M. A. Humbert on Niphargus puteanus, var. Forelii. 251 



eyes, and over and above these an aecessory eye on each of 

 the maxillas of the second pair. 



" The second solution, founded on the theory of transfor- 

 mation, assumes that these bhnd creatures originate from 

 ancestors furnished with eyes, which liave by degrees, under 

 the influence of disuse, lost these organs, which have become 

 useless. One of the best proofs in favour of this view may be 

 drawn from the transitions observed in certain species. In 

 some Gammarids of Lake Baikal we can ascertain a tendency 

 on the part of the visual organs to become less perfect in pro- 

 portion as the animal inhabits greater depths. We may cite, 

 as examples, Oammarus Ussolzeion (var, ahyssoru7)i) and O. 

 Borowshil (var. dichrous^ subvar. ahyssalis). But tliis trans- 

 formist explanation, which is now-a-days generally accepted, 

 and which appears to me to be the true one, does not settle 

 the whole problem. It may be asked, among other things, 

 with regard to this or that blind species, whether its origin 

 dates back to a very ancient epoch or is comparatively 

 recent, whether it originated from extinct or from still ex- 

 isting forms. These questions have sometimes been settled 

 by a dash of- the pen; and, amongst others, this has been the 

 case with the Niphargi of the caves and wells. Nevertheless 

 the problem presents itself in a very complex fasliion, and 

 appears to me to necessitate a greater number of observations 

 than we possess at present before it can be regarded as com- 

 pletely solved. I even believe that it is impossible now to 

 arrive at any thing precise with regard to the origin of the 

 Niphargi) we can only claim to indicate the probabilities, 

 and to clear the ground by getting rid of some erroneous 

 notions. 



" The Ntjjhargus of the Lake of Geneva lives at a depth of 

 from 30 to 300 metres. Now, according to tlie observations 

 of M. Forcl*, the chemical action of the solar rays in the 

 waters of the lake ceases to make itself felt in summer below 

 40 or 50 metres, and in winter below 80 or 100 metres. Con- 

 sequently, although inhabiting an open sheet of water, this 

 crustacean is subjected, throughout the greater part of the zone 

 occupied by it, to the same conditions of obscurity as its con- 

 geners enclosed in wells or caverns. We seem, therefore, to be 

 justified in concluding that it is under the influence of this 

 obscure medium that our species has lost its visual organs. 

 This is the explanation that has been proposed by the 

 naturalists who have sought to account for the origin of the 

 Niphargi of wells and caverns. Some have even gone still 



* F. A. Forel, " Rccherches photographiques sur la transparence de 

 I'eau," Bull. Soc. "S'aud. des Sci. jNat. 2" sdr. tome xiv. (1874) p. 24. 



17* 



