ITS 



THE AMERICAN NATURALIST [Vol. XLIX 



is no unanimity of opinion amongst biologists as to what 

 a species is. Lamarck insisted that nature recognizes 

 no snch things as species, and a great many people at 

 the present day are, I think, still of the same opinion. 

 In practise, however, every naturalist knows that there 

 are natural groups to which the vast majority of indi- 

 viduals can be assigned without any serious difficulty. 

 Charles Darwin maintained that such groups arose, 

 under the influence of natural selection, through gradual 

 divergent evolution and the extinction of intermediate 

 forms. To-day we are told by de Vries that species 

 originate as mutations which propagate themselves with- 

 out alteration for a longer or shorter period, and by 

 Lotsy that species originate by crossing of more or less 

 distinct forms, though this latter theory leaves quite un- 

 solved the problem of where the original forms that 

 crossed with one another came from. 



I think a little reflection will convince us that the origin 

 of species is a different problem from that of the cause 

 of progressive evolution. We can scarcely doubt, how- 

 ever, that Darwin was right in attributing prime im- 

 portance to divergent evolution and the disappearance 

 of connecting links. It is obvious that this process must 

 give rise to more or less sharply separated groups of in- 

 dividuals to which the term species may be applied, and 

 that the differences between these species must be at- 

 tributed ultimately to differences in the response of the 

 organism to differing conditions of the environment. It 

 may be urged that inasmuch as different species are 

 often found living side by side under identical conditions 

 the differences between them can not have arisen in this 

 way, but we may be quite certain that if we knew enough 

 of their past history we should find that their ancestors 

 had not always lived under identical conditions. 



The case of flightless birds on oceanic islands is par- 

 ticularly instructive in this connection. The only satis- 

 factory way of explaining the existence of such birds is 



