114 



may be relatively few, because the dominating conditions are so thor- 

 oughly established and extreme. But near the eastern margin of this 

 dominance, upon the prairies — the "long grass country" — the number 

 of limiting factors increases greatly, and even a relatively trivial local 

 influence is able to overcome the slight momentum which this domi- 

 nance possesses. In Illinois, then, the causes of the prairie biota, men- 

 tioning only the larger groups of influences, seem to be as follows : a, 

 a sandy character of the soil, resulting in sand prairie; b, loam and 

 good drainage, resulting in black soil prairie ; c, very imperfect drain- 

 age, resulting in wet prairie. A shallow soil underlaid by rock might 

 also produce prairie, but I have not seen any large area of this kind in 

 Illinois. 



We have, then, in the wetness and the dryness of the prairie two 

 of the important controlling influences upon the prairie associations. 

 On the prairie aquatic animals may thrive, particularly those which 

 develop early and mature rapidly, and possess some power to resist 

 or tide over the dry season, either as adults of non-aquatic habits by 

 estivation, or in some resistant immature stage. We can see how 

 aquatic animals, in this manner, are capable of enduring these extreme 

 conditions and remain numerous upon the prairie. Where crawfish 

 holes are abundant, many small aquatic animals are able to utilize them 

 and thus escape drying. Crawfish holes should be examined during 

 dry seasons with this idea in mind. On the other hand, the prairie 

 is inhabited by many animals which can not endure much moisture, and 

 live best in conditions of moderate or extreme dryness. These are the 

 kinds which find their optimum during the driest part of the season, 

 and in very dry years. When there is an abundance of moisture, some 

 of these, for example the chinch-bug, are particularly susceptible to 

 disease. The maximum development of this arid type as seen on the 

 Illinois sand prairie has been studied by Hart ('07) ; more recently by 

 one of my students, Vestal ('13b, '14) ; and about Chicago and north- 

 ern Indiana by Shelf ord ('13a). An examination of the lists of sand 

 invertebrates given by Hart (1. c, pp. 230—257) and Vestal ('13b, 

 pp. 14—60), in comparison with those for the black soil prairie at 

 Charleston, will show many differences, not only in kinds but also in 

 their relative abundance. Some allowance must also be made for the 

 fact that the animals of the black soil prairie are not as fully pre- 

 served as those of the sand areas. 



1. The Black Soil Prairie Community 



The soil population of both sand and black soil prairie has never 

 received thorough study, although observations from the sand areas 



