66 



roller (Campostoma anomalum). Frogs, toads, and salamanders were 

 also taken in the vicinity by Mr. Hankinson, who dug from their bur- 

 rows specimens of Cambarus diogenes, and also secured immunis and 

 propinquus. On the surface of the pools were numerous specimens 

 of a water-strider, Gerris rcmigis. The forest cover is undoubtedly an 

 important factor in the preservation of such pools, as it controls the 

 evaporating power of the air. 



Mr. Hankinson tells me that during the summer of 19 12 this tem- 

 porary stream was completely dry, and that no fish have been taken 

 from it since the earlier collection mentioned above. From the mouth 

 of the ravine across the bottom to the river it is only a few hundred 

 feet, and in time of heavy or prolonged rains these pools are in direct 

 communication with the river. Such a stream is an excellent example 

 of an early stage in the development of the stream habitat, and shows 

 its precarious character, and the liability to frequent extermination 

 of these pioneer aquatic animals which invade it in its early stages. 

 This applies particularly to those animals which have no method of 

 tiding over dry periods. On the other hand, those animals which live 

 in the pools, those parts of temporary streams which persist longest 

 between showers, have better chances of survival, particularly bur- 

 rowing animals, like the crawfish and its associates. It seems prob- 

 able that crawfish burrows harbor a varied population; not only the 

 crawfish leeches (BrancJiiobdcllidcc) but also the eggs of certain Cor- 

 ixidcB (Forbes, '76:4-5; '78, p. 820; Abbott, '12) may almost cover 

 the body of some crawfishes. By means of this burrow ground-water 

 is reached, and a subterranean pool is formed. For the elaboration of 

 the stream series see Adams ('01) and Shelf ord ('11 and 13a). 



This temporary stream shows how, by the process of erosion, the 

 upland forest area is changed into ravine slopes, and, later, even into 

 the bed of a temporary stream. Thus progresses the endless transfor- 

 mation of the habitat. 



GENERAL CHARACTERISTICS OF THE GROSS 

 ENVIRONMENT 



1. Topography and Soils of the State 



Illinois lies at the bottom of a large basin. This is indicated in 

 part by the fact that so many large rivers flow toward it. The mean 

 elevation of the state is about 600 feet, and about a third of it lies be- 

 tween 600 and 700 feet above sea-level. Except Kentucky, the bor- 

 dering states are from 200 to 500 feet higher. Iowa and Wisconsin 

 are considerably higher, so that winds from the north and northwest 



