Ghdick — Divergent Evolution and the Darwinian Theory. 23 



the crossing would be chiefly between the individuals of the 

 in w '• living together in th< same place. . . . 



" Even in the case of animals which Breed slowly and nnite 

 for each birth, we must not assume that the effects of natural 



lection will always be immediately overpowered by free inter- 

 crossing; for I can brings considerable body of facts showing 

 that within the same area, varieties of the same animal may 

 long remain distinct, from haunting different stations, from 

 breeding at slightly different seasons, or from varieties of the 

 same kind preferring to pair together. . . . 



" Isolation, also, is an important element in the changes 



effected through natural selection. /// a confined or isolated 



area, if not very large, tin- organic and inorganic conditions 



7 l» almost uniform : so that natural selection will 



^[ tend to modify all the varying individuals of the same species 



in tin s<n,t>' manner. Intercrossing with the inhabitants of 



the surrounding districts will, also, be prevented. Moritz 



Wagner has lately published an interesting essay on this sub- 



^ ject, and has shown that the service rendered by isolation in 



preventing crosses between newly formed varieties is probably 



eater even than 1 have supposed. But from reasons already 



-igned, I can by no means agree witli this naturalist that 

 migration and isolation are necessary for the formation of new 

 species." [Origin of Species, fifth edition,* Chapter IV, Sec- 

 tion on ki Circumstances favorable for the production of new 

 forms through Natural Selection."] 



Again in the same chapter, in the section on " Various 

 Objections," in answer to the question, " How, on the principle 

 of natural selection, can a variety live side by side with the 

 parent-species?" he replies, "If both have become fitted for 

 slightly different habits of life or conditions, they might live 

 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, etc., 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 



s, lie cannot regard migration and isolation as necessary for 



* The same passages occur in the sixth edition, pp. 80, 81. 

 f In the sixth edition this passage will be fouu<l. slightly modified, in Chapter 

 VII, p. 169. 



