250 DRY AND MESOPHYTIC FOREST COMMUNITIES 
spring. Evaporation in the various associations varies directly with the order 
of their occurrence in the succession. The differences in the rate of evapora- 
tion in the various plant associations studied are sufficient to indicate that 
the atmospheric conditions are most efficient factors in causing succession 
(Fuller, 131). 
A comparison of Fuller’s (131) data with the tables or lists of ani- 
mals shows that the distribution and succession of animals is clearly 
correlated with the evaporating power of the air. Further comparison 
with the description of different forest stages shows that the evaporating 
power of the air may be taken, in this case, as an index of the materials 
for abode, etc. 
2. CHARACTERS OF THE COMMUNITIES 
It is possible to characterize the formations of the forest in physio- 
logical terms, though these cannot be of a very definite kind until the 
mores have been studied in detail, and accurate measurements made. 
Taking them stratum by stratum, we may note the following obvious 
characters: 
a) Pioneer communities —The communities of the cottonwood, pine, 
and black-oak stages may be designated as pioneer because of the 
presence of bare mineral soil. 3 
Subterranean and ground strata: (a) The cottonwood’ community 
is characterized by animals which breed and spend the dark and cloudy 
days chiefly below the surface of the sand. They are very largely 
diurnal and predatory, and are exceedingly swift and wary. The bur- 
rowing spider (Geolycosa pikei) is one of the few nocturnal animals. 
(b) The pine community is characterized by similar mores, but is 
to be distinguished from the preceding by the presence of many animals 
which prefer sand that is less shifting and which is slightly darkened by 
humus (170). Animals requiring “cover,” such as the lizard, the blue 
racer, a few ground squirrels, etc., give character because of their absence 
from earlier and later communities. 
(c) The black-oak community represents the climax of diversity 
of the subterranean and ground strata. The bare-sand mores continue 
in the open spaces, which we have designated as transition areas. Leaf- 
cutters are now present, while among the burrowers the root-borers 
(prionids and lucanids) work on the roots of the decaying trees. The 
behavior differences between this and the preceding community are 
differences of detail which, for the making of deductions, would require 
much careful study. 
