INTRODUCTION. Xl 
This genus has been incorrectly assumed to have been evolved in Central 
Asia and to have migrated therefrom, reaching Ireland by way of the 
Mediterranean region; while the ancient Arionid, described as Tetraspis 
by Dr. Hagenmiiller, is according to present knowledge, confined to the 
grotto of Planina, in Carniola; it presents the earlier stage in the slug 
evolution, before the mantle had finally closed and fused above the vestigial 
shell, as it still retains a ventral opening in the mantle, recalling the space 
left uncovered by the expanded mantle lobes in Amphipeplea glutinosa. 
Dr. Simroth, is, however, disposed to regard 7vtraspis as founded upon 
abnormal Arion intermedius or young A. subfuscus ; but, even if this con- 
jecture be correct, the atavism is equally remarkable and instructive. 
GEOGRAPHICAL DistRIBUTION is closely interwoven with the problems of 
evolution, and when studied compels the acceptance of the existence of 
zoological regions, these being immense areas or districts more or less 
isolated from each other by natural obstacles to dispersal, and each 
possessing a fauna which to a great extent is peculiar to it; yet the 
forms of life they now harbour will inevitably, in process of time, be 
slowly and gradually changed to species of a higher type, drawn directly 
from a neighbouring more advanced area, but primarily emanating from 
the European region, where it is fairly well established as a general law 
that the more ancient the geological formation containing the fossil 
remains of any group of organisms, the further removed geographically 
is their present abode and in proportion as they are found in more and 
more recent deposits, the nearer to the European area are their living 
representatives to be found at the present day. 
This is corroborated by Dr. Wallace and others who have affirmed that 
the Eocene period is represented faunally in the Africa of to-day, and the 
Miocene in that of Madagascar, while the faunal and floral features of 
New Zealand, Australia, and other distant southern countries represent 
the European at still earlier epochs. 
Similarly the isolated districts now occupied by species of a more 
generalized type than those occupying the surrounding areas, must not 
be regarded as the cradle of those species, as some have so confidently 
declared, but must rather be regarded as a sanctuary to which the more 
generalized and consequently weaker forms have retreated to escape the 
severe competition of the improved stronger forms evolved from their 
descendants. 
Nor is it more accurate to regard the present metropolis or head- 
quarters of a species or group as necessarily disclosing the theatre of its 
origin. If the species or genus be an ancient one, it is very improbable 
indeed that such is the case, and much more likely that it is merely one of 
