MEANS OF DISPERSAL. I43 



tion to this explanation of the presence in the Pacific 

 Province of Central Province species, in a letter^ con- 

 cerning H. strigosa^ from which the following is an 

 extract. The same considerations, according to Mr. 

 Binney, ^pply to Helix solitaria^ the group of Helix 

 mullaniy and Helix ptychophora : 



" The Cascade range of mountains in Oregon is, as you 

 are aware, a continuation of the Sierra Nevada. It 

 crosses the Columbia River between the Dalles and 

 Portland, and continues its northerly course on the west 

 side of the Columbia. Numerous spurs, however, break 

 off from the main range, and pass north through East 

 Oregon into Utah and Idaho. One of these spurs, 

 called the Blue Mountains, shoots off the Cascades near 

 Mount Hood, and runs nearly parallel with the 

 Columbia, forming the eastern boundary of its valley, 

 and is about forty miles from the river, and terminates 

 about abreast of the mouth of Salmon River, Idaho, 

 and on the south side of Snake River. On the north 

 side of Snake River these mountains have local names, 

 but are known by the general name of Bitter Root 

 Mountains. ... By tracing the course of Snake River 

 and its tributaries you will see it drains the northern 

 part of the great central basin, and when it cut its way 

 through these mountains it very likely drained the 

 great system of lakes that once covered a great part 

 of this central basin. Now the mountain rang-es in 

 this portion, north-east, are the metropolis of strigosa 

 so far as we know at present ; and it is not improbable 

 that many individuals, and quite likely whole colonies^ 



