I909-] FRESH-WATER FAUNA IN PENNSYLVANIA. 95 



is given by the disappearance of the Unionidse, and, generally, this 

 fauna is irreparably lost. Close upon this follows the disappear- 

 ance of the fishes, yet in times of recovery of the rivers (at high- 

 water stages), fishes reappear, coming from tributaries, etc., which 

 have acted as preserves, and this may go on indefinitely as long as 

 the river is recovering again at times, since the fishes possess a high 

 power of locomotion (as we shall see below, the construction of 

 dams in a river puts an end also to this). Crawfishes stand it a 

 little longer than fishes, but they also disappear finally, and the tem- 

 porary restocking of a stream takes place only in a limited degree.^ 

 With the crawfishes, or soon after them, the Gasteropods of the 

 family Pleuroceridce are driven out. When the process has reached 

 this stage, the higher forms of life, which subsist on these various 

 forms are compelled to abandon the stream: tailed Batrachians, 

 Snakes, and part of the Turtles. Finally, only Lymncea, Planorbis 

 and Physa, and the muskrat survive. Of these, Physa disappears 

 last, while the muskrat may stay indefinitely, being not entirely 

 dependent upon animal or aquatic food. 



II. The Causes of the Destruction of the Fauna. 

 A. Direct Extermination by Man. 



A number of fresh-water animals are directly killed by man, and 

 thus disappear in streams, the character of which has not been 

 changed unfavorably for life. This is true in the first line for the 

 fishes. Fishes, forming part of human food, are sought for every- 

 where, and in consequence of the increase of the population neces- 

 sarily must bfe decimated in number. Yet a complete destruction 

 of the fish life hardly has ever been brought about by man alone, 

 •chiefly so, if the fishing is carried on under the restrictions put upon 

 it by law. The fact is that there are many places where " fishing is 

 good," and where fishermen freely avail themselves of this chance, 

 but where fishes are still abundant (upper Allegheny River, for 



^ It happens sometimes that restocking of the lost territory is done by 

 a different species. Thus in the Mahoning Creek at Punxsutawney, Jefferson 

 Co., and in Slipperyrock Creek at Branchton, Butler Co., the original species, 

 which was destroyed, was Cambarus ohscurus, and subsequently, C. bartoni 

 entered the creek. 



PROC. AMER. PHIL. SOC. XLVIII. I9I G, PRINTED JULY 6, I9O9. 



