186 ADAPTATION 



near the Ohio River, was covered a few thousand 

 years ago with a sheet of ice. It contained no 

 environment suitable for fishes. The entire 

 fauna and flora of this area, including the fish- 

 fauna, are composed of immigrants that moved 

 in as the ice moved out, and selected the places 

 adapted to each species. While a few of them 

 have become modified since their advent into this 

 area, their fundamental and even their minor 

 adaptations were acquired elsewhere than in their 

 present home. Their adaptation is due to the 

 selection of an adapted environment.^ The entire 

 area is unsuitable as a place for the study of the 

 origin of all but a few minor adaptations. 



The check by cold has not been placed on any 

 individual migration or set limits to the adult. 

 RMnichthys dulcis living in glacial waters and 

 warm springs and the many species adapted to 

 the great range of variation in the temperature 

 in any of our temperate lakes show this. The 

 temperature factor determining distribution is 

 set rather by the adaptation of the eggs to warm 

 or cold water. Our trout, salmon, and white 

 fishes breed largely in winter when the tempera- 

 ture is low. The rate of development of their 

 eggs, like that of all cold-water eggs, is slow. 



' Of the 1S2 species of fishes of the Great Lake basin, only 

 36 species and varieties, 17 per cent of the total, are peculiar to 

 the area. Five of these are but varieties of more southern species, 

 and the other 21 more than represent the extent to which the 

 fauna has become adapted in tWs area, for eight salmonids and 

 eight cottids are cold water species that may have been crowded 

 out of the region to the souili of the basin, by the encroaching 

 heat after the passing of the last glacial epoch. 



