402 
■WESTERN OR CELTIC BRITAIN. 
more adaptable to its modifications than their weaker predecessors, 
gradually overcome or dispossess them of the more favourable regions, 
starving out and driving the previous possessors to remote areas or com- 
pelling them to take refuge in the less desirable districts to which their 
range becomes more and more restricted and conseiipiently isolated. 
Viewed in regard to its mollusca, the British Isles may be divided 
into a Western or Celtic ami an Eastern or Teutonic province ; and 
although these districts cannot be rigorously defined, as some species 
have advanced beyond their fellows, while others, owing to later 
advent or greater susceptibility to changes of environment have not 
advanced so far, yet they undoubtedly express the salient feature of 
our molluscan fauna and its connection with the general scheme of 
distribution. 
The Western or Celtic province comprises the whole of Ireland 
and the western portions of England, Scotland and AVales as delimited, 
its characteristic feature consisting of the presence of a number of 
weaker species, whose originally conterminous areas of occupation 
have been cleft in twain by their more or less complete expulsion from 
the South-Eastern and Midland districts of England by the advancing 
general body of improved .s2)ecies, as although the later developed 
Pentatreniate species have outstripped their less highly organized 
brethren and overspread the whole of the British Isles, this is due to 
their marked superiority to the bulk of our fauna in organization and 
adaptability. 
Probably the oldest, as it is also geographically the most remote 
section of this AVestern fauna, comprises the peculiar forms of life 
occupying the extreme south-west of Ireland, many of whose component 
si)ecies are also found inhabiting the west of the Iberian peninsula ; 
these forms, representeil among the mollusca by Geomalncus macnlosm^, 
are not imi)robably on the verge of extinction, being now reduced to 
and confined within these restricted districts by the pressure of the 
more highly organized forms of life which have already driven them 
from the more easterly tracts which they probably formerly inhabited, 
and are now gradually invading and dispossessing them of the terri- 
tory upon which they still linger. 
The older notion, which regards as the original home of a primitive 
.species the districts wherein it now exists, is in my view manifestl}^ 
inaccurate, and we must regard the regions now occupied by weaker 
groups as one of the various stages of their retreat before the advance 
