146 DARWINISM 



isolated from their parent form. Let us suppose, for example, 

 that one portion of a species usually living in forests ranges 

 into the open plains, and finding abundance of food remains 

 there permanently. So long as the struggle for existence is 

 not exceptionally severe, these two portions of the species may 

 remain almost unchanged ; but suppose some fresh enemies are 

 attracted to the plains by the presence of these new immi- 

 grants, then variation and natural selection would lead to the 

 preservation of those individuals best able to cope with the 

 difficulty, and thus the open country form would become 

 modified into a marked variety or into a distinct species ; 

 and there would evidently be little chance of this modifica- 

 tion being checked by intercrossing with the parent form 

 which remained in the forest. 



Another mode of isolation is brought about by the variety 

 — either owing to habits, climate, or constitutional change — 

 breeding at a slightly different time from the parent species. 

 This is known to produce complete isolation in the case of 

 many varieties of plants. Yet another mode of isolation is 

 brought about by changes of colour, and by the fact that in a 

 wild state animals of similar colours prefer to keep together 

 and refuse to pair with individuals of another colour. The 

 probable reason and utility of this habit will be explained 

 in another chapter, but the fact is well illustrated by the 

 cattle which have run wild in the Falkland Islands. These 

 are of several different colours, but each colour keeps in a 

 separate herd, often restricted to one part of the island ; 

 and one of these varieties — the mouse-coloured — is said to 

 breed a month earlier than the others ; so that if this 

 variety inhabited a larger area it might very soon be estab- 

 lished as a distinct race or species. 1 Of course where the 

 change of habits or of station is still greater, as when a ter- 

 restrial animal becomes sub-aquatic, or when aquatic animals 

 come to live in tree -tops, as with the frogs and Crustacea 

 described at p. 11 8, the danger of intercrossing is reduced to 

 a minimum. 



Several writers, however, not content with the indirect 

 effects of isolation here indicated, maintain that it is in itself 

 a cause of modification, and ultimately of the origination of 

 1 See Variation of Animals and Plants, vol. i. p. 86. 



