VERTEBRATES OF WALES AND 1RELAND. 455 
Mr. R. J. Ussher points out that, whilst the majority of pre- 
historic mammals found in Welsh cave-deposits occur also in 
Ireland, the Lion, Glutton, Rhinoceros, and Bison have not yet 
been identified in Ireland. 
In connection with the theory advanced to account for the 
absence of certain species from Ireland, the following are factors 
of some importance :— 
(1) It is evident from the contour of the sea-bottom that a 
narrow land-connection between Scotland and the North of 
Ireland persisted for some time after Ireland had become sepa- 
rated from England by the sea. 
(2) At the time when Great Britain and Ireland were still 
part of the Continental mainland, a great river existed which 
discharged its waters into the Arctic Ocean between the Shet- 
land Islands and Norway; the Rhine, Thames, and other rivers 
of this region were all tributaries to this great river. 
With regard to the first of these statements, I hardly think 
that the existence of this land-connection would affect the question 
of distribution of species. Any land animal extending its range 
into Great Britain after the sea had severed Ireland from England 
would not be likely to travel into North Ireland from Scotland, 
because that would involve a movement southwards, i. e. ina 
direction contrary to the general trend of all such species towards 
the north-west. 
As to the second statement above, it is evident that the exist- 
ence of a big river with tributaries extending over the entire 
district might have an important bearing on the distribution of 
freshwater fishes, because it would form a means of communi- 
cation between streams and lakes now entirely severed from one 
another. In this way we may imagine that the various species 
(or subspecies) of Char and the different members of the genus 
Coregonus (Gwyniad, Pollan, &c.) may have each originated from 
a single species spread throughout this great river system. The 
local races subsequently acquired special characters owing to 
isolation and the influence of different environment in each 
case. 
