CONVERSION OF MARINE INTO FRESHWATER ANIMALS, 327 
so that they 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 Norway, 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 marine 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 : 
Lovén. “Ueber einige im Wetter und Wener See gefundene Crustaceen.” 
Halle Zeitschrift fir die Gesammten Wissenschaften, xix,, 13862: G. O. 
Sars, “Histoire Naturelle des Crustacés d’eau douce de Norvége,” 1867. 
