322 THE ZOOLOGIST. 
When the subsidence of the land began, this deep channel would 
be the earliest portion to sink beneath the waves, so that Iveiand 
would become an island whilst Great Britain was still a part of 
the mainland. A further subsidence submerged more low-lying 
land, and converted Great Britain also into an island. 
The land animals of these islands would naturally be identical 
with those of Western Europe so long as they were actually 
united with the mass of the continent, but the species have 
changed somewhat from time to time. For instance, the Lem- 
ming was at one time common over the whole of Western 
Kurope, including the British Isles, but has since vanished from 
the greater portion of its ancient habitat. 
It is evident that any changes in the species of land animals 
would affect these islands so long as they were connected with 
the Continent, but not afterwards. As Ireland was separated 
from the mainland before Great Britain, any species which 
reached the latter at a later period would be unable to reach the 
former on account of the sea intervening, whilst the occurrence 
of a species in Great Britain but not in Ireland would be pre- 
sumptive evidence that it reached here whilst the latter was still 
connected by land with the Continent, but after Ireland had 
become an island. It also follows that species found in both 
countries are longer established, or ‘“‘of older family,” than 
those found only in Great Britain. 
These considerations add largely to the interest and import- 
ance of the facts to be now adduced.* 
FISHES. 
Porr or Rurr.—Occurs in the Severn and tributaries, the 
Dee, and canals in the border counties of Wales. Unknown in 
Treland. 
RoacH.—Common in the eastern counties of Wales, rarer in 
the west. Unknown in Ireland. 
Rupp.—Occurs in a few scattered localities in Wales. Abun- 
dant in Ireland, where it takes the place of the Roach. 
Dace.—Common in the Severn and Dee with their tribu- 
taries. Unknown in Ireland. 
** Full local details are given in the writer’s ‘ Fauna of North Wales.’ 
