^68 The Theory of Natural Selection 



due to the impinging upon each organism of a multitude of 

 influences. Darwin meant to avoid any commitment as to 

 an ulterior purpose as guiding the variations, and so the 

 course of evolution. He sought to avoid the mysticism of 

 an innate " principle " or tendency to vary. 



Darwin's theory leaves us with the necessity for a fur- 

 ther study of variations. We have to find out why there 

 is variability both of heritable and of non-heritable qualities. 

 We have also to find out in detail just what the mechanism 

 is that results in heritable variations (mutations) of an 

 adaptive kind. If the theory of natural selection may be 

 taken to account for adaptation, it fails to tell us anything 

 about how modifications arose in the course of descent. On 

 the other hand, whatever knowledge we have about the 

 sources of variations tells us nothing about the origin of 

 adaptations. It is probable that further speculation, taking 

 into consideration newly established facts, will have to in- 

 clude both Lamarck's emphasis upon the adjustability of 

 the individual and Darwin's emphasis upon the elimination 

 of the unfit. 



With all the criticisms of the special theory of natural 

 selection, there is a growing appreciation of Darwin's con- 

 tribution in making the general theory of transformation a 

 commonplace among all scientists and indeed all thinkers. 

 If many of us often say " evolution " when we mean 

 " natural selection " or if we sometimes think that the latter 

 is as well established a " fact " as is the former, it is because 

 the evolution of human thinking has not yet gone far enough. 

 Darwin has at least made it possible for more and more 

 persons to consider the entire question of organic evolution 

 from a fresh start. The inclusion of man among the living 

 forms that have undergone modification along with descent 

 from their ancestors has become something to think about 

 without fear and without reproach. That is to say, more 

 and more of us can accept the notion of human evolution 

 from a more primitive type, even if leading anthropologists 

 cannot agree on the hypothetical reconstruction from fossil 



