102 



tinental climates. These extreme conditions are characteristic of many 

 habitats. 



It is readily seen how the general principles just summarized apply 

 to the land animals of the prairie. Many of these are active during the 

 day, live in the bare exposed places, or near the level of the vegetation, 

 where evaporation is greatest and water-loss is correspondingly large, 

 and feed upon the dry haylike vegetation. Others remain among the 

 humid layers of the vegetation or in the moist soil, and feed upon 

 juicy plants and other moist food. Predaceous and parasitic animals, 

 deriving their moisture from their prey, occupy both the dry and humid 

 situations. These are representative cases, between which there are a 

 large number of intergradations. 



In the forest, where evaporation is more retarded than in the open, 

 a large number of animals live in the forest crown, at the forest mar- 

 gin, in glades, and in wood, of all degrees of dryness, and eat food 

 varying similarly from juicy leaves to dry wood. On the other hand, 

 some live in moist logs, among damp humus, or in the soil, and feed 

 upon dripping fungi or soggy wood. Many of these animals possess 

 little resistance to drying. 



The optimum for prairie and forest animals thus involves a 

 dynamic balance between the intake of water and its loss by evapora- 

 tion and excretion. 



ANIMAL ASSOCIATIONS OF THE PRAIRIE 

 AND THE FOREST 



I. Introduction 



In an earlier chapter of this paper the habitats and animals found 

 at the different stations were discussed, and in the preceding section 

 the general characteristics of the physical and vegetational environ- 

 ment of the prairie and forest have been described and summarized. 

 We are now in a better position to consider the relations of the inverte- 

 brates, not only to their physical environment, but also to the vege- 

 tation, and, furthermore the relations which these animals bear to one 

 another. We wish also to consider both the prairie and the forest as 

 separate units, and to see how the animals are related to their physical 

 and biological environment. As previously stated, the special locali- 

 ties studied were described by stations both to give a precise and con- 

 crete idea of the prairie and its animals, as now existing in a limited 

 area, and also to preserve as much of the local color as the data would 

 permit. I wish now to reexamine these animals from another stand- 

 point, that of the animal association as a unit. The prairie as a whole 



