48 IOWA ACADEMY OF SCIENCES. 



between cause and effect, or to avoid the substitution of a mere 

 coincidence for a cause, as it was in the days of the earliest 

 observers — in fact, while we now have knowledge of a greater 

 number of possible causes, we are by that very fact exposed to 

 the danger of a greater number of possible misinterpretations 

 of effects. It is, therefore, well for the student of our forest 

 and prairie problems to approach his subject with his mind in 

 a position of receptive neutrality toward the various theories 

 of the origin of prairies which have been advanced, but com- 

 mitted to none of them. For it seems that the chief weakness 

 of the majority of the discussions which have been published 

 thus far, is the fact that they are based, for the most part, 

 upon single causes, I:; is well to bear in mind that the growth 

 of trees may be prevented or influenced by a variety of causes, 

 and that, therefore, there is at least a strong probability that 

 a combination of these causes produced our prairies. For the 

 prairies are not uniform in topography, nor in character of 

 soil, nor in humidity. We have here in Iowa, prairies upon 

 the flat, comparatively wet, north-central Wisconsin drift plain, 

 and upon the adjacent dry, loess hills of the western part of 

 of the state. In fact, so far as their physical features are con- 

 cerned, these areas agree only in being treeless. And even 

 in more restricted areas differences may be observed. We 

 find one side of a hill treeless, the other clothed with forest. 

 One shore of a lake or stream is skirted with trees, while the 

 other is unobstructed by tree or shrub. Sometimes it is the 

 lowland, and sometimes the adjacent hill, which forms the 

 promising nucleus or the last remnant of a forest. It is, there- 

 fore, not wise to assume that one cause alone is responsible for 

 this condition, nor that in every restricted locality the predom- 

 inating cause, or combination of causes, was the same. It is 

 the purpose of this paper to present a discussion chiefly of 

 a neglected agency which operates against the development of 

 forest trees. In order, however, that it may not seem like 

 another attempt to introduce a siogle cause explanation, a brief 

 resume of the causes which have been prominently discussed 

 is here presented. 



