850 THE AMERICAN NATURALIST. [VoL. XXXV. 
a narrow neck of lowland being carried back into the uplands 
an isolated, elongated land habitat is developed which reminds 
one of river isolation. Merriam’s Life Zone Map (98, plate) 
shows that there is an extension of the Upper Austral Zone 
into the Transition Zone due to the Tennessee valley. With 
the baseleveling process the isotherms gradually migrate up 
the valley and thus help to make it possible for the fauna to 
follow. Of course valleys, of whatever origin, may act as 
highways of dispersal. 
It is important in this connection to recall the more or less 
definite succession of forms up the valley, because the problem 
of dispersal among some animals is largely a problem of the 
continuous distribution of the habitat. 
Segregation of the Upland Faunas by Valleys. 
Correlated with the extension of the valleys into uplands is 
the segregation of the uplands by the growth of the valley. 
This influence is beautifully shown by the land shell fauna 
on each side of the Tennessee valley. Since the Tennessee 
valley divides the uplands of the Cumberland plateau from 
those of the Smoky Mountains, I was led to anticipate that 
this isolation, which has existed for a considerable period of 
time, would show upon the land shell fauna, because these 
animals are very sensitive indicators of such influences. Later 
I was interested to learn that Pilsbry (00) had very recently 
divided the «Cumberland sub-region” of Binney into an east- 
_ern and western division, corresponding to the uplands segre 
gated by the Tennessee valley. He gives about twenty 
species and varieties characteristic of the Cumberland plateau, 
and about twenty-five to the Smoky Mountains. The French 
Broad River has for a long time separated the Great Smokies 
from Roan Mountain, and corresponding with these facts it has 
been found that six forms are peculiar to Roan Mountain and 
about a dozen to the Smoky Mountains. 
