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On the Origin of Species

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.
FRESH-WATER PRODUCTIONS.
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 formidable barrier, that they would
never have extended to distant countries. But the case is exactly the reverse. Not only
have many fresh-water species, belonging to different classes, an enormous range, but
allied species prevail in a remarkable manner throughout the world. When first collecting
in the fresh waters of Brazil, I well remember 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 the wide ranging power of fresh-water productions 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, within their own
countries; and liability to wide dispersal would follow from this capacity as an almost
necessary consequence. We can here consider only a few cases; of these, some of the
most difficult to explain are presented by fish. It was formerly believed that the same
fresh-water species never existed on two continents distant from each other. But Dr.
Gunther has lately shown that the Galaxias attenuatus inhabits Tasmania, New Zealand,
the Falkland Islands and the mainland of South America. This is a wonderful case, and
probably indicates dispersal from an Antarctic centre during a former warm period. This
case, however, is rendered in some degree less surprising by the species of this genus
having the power of crossing by some unknown means considerable spaces of open
ocean: thus there is one species common to New Zealand and to the Auckland Islands,
though separated by a distance of about 230 miles. On the same continent fresh-water
fish often range widely, and as if capriciously; for in two adjoining river systems some of
the species may be the same and some wholly different.
It is probable that they are occasionally transported by what may be called accidental
means. Thus fishes still alive are not very rarely dropped at distant points by whirlwinds;
and it is known that the ova retain their vitality for a considerable time after removal from
the water. Their dispersal may, however, be mainly attributed to changes in the level of
the land within the recent period, causing rivers to flow into each other. Instances, also,
could be given of this having occurred during floods, without any change of level. The
wide differences of the fish on the opposite sides of most mountain-ranges, which are
continuous and consequently must, from an early period, have completely prevented the
inosculation of the river systems on the two sides, leads to the same conclusion. Some
 
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