The first of these facts which I shall adduce is, that although the geographical range of any given species is, as a rule, continuous, such is far from being always the case. Very many species have more or less discontinuous ranges--the mountain-hare, for instance, extending from the Arctic regions over the greater portion of Europe to the Ural Mountains and the Caucasus, and yet over all this enormous tract appearing only in isolated or discontinuous patches, where there happen to be either mountain ranges or climates cold enough to suit its nature. Now, in all such cases of discontinuity in the range of a species the theory of evolution has a simple explanation to offer--namely, either that some representatives of the species have at some former period been able to migrate from one region to the other, or else that at one time the species occupied the whole of the range in question, but afterwards became broken up as geographical, climatic, or other changes rendered parts of the area unfit for the species to inhabit. Thus, for instance, it is easy to understand that during the last cold epoch the mountain-hare would have had a continuous range; but that as the Arctic climate gradually receded to polar regions, the species would be able to survive in southern latitudes only on mountain ranges, and thus would become broken up into many discontinuous patches, corresponding with these ranges. In the same way we can explain the occurrence of Arctic vegetation on the Alps and Pyrenees--namely, as left behind by the retreat of the Arctic climate at the close of the glacial period.