SNAILS AND SLUGS OF CALIFORNIA 213 



or have crossed over by a former North Atlantic land bridge. 

 In western Europe and north Africa we have the four genera 

 Arion, Geomalacus, Ariunculus and Letourneuxia. Only the 

 first of these has a wide range, one species extending through 

 Russia to northern Siberia. In Asia only a single genus 

 (Anadenus) of the family is known, inhabiting the Hima- 

 layan Mountains and China. The Arionidae thus have a most 

 discontinuous range, and their origin and dispersal form an 

 interesting problem. Since Geomalacus and Letourneuxia 

 are confined to the western borders of Europe and North 

 Africa, and Arion obviously has its headquarters in the same 

 region, Professor Simroth * argued that the European Ario- 

 nidae had either originated on a sunken land which lay out 

 in the Atlantic, or wandered across an ancient Atlantis from 

 western America. Dr. Pilsbry f also is in favour of an 

 American genesis for the Arionidae, but he believes that 

 the ancestors of the Old World genera just alluded to 

 must have crossed over to Asia by means of a former 

 Alaskan land bridge, and then have wandered along, dropping 

 Anadenus on the way, until they finally reached western 

 Europe. Of the two theories Professor Simroth's appears; 

 to me the more plausible one. And his hypothesis is 

 strengthened by the occurrence in south-western North 

 America of a species of slug either identical or very closely 

 related to a European one, viz., Amalia hewstoni. The Euro- 

 pean Amalia (Milax) gagates must be regarded as an exceed- 

 ingly ancient species, certainly dating back to beyond Tertiary 

 times. Geological evidence for such a belief there is none, 

 nor could we expect to find the remains of slugs in ancient 

 deposits. In a matter of that kind we have to rely purely on 

 distributional evidence. That Amalia gagates could be trans- 

 ported to any island from the mainland by the ordinary means 

 of dispersal is out of the question. It can only have 

 reached its present habitat on many very remote islands by 

 human introduction or by former land connections with the 

 mainland. It is by no means a slug that congregates near 

 human habitations, nor is it commonly found among vegeta- 

 bles like some of the Limaces and Arions. Yet it occurs in 



* Simroth, H., " Nacktschnecken Russlands," p. 60. 

 t Pilsbry, H. A., " Phylogeny of Arionidae," p. 103. 



