Dispersion of Fresh-water Fishes 285 



" The study of these features [of distribution] is of the greatest 

 importance, inasmuch as it may eventually lead to a better 

 understanding of the intentions implied in this seemingly arbi- 

 trary disposition of animal life. ... 



"There is still another very interesting problem respecting 

 the geographical distribution of our fresh-water animals which 

 may be solved by the further investigation of the fishes of the 

 Tennessee River. The water-course, taking the Powell, Clinch, 

 and Holston Rivers as its head waters, arises from the moun- 



FiG. 1S5. — Homed Dace, Semohlus atromaculatiui (Mitchill). Aux Plaines River, 

 Ills. Family Cyprinida:. 



tains of Virginia in latitude 37°; it then flows S.W. to latitude 

 34° 25', when it turns W. and N.W., and finally empties into 

 the Ohio, under the same latitude as its source in 37°. 



"The question now is this: Are the fishes of this water sys- 

 tem the same throughout its extent? In which case we should 

 infer that water communication is the chief condition of geo- 

 graphical distribution of our fresh-water fishes. Or do they 

 differ in different stations along its course? And if so, are the 

 differences mainly controlled by the elevation of the river above 

 the level of the sea, or determined by climatic differences cor- 

 responding to dift'erences of latitude? We should assume that 

 the first alternative was true if the fishes of the upper course 

 of the river differed from those of the middle and lower courses 

 in the same manner as in the Danube, from its source to Pesth, 

 where this stream flows nearly for its whole length under the 

 same parallel. We would, on the contrary, suppose the second 

 alternative to be well founded if marked dift'erences were ob- 

 served between the fish of such tracts of the river as do not 



