132 GEOGRAPHICAL DISTRIBUTION. 



Although the fauna, taken as a -nhole, may be said to possess 

 certain special characters, j^et, broadly considered, it is only the 

 representative, by slight modification, of the fauna of the littoral 

 zone. It possesses no really well-defined or abnormal features of 

 its own. Most of the forms are of small size, and a number of 

 them, whose surface representatives are active and good swimmers, 

 appear to have taken to sluggish habits ; neither Cyclops nor Lynceus 

 would rise when placed in an aquarium. Blindness is exceptional, 

 and it is a surprising fact that the animals suffering from this defect 

 (Gammarus coscus, Asellus caecus) are comparatively shallow-water 

 forms (thirty metres), whereas those living at the greater depths, 

 down to three hundred fathoms, are well provided with visual 

 organs. The faunas of different zones of depth do not appear to 

 differ sensibly from one another, except in the elimination or excess 

 of a number of species. The greater number of these would seem 

 to be distinct from their analogues of the littoral zone.* 



Many of the species found in Lake Geneva are identical with 

 forms found in the other Swiss lakes, and in the lakes of Savoy, as 

 identified by Imhof, and there is good reason for supposing that 

 a general analogy, if not absolute identity, unites the different deep 

 lacustrine faunas of the same region. Professor Smith obtained 

 from deep water (exceeding fifteen fathoms) in Lake Superior^* 

 Hydra carnea (from eight to one hundred and forty-eight fathoms), 

 a Pisidium (from four to one hundred and fifty-nine fathoms), several 

 species of worms (SiTnuris, Nephelis, Tubifex, &c.), the larvae of 

 various tipulids and ephemerids, and among crustaceans Mysis 

 relicta and Pontoporeia affinis (from shallow water to one hundred 

 and fifty-nine fathoms). The last two, which were also found by 

 Stimpson in Lake Michigan, are forms belonging to Lake Wetter 

 in Sweden, supposed by Loven to have been derived by modifica- 

 tion from marine species. 



* So stated by Forel in liis report of 18T6, although in 1874 be and Plessis 

 appear to have maintained the opposite view. 



