214 EVOLUTION, GENETICS, AND EUGENICS 



running forms that prefer the surface life and swift motion; subter- 

 ranean burrowing t5^es that sometimes are limbless like snakes, and 

 are blind; many arboreal or climbing types; a few volant or flying 

 types; a few ant-eating types; and several more or less completely 

 aquatic types. Each of these types has the customary adaptations 

 for its own mode of life. 



We see, then, that whether divergent structures are molded into a 

 semblance of similarity to fit a definite environment, or whether 

 similar structures are modified in diverse ways to fit various divergent 

 environments, the adaptation is related very definitely to the environ- 

 ment and to the functional fife of the organism. No wonder, then, 

 that so many biologists consider that the environment has been a 

 molding force in the evolution of adaptations. 



General considerations. — Adaptations are characteristic of all 

 living organisms and must be accounted for by any evolutionary theory 

 that is to be acceptable. Any theory that claims to account for new 

 species but does not account for adaptations is at best only a partial 

 explanation. All of the phenomena which have been briefly men- 

 tioned in this chapter, together with the more intricate phases of 

 general adaptiveness involved in the idea of "the web of Ufe," are part 

 of the background of Darwinism and were in the mind of Darwin when 

 he thought out the great generalization called "natural selection." 

 The "web of life" idea has been admirably presented by Professor 

 Thomson, Scotland's most skilful and prolific biological writer. The 

 present writer feels that no student of evolution should miss the oppor- 

 tunity of getting into the spirit of Darwinism with this distinguished 

 author, and to make this desideratum easily attainable, the chapter 

 is quoted unchanged as part of the general text and immediately 

 follows this discussion. 



