M. A. Iluinbeit on Nipliargus puteanus, var. Forelii. 253 



double in Gaynmarus^ of a single piece but deeply cleft in 

 Niphargus^ and completely entire in Crangonijx. By adopting 

 the theory of M. de Uougemont it would therefore be neces- 

 sary to assume here that the Nijyhargi differ more in their 

 youth than in their adult state from the Gammarus pulex 

 from which they originated. 



" 3. If we consider that Eriopia ought to be united with Xi- 

 plmrgus^ it is ditHcult to understand how these marine Gamma- 

 rids could have originated from the Gammarus pulex of the 

 fresh waters, and get into the North Sea and the Black Sea. 



"4. We find the Xi'phargi distributed over a great part of 

 Europe in Avaters deprived of light, both in wells and caverns 

 and at the bottom of lakes. On the other hand, in Lake 

 Baikal, so well explored by M. Dybowsky, who has found 

 there ninety-seven species of Gammarids, including Gam- 

 marus pulex, no species of Niphargus appears to exist *. 

 Nevertheless this immense lake presents depths much greater 

 than those of the Lake of Geneva and the Lake of Neuchatel ; 

 and the solar rays, which are more oblique in Siberia than in 

 Switzerland, must make tlieir action felt to a still less depth 

 than in our waters. It may be added that the astonishing 

 number of species which inhabit Lake Baikal, and the variety 

 of their forms, would tend to make us suppose that this vast 

 sheet of water has a fauna more ancient than that of the Swiss 

 lakes, and that the modificatory causes have consequently had 

 more time there to act upon the species. 



" These various considerations lead me to believe that 

 Niphargus is an ancient genus descended from a form now 

 extinct, as is evidently the case with Proteus, Leptoderus, 

 Anopldhalmus, &c. As to the question whether the AV^y/iarr^t 

 of the lakes are colonies originating from animals of the same 

 genus which inhabit subterranean waters, or whether the 

 reverse is the case, it is difiicult to solve, and its solution is 

 even complex. Assuming that the genus Niphargus appeared 

 before the glacial epoch, it is impossible to say any thing about 

 its place of origin. But, not to carry the question so high, and 

 considering only the existing fauna, I should be disposed to 

 think that our Niphargi of the Swiss lakes have originated 

 from those which inhabit subterranean waters. Having reached 

 the lakes, they would have acclimatized themselves in the 

 depths which present the darkness that they seek, in this 

 more or less obscure zone they found themselves under condi- 

 tions which allowed them to exist ; whilst in the illuminated 

 zone they could not have escaped their enemies, or maintained 

 the struwwle ao:ainst their near allies furnished with visual 

 * A Cran<jo7iyx is known from the subten'anean waters of Kamtschatka. 



