It is well known that animals and plants inhabiting freshwater have, as a general rule, a very wide distribution; yet each river system, with all the pools and lakes in connection with it, seems completely cut off from every other system of the same country. Still more complete is the separation between the freshwaters of distinct continents or of islands; nevertheless they often possess freshwater species in common. In my “Origin of Species” I have suggested various means of transportal; but as few facts on this head are positively known, the case given in the adjoined letter of a living Unio, which had caught one of the toes of a duck' foot between its valves, and was secured in the act of being transported, seems to me well worth recording.
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