Summary 367 



general agency in bringing about sex differentiation is not 

 considered seriously by most naturalists. 



SMinmary 



No matter how variations arise, no matter how they 

 are inherited, no matter how severe or how mild the struggle 

 for existence may be, organisms can continue to live only 

 if and only in so far as they meet the conditions imposed by 

 the environment. Natural selection thus remains from the 

 common sense point of view a permanent factor in de- 

 termining survivals and in insuring that the species which 

 do actually exist at any time shall be more or less fit. Indeed, 

 so axiomatic is this that the question may reasonably be 

 asked whether it tells us anything at all that will help us 

 to solve the problem of evolution. We know that living 

 things have among other characteristics the ability to adjust 

 themselves more or less adequately. Each species is adapted 

 to live in a relatively restricted type of environment. Does 

 the doctrine of natural selection tell us anything more than 

 that? Perhaps it is not always the fittest, or only the fittest, 

 that survive; but obviously the unfit cannot carry on. 



Natural selection cannot originate anything if it is, as 

 defined, a truly negative agency, a destruction of the unfit. 

 This process has been compared to the action of a mighty 

 sieve which lets pass all below a certain standard and re- 

 tains only those that meet a certain minimum requirement. 

 The variations which appear in every array of individuals 

 tend indefinitely in all directions, according to Darwin. 

 There does not appear among them anything determinate, 

 anything pointing definitely in a specific direction. There 

 is a vague tryout of novelties without predisposition, with- 

 out prejudice. And some of the trials happen to be lucky. 

 The whole process is one of trial and error. 



In describing this process as one of chance variations, 

 Darwin did not of course intimate that variations were with- 

 out cause. On the contrary, this " chance " characteristic is 



