Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/402

Rh to the depths of our seas, is exactly comparable to the change in our flora, brought about by the same geological events. During the Glacial epoch, the group of islands now forming the mountains of Britain, were doubtless clothed with a vegetation correspondent in character to the northern and Scandinavian assemblage of marine animals which then inhabited the surrounding sea (diagram ). The origin of this flora has been inquired into in the earlier part of this essay. The subsequent change of level converted these islands into mountain summits, on which the original flora was preserved and isolated (diagram ), whilst new and warmer climatal conditions introduced over the middle and low ground a vegetation of a more temperate character to become the general flora of the new-formed land.





I have already shown that in the law of representation by plants of latitudes by zones of elevation, the element of representation has been confounded with that of identity. If this be true in the vertical distribution of terrestrial creatures, it should also be true with respect to that of marine, which I have elsewhere shown obey a similar law in their vertical distribution, representing parallels of latitude by zones of depth. This I have demonstrated in the case of the Ægean mollusca.

The deepest of the submarine localities, which may be considered as Arctic outhers, with the fauna of which I am acquainted, is one in Loch Fyne, examined by Mr. MacAndrew and myself on the 16th of August, 1845. The dredge brought up eight species of testaceous mollusca, one crustacean, and two echinoderms. So small a number of species among the contents of a full dredge is remarkable and rare, and itself a fact indicative of great depth. Of these mollusca five species were alive. One, a minute species of Rissoa, was new; and had been previously taken by Mr. MacAndrew in the British Seas, but only at great depths. The remaining four were Nucula nuclea (a northern variety), Nucula tenuis, Leda minuta, and Lima subauriculata. Of these the number of examples of Nucula tenuis and Leda minuta exceeded greatly those of their companions; they are both essentially northern and Arctic forms, ranging