No. 424.] NOTES AND LITERATURE, 331 
of the origin of the Helvetian fauna in general, but preéminently 
applies the views gained by previous studies to the lower forms of 
animals that inhabit the fresh-water lakes, streams, etc., of the country. 
Professor Zschokke points out that the present terrestrial fauna 
and the present flora belonging to the high mountain regions of the 
Alps find a counterpart in the arctic regions, the identical species 
being in many cases represented there. These two main ranges are 
in some degree connected by intermediate, but isolated and discon- 
tinuous, stations on the highest elevations of the intervening low 
countries, or in localities which, owing to local conditions, possess a 
particularly cool. climate (for instance, peat bogs). It is generally 
agreed that this peculiar distribution is to be explained as a remnant 
of the glacial period. During the latter the originally alpine and 
arctic fauna and flora was widely distributed over the low countries 
of central Europe, where it found congenial climatic conditions. 
After the close of the glacial period, at the commencement of a 
warmer climate, this glacial fauna and flora retreated in two direc- 
tions : toward the north pole, and toward the high mountains to the 
south; while in the intervening parts it remained only in a few scat- 
tered colder localities, such as the highest summits of the mountains 
of central Germany. The parts evacuated by this arctic element 
were occupied by a new stock of life that immigrated chiefly from 
the east (Siberia). 
This is true for plants and terrestrial animals, such as mammals, 
land mollusks, insects, reptiles; and in many cases we have direct 
means of ascertaining and verifying this process, since we know 
glacial and interglacial remains of the respective forms that have 
been found in the low parts of central Europe. 
While it thus seems positively assured, by paleontological facts, 
that the origin of the alpine flora and terrestrial fauna of Switzerland 
is to be traced back to glacial times, and lies in the vast country bor- 
dering the ice sheet that covered north Europe to the south, we have 
no such evidence as regards the fresh-water fauna. Nevertheless, 
Professor Zschokke demonstrates that the distribution. of many fresh- 
water animals of the alpine lakes and streams of Switzerland exhibits 
the identical peculiar features that are known in terrestrial animals ; 
they reappear in arctic fresh-water basins offering the same climatic 
conditions, and are wanting in the intermediate tracts except for a 
few scattered and exceptionally favorable localities. From this sim- 
ilarity in distribution we are to infer that the origin of the distribu- 
tion of these fresh-water forms was subject to the same laws. 
