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THE AMERICAN NATURALIST [Vol. XLII 



nearly parallel directions. Divergence may be greater than evo- 

 lution when changes are not progressive but sideways or back- 

 wards. Mutations, reversions, or degenerations, can take place 

 suddenly, without the slow and gradual weaving of new char- 

 acters in the network of descent of a species: they involve only 

 the suppression of characters or the return to expression of old 

 characters that continue to be transmitted in latent form. 



As long as Dr. Gulick lets it appear that the divergencies of 

 his snails arise through isolation. I fully agree with him, but 

 not when he seems to surest that isolation and selection produce 

 new characters. The fact that isolated groups have no mutual 

 sharing of evolutionary progress leaves them free to become 

 more and more different, but the isolation does not explain the 

 progressive changes to which the differences are due. Isolation 

 explains speciation. but does not explain evolution. 



This is the same objection that Darwin made to Wagner's 

 theory of isolation, that it did not help him to understand "how 

 or why it is that a long isolated form should almost always 

 become slightly modified." Dr Gulick explains that his inter- 

 pretation differs essentially from that of Wagner, who held that 

 even natural selection must have an "isolated colony" to work 

 upon. Nevertheless, it appears that Darwin's objection ap- 

 plies to Gulick 's doctrine as well as to Wagner's, for isolation 

 only reveals the fact of evolution, while a genuine "factor" 

 should do something toward explaining it. 



Such a factor Darwin believed that he had found in natural 

 selection, but he saw in isolation alone nothing to aid evolution. 

 Darwin took it for granted that species were normally stationary 

 and most of his successors still accept this un evolutionary as- 

 sumption. With Darwin evolution was a definite process by 

 which the characters of a species are changed, but with some 

 of our later writers it has become merely a general name for 

 a subject of study, whose various phases or branches are loosely 

 called "factors."' though they have no apparent relation to the 

 original concrete idea of evolution as a process of change in 



ihe failure to give a more definite recognition and a name 

 ( evolution or otherwise! to the processes of spontaneous, pro- 

 gressive change m species, appears to me to have prevented the 

 attainment of the complete clearness sought by Dr. Gulick in 

 the presentation of his elaborate and valuable evidence that only 



