113 



blossom of Asclepias incarnata may explain this paucity at this time 

 and place. The milkweed butterfly, Anosia plexippus, is of course a 

 member of this community. 



W. Hamilton Gibson ('oo, pp. 227-237) has discussed, in a very 

 interesting manner, the relations of this plant to its insect pollinators, 

 and calls attention to the variety of insects which are entrapped and 

 killed by its flowers. He also points out that the dogbane (Apocy- 

 iiuiii) has a similar habit. 



Robertson, our leading American authority on the relations of 

 flowers and insects, has published extensive lists of the flower visitors, 

 not only of A. syriaca (cornuti) but of other Illinois milkweeds 

 (Bot. Gaz., Vol. XI, pp. 262-269; Vol. XII, pp. 207-216, 244-250; 

 and Trans. St. Louis Acad. Sci., Vol. V, No. 3, pp. 569-577). 



III. Relation op Prairie Animals to their Environment 



The relation of prairie animals to the major features of their phys- 

 ical and biotic environment presents several facts of unusual interest. 

 On account of the relatively heavy precipitation during June, the slight 

 topographic relief of the region, and its imperfect drainage, unusually 

 large areas of the original black soil prairie are wet or swampy. Cer- 

 tain animals are able to tide over this early, unfavorable wet-summer 

 period because they are not fully roused from their winter inactivity ; 

 others, in their immature stages of development, require less food than 

 later; still others survive by migration to the drier uplands. At the 

 same time, other animals, preferring moist or wet habitats, flourish, 

 and then decline in numbers as the season advances. Toward August, 

 on account of the eastward migration of the continental peninsula of 

 aridity and intense evaporation, those animals whose activity is re- 

 tarded by the earlier wet season find the conditions progressively more 

 favorable, and thrive and grow accordingly. This is the acme of the 

 season for dry-prairie animals, and great numbers of slowly maturing 

 composite plants now make the landscape yellow with their flowers. 

 The Ortlwptcra are now mature, and when flushed, or, when not 

 flushed, by their sounds, are noticeable. That these conditions cause 

 these animals to thrive, is only too evident during exceptionally dry 

 seasons, when the ordinary August drouth begins in July and extends 

 into September. 



In the conditions just indicated, the imperfect drainage, the wet 

 season followed by the dry, we are touching closely upon the real causes 

 of the prairie. Yet to me it seems fruitless to search for the cause of 

 the Illinois prairie ; the causes are probably multiple. In the midst of 

 the Great Plains, the "short grass country" the causes of grass-land 



