EASTERN OR TEUTONIC BRITAIN. 
403 
of the improved and improving races, which invade this country 
in such a way as tends to divide its previous occupants into a northern 
and a soutliern section, as exemplified by the distribution of (Jnio 
margaritifer, Pupa anglka and other species ; the northern section 
being driven towards tlie nortli, and not spreading or having spread 
soutliward as is the prevalent belief. 
This waning western fauna is exemplified by Heli.v fnsca, Helix 
lamellata, Papa angliai, Saccinea oblonga, Pnio margaritifer, etc. 
The Eastern or Teutonic province, which embraces the whole 
eastern districts of Britain, consists chiefly of the more highly 
organized and later immigrant species, which have invaded our South- 
Eastern shores and advanced mainly in a westerly and north-westerly 
direction, driving liefore them the feebler western si)ecies and 
gradually isolating them or restricting the area they inhabit to Ireland 
and to the southern or northern extremities of Britain. In Ireland 
there is a less severe competition, owing to the later advent and limited 
number of the more vigorous eastern types, so that many weaker 
species are still flourishing, although the feeble remnant of the 
Iberian molluscan fauna, now restricted to narrow limits in the more 
remote south-western district, is on the point of e.xtinction. 
Among these more dominant eastern species may be enumerated • 
Helix nemoralis, H. aspersa, H. pomatia. Pupa secale, Planorhis 
corneas, Limncea peregra, Unio pictorum, etc. 
The Geological history or distribution in time of the mollusca, 
forcibly displays the operation of the same laws as govern their 
distribution in space, furnishing evidence by their fossil remains, not 
only of the successive evolution of new forms, but also of the eventual 
extinction or gradual expulsion from the vicinity of the evolutionary 
area by the more vigorous and later developed species of the weaker 
and more primitive previous inhabitants. 
Many of these weaker forms, however, still survive, their survival 
being probably due to the adoption of dissimilar habits or foods to those 
of the more dominant species among which they live, others though 
locally extinct, still exist in other regions, finding a respite from their 
inevitable and impending extinction by continual retreat to more and 
more distant regions before the advancing hosts of improved forms. 
Many of the older rocks have lost all traces of the fossils they un- 
doubtedly originally contained and this obliteration of the precious 
relics of a byegone life, leaves us, so far as direct evidence is con- 
