50 ORIGIN OF LIFE IN AMERICA 



Other species of fishes related to marine forms live in the 

 Great Lakes. But we need not suppose that typical fresh- 

 water fishes are absent. The family of Cyprinids, those 

 essentially fresh-water fishes to which the carp belongs, is 

 well represented there. Hence if we suppose that they were 

 destroyed during the invasion of that region by the sea, the 

 lakes must, at a subsequent period, have been freely connected 

 with rivers which were stocked with fresh-water fish. And 

 it appears that this actually has been the case. 



Whether the St. Lawrence was choked by obstructing ice 

 masses, or whether it was filled with detrital matter carried 

 down from the neighbouring eastern heights, does not affect 

 our present inquiries. It seems that after the ice, or, if we 

 prefer it, after the retreating sea, had left the district, there 

 were only three lakes instead of the five as at present. These 

 three lakes, which have been called " Duluth," " Chicago " 

 and "Maumee," were independent of one another. Lake 

 Duluth emptied its waters into the Mississippi, Lake Chicago 

 excavated an outlet which has since become the Chicago 

 drainage canal, while Lake Maumee communicated with the 

 Wabash River. All, of course, eventually helped to swell the 

 waters of the Mississippi. Lake Maumee gradually developed 

 into the huge Lake Warren. Later on the other lakes joined, 

 and poured their waters into Ontario, which then communi- 

 cated with the Hudson by means of the Mohawk River. It 

 was only then that the sea finally broke down the barrier 

 which had prevented an overflow into the St. Lawrence, and 

 advanced as far as the western end of Lake Ontario, one 

 marine channel communicating with the Atlantic by way 

 of the Hudson Valley. 



We can easily imagine how the fresh-water fishes of the 

 north, which were either killed or driven south, such as the 

 redfin (Notropis cornutus) and Notropis atherinoides (it does 

 not seem to possess a common name) managed to return to 

 their northern stations after the Glacial Epoch. Yet I feel 

 sure that a survival even of some fresh-water fishes took place 

 within the drift area in some of the eastern mountain ranges. 

 Near Freeport in Maine, quite an isolated colony of the 

 beautiful red-bellied dace (Chrosomus erythrogaster) occurs. 

 This is probably a pre-Glacial relict. Nor is it likely that 



