306 ECOLOGY 
animals occur. Plants grow from seeds only under a very limited range 
of conditions. However, if trees are given a few years’ growth under 
favorable conditions they will be successful under a great range of con- 
ditions. The great age to which trees often live and the slowness with 
which they grow make it possible for conditions to change while the 
trees still live on with changes only in leaf structure. It is to be expected 
that the distribution of animals is correlated with the occurrence of 
seedlings or of quick-growing plants or at least with leaf structure types 
rather than strictly with species of trees. These facts suggest that 
there are two types of cases in which physical conditions and forest 
conditions are not in accord. In the first case atmospheric conditions 
become favorable for forest animals before any woody plants have been 
able to grow; in the second, woody plants remain after conditions have 
become unfavorable for forest animals; both are due to lagging behind 
of vegetation; both are very local and of minor significance. 
The reasons for the wide distribution of some animals in the forest 
stages which we have considered are no doubt various. For example 
Zonitoides arboreus (Table L, p. 252) is rare in the early stages and is 
confined to the lower and moister localities. If Epeira domicilorum is a 
species of stable physiological makeup we can offer no explanation for 
its peculiar distribution (Table LVI, p. 257). A species may have its 
critical period in the early spring when the leaves are off the trees and 
the condition of the atmosphere similar in all stages (see Fig. 251, p. 248) 
or may live at higher levels in the denser and older stages, and thus be 
surrounded by similar atmospheric conditions, but we are not warranted 
in assuming either of these causes here. 
Another striking feature of the distribution of many beetles, bugs, 
spiders, and Orthoptera is the fact that they are found in open woods, 
edges of woods, on the vegetation of marshes, and over the water of small 
ponds in which vegetation is growing. In this way many species are 
found to occur in what at first appear to be very unlike situations. 
Lygus pratensis, Triphleps insiduosus, and Euschustus variolarius, 
which occur on the vegetation of the margins of swamps, of the 
black-oak forest dunes, and on prairies and agricultural lands, may 
serve as examples. Shull has pointed out similar facts as one of the 
difficulties in the way of ecological classification of Orthoptera and 
Thysanoptera. Such species as the bugs mentioned above are said to 
occur “everywhere,” although they are rarely found in moist woods or 
in any situation in which they are not fully exposed to the sun and 
may always live in similar conditions. 
