24 VICTOR E. -HKLFORD. 



than that i the older streams, and the ditYerent conditions accord- 

 ingly closer to-eiher. 



Turning to the other t\vi> >t reams studied (Hickory Creek and 

 Thorn-Butt uTtield Creek), we see that the same species, the 

 horned dace, is at the headwaters of these, as at the headwater 

 of the north shore streams. It is accompanied, however, by 

 eral other species, a part of which are found in the north shore 

 >trcams at points further down stream and in a larger volume of 

 water than some individuals of the horned dace. In other words 

 the species of the north shore streams are crowded together in 

 streams \\here the volume of water is greater at the headwaters 

 than in the north shore streams, 

 (a) Causes of the Definite Arrangement of the Fishes in the 



Streams. 



The arrangement of the fish in these streams suggests definite 

 reactions to some factor or factors in the stream. Rheotaxis 

 is suggested as a cause of the upstream movement, and water 

 pressure and size of stream a factor limiting the upward move- 

 ment. This should be studied experimentally. 

 (b} Origin of the Fish Communities. Migration. 



A discussion of the mode of origin of the fish communities is 

 concerned with the mode of entrance of the fish into the habitat. 

 \Yhile manner of entrance of fish into a stream is not of particular 

 importance to us, it follows from a reference to Fig. I that fish may 

 enter when the stream is young and keep pace with erosion. Fish 

 entering when a stream was at the youngest stage indicated in 

 Fig. I, need only maintain their position against the current and 

 they would be carried inland as the source of the stream migrated 

 inland. In the case of Bull Creek, the horned dace is absent 

 from the lower portion (Dead River) so far as our collections 

 show. Did this species enter when Bull Creek was similar to 

 Glencoe Brook." This is improbable for the condition in Bull 

 < 'reek, \\ hen it was similar in size to Glencoe Brook, mn>t ha\e 

 been Hinilar to conditions now found ne.tr glaciers, e. g., Green- 

 land. 



There is further evidence as to the improbability ol such early 

 entrance in the data of Hickory Creek fishes. \\V have noted that 



