CHAPTER XIII. GEOGRAPHICAL DISTRIBUTION--continued.
1. 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.