328 THE THEORIES OF EVOLUTION 



same purpose, but an organ which has the same 

 embryogenetic origin. 



For instance, we do not seek the origin of the bird's 

 wings in the insect's wings but in the anterior mem- 

 bers of reptiles, in spite of the reptiles' widely differ- 

 ent mode of locomotion. And if we list the whale 

 with mammals, not with fishes, it is because we con- 

 sider elements other than the whale's characters of 

 adaptation to its environment. 



We therefore face two distinct problems for which 

 any theory that presumes to explain the entire pro- 

 cessus of evolution should offer two distinct solutions. 

 We fail to find any system that is inclusive enough to 

 cover both points ; every known system confines itself 

 to the solution of one of the two problems. Some 

 systems disregard the question of adaptation entirely. 

 Such are De Vries' theory of mutation, Eimer's and 

 Naegeli's theory of heterogenesis, Wagner's theory 

 of geographical isolation, Romanes' theory of physi- 

 ological selection. On the other side we find Dar- 

 win's theory which is solely a theory of adaptation, 

 for it holds that natural selection fosters only useful 

 variations; by describing the method of development 

 of those variations it explains that of the differentia- 

 tion of species. 



The Lamarckian theory is generally characterised 

 as a theory of adaptation, but a distinction should be 

 made: it is an adaptation theory when it deals with 



