Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/356

Rh that they are mainly due to—indeed, the only course left—migration before isolation of the area, it is necessary, if possible, to ascertain two fixed points in time, between which the migration or migrations must have taken place.

The eocene tertiary epoch—that of the deposition of the London clay—affords a first or most ancient point, after which only such migrations could have taken place; for we have abundant evidence that both the flora and fauna of such parts of the area under examination as were above water, were then very distinct from those which now occupy it, and enjoyed a climate far warmer than suitable to its present (terrestrial) inhabitants.

The epoch usually designated "historical"—that during which man has been a known inhabitant of the earth—affords a last point, one before which the migrations (at least, for the most part) took place. For the great deposits of peat, formed in part out of the remains of vast forests, which probably, during the earliest stages of the true historical epoch, covered a great part of the existing area of the British Isles, in many places overlie the fresh-water marls of the post-pliocene epoch, during which the Cervus megaceros flourished, themselves overlying and occupying depressions in the pleistocene tertiaries, formed of the upheaved bed of the sea of the glacial period.

During the post-pliocene epoch, over the elevated bed of the glacial sea, the great mass of the flora and fauna of the British Isles migrated from the Germanic regions of the continent The whole of the flora I have numbered V., including the great mass of British plants, is Germanic. Every plant universally distributed in these islands is Germanic; every quadruped common in England, and not ranging to Ireland or Scotland. The great mass of our pulmoniferous mollusca have also come from the same quarter. Certain botanical and zoological peculiarities are presented by the eastern counties of England. In every case we find these to depend on Germanic plants and animals arrested in their range. The number of species of the Germanic type diminishes as we go westwards, and increases when we cross the German Ocean. On the other hand, the peculiarities of the Irish and Scottish faunas and floras depend either on the presence of animals and plants which are not of the Germanic type, or on the absence of English species, which are. When we turn to plants and mollusks which affect localities presenting certain mineral peculiarities, such as limestone plants and animals, as they are called, and which are consequently never generally diffused, we find that the species of the Germanic type are deficient westwards—in Ireland, for instance, where conditions favourable to their presence and diffusion occur abundantly. To what else can we attribute such peculiarities, unless to arrestment of the