126 GEOGRAPHICAL DISTRIBUTION. 



fauna: " The fauna of the coast has not only given origin to the 

 terrestrial and fresh-water faunas, it has throughout all time, since 

 life originated, given additions to the pelagic fauna in return for 

 having received from it its starting-point. It has also received 

 some of these pelagic forms back again to assume a fresh littoral 

 existence. The terrestrial fauna has returned some forms to the 

 shores, such as certain shore-birds, seals, and the polar bear ; and 

 some of these, such as the whales and a small oceanic insect, Halo- 

 bates, have returned thence to pelagic life. 



" The deep-sea fauna has probably been formed almost entirely 

 from the littoral, not in most remote antiquity, but only after food, 

 derived from the debris of the littoral and terrestrial faunas and 

 floras, became abundant in deep water. It was in the littoral region 

 that all the primary branches of the zoological family-tree were 

 formed ; all terrestrial and deep-sea forms have passed through a 

 littoral phase, and amongst the representatives of the littoral fauna 

 the recapitulative history, in the form of series of larval conditions, 

 is most completely retained. " 



Lake Faunas. It would appear that in all large lakes three 

 distinct faunas can be recognised: 1. The littoral fauna, comprising 

 the animals of the shore-line, which do not habitually descend to a 

 much greater depth than fifteen or twenty feet. 2. The deep fauna, 

 whose representatives live along the floor of the lake, at depths 

 usually exceeding sixty to a hundred feet, a limited number of 

 forms occasionally rising to the surface ; and 3. The pelagic fauna, 

 whose members occupy the free surface of the lakes, rarely or never 

 reaching the shore-line or descending to the bottom. The zone 

 inhabited by the last measures from fifty to a hundred metres in 

 depth. 



Our general knowledge respecting the pelagic fauna is still very 

 limited, and is based almost exclusively upon observations made 

 upon the European lakes. From these it would seem that the 

 fauna is a very restricted one, consisting, as far as is known, of 

 some twenty-five species of entomostracous crustaceans (ostracods, 

 cladoceres, and copepods), a fresh-water mite (Atax crassipes), about 

 six species of rotifers, and a limited number of infusorians. No 

 lake has thus far yielded all these forms, and the majority of lakes 

 are largely deficient. Between the years 1874 and 1878 Forel 49 ob- 

 tained in the Lake of Geneva only eight species : Diaptomus castor, 



