36 MODERN IDEAS OF EVOLUTION 



quotes Cope and Packard in support of his view on 

 this point. The latter we shall have occasion to refer 

 to in the sequel in connection with cave animals. 

 Cope has, in a series of brilliant essays, 1 endeavoured 

 to illustrate what he terms * causes of the origin of the 

 fittest. Of this kind are growth-force modified by 

 retardation or acceleration of development produced 

 by unfavourable or favouring conditions, the effects of 

 use and disuse on modifying structures, the law of 

 correlation of parts and the effects of animal intelli 

 gence. These are all causes ignored by the genuine 

 Darwinian. Nevertheless they exist in nature, though 

 rather as causes of mere adaptive variation than of 

 specific difference. 



Another modification of orthodox Darwinism is 

 that of Romanes, who may almost be regarded as 

 Darwin s most prominent successor. He has intro 

 duced the idea of physiological selection, that is, of 

 the occurrence accidentally or from unknown causes 

 of reproductive changes which render certain indi 

 viduals of a species infertile with others. The effect 

 of this would be an isolation amounting to the erection 

 of two forms not reproductive with each other ; or, in 

 other words, of two species not gradually differentiated, 

 but distinct from the first. This is really an inversion 

 of Darwin s theory, in which the initial stage of 

 Romanes is necessarily the culmination of the develop 

 ment. It differs also essentially in eliminating the 



1 Origin of the Fittest, American Naturalist. 



