 

 so, both adaptation of species and progressive evolution can 

 be accounted for — at least with respect to such characters 

 as appear in the individual because of some active processes. 

 Unfortunately for this special theory, it not only fails to 

 account for many characters in animals as well as in plants 

 that are purely " passive," but it fails to find support in 

 any positive evidence that the acquired traits are ever ac- 

 tually reproduced in the offspring. Even the newer modifi- 

 cations of the theory, which look to a parallel modification 

 in the germ and in the parent's body, have so far failed to 

 marshal supporting evidence of a satisfactory kind. There 

 is, however, the adjustment of the individual during his life- 

 time; and there is the progressive adaptation of species by 

 modification in successive generations. The parallelism re- 

 mains as a matter of observation, even if we cannot explain 

 just how it or either of its members comes about. 



Darwin's theory of natural selection also stands to 

 reason. After all, the great variations among individuals, 

 including variations in adaptiveness to given surroundings, 

 cannot be without effect in determining survival and re- 

 production. Perhaps the struggle for existence is not so 

 severe as Darwin and Wallace supposed. Perhaps it is not 

 along the lines that had been assumed. But surely there 

 must be a differential death rate or survival rate. That is 

 so obvious that it leaves no room for argument. As we 

 have seen, however, the fact of differential survival, or of 

 selection, accounts neither for the origin of variations, nor 

 for the origin of adaptations. The theory may be but another 

 way of restating the fact that organisms are adapted, and 

 that only organisms which are more or less " fit " can pos- 

 sibly continue to live and reproduce. 



