CHAP. XXIII.] 
SUMMARY AND CONCLUSION. 
549 
Fresh-water Fishes. 
Although it would appear, at first sight, that the means of 
dispersal of these animals are very limited, yet they share 
to some extent the wide range of other fresli-water organisms. 
They are found in all climates ; but the tropical regions are 
by far the most productive, and of these South America 
is perhaps the richest and most peculiar. There is a certain 
amount of identity between the two northerly continents, and 
also between those of the South Temperate zone ; yet all are 
radically distinct, even North America and Europe having but 
a small proportion of their forms in common. The occurrence 
of allied fresh-water species in remote lands — as the Aphritis 
of Tasmania and Patagonia, and the Comephorus of Lake 
Baikal, distantly allied to the mackerels of Northern seas — 
would imply that marine fishes are often modified for a life in 
fresh waters; while other facts no less plainly show that per- 
manent fresh-water species are sometimes dispersed in various 
ways across the oceans, more especially by the northern and 
southern routes. 
The families of fresh-water fishes are often of restricted 
range, although cases of very wide and scattered distribution 
also occur. The great zoological regions are, on the whole, very 
well characterized ; showing that the same barriers are effectual 
here, as with most other vertebrates. We .conclude, therefore, 
that the chief lines of migration of fresh-water fishes have been 
across the Arctic and Antarctic seas, probably by means of float- 
ing ice as well as by the help of the vast flocks of migratory 
aquatic birds that frequent those regions. On continents they 
are, usually, widely dispersed; but tropical seas, even when of 
small extent, appear to have offered an effectual barrier to their 
dispersal. The cases of affinity between Tropical America, 
Africa, Asia, and Australia, must therefore be imputed either to 
the survival of once widespread groups, or to analogous adap- 
tation to a fresh-water life of wide-spread marine types ; and 
these cases cannot be taken as evidence of any former land 
connection between such remote continents. 
