116 INTRODUCTION. 



during the summer, a temperature which is sufficiently 

 mild to permit any of the species to exercise their usual 

 functions, and to reproduce their kind, although the 

 length and severity of the winters retard their maturity 

 and prevent a rapid increase of numbers. Hence, 

 species are less numerous, and individuals are reduced 

 in size and beauty, on the highest levels, in the same 

 manner as they are in northern localities. A striking 

 instance of this effect is seen in the mountainous region 

 of New Hampshire and Vermont, where Helix triden- 

 tata, and IT. sayi, although frequently met with, do not 

 reach one half the magnitude which they attain in the 

 lower levels of western New York and Ohio. 



The observations hitherto made on this interesting 

 subject are few, and do not authorize any confident 

 inference ; but so far as they go, they show that nearly 

 all the species which inhabit the country on either side 

 of the mountains, exist also on the high table-lands, 

 and that if there are any species peculiar to the extreme 

 high points, they must occupy very hmited locaUties on 

 the few peaks which rise more than 5000 feet above the 

 sea. It is not unlikely indeed, that the genus Vitrina, 

 which has been found elsewhere at high elevations, may 

 be discovered in these situations. 



The relations which the different levels of elevation 

 bear to the parallels of latitude, although as interesting 

 to the zoologist as to the botanist, have not yet been 

 made the subject of examination in this country. But 

 the Rocky IMountains, towards and beyond which the 



