CONVERSION OF MARINE INTO FEESHWATEE ANIMALS. 327 



BO that thej' are no longer absolutely identical with those 

 descendants of their ancestors which have continued to 

 live in the sea.* 



In several of the lakes of Norwa.y, Sweden and 

 Finland, and in Lake Ladoga, in Northern Europe ; in 

 Lake Superior and Lake Michigan, in North America; 

 a small crustacean, Mysis relicta, occurs in such abund- 

 ance as to furnish a great part of the supply of food to 

 the fresh water fishes which inhabit these lakes. Now, 

 this Mysis relicta is hardly distinguishable from the 

 Mysis oculata which inhabits the Arctic seas, and is 

 certainly nothing but a slight variety of that species. 



In the case of the lakes of Norway and Sweden, there 

 is independent evidence that they formerly communicated 

 with the Baltic, and were, in fact, fiords or arms of the 

 sea. The communication of these fiords with the sea 

 having been gradually cut off, the maiine animals they 

 contained have been imprisoned ; and as the water has 

 been slowly changed from salt to fresh by the drainage 

 of the surrounding land, only those which were able to 

 withstand the altered conditions have survived. Among 

 these is the Mysis oculata, which has in the meanwhile 

 undergone the slight variation which has converted it 

 into Mysis relicta. Whether the same explanation ap- 



* See on this interesting' subject: Martens, "On the occurrence of 

 marine animal forms in fresh water." Annals of Natural History, 1858 : 

 Lov6n. " TJeber einige im Wetter und Wener See gef undene Crustaoeen." 

 Halle Zeitsohrift fur die Gesammten Wissenschaften, xix., 1862 : G. 0, 

 Sars, " Histoire Naturelle des Orustaces d'eau doaoe de Norvege," 1867. 



