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CHAP. XXIII.] SUMMARY AND CONCLUSION. 549
Fresh-ivater 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 fresh-water organisms.
They are found in all climates ; but the tropical regions are
hy 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 northern 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 Ajphritis
of Tasmania and Patagonia, and the ComepTiorus of Lake
Baikal, distantly allied to the mackerels of Northern seas
Avould 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 off'ered 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 fonner land
connection between such remote continents.
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