Gunther has recently been led by several considerations to infer that with fishes the same forms have a long endurance. Saltwater fish can with care be slowly accustomed to live in fresh water; and, according to Valenciennes, there is hardly a single group of which all the members are confined to fresh water, so that a marine species belonging to a freshwater group might travel far along the shores of the sea, and could, it is probable, become adapted without much difficulty to the fresh waters of a distant land.

Some species of freshwater shells have very wide ranges, and allied species which, on our theory, are descended from a common parent, and must have proceeded from a single source, prevail throughout the world. Their distribution at first perplexed me much, as their ova are not likely to be transported by birds; and the ova, as well as the adults, are immediately killed by seawater. I could not even understand how some naturalised species have spread rapidly throughout the same country. But two facts, which I have observed⁠—and many others no doubt will be discovered⁠—throw some light on this subject. When ducks suddenly emerge from a pond covered with duckweed, I have twice seen these little plants adhering to their backs; and it has happened to me, in removing a little duckweed from one aquarium to another, that I have unintentionally stocked the one with freshwater shells from the other. But another agency is perhaps more effectual: I suspended the feet of a duck in an aquarium, where many ova of freshwater shells were hatching; and I found that numbers of the extremely minute and just-hatched shells crawled on the feet, and clung to them so firmly that when taken out of the water they could not be jarred off, though at a somewhat more advanced age they would voluntarily drop off.