THE BACKGROUND OF DARWINISM—ADAPTATIONS 195 
running forms that prefer the surface life 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 environment 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 CLASSIFIED 
Adaptations are variously classified by different authors, and that 
of Jordan and Kellogg is as good as any: “‘(a) food-securing; (0) self- 
defense; (c) defense of young; (d) rivalry; (e) adjustment to sur- 
roundings.” 
