Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/359

346 also the peculiarities of the British alps, there still remain certain limited assemblages of organized beings which present distinctive characters, and for which the geological operations just referred to would not account. These are three in number. 1st. The animals and plants which give a peculiar character to the south-east of England, and inhabit, for the most part, the chalk districts. 2nd. The animals and plants which distinguish the south-west of England and the south- east of Ireland, species mostiy indigenous also in the Channel Isles: and, 3rd. The assemblage of plants (small as to number of species but playing an important part in the general vegetation) giving a peculiar and very remarkable character to the flora of a considerable part of the west of Ireland.

These three sub-floras are all allied to, and derived assemblages of European plants south of the great Germanic group. As the south of England and of Ireland were in all probability unsubmerged during the Glacial epoch, they may have come over either before, or during, or after that period. There are strong reasons for believing they migrated before. As a general rule, we may regard the most southern floras to be oldest, especially when, as in these cases, they are more and more fragmentary, and their character is more and more southern.

That which I have numbered III. is the most extensive, and from the number of species which are exclusively or chiefly found in chalk districts in this country, I have called it the Kentish flora. But the attachment of such plants to chalk is an accident, and not an essential habit of the species; the preference is simply for calcareous districts. In other countries they are found indifferently on most forms of limestone, and on calcareous sands and clays. Botanists who write of such and such plants being present because the chalk is present, forget that, unless they be species peculiar to the locality examined, they must owe their presence to diffusion from some other, since chalk of itself has no power to call up any species, by an equivocal or spontaneous generation of it And when a so-called chalk plant is not found on limestones adapted to it beyond (in our case, north of) the chalk, we must attribute its absence rather to geographical causes and impediments to its extension in that direction. This flora is evidently derived from the north-western provinces of France, and as no geologist doubts the ancient union of the two sides of the Channel, the course it pursued in migrating to England is sufficiently obvious. The epoch of the formation of the Straits of Dover would mark the period of its isolation, and if that breach of continuity was effected before the destruction of the great Germanic plain, as is probable, we may regard the Kentish flora as very ancient. Still more ancient appears to have been the flora numbered II., the