o
a great height under the equator. The various beings thus left stranded
may be compared with savage races of man, driven up and surviving in the
mountain-fastnesses of almost every land, which serve as a record,
full of interest to us, of the former inhabitants of the surrounding
lowlands.

12. GEOGRAPHICAL DISTRIBUTION--continued.

Distribution of fresh-water productions. On the inhabitants of oceanic
islands. Absence of Batrachians and of terrestrial Mammals. On the
relation of the inhabitants of islands to those of the nearest mainland.
On colonisation from the nearest source with subsequent modification.
Summary of the last and present chapters.

As lakes and river-systems are separated from each other by barriers of
land, it might have been thought that fresh-water productions would not
have ranged widely within the same country, and as the sea is apparently
a still more impassable barrier, that they never would have extended to
distant countries. But the case is exactly the reverse. Not only have
many fresh-water species, belonging to quite different classes, an
enormous range, but allied species prevail in a remarkable manner
throughout the world. I well remember, when first collecting in the
fresh waters of Brazil, feeling much surprise at the similarity of
the fresh-water insects, shells, etc., and at the dissimilarity of the
surrounding terrestrial beings, compared with those of Britain.

But this power in fresh-water productions of ranging widely, though so
unexpected, can, I think, in most cases be explained by their having
become fitted, in a manner highly useful to them, for short and frequent
migrations from pond to pond, or from stream to stream; and liability
to wide dispersal would follow from this capacity as an almost necessary
consequence. We can here consider only a few cases. In regard to fish, I
believe that the same species never occur in the fresh waters of distant
continents. But on the same continent the species often range widely and
almost capriciously; for two river-systems will have some fish in common
and some different. A few facts seem to favour the possibility of their
occasional transport by accidental means; like that of the live fish
not rarely dropped by whirlwinds in India, and the vitality of their
ova when removed from the water. But I am inclined to attribute the
dispersal of fresh-water fish mainly to slight changes within the recent
period in the level of the lan