GEOGRAPHIC ISOLATION 



85 



pean forms. 44 Many of the rhabdocoel turbellarians of fresh waters are 

 very widespread. 45 These are all, to be sure, adapted for wide distribu- 

 tion by their resting stages which protect them against the drying up 

 of their habitats. 



Even the snails and mussels of inland waters have surprisingly 

 large generic and even specific ranges, though they are incapable of 

 active migration across land barriers. The genera Limnaea, Physa, 

 Planorbis, Ancylus, and Unio have a world-wide range. Even the 

 exceptionally isolated New Zealand, with its highly specialized fauna 



•#? 



*J 



Fig. 7. — Tanganyika snails: a, Tiphobia horei; b, Paramelania damoni; c, Lavi- 

 geria diademata; d, Limnotrochus kirki. After Lauterborn. 



of ten genera of fresh water mollusks, has only one endemic genus. 

 The same phenomenon characterizes fresh-water insects. The tropical 

 and temperate representatives of the Dytiscidae are more alike than 

 in any other family of beetles. Among species, the dytiscid Eretes 

 sticticiis is present on five continents, and Cybister tripunctatus has a 

 range nearly as extensive. 46 Some genera of water bugs, such as 

 Ranatra and Notonecta, also have a wide distribution. 



There is thus a contradiction between the high degree of isolation 

 of bodies of fresh water and the degree of differentiation of their 

 faunae. The presence of the same species in the tropics and in the 

 temperate zones is not explainable by a greater uniformity of environ- 

 ment in fresh waters as compared with terrestrial environments. 47 

 A satisfactory explanation of this contradiction was presented by 

 Thomas Belt by calling attention to the transitory nature of lakes and 



