844 THE AMERICAN NATURALIST. [VoL. XXXV. 
correlations and to some of the principles involved, that others, 
more familiar with special groups of animals, may be led to 
make similar comparisons and correlations, and also to empha- 
size the fact that causal and genetic faunal studies must in the 
future take into account these factors, if work of permanent 
value is to be done. Biological surveys will, carried out in 
this way, become decidedly more than faunal lists, with miscel- . 
laneous biological information. In order to emphasize certain 
aspects of the baseleveling processes special phases will be 
discussed under separate heads. 
Migration of Divides. 
The migration of divides has long been recognized as an 
important factor in the mixing of river faunas, but without a 
distinct recognition of it as a phase of the baseleveling process. 
The following examples will show the influence of the migra- 
tion of divides upon faunas. But first, attention will be called 
to some of the facts of the distribution of the family Pleu- 
roceride, or Strepomatidze of the older authors. This family 
is distinctly North American, living forms occurring here only. 
Tryon’s monograph ('73) on this family, from which I take my 
data, recognizes four hundred and sixty-four species. Excepting 
those species whose exact locality is not definitely known and 
certain Pacific coast forms, most of these are limited to the 
Tennessee River and its tributaries above Florence, Alabama, 
and to the Coosa River. Two-thirds, or about three hundred, 
of these species are confined to these two river systems. 
The region in which this family reaches its maximum 
development in both individuals and species is remarkably 
limited and is bounded by Tryon as follows. On the “North, by 
the Tennessee River and tributaries. The Cumberland Moun- 
tains prevent the dispersion of the species of this river to the 
northward until its course is directed into Alabama. Here the 
character of its species (which we will again allude to further 
on) changes, and they become gradually less numerous and of 
greater geographical dispersion as the river runs toward the 
west. Eas/,the mountain range of the Blue Ridge, running 
