26 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



All the marine fishes, crustaceans, mollusks, and nearly all the an- 

 nelids, are more or less altered forms of North Atlantic species. 



The reduction in number of species in such basins as the Baltic 

 corresponds to a still greater impoverishment of the fauna of fresh 

 waters. Leaving out of account the secondarily aquatic forms in fresh 

 water, which have entered it from the land, i.e., all of the insects, 

 arachnids, and pulmonate snails, there is a vanishing remnant of 

 primarily aquatic forms. A few coelenterates, two genera of sponges, 

 a few turbellarians, annelids, and bryozoans, and a very few genera 

 of clams and prosobranchiate snails, enter fresh water. Fresh-water 

 fishes, protozoans (especially ciliates) , rotifers, and Gastrotricha have 

 found the fresh-water environment favorable. Even the fresh-water 

 fishes, however, are far behind the coastal marine fishes in number of 

 species. Gunther 7 reckoned 2286 of the former against 3587 of the 

 latter. Although these figures have been greatly increased, their rela- 

 tive value remains approximately correct. 



The size of shad of the lakes of North Italy shows that fresh water 

 is less favorable than that of the ocean. The two forms in the Tessin 

 Lakes are known to the fishermen as the cheppia and the agon. The 

 cheppia (Alosa finta) is a migratory fish going up to the lakes to 

 spawn, but otherwise marine. The agon (Alosa finta var. lacustris) 

 has become permanently resident in the lakes. The cheppia reaches 

 45 cm., the agon usually only 25 cm. 8 



In fresh waters, the optimum temperatures of the tropics have the 

 same effect as in the seas. Of Giinther's 2286 species of fresh-water 

 fishes, 1552, or more than two-thirds, are tropical. This relation be- 

 comes still more evident by the comparison of the fish faunae of trop- 

 ical and northern rivers. The Ganges with the Brahmaputra, draining 

 1,750,000 sq. km., has 170 species of fishes, while the Mackenzie, drain- 

 ing 1,500,000 sq. km., has about 23. The Indus with 113 species com- 

 pares with the Saskatchewan with 22, with approximately equal 

 basins. The Nile may be compared with the Obi, both with drainage 

 areas of about 3,000,000 sq. km., the former with 101 species, the 

 latter with 45. 



Increase of temperature above the optimum impoverishes the fresh- 

 water fauna in the same way. 



Additions to the water such as carbon dioxide, humus acids, hydro- 

 gen sulphide, iron oxide, etc., require special adaptations and thereby 

 react selectively upon the fauna. Only a few fishes, such as Gasteros- 

 teus, Cobitis, and Tinea, live in bog waters, rich in humus acids. The 

 carp find these waters less favorable, and pike and trout avoid them. 

 Rotifers, on the other hand, flourish in such water. Of the 186 species 



