1887.] NATURAL SCIENCES OF PHILADELPHIA. 393 



Our own prairies afforded evidence of the soundness of these views. 

 There would often be seen elevated peninsular-like arms stretching 

 from the woody area into the grassy basins, or even little islands of 

 elevated ground in the midst of the flat sea of soddy land, covered by 

 trees of various kinds. These elevations by reason of drouth, lighter 

 soil, or other sj)ecial conditions, were unfavorable for the growth of 

 the thick mass of herbaceous vegetation that possessed the land be- 

 low. The seeds could not only sprout and become trees, but the ab- 

 sence of much grassy undergrowth saved them from serious eflTects 

 from fire on lower prairie ground. 



There could be no doubt these considerations fully accounted for 

 the perpetuity of the grassy areas, and the inability of the forests to 

 encroach thereon. 



If we are now" asked how these extensive areas were given over to 

 grass in the first place, we may safely reply that the trees were not 

 there to dispute with it for the possession of the ground, or they 

 would have conquered. We may imagine the prairie region as in a 

 state of transition from the paludose to the limose state, with ligneous 

 or arborescent vegetation on the higher lands, many miles away. 

 The tufty grasses would undoubtedly take possession long before their 

 woody neighbors could come down from the hills and spy out the 

 land. The struggle for life would be at the boundaries where the 

 two forces met. The ti-ees could not gain rapid advances, but by 

 the overshadowing of their branches would weaken the grass be- 

 neath, and thus, by slow approaches, gradually conquer their 

 weaker neighbors. In meadows, where cattle kept the coarse grass 

 down, or where briars or light bushes kept tough grass from spread- 

 ing, or where the ground was too gravelly or sandy or the native 

 grasses not of a close tufty character, trees found no obstruction 

 whatever in their endeavor to take possession of the soil. 



December 13. 

 The President, Dr. Jos. Leidy, in the chair. 

 Twenty-three persons present. 



Bot-larvae in the Terrapin. Prof. Leidy remarked that the 

 habits of a naturalist often led him to observe things in our daily 

 life which usually escape the notice of others. In our food he had 

 frequent occasion to detect parasites which he preferred to reject 

 but which are unconsciously swallowed by others. While he liked 

 a herring, he never ate one without first removing the conspic 

 uously coiled worms on the surface of the rows; and he had 

 repeatedly extracted from a piece of black bass or a shad a thread 

 worm which others would not distinguish from a vessel or a 

 nerve. While he did not object to the little parasitic crab of the 

 oyster, he made it a point to remove the equally frequent leech from 

 the clam. It was in a piece of ham, he was eating, that he first 

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