MATTHEW, CLIMATE AND EVOLUTION 303 



slowly into Potamobius and spreading east and west from that center, 

 as the American group spread southward. 



DISTRIBUTION OF HELIX HORTENSIS 



Dr. Scharff^^^ regards the distribution of Helix Iwrtensis as an im- 

 portant part of the evidence in favor of a late Cenozoic bridge connecting 

 Europe with eastern North America. The species is well known in 

 Europe and has always been regarded as indigenous there. It occurs 

 along the North Atlantic coast, and in Labrador, Greenland, Iceland and 

 the Shetland and Faroe Islands. It was formerly considered as intro- 

 duced on this side of the Atlantic by human agency; but it has been 

 found in old Indian shell-mounds and more recently in undoubtedly 

 Pleistocene deposits in Maine. It is unknown in Asia or western North 

 America. Hence, Dr. Scharff concludes that it must have migrated from 

 Europe to America across a land bridge via Iceland and Greenland in 

 Pliocene or Pleistocene times. 



The early opinion that Helix hortensis is an introduced species in this 

 country was founded, so far as I recall, mainly upon the peculiar local 

 range and habitat of the species, very different from the truly indigenous 

 New England land-snails, and my early experiences in land-snail collect- 

 ing in southern New Brunswick were quite in accord with this evidence. 

 It is quite possible that Helix hortensis, like the genus Equus, is both 

 introduced and indigenous. 



Granting that it is at least partially indigenous, what evidence is there 

 that the present distribution is not the remnant of a Tertiary circumpolar 

 distribution? The fact that it is not recorded in the Tertiary of Asia? 

 But what proportion of the presumably abundant Miocene or Pliocene 

 land-snails of Asia is known to us? It can only be a minute fraction 

 at the best — less than one per cent. So the chances are a hundred to one 

 that if Helix hortensis or an ancestral form of the species existed in the 

 Tertiary of North Asia, we should have no record of its existence at 

 present. We do, however, have a good deal of indirect evidence that an 

 environment favorable to the present habits of the species existed during 

 the later Tertiary in the region intervening between its present discon- 

 tinuous distribution areas, and that the environment became unfavorable 

 in that intervening region at the close of the Tertiary. I can see no 

 need for assuming a transatlantic land bridge to account for the distribu- 

 tion of this species. And the explanation here suggested is in harmony 

 with the known course of distribution of those members of the northern 



^- R. F. ScHARFF : rroc. Roy. Irish Acad., vol. xxvlli, p. 19. 1909. 



