342 W. C. ALLEE 



does exactly express the difference between the pond and stream 

 isopods. Thus we are dealing with the pond mores and the 

 stream mores of Asellus communis, which depend on environ- 

 mental rather than on hereditary differences for their distinctive 

 features. 



In the isopods used, these two mores occur in the same taxo- 

 nomic species, but in the amphipods, Mr. Saunders (I.e.) found a 

 different species in the ponds from that in the streams. This 

 means that with the amphipods studied the natural mores would 

 run parallel with the taxonomic species. But the pond mores 

 could be transferred directly into stream mores without affecting 

 their structural characters. Thus the mores or ^ecological 

 species' are independent of taxonomic species, and a single eco- 

 logical species may be composed of half a dozen taxonomic species 

 or of only a fraction of one. 



The general conclusion to be drawn from this series of experi- 

 ments is that in the isopod, Asellus communis, the rheotactic 

 reaction is dependent upon the metabolic state of the animal 

 for its degree of positiveness and that the natural or experimental 

 conditions which affect the metabolic state of the animal, change 

 its rheotactic response. That is to say, the rheotactic reaction 

 is here an expression of the metabolic condition (physiological 

 state) of the isopod and may be controlled by those factors 

 known to control animal metabolism. 



It is a pleasure to acknowledge my indebtedness to Dr. C. M. 

 Child for his many helpful criticisms but my best thanks are due 

 to Dr. V. E. Shelf ord, who first suggested this problem and under 

 whose direction the work has been done. 



BIBLIOGRAPHY 



Allee, W. C. 1911 Seasonal succession in old forest pond^. Trans. 111. Acad. 



Sci., vol. 4, pp. 126-131. 

 Allen, G. D. 1910 Rheotaxis of Planaria dorotocephala. Unpub. Master's 



Thesis; Library of Univ. of Chicago. 

 Banta, a. M. 1910 A comparison of the reactions of a species of surface isopod 



with those of a subterranean species. Jour. Exp. Zool; vol. 8, pp. 243- 



310; 439-488. 



