324 DISTRIBUTION AND AETIOLOGY OF THE CRAYFISHES. 



the stock from which they have been evolved. Even 

 nnder existing geographical conditions, an affluent of the 

 Mississippi, the St. Peter's river, communicates directly, 

 in rainy weather, w4th the Red river, which flows into 

 Lake Winnipeg, the southernmost of the long series of 

 intercommunicating lakes and streams, which occuj)y the 

 low and flat water-parting between the southern and the 

 northern watersheds of the North American Continent. 

 But the northernmost of these, the Great Slave Lake, 

 empties itself by the Mackenzie river into the Arctic 

 Ocean, and thus provides a route by which crayfishes 

 might spread from the north over all parts of North 

 America east of the Eocky Mountains. 



The so-called Rocky Mountain range is, in reality, an 

 immense table-land, the edges of which are fringed by two 

 principal lines of mountainous elevations. The table- 

 land itself occupies the place of a great north and south 

 depression which, in the cretaceous epoch, was occupied by 

 the sea and probably communicated with the ocean at its 

 northern, as well as at its southern end. During and 

 since this epoch it became gradually filled up, and it 

 now contains an immense thickness of deposits of all 

 ages from the cretaceous to the pliocene — the earlier 

 marine, the later more and more completely freshwater. 

 During the tertiary epoch, various portions of this area 

 have been occupied by vast lakes, the more northern of 

 which doubtless had outlets into the Northern sea. That 

 crayfish existed in the vicinity of the Eocky Mountains 



