128 



stream Mr. Hankinson secured Cambarus diogenes. Thus by the 

 growth of this ravine a new community is developing at this place — 

 that of a temporary stream. 



In time such a stream will cut down to ground-water level, the 

 pools will become permanent, and a constant current will be main- 

 tained between the pools, and a permanent stream will become estab- 

 lished. The manner in which this ravine and stream grow, at the 

 expense of the upland forest, is an indication of how the upland for- 

 est may be changed and by degrees become converted into a lowland 

 forest and even into an aquatic habitat. 



V. Relation of the Deciduous Forest Invertebrates to their 



Environment 



We have seen that the forest should be looked upon as a thick 

 layer of vegetation in its effect upon the physical conditions which in- 

 fluence animal life. This thick layer is of relatively slow growth, and 

 in its early stage it is composed of shrubs and young trees. But "as 

 the vertical extent of the forest increases and the forest crown mi- 

 grates upward, the intervening trunk, bark and branch habitat .... 

 enlarges and the leaf-eating inhabitants of the forest crown rise up- 

 ward. This crown fauna retains or rather continues some of the char- 

 acteristics found at the marginal zone, with which it retains direct con- 

 tinuity" (Adams, '09, p. 162). In addition to this vertical upward mi- 

 gration of the forest crown, the forest also tends to spread laterally, 

 by arms or peninsulas of forest, which expand upon the open, or by 

 the excentric growth of groves, which in time fuse and form a contin- 

 uous forest. The original forest margin and adjacent prairie was 

 characterized by "groves", as they were commonly called by the early 

 settlers, and also' by more or less open woods or "oak openings," which 

 are the homologs of the open oak forests yet found on the Illinois 

 sand areas. This interdigitation of forest and prairie produced penin- 

 sulas of forest extending into the prairie, peninsulas of prairie ex- 

 tending into the forest, islands of prairie surrounded by forest, and 

 islands of forest surrounded by prairie. Where the forest was advanc- 

 ing, the open places or glades are to be considered as prairie relics; 

 and when the prairie was for any reason encroaching on the forest the 

 forest is to be considered the relic. The glade and the grove are thus 

 comparable communities, and are to be considered as relics or pio- 

 neers according to the direction of advance of the local association. 

 The development of adequate drainage and all that is associated with 

 this process, the character of the soil, the extension or retreat of the 

 forest, the changes in composition of the forest, and the kinds of 



