LAKE FAUNAS. 129 



Geneva at one hundred and ninety-two metres. At this depth, 

 however, the impression produced upon the sensitised plates was 

 of no greater value than that which would have been produced, 

 under ordinary conditions, on a clear night, without a moon. 



A remarkable feature of the lacustrine fauna is the very broad 

 distribution of most of the species. Not only is there a general 

 resemblance between the pelagic faunas of all the European lakes 

 that have thus far been examined, from Scandinavia to Italy, and 

 from Italy to Bohemia and the Caucasus, but a strict identity, at 

 least as far as the species of Entomostraca are concerned. The 

 species that occur in the one lake are also the species of the other 

 lakes, although the respective littoral and deep faunas may be 

 largely distinct. Further, it would appear that the same species 

 are constituents of the pelagic faunas of American lakes as well, 

 and not improbably make up the greater part of them. Professor 

 S. I. Smith, *^ in his investigations of the fauna of Lake Superior, 

 determined the presence, in the surface waters, of Dajibnia galeata 

 and Leptodora hyalina, common forms in the lakes of both Southern 

 and Northern Europe, and of Daphnia pellucida, which was de- 

 scribed by Mtiller as a pelagic inhabitant of some of the Danish 

 waters. 



As to the origin of the pelagic fauna little positive is known. 

 That it is not a direct derivative of the different littoral faunas is 

 very nearly certain, for were this the case we should expect to meet 

 with largely differing assemblages of pelagic forms in all lakes 

 where the littoral or deep faunas likewise differ; but, as has been 

 seen, this is not the case. Yet there can be little or no question 

 that it really represents a modification of some primary shore- fauna, 

 whose members, through force of circumstances, were compelled 

 to adapt themselves to new conditions of existence. The supposed 

 method of its differentiation and further distribution is thus indi- 

 cated byForel: "I believe we must find the cause of the differ- 

 entiation of the pelagic fauna in the combination of two different 

 phenomena — namely, the daily migrations of the Entomostraca, 

 and the regular local winds of the great lakes. It is well known 

 that on the borders of great masses of water two regular winds 

 prevail, one of which blows at night from the land towards the 

 water, the other by day from the water to the land. The nocturnal 

 animals of the shore-region, which swim at night at the surface, 

 7 



