12 HEREDITY AND SOCIETY 



and it finds itself more and more able to overcome 

 the difficulties which beset its upward path. 



Such, in broad outline, is the scheme of natural 

 selection as put forward by Darwin to explain the 

 process of organic evolution, and to account for the 

 origin of different species of plants and animals. 

 Whether or no it be capable of working all the 

 wonders of creation which naturalists have assigned to 

 it, or whether eventually we shall have to look behind 

 its mechanism to some non-mechanical creative energy 

 which uses matter as its medium, natural selection, be 

 it cause or means, must still be effective in modifying 

 the average character of existing species. Hence arises 

 the importance of tracing its action in civilized com- 

 munities, and of investigating the causes which are 

 now at work as efficient selective agencies. 



For the sake of convenience, if for no more funda- 

 mental reason, variation may be divided into two kinds : 

 continuous and discontinuous. In continuous variation 

 we have every possible value of the character through- 

 out wide limits. Thus it would be possible to find 

 men for every tenth of an inch of stature between 

 5 feet and 6 feet 4 inches, though the numbers are 

 greatest for those values nearest the average height of 

 5 feet 8 inches. On the other hand, variation some- 

 times occurs in definite steps. A flower has either four 

 petals or five, a man either has or has not brown 

 pigment in his eyes. 



Continuous variation is much the more common, 

 and, till recently, it alone was studied by biologists. 

 Of late years, however, much attention has been paid 



