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 arbor eus (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 insidiosus, and Euschistus 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 full""" exposed to the sun and 

 may always live in similar conditions. 



