THE ARCTIC FAUNA. 1 77 



organisms, which are supposed to have been 

 flooded by, or to have been in close communication 

 with the sea at some former period, like the lakes 

 Ladoga and Onega in Russia. His views have been 

 worked out subsequently in greater detail by Loven 

 and O. Peschel, who gave them their strong adherence. 

 Many leading zoologists, such as Professor Sars and 

 others, have since adopted them, and though dis- 

 credited by Professor Credner, the theory still 

 offers the best explanation for the origin of marine 

 animals in freshwater lakes. 



Professor Credner's contention, that marine mollusca 

 are always absent from these relict lakes, seems at first 

 sight a stumbling-block to the theory. But the ex- 

 planation is really simple enough. It is to Dr. Sollas 

 that we owe a very ingenious explanation of the origin 

 of freshwater faunas. He showed that all freshwater 

 organisms in their early stages of development are pro- 

 vided either with some process enabling them to attach 

 themselves to a foreign object, or that they pass this 

 period within the body of the parent. This is a 

 provision of nature to prevent freshwater organisms 

 from being floated out to sea, where they would 

 perish, until they reach maturity and can cope with 

 floods and currents. Had Professor Credner been 

 aware of Dr. Sollas's views, no doubt he would have 

 modified his criticisms, for, as most marine mollusca 

 have free- swimming larvae, they would have little 

 chance of becoming permanent residents of lakes. 

 During their larval stage, marine molluscs are quite a 



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