110 ORIGIN, ETC., OF THE FAUNA. 
It is probable that the distinctness of the North and South American faunas 1s 
more marked, and that where they meet there is less overlapping in the case of the 
fresh-water fishes than in any other group of animals. 
Fresh-water Fishes——General Conclusions.—The South American fresh-water 
fish-fauna is as distinct as the Mammalian fauna would have been if a large part of 
the endemic fauna had not died out in late Tertiary times, and been replaced by the 
invasion of types that had evolved elsewhere. 
Turning to the other parts of the world, we find that the distinctness of the 
Australian Region, marked in the Mammalia, 1s emphasized by the Fishes. ‘The 
effectiveness of the sea asa barrier is strikingly illustrated by a comparison between 
Borneo, with its hundreds of species of Cyprinoids, Siluroids, Anabantoids, etc., and 
Celebes, without a single indigenous true fresh-water fish. The other four regions are 
well characterized, although less distinct, for the Indian has certain relationships with 
the Ethiopian and Palearctic, and the last with the Nearctic ; there has been a certain 
interchange, but in each case the endemic groups preponderate. 
The general impression derived from a study of the fresh-water fishes of the world is 
that many of the families were in existence at the beginning of the Tertiary, that for the 
most part they have evolved in the areas they now occupy, have dispersed slowly, and 
have never had a distribution much wider than at present. For Mammals, it may be gene- 
rally stated that they have evolved rapidly and spread rapidly, and have found it easier 
to reach a country than to live in it. For fresh-water fishes migration has been much 
more difficult, survival relatively easy ; hence their great importance in zoogeograpby. 
BATRACHIA. 
Batrachians.—These resemble fresh-water fishes in their inability to swim across 
the sea, and their eggs are no more likely to be transported over the sea than those of 
fresh-water fishes. But it is probable that accidental transmission has played a part in 
the dispersal of some arboreal and terrestrial frogs ; at any rate, this seems the most 
reasonable explanation of the distribution of certain genera and species in the Indo- 
Australian Archipelago. Even the most aquatic types can travel overland from 
one stream to another, and consequently are able to migrate rapidly when conditions 
are favourable. 
Nearctic and Neotropical Batrachians.—The Batrachian faune of these two 
regions are less distinct than the fish-faune, but are, nevertheless, very different. 
Urodeles are scarce in South America, Ceecilians are absent from North America; the 
majority of the Neotropical Frogs are Cystignathide and Hylide, families but sparingly 
represented in the Nearctic Region. 
The mountains that fringe the Mexican Plateau form the boundary between the 
