840 THE AMERICAN NATURALIST. [Vor. XXXV. 
His paper is of great value to the general student as well as 
to the special student of faunal problems. I find that he 
had anticipated many of the conclusions to which I had come, 
but, on account of their imperfect recognition, they will be 
well worth repeating. Geologists have been primarily con- 
cerned with the submarine aspect of physiographic changes. 
Chamberlin has discussed this in a very suggestive paper ('98). 
Since there is a very evident correlation between the erosion 
of the land and the deposition of the eroded material in the 
sea, we are able to see that under some circumstances both 
the submarine and subaerial factors of physiographic change 
must be taken into consideration in explaining habitats and 
the struggle for existence of aquatic organisms. This may be - 
illustrated if we suppose that as a continental shelf becomes 
reduced in area the crowding of its fauna might force some 
of its members into fresh water, a habitat with a relatively 
poor fauna, which consequently is open territory. 
It appears that many biologists are unfamiliar with the idea of 
baseleveling. For this reason the following brief account is 
given, showing how rivers cut down land and tend to reduce it 
to a baselevel. If a comparatively level country be elevated 
out of the sea a few hundred feet, and rains begin to work 
upon it, the slight inequalities of the surface will cause the 
waters to collect in the hollows and then run off, carrying 
detritus and cutting a trench or gully. This trench becomes 
with succeeding rains deeper, wider, and longer as it cuts its 
way into the uplands, and thus valleys may be formed. With 
this growth or increase of the area of valleys there is an 
increase of lowland and cliff habitat and a decrease in the 
upland habitat. These progressive changes are beautifully 
shown in Figs. 1, 2, and 3, after Salisbury. It is important 
to notice here the changes in the relative proportions of the 
habitats, with a premium placed upon those forms which can 
1 Dr. H. C. Cowlethas recently applied these principles to the study of plants: 
The Physiographic Ecology of Chicago and Vicinity; A Study of the Origin, 
Development, and Classification of Plant Societies, Bot. Gazette, 
(1901), pp. 73-108, 145-182. 
The Plant Societies of Chicago and Vicinity, Bull. of the Geographic Society f 
Chicago, No. 2, 1901. 
