146 
DARWINISM 
CHAP. 
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 crnstacea 
described at p. 118, 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. 
