No. 418.] BASELEVELING. 843 
and thus a period of swamping by intercrossing may be pro- 
duced. The cliff fauna would have its maximum distribution 
and area of habitat when the country was at its maximum 
roughness. 
It will not be necessary here to go further into the details 
of these processes, as the preceding remarks, with an exami- 
nation of the diagrams, will suffice to show that these princi- 
ples may be applied almost indefinitely, but the details vary 
greatly in different localities. One point perhaps needs special 
emphasis, and that is, we must think of these processes as 
active and taking place before our eyes. - 
The bearing of these factors upon the large amount of 
endemism seen in southeastern United States will be illus- 
trated by a few examples. The physiographic changes of this 
Fic, 4. — Valleys of further development than shown in Fig. 2. (After Salisbury.) 
region have been very carefully studied, but comparatively little 
3 been done to correlate these changes with the geographic 
'stribution of the fauna of this region, and especially with its 
endemism. I hope, by calling attention to a few of these 
