Mutation and Darwinism 393 



Mutation and Natural Selection 



At the time of the pubUcation of The Origin of Species 

 it was assumed that heredity is a process in which the 

 characters of the two parental hnes are blended. It was 

 then possible to show that a new character would necessarily 

 become so diluted after a few generations that, unless it had 

 outstanding advantages in the struggle for existence, from 

 the very first, there was no chance for it at all. If, how- 

 ever, the new characters are already of such outstanding 

 advantage, it was argued, we do not need to assume that 

 " natural selection " is responsible for the accumulation of 

 minute advantages, to account for the origin of new species. 

 The mutation theory, with the known facts of genetics, 

 obviates this difficulty of the selection theory. 



The observation has been made that each mutation con- 

 cerns not merely the outstanding character that holds the 

 attention of the experimenter but many other physiological 

 as well as structural details. This fact simplifies our under- 

 standing of the origin of species by mutation, for it solves 

 Darwin's problem of " correlation." The marks by which 

 we distinguish species are not, in general, the characters that 

 are significant in the *' struggle for existence." Yet they 

 are as constant as the essential organs and processes for 

 maintaining life. The theory of origin of mutations through 

 a change in a gene reconciles these two sets of facts. The 

 classifiers and experimenters have naturally emphasized 

 obvious and superficial traits. Survival has depended upon 

 more deep-seated structures and capacities. In so far as a 

 plant or animal is fit to live and reproduce, it is because 

 of the latter class of characters; but these are also manifesta- 

 tions of the same genes as bring about the superficial struc- 

 tures and markings. A mutation in such a gene may bring 

 about more or less far-reaching changes in both kinds of 

 characters. This would explain why most mutations are 

 actually injurious rather than useful. An established species 

 must have survived, with its characteristic structures and 



