208 FRESHWATER FAUNA 



fresher than that on the west ; opportunity is thus 

 afforded to the marine inhabitants to adapt themselves 

 to brackish or even freshwater conditions ; many of them 

 have succeeded in doing so, more have failed ; the former 

 have paid for their success by a diminution in bodily 

 bulk. Moebius and Heincke remark that the charac- 

 teristics of the brackish as opposed to the saltwater 

 fauna are fewer species, represented by individuals of 

 comparatively small size, which are present, however, in 

 immense numbers. 



These, it will be recalled, are the distinctive features of 

 the Sarmatian fauna. 



We have now completed our review of the successive 

 transformation of marine into lacustrine areas, and have 

 found the freshwater fauna increasing in complexity with 



FIG. 75. The Freshwater Limpet, 

 Ancylus fluviatilis, Miill. 



the flow of time. The passage from Silurian to Devonian 

 would seem to have brought into existence Uiiio and its 

 allies, that from Permian to Trias may have supplied the 

 greater part of the Mollusca, which are now almost 

 universally distributed. With each successive later trans- 

 formation these enforce their claims to the new fresh- 

 water domain, but some few additional marine forms 

 contrive to adapt themselves to the changed conditions 

 and contribute a slight accession to the freshwater fauna. 

 There still remain a few lakes which have been 

 regarded as relic seas, and foremost among these is lake 

 Tanganyika, concerning which a large volume by Mr. 

 J. E. S. Moore was published in 1903, under the title of 

 " The Tanganyika Problem." This lake contains, in 

 addition to the common and widespread freshwater 

 genera, which are common to it and the other great lakes 



