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, II., “ Nacktschnecken Russlands,” p. 60. 
\ Pilsbry, H. A., “ Phytogeny of Arionidae,” p. 103. 
