INTRODUCTION. xi 



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 Tetraapis 

 by Dr. Hagenmiiller, is according to present knowledge, confined to the . 

 grotto of Planina, in Carniola ; it presents the earlier stage in the shig 

 evolution, before the mantle had finally closed and fused above the vestigial 

 shell, as it still retains a central opening in the mantle, recalling the space 

 left uncovered by the expanded mantle lobes in AmpMpepka glutinosa. 

 Dr. Simroth, is, however, disposed to regard Tetraspis as founded upon 

 abnormal Avion intermedins or young A. sttbfuscus ; 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 fi-om 

 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 



