520 LOUIS AGASSIZ. 



see whether they are identical with those of 

 the island. Whatever may come out of such 

 an investigation it will, at all events, furnish 

 interesting data upon the local distribution of 

 the species. ... I am almost confident that 

 it will lead to something interesting, for there 

 is one feature of importance in the case ; the 

 present surface of Long Island is not older 

 than the drift period ; all its inhabitants must, 

 therefore, have been introduced since that 

 time. I shall see that I obtain similar col- 

 lections from the upper course of the Con- 

 necticut, so as to ascertain whether there, as 

 in the Mississippi, the species differ at differ- 

 ent heights of the river basin. . . . 



TO PROFESSOR S. S. HALDEMAN, COLUMBIA, PENNSYL- 

 VANIA. 



CAMBRIDGE, July 9, 1853. 



. . . While ascending the great Mississippi 

 last spring I was struck with the remarkable 

 fact that the fishes differ essentially in the 

 different parts of that long water-course, 

 a fact I had already noticed in the Rhine, 

 Rhone, and Danube, though there the differ- 

 ence arises chiefly from the occurrence, in the 

 higher Alpine regions, of representatives of 

 the trout family which are not found in the 



