HEREDITY AND EVOLUTION 



185 



dis flies is in existence today. Natural selection results in 

 "the survival of the fittest" for the particular environment, 

 and the principle is used to explain the degeneration due to 



parasitism, as well as the development of 

 increased complexity in animal structure. 

 Zoologists of the generation following 

 Darwin attempted to explain all evolu- 

 tion as the result of natural selection. 

 With the discovery of instances where 

 natural selection obviously could not 

 explain the course of evolution many at 

 once declared the whole idea of natural 

 selection worthless. The more we learn 

 of nature the more does it appear that 

 the operations of the universe are in 

 accord with general plans or laws. But 

 there is neither necessity nor reason to 



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Fig. 99. Caddis flies. (Enlarged) 



