30 VERTEBRATE ZOOLOGY 



induction, which means that both soma and germ are affected simul- 

 taneously and in the same way. Perhaps it would be best to think 

 of the organism as a whole and view the response as a general organ- 

 ismal change that becomes fixed only after oft-repeated exposure 

 generation after generation. In some cases it may be said that ex- 

 ternal factors simply accelerate or retard processes that were already 

 under way in the germ-plasm, so that the response appears to be 

 something new in kind when it is only the result of a sudden accelera- 

 tion of a character evolution already under way. Whatever be the 

 underlying mechanism involved in adaptive changes, there is no hope 

 of explaining adaptations on the Darwinian basis, through the selec- 

 tion of the best out of a vast array of purely fortuitous variations; 

 for if the historical study of vertebrate evolution reveals one thing 

 more clearly than any other, it is that evolutionary changes are orderly, 

 progressive, and determinate in character, and that in many re- 

 spects these orderly processes of evolution are independent of each 

 other and of environmental changes. 



