Two types of changefulness affect a countrys animals one temporary in.

Incidence and local in effect, a function of circumstance ; the other persistent and general, a function of time.

Within itself a fauna is in a constant state of uneasy restlessness, an assemblage of creatures which in its parts ebbs and flows as one local influence or another plays upon it.

It may be that a succession of favourable seasons breeds many field-voles, and the tide of the field-vole race flows to its high water-mark of numbers.

But this new food-supply brings to the feast hungry owls, hawks, stoats and others, and as the tide of the beasts and birds of prey flows, that of the voles ebbs.