196 The Scottish Naturalist. 



which are completely isolated from their co-species of .'the 

 southern coast of New England, and surrounded on all sides by 

 more northern forms. But more than this, when we examine 

 the post-glacial marine beds, we find these marked by the pres- 

 ence of southern species in still greater numbers. Thus in the 

 raised beaches of Scandinavia, southern species are not only 

 individually abundant, but their shells are larger and better 

 developed than those of their descendants that still linger in 

 diminished numbers in the adjoining seas. Some of the southern 

 species, indeed, no longer live in Norwegian waters. Again, in 

 Spitzbergen, there occur considerable post-glacial deposits,^ which 

 are made up in large measure of shells of the common mussel 

 — a mollusc which is rarely or never met with now so far north. 

 In the same beds, again, there are found two species {Cyprina 

 islaiidica and Litorina litored) which no longer live round the 

 coasts of Spitzbergen. The other shells with which these sub-fossils 

 are associated in the Spitzbergen deposits are, indeed, all species 

 still occupying the adjoining seas, but the evidence, nevertheless, 

 is clearly in favour of a somewhat warmer sea than the present, — 

 a conclusion which derives additional support from the fact that 

 Fiiais cajialictilatus, which is very common in the mussel-beds, is 

 not now found living in the Spitzbergen fiords. We are not with- 

 out similar indications in the marine post-glacial beds of Scotland 

 of a formerly more genial climate. Mr Crosskey has drawn special 

 attention to the so-called Fecten maxiitius\>t^ of the Clyde, which 

 contains such shells as Fsaj?imobia ferromsis and Tellina incar?iata, 

 of larger size and in greater numbers than they at present occur 

 living in the neighbouring seas. These facts plainly show us 

 that the temperature of our seas has been exceptionally high at 

 some recent period. In no other way can we account for the 

 northern immigration of the southern species. These species 

 tell of a time when the Gulf Stream carried into the North 

 Atlantic a much greater body of heated water than now reaches 

 such high latitudes. I have hitherto been inclined to assign that 

 latest immigration of southern forms to the last inter- glacial epoch, 

 and have therefore looked upon the isolated colonies, and in- 

 dividual species in our post-glacial deposits and present seas as 



^ There can be no doubt about these beds being post-glacial. I was in- 

 formed by Mr Nathorst, a well-known vSwedish naturalist, that he had seen 

 a bank of mussel-shells resting upon a striated rock-surface, at the mouth of 

 Dickson's Bay, on the north side of Ice Sound. The post-glacial deposits of 

 Spitzbergen have been referred by Ileer to an inter-glacial period. 



