GREAT LAKES. 



121 



The Great Lakes of Canada and the United States form a group 

 too famiHar to need description. Their date and mode of origin are 

 as yet unknown, for the glaciation which so influenced the temperate 

 parts of the northern hemisphere has swept away most of the old lacus- 

 trine deposits in which was written the ancient history of these lakes. 

 The same glaciation, or the arctic climate that accompanied it, has 

 obliterated the old lacustrine fauna and flora, if a peculiar one ever 

 existed. We thus find that at the present day the inhabitants of the 

 Great Lakes consist merely of the ordinary lacustrine species of 

 North America, with the admixture of a few forms perhaps of marine 

 origin. Too short a time has yet elapsed since the passing away of 

 the arctic climate for a special lacustrine fauna and flora to be evolved 

 in this region, or apparently in any other of the areas which felt the 

 rigour of the cold during the Glacial epoch. 



Fjg. 2. — The Great Lakes of North America. 



From what we have now learnt of the sweeping climatic changes 

 that occurred during the Pleistocene period in temperate regions, it 

 is evident that only in tropical or sub-tropical countries can we hope 

 to find a well-marked and peculiar fauna and flora in each detached 

 basin. Lakes are like oceanic islands. If their fauna and flora has 

 recently been obliterated, they become peopled from the nearest land 

 and will contain no endemic species. Such is the case with the Faroe 

 Islands and with the lakes of Cumberland. If, however, there has 

 been no break in the continuity during long periods, the result is 

 quite different. Accidental introductions will have little chance of 

 establishing themselves in a country already occupied, and each 

 island of a small archipelago, like the Madeira or Galapagos, will 

 contain peculiar species. 



The same rule will probably apply to tropical lakes ; but, 

 unfortunately, one group only, and that almost unexplored, seems to 

 fulfil the necessary conditions of long-continued isolation and absence 



