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The excerpt represents the core issue or deciding factor on which you must meditate, and is drawn from On the Origin of Species by Charles Darwin: mountain-ranges, which from an early period must have parted river-systems
and completely prevented their inosculation, seems to lead to this same
conclusion. With respect to allied fresh-water fish occurring at very
distant points of the world, no doubt there are many cases which cannot at
present be explained: but some fresh-water fish belong to very ancient
forms, and in such cases there will have been ample time for great
geographical changes, and consequently time and means for much migration.
In the second place, salt-water fish can with care be slowly accustomed to
live in fresh water; and, according to Valenciennes, there is hardly a
single group of fishes confined exclusively to fresh water, so that we may
imagine that a marine member of a fresh-water group might travel far along
On the Origin of Species |
