CHAPTER I 



INTRODUCTORY 



It is now more than fifty years since Darwin gave 

 the theory of natural selection to the world, and the 

 conception of a gradual evolution has long ago become 

 part of the currency of thought. Evolution for Darwin 

 was brought about by more than one factor. He 

 believed in the inherited effects of the use and disuse 

 of parts, and he also regarded sexual selection as 

 operating at any rate among the higher animals. Yet 

 he looked upon the natural selection of small favour- 

 able variations as the principal factor in evolutionary 

 change. Since Darwin's time the trend has been to 

 magnify natural selection at the expense of the other 

 two factors. The doctrine of the inherited effects of 

 use and disuse, vigorously challenged by Weismann, 

 failed to make good its case, and it is to-day discredited 

 by the great majority of biologists. Nor perhaps does 

 the hypothesis of sexual selection command the 

 support it originally had. At best it only attempted 

 to explain those features, more especially among the 

 higher animals, in which the sexes differ from one 

 another in pattern, ornament, and the like. With 

 the lapse of time there has come about a tendency to 



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