332 THE AMERICAN NATURALIST. [VoL. XXXVI. 
But there is an important difference of the fresh-water fauna from 
the terrestrial. In the latter, the different faunas, the old glacial 
and the newer postglacial, are, in the mountains, so to speak, on top 
of one another, the postglacial fauna occupying the lower parts and 
ascending the mountains only to a certain elevation, where it gives 
room to the older fauna, which has taken to the summits. In the 
fresh-water fauna, however, there is a much larger degree of inter- 
mingling ; indeed, the postglacial immigrants have been able to reach 
the most exposed and highest stations in the mountains, and are 
found everywhere in close vicinity and association with the older 
glacial elements. According to Zschokke, this fact is due, on the 
one hand, to the character of the immigrants, which are cosmopolitan, 
eurytherm forms of life, which may find congenial conditions of exist- 
ence anywhere ; on the other hand, it is due to the peculiarities of 
temperature in the fresh-water bodies, where the extremes are not 
so much contrasted and lie closer together than those of the air 
influencing the terrestrial animals. 
The old glacial fauna of the glacial time did not only retreat in 
these two directions mentioned, but there is a third refugium for 
those that belong to the fresh water. In the depths of the great 
lakes that fringe the northern foot of the Alps, similar conditions of 
temperature prevail as in glacial times everywhere in fresh water. 
These cold depths contain a fauna consisting of peculiar fishes: (Sal- 
monidz) and crustaceans (the latter chiefly pelagic) that point dis- 
tinctly to a northern origin. These forms apparently immigrated at 
the close of the glacial period, when there was an abundance of 
cold water derived from the receding ice. Later, after the abundant 
supply of water had given out, these forms were cut off from the 
communication with their northern homes, and their only chance to 
survive was in these large and cold fresh-water lakes, which resemble 
their original home at least in temperature. 
Thus we may distinguish, among the fresh-water fauna of Switzer- 
land as well as among the terrestrial fauna, two elements: (1) an 
old glacial stock that lived during the’ glacial: period all over the 
inhabitable parts of central Europe; and (2) a later immigration in 
postglacial times.. The first one retreated out of the plains in two 
directions : toward the north pole and toward the mountains. But 
while for the terrestrial animals which fled to the'highlands no other 
way was left than to go upward, for some of the fresh-water animals 
a third asylum was open: the deep and cold fresh-water lakes on the 
northerm:batder of thie jes <i» saw amot topo deat oegliio sio 



