524 THE CHANGING WORLD 



Dr. Vernon L. Kellogg, an authority on bird-lice (Mallophaga), is 

 acquainted with several hundred species that live parasitically among 

 the feathers of birds. He finds that nearly every kind of bird enter- 

 tains its own particular species of bird-lice. Since birds of different 

 species in their aerial activities do not often come into bodily contact, 

 these wingless bird-lice are isolated, as if on an island, and each species 

 is passed around among nest mates of bird hosts of one kind. This 

 peculiar type of isolation on specific hosts helps to explain why 

 so many different sorts of Mallophaga have evolved and maintained 

 their distinctive differences. 



The Mutation Theory of DeVries 



Darwin devoted a large portion of The Origin of Species to a dis- 

 cussion of anticipated objections to the theory of natural selection. 

 An attempt to review these controversial matters is aside from the 

 purpose of this book. They form a pile of straw that has been thor- 

 oughly threshed over, not only by Darwin himself but by biologists 

 generally. Suffice it to say that, after all the objections to this theory 

 have been considered, Darwin's contribution to the fundamental 

 problem of evolution remains an enduring monument to his genius, 

 the influence of which extends far beyond the realm of biology. 



One of the difficulties that has often been emphasized has to do with 

 variations, which are the indispensable materials for selection to act 

 upon. The kind of variation on which Darwin depended was the 

 minute modifications everywhere evident. Natural selection does not 

 satisfactorily explain how such slight variations can become life- 

 determining. In order to assume importance in the survival of the 

 individuals possessing them, that is, to become of selective value, 

 these slight variations must accumulate and increase until they 

 acquire a life-and-death significance in the struggle for existence. 

 The greenness of a katydid, for example, is a life-saving feature which 

 renders its possessor largely invisible to its bird enemies, against a 

 background of green leaves. A slight departure towards greenness 

 from the ancestral conspicuous brown color of the species would be 

 of no use in concealment. Natural selection cannot take hold until 

 there is enough greenness developed to provide safety by concealment. 

 One suggestion is that useless variations are often correlated with 

 useful ones and so are rescued from oblivion, just as in a "landshde" 

 during a political election many insignificant minor officials arrive 

 in office on the coat-tails of the real winner. 



