THE BACKCxROUND OF DARWINISM— ADAPTATIONS 195 



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

 ranean burrowing types 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 life of the organism. No wonder, then, 

 that so many biologists consider that the ehvironment has been a 

 molding force in the evolution of adaptations. 



One of the most interesting discussions of adaptations is that of 

 Weismann, who, it appears, is greatly impressed with the uni- 

 versality of adaptation. His thesis apparently is that if we had 

 complete knowledge of the field of biology, we would discover that 

 everything is adaptive, and that many structures or habits that now 

 appear to us useless or non-adaptive, would be found to have a definite 

 value to the organisms possessing them. Some authors take the 

 opposite extreme and claim that adaptation has been merely read into 

 a vast number of so-called adaptive structures and that when these 

 structures shall have been adequately investigated they will be found 

 to lack the value imputed to them by uncritical observers. Some- 

 where between these extreme views lies the truth. 



Thus we see that a certain amount of adaptation is inevitable and 

 needs only a formal physiological explanation. The fitness of the 

 environment, habitat selection, and the relationship that exists 

 between function and structure, are adequate explanations for the 

 general fact of adaptation and thus take away much of the mystery that 

 has shrouded the concept of fitness. There are, however, very many 

 types of special adaptation which do not yield so readily to the general 

 explanation given. Some of the most important of these will be de- 

 scribed below. 



ADAPTATIONS CLASSIPIED 



Adaptations are variously classified by different authors, and that 

 of Jordan and Kellogg is as good as any: " {a) food-securing; {b) self- 

 defense; (c) defense of young; {d) rivalry; (e) adjustment to sur- 

 roundings." 



