348 ELEMENTS OF BIOLOGY 



into two classes, structural and functional adaptations. Struc- 

 tural adaptations in an animal may be compared with the thread in 

 a nut to engage and follow the thread on a bolt; functional adapta- 

 tions may be thought of as the energy expenditure both as to direc- 

 tion and quantity, required to advance the nut along the bolt. As 

 the thread of a nut is not perfectly fitted to that of the bolt, so the 

 adaptations of the organism do not fit perfectly with the demands 

 of the environment, and as energy is absorbed by friction as the 

 nut advances, so energy is expended as the organism overcomes its 

 obstacles. 



The Preservation of Mutations. Present views are that spe- 

 cies adaptations, whether structural or functional, have their origin 

 in chance germinal mutations and that their continuance as a part 

 of the inheritance of the individual and of the species is conditioned 

 upon the fact that some mutations better fit, and others unfit the 

 organism for its struggle to maintain existence; some tend to true 

 the thread, others to distort it. By the constant pressure of the 

 environment the unfit are eliminated; the fit are preserved. Thus, 

 according to this point of view, the adaptive characters of the 

 species are determined, in last analysis, by the environment and the 

 survival value of mutations; the unfit fail in competition for food, 

 are destroyed by enemies, or are unable to perpetuate the species. 

 The principle is illustrated by an example. The fruit fly, Drosoph- 

 ila, has given rise to several hundred mutations in the two 

 decades that it has been reared for experimental purposes. Some of 

 these mutations are obviously of a nature that makes it difficult for 

 the animal to compete for food, and for mating. For example, one 

 mutant race is wingless. In Nature this mutant would be promptly 

 stamped out, or at least obscured in a heterozygous state. 



Adaptive Structural Species Characters. Adaptive struc- 

 tural species features in animals include all those anatomical char- 

 acters that are characteristic of the species and fit it for its 

 environment; for examples, the fins and gills of fishes, the human 



