ORIGIN OF SPECIES 143 



mutant types of Drosophila, that visible, superficial 

 changes are nearly always accompanied by other 

 changes tliat have a more physiological aspect, such 

 as the vigor, or length of life, or productivity of the 

 individual. From the j^oint of view of evolution 

 these physiological effects are those of most sig- 

 nificance, while the superficial changes are trivial in 

 comparison. 



Xow it is highly probable, if definite structural 

 clianges have definite accompanying physiological 

 changes, that other nmtations that bring about phys- 

 iological changes produce, at the same time, superfi- 

 cial structural effects. If so, we mav find here an 

 explanation of the constancy of the latter when they 

 are by-products of important physiological charac- 

 ters. Hence their constancy and their value as diag- 

 nostic characters of species. 



The study of the mutation process has to a large 

 extent also concerned itself with superficial charac- 

 ters, while the concomitant physiological modifica- 

 tions are referred to only in passing; but from the 

 evolutionist's point of view it is the internal phys- 

 iological accompaniments of the superficial effects 

 that are of much greater significance. It is not sur- 

 j^rising, therefore, that a good deal of the discussion 

 of the bearing of mutants on the theory of evolution 

 may seem rather far afield. If the mutation process 

 were studied as contributorv to the theorv of evolu- 

 tion rather than in its genetic bearings we would re- 



