254 CAUSES OF DEGENERATIVE EVOLUTION 



are useless they are harmful as they use nutrition 

 without conferring any advantage upon the whole 

 organism. Darwin pointed out that when through 

 changed environment a structure became useless, 

 its degeneration became certain, as it was a dis- 

 advantage to the individual to squander nourish- 

 ment upon a useless part. Weismann has shown 

 that such a reduction or disappearance of a useless 

 organ is the result of variation and natural selec- 

 tion. 1 



Variation results in the appearance of individuals 

 with the useless organ in various stages of imperfect 

 development ; natural selection perpetuates these 

 advantageous stages by giving advantage to indi- 

 viduals which tend to produce the organ in the 

 most degenerate condition. 2 



1 See, however, Herbert Spencer's Social and Moral Problems, as 

 he differs from Weismann on this point. 



2 The following are good examples of the operation of variation 

 and selection in producing atrophy in a species : 



(1) Loss of constant colour among domesticated animals. Wild 

 animals, especially birds and mammals, have a colour which is 

 constant for a whole species. Frequently the colour is protective 

 in rendering the animal little distinguishable from the environ- 

 ment in which it lives. As soon as such a species has been 

 domesticated man becomes its protector, and protective colouration 

 is no longer necessary, and soon disappears. In the wild state, 

 the colour is quite as variable as in the domesticated state, but 

 the abnormal individuals become the prey of enemies and are 

 removed from the species. This applies not only to animals 

 which are preyed on by others, but to predatory animals them- 

 selves. The wild-cat, for instance, will have less difficulty in 

 stalking its prey if its colour makes concealment easy. In the 



