No. 418.] BASELEVELING. 851 
Influence of Baseleveling upon Tension Lines. 
The relation of the faunas of the different levels or base- 
levels to each other is also important from the standpoint of 
tension zones. On account of the large area which they may 
occupy at certain stages of topographic development these 
zones have great importance, for in them there is a large 
population subject to a peculiar environment and hence liable 
to important evolutionary changes. A stage of equilibrium is 
not reached here, as erosion makes this area relatively unstable 
and subject to continual changes. It is important to determine 
which is the more powerful, the physiographic or organic 
influences, in the production of these lines. The. powerful 
influence of the physiographic processes in general leads one 
to expect that at first the physiographic factors will dominate, 
but later, in a fairly uniform habitat, the struggle will become 
more organic. In the study of dispersal and distribution of 
animals, it is important to see that the physical conditions 
lead, and that in a more or less definite succession the flora 
and fauna follow ; thus the fauna comes to fit the habitat as a 
flexible material does a mold. 
The time is passed when faunal lists should be the aim of 
faunal studies. The study must not only be comparative, but 
genetic, and much stress must be laid on the study of the habi- 
tat, not in a static, rigid sense, but as a fluctuating or periodi- 
cal medium. The bearing of faunal studies upon the problem 
of differentiation and the origin of species, is very close, and 
in our search for the factors we must not lose the perspective, 
and overlook those factors which are fundamental and work 
through long periods of time. 
HULL ZoéLocicaL LABORATORY, 
UNIVERSITY OF CHICAGO. 
