Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/417

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In this diagram the Roman numerals within circles indicate the terrestrial provinces in which we find the several sub-floras composing the British flora chiefly developed, and the arrows with coloured flukes indicate the directions they took in their migration from the continent. I. is the region of the west Irish flora, or Asturian type. II. is the region of the Devon flora, or Norman type. III. is the region of the Kentish flora, or the north French type. IV. is the region of the Alpine flora, or Scandinavian type. V. is the sign of the great Germanic or central European type, which must be understood as mingled up in the other regions—overspreading, as it were, the country. The orange tint represents I.; the pink, II.; the yellow, III.; the blue, IV., and the green, V.

The marine molluscan faunas are indicated by Roman numerals, not surrounded by circles, and by arrows with barred tails; the number of the bars on the tail of an arrow indicates the number of the type, the direction of which, in the course of its migration, is marked by the direction of the arrow, and the turning point by curved arrows. The types which are nonmigratory are indicated by numerals only, and those which are universally distributed over the area represented in the map are omitted.

In this map I have endeavoured to convey a graphic idea of certain geological, zoological, and botanical features presented by the region of the north Atlantic, either now or at former epochs, and which throw light on the history of the Pleistocene formations, and of our existing fauna and flora.

The space coloured blue represents the portion of the northern hemisphere anciently, and in part at present, under the condition of which a glacial or boreal fauna, is characteristic. The dark blue represents such a region as now existing; the pale and dark together indicate the ancient glacial region. Throughout this region an uniform assemblage of marine testacea lived during the glacial epoch. Two-thirds of the same species now live assembled together in the region coloured dark blue, including all the forms of northern origin now extinct in the European portion of the pale-blue area. The bound of the dark-blue region indicates the line within which ice floated in summer, and which is constantly inhabited by the larger cetacea. The bound of the pale-blue region agrees very nearly with the limit of the occasional range of whales now.