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 established as a distinct race or species.[48] Of course where the change of habits or of station is still greater, as when a terrestrial animal becomes sub-aquatic, or when aquatic animals come to live in tree-tops, as with the frogs and Crustacea 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 new species.

This was the keynote of Mr.Vernon Wollaston's essay on "Variation of Species," published in 1856, and it is adopted by the Rev.J.G.

Gulick in his paper on "Diversity of Evolution under one Set of External Conditions" (_Journ.Linn.Soc.