110 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 R. 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 limited localities 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 Mountains, towards and beyond which the 
