Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/351

338 natural history, well known and placed on record by British botanists and zoologists. The consideration of them has caused me to make this attempt towards ascertaining the causes by which they were produced.

The vegetation of the British Isles presents a union of five well-marked floras, four of which are restricted to definite provinces, whilst the fifth, besides exclusively claiming a great part of the area, overspreads and commingles with all the others.

I. Commencing with the smallest, we find the mountainous districts of the west and south-west of Ireland characterized by botanical peculiarities which depend on the presence of a few prolific species belonging to the families Saxifrageæ, Ericaceæ, Lentibulariæ and Cruciferæ. The nearest points of Europe where these plants are native is the north of Spain. The species are—

There are two or three other species, including Allium Babingtonii, whichpossibly belong to the same assemblage.

There is no evidence of any local assemblage of animals corresponding to this flora.

II. In the south-west of England and south-east of Ireland we find a flora which includes a number of species not elsewhere seen in the British Isles, and which is intimately related to that of the Channel Isles, and the neighbouring part of France. In the Channel Isles we find them associated with a number of plants which are not natives of England or Ireland. Such are Ranunculus ophioglossifolis, Sinapis cheiranthus, Erucastrum incanum, Arthrolobium ebracteatum, Centaurea Isnardi, Linaria pelisseriana, Echium violaceum, Orchis Laxiflora, Allium sphærocephalum, &c., plants which mark the commencement of the type of vegetation characteristic of Southern Europe. They are accompanied by terrestrial mollusca of the same climatal stamp, such as Helix aperta and Helix revelata, both first noticed in Guernsey by myself in 1839; the former is there at its most northern limit, and the latter only extends further into Devonshire. Thither it accompanies a