482 Proceedings of the Royal Irish Academy. 



The animals, such as the Irish hare and others of a northern origin, 

 ■which reached Ireland, might have passed from the Arctic regions to 

 I^orth America, then crossed over to Asia, migrated across that conti- 

 nent and Europe, and then passed over the old land connection to 

 England, and ultimately to Ireland. If they came by a more direct 

 route, the distribution of land and water in Arctic Europe, too, must 

 have undergone some changes within recent geological times. 



In the English Pleistocene (glacial) deposits, we find the remains 

 of an extraordinary assemblage of animals which at present chiefly 

 inhabit the Tundras of Siberia — a fact which has naturally given 

 rise to the belief that England must have passed through a period of 

 intense cold. As we cross over the Channel into Northern Germany, 

 we again meet with the same species in similar deposits, and with 

 many more whose present habitat agrees with the others. A good 

 many of these Siberian species have also been found in Erance. But 

 not a single one occurs in the Pleistocene deposits of Ireland, although 

 we have, as I mentioned before, some species, such as the Irish hare, 

 to which may be added the extinct reindeer, which are of undoubted 

 northern origin. I have reason to believe that the latter belong to a 

 different and independent migration, and came to this country by a 

 route which the Siberian mammals were unable to utilize. That 

 Ireland was already severed from England when these vast herds of 

 Siberian immigrants entered England, seems to me certain. It is pos- 

 sible, though, that Ireland was at that time still connected with 

 Scotland, and that some barrier existed which prevented a free 

 migration to the latter country from England. The condition of the 

 British Isles may have been something like that represented by 

 Mr. Jukes-Browne^ on a map illustrating Pleistocene geography. 



A study of the geographical distribution of the British Land 

 and Freshwater Mollusca supports the view just indicated. We 

 can roughly divide the whole of the Irish species into those 

 inhabiting the plain and into the mountain forms. Among the latter 

 we have some of northern, and others of southern origin, but those of 

 the plain are mostly southern species. l!^ow among the latter, some, 

 such as Selix pisana, occur in England only in the south-west, others 

 such as Selix acuta, Selix interseda, and Selix ericetorum^ are not so 

 much confined to the south of England, but they become much rarer 

 as we go north, and in Scotland they occur in the south-west or west 

 only. Altogether the majority of the Irish species occupy in Great 



1 Jukes-Browne, A. J., The Building of the British Isles (PI. xv.), 2nded. 1892. 



