and the Darwinian Theory. 159 



together, though in the case of animals which freely cross and 

 wander much about varieties seem to be almost always con- 

 fined to distinct localities. But if we put on one side poly- 

 morphic species, in which the variability seems to be of a 

 peculiar nature, and all mere temporary variations, such as 

 size, albinism, &c, the more permanent varieties are generally 

 found, as far as I can judge, inhabiting distinct stations, 

 high land or low land, dry or moist districts, or distinct 

 regions'''' *. 



In the portions of these passages which I have distinguished 

 by italics Darwin seems clearly to maintain that for the for- 

 mation of coexistent permanent varieties some degree of local 

 separation is necessary. I therefore conclude that when he 

 says he cannot regard migration and isolation as necessary 

 for the formation of new species he intends to express, in 

 opposition to Moritz Wagner, the opinion that a species may 

 be transformed into a new species without leaving its original 

 locality, but that he does not intend to say that two or more 

 divergent species can arise in the same locality from the same 

 stock. If I interpret him rightly he considers the partial 

 separation described in the first of the paragraphs just quoted 

 as sufficient to allow of the formation of divergent species, 

 when the external conditions of the separate districts are 

 sufficiently different and sufficiently permanent to secure long- 

 continued divergent natural selection. That the second para- 

 graph is to be interpreted in accord with this meaning I judge 

 from the fact that natural selection is mentioned here as the 

 cause of the divergence which crossing tends to overpower, 

 and in the third paragraph uniformity in the environment is 

 represented as ensuring uniform natural selection. The 

 varieties that are restrained from crossing with each other by 

 diverse times and habits of breeding he must regard some- 

 times as slightly divergent forms tending to disappear under 

 the pressure of uniform natural selection, and therefore never 

 becoming separate species, though one of them may prevail 

 and be established as a new species, and sometimes as forms 

 that are becoming more and more divergent, because they 

 have found their way into districts or stations where they are 

 somewhat separated from each other, and where the conditions 

 are somewhat different, and the natural selection, therefore, 

 somewhat diversive. 



If this is not his meaning, if he intends to teach that forms 

 arising in one place and not locally separated from each other 

 can continue to diverge till they become separate species, 



* In the sixth edition this passage will be found, slightly modified, in 

 Chapter VII. p. 169, 



12* 



