364 PRINCIPLES OF ANIMAL BIOLOGY 



same genus, almost never do the distinguishing characters seem to l)e of 

 2k,ny particular vakie to the individuals possessing them. This lack or 

 infrequency of adaptiveness of the so-called species and genus characters 

 is one of the principal reasons for adopting the view, just described, 

 that varieties and species may become separated from one another by 

 accidental changes in genetic composition, while natural selection does not 

 exert its most powerful influence until some degree of differentiation has 

 been attained. 



Pointing out the adaptations of animals has been one of the favorite 

 pastimes of naturalists. Books and articles on natural history are full 

 of examples, and recitation of the marvelous fitness of organisms to some 

 special niche in the environment never fails to excite wonder. The 

 several decades following the publication of Darwin's "Origin of Species" 

 were marked by inordinate attention to the features of living things that 

 enable them to cope with the environment, for to explain the develop- 

 ment of any character through natural selection it was only necessary 

 to find a use for it. The things most often regarded as making for suc- 

 cess were ability to secure food, escape enemies, resist conditions of the 

 physical environment, and attract the opposite sex. The supposed uses 

 of spots and spines, colors and habits, to attain these ends were exceeded 

 in marvelousness only by the ingenuity of the naturalists in devising 

 them. In this period, what are probably the things of greatest impor- 

 tance, the physiological qualities, were relegated to minor roles. Com- 

 paratively little attention was given, for example, to resistance to disease 

 and exceptional fertility. Either of these should influence the number 

 of descendants more than most of the structural characters whose origins 

 were sought. Plasticity, or the capacity of either an individual or a 

 species to adjust itself to many types of environment, must be highly 

 important but was seldom considered then. These mistakes of the early 

 followers of Darwin led to a reaction against the natural selection theory 

 over the end of the last century, but the doctrine has emerged again with a 

 very different type of support, based on knowledge of mutations, the laws 

 of heredity, and the mathematics of chance. 



It should be pointed out that adaptation is a quality of an organism 

 as a whole. While in some instances one feature of an organism stands 

 out as supremely important, so that other characters all yield to it in 

 determining success, in most living things fitness is composed of many 

 things. The success of an individual is a product of them all. An animal 

 has only one life to lose or preserve. If a frog perishes in the tadpole 

 stage because it has not the requisite power to withstand desiccation, it 

 cannot be preserved in the adult stage by any special agility in escaping 

 from enemies. Likewise, an animal gives rise to only one set of descend- 

 ants. If these are few because the animal's life is short, they cannot be 



