PART II. 



CHAPTER I. 



The Evolution of Species— Natural Selection — Sexual Selection — The Action 

 of the Environment independently of Selection. 



Species are evolved in a threefold manner — (i) By natural 

 selection ; (2) by sexual selection ; (3) by the action of the 

 environment, independently of either natural or sexual selec- 

 tion. 



It is necessary to inquire how far man comes under the 

 influence of these three methods, for a knowledge of the laws 

 by which the human organism is built up must be of use to 

 us in our search after the causation of disease, seeing that S 

 plays such an important part in it. In our study of this sub- 

 ject, moreover, many important side issues will be raised, which 

 might otherwise be passed over unnoticed. 



The first requisite is a clear understanding of the methods 

 in question. 



Natural Selection is, so to speak, the selection by " nature" 

 of those organisms most fit to live — that is, best adapted to 

 their own particular environments. This is the term by which 

 Darwin himself denoted the great law he discovered. Herbert 

 Spencer's Survival of the Fittest expresses the same idea, 

 and it has found greater favour among biologists, because the 

 term M natural selection " seems to imply (what Darwin cer- 

 tainly did not mean it to do) a voluntary choice on the part 

 of a personified nature. 



There are two main factors underlying natural selection — 

 (1) The existence of natural variations; (2) a struggle for 

 existence. Were there no natural variations, it is obvious 



