GENERAL DISCUSSION 7 



II. SUMMARY OF THE DISTRIBUTION OF THE LAND SHELLS 

 OF ALASKA AND ADJACENT REGIONS. 



I have summarized the distribution of the fresh water shells 

 by drainage areas, as perhaps the least objectionable method of 

 connecting the facts of distribution. But the land shells require 

 a somewhat different treatment, since their distribution has noth- 

 ing to do with currents of water, though sometimes a snail may 

 be carried in the spring freshets under the bark of a floating 

 log, and by rare chance survive to be stranded by the falling 

 waters somewhere down stream. A certain amount of move- 

 ment of the minute forms may result from the distribution by 

 high winds of dead leaves and other light material to which the 

 smaller land shells are accustomed to adhere. Pieces of ice 

 from smaller brooks carried by freshets may also convey a cer- 

 tain distance and deposit, when stranded by falling water, pieces 

 of bark or leaves containing snails or their eggs. Such chances 

 are too rare to be made much account of, and doubtless the dis- 

 tribution of our smaller snails is brought about in the main by 

 the slow movement of individuals. 



The Pulmonate fauna of Alaska is composed of four elements : 

 contributions from the faunas of Asia, of the Pacific Coast of 

 America, of the Canadian (or Hudsonian) region, and of the 

 circumboreal or common subarctic fauna of the whole northern 

 hemisphere. 



In tabulating the distribution of the species a column may be 

 reserved for each of these elements : the circumboreal column 

 being headed ' Europe.' A column may be reserved for Green- 

 land, and another for the approximate highest north latitude 

 which the species is known to attain. This means for the snail 

 not so much differences of temperature corresponding to latitude, 

 as differences of period in activity, which diminish as one pro- 

 ceeds northward. Snails at Point Barrow must remain in a 

 state of hibernation at least nine months in the year, and I sus- 

 pect that this more probably brings a limiting strain on the 

 vitality of the organism than would the mere occurrence at times 

 of a specially low temperature. 



