120 BIOLOGY AND SOCIAL PROBLEMS 



wings was less advantageous under stress of 

 circumstances than a body of average weight 

 carrying wings of average extent. In this in- 

 stance, then, natural selection could be said 

 to take place in that extreme inclemency tended 

 to eliminate the less fit. Other instances of a 

 like kind have shown that this process is of 

 common occurrence in nature. 



But though we may feel confident that nat- 

 ural selection is of actual occurrence, it is by 

 no means certain that it is of first importance. 

 Numerous serious and valid objections to it 

 have been raised from time to time. The most 

 telling of these is the difficulty experienced in 

 understanding how in a given species new 

 characters can get a foothold. The slight in- 

 dividual differences that Darwin assumed to 

 be the beginnings of new traits are altogether 

 too insignificant to be of selectional value, and 

 hence it seems impossible that the initial step 

 in the origin of such a trait could be taken 

 by natural selection. This objection has been 

 fairly met by the mutation theory of De Vries. 

 According to this theory the differences ef- 

 fective in evolution are not the small individ- 

 ual variations, such as were considered so 

 important by Darwin, but the considerable 



