264 BRITISH FRESHWATER FISHES 



identical with continental species, and which came to 

 us at the time of our last union with the Continent. 

 For the most part these belong to families which 

 consist wholly of freshwater fishes,^ which have been 

 long established and have evolved their genera and 

 species in fresh water; such families are of the 

 greatest importance in the solution of the problems 

 of geographical distribution, especially in pointing 

 out the existence of former land connexions or of 

 ancient lines of severance. 



We may here indicate that the freshwater fishes 

 call for a primary separation of the world into two 

 main zoogeographical divisions, on the one hand 

 America, Africa, Europe, and Asia, including the 

 Philippines, Java, Sumatra, Borneo, and Bali, on the 

 other New Zealand and Australia, with New Guinea, 

 Celebes, and Lombok. Wallace's Line, drawn between 

 Borneo and Celebes and Bali and Lombok, probably 

 represents the line of the final separation of the 

 Asiatic and Australian land-masses at or before the 

 beginning of the Tertiary epoch. The true fresh- 

 water fishes, which have evolved in the rest of the 

 world, have never found their way into the Australian 

 region, wherein, except for two persistent archaic 

 types, the freshwater fishes are closely allied to the 

 marineones; peculiar genera and species of Gobies, Sea- 

 perches, Grey Mullets, Sand-smelts, etc., are plentiful 

 in the rivers of Australia and New Guinea, but there 

 are no peculiar families confined to fresh waters in 

 this area. The contrast between Celebes, without a 



' Many species, of course, can stand brackish water ; Roach, Perch, 

 etc., occur in parts of the Baltic and Caspian, where the saUnity is very 

 low. Species of Pike-perch {Lzicioperca), fishes of the Perch family, 

 even thrive in the Black Sea, where, however, the salinity is only half 

 that of ordinary sea water. 



