Darwin's On the Origin of Species: A Modern Rendition

12. The Geographical Distribution of Life, Continued

238 Lakes and river systems are separated from one anotherbybarriersofland,soitmightseemobviousthatfreshwaterspecies do not range across many bodies of water and that because the sea is an even greater barrier, they would never spread to distant regions. But actually the reverse is true. Not only do many freshwater species from quite different classes have enormous ranges, but interrelated species prevail throughout the world! When I first collected freshwater insects, shelled mollusks, and other freshwater species in Brazil, I remember being surprised at their similarity to those in Britain, and at the dissimilarity of the surrounding terrestrial organisms. The capacity of freshwater organisms to range widely may be unexpected , but in most cases it can be explained by their being adapted to make short and frequent migrations from pond to pond or stream to stream–a tendency for wide dispersal follows as an almost necessary consequence. I can here consider only a few cases. With respect to fish, I believe that the same species never occurs in freshwater on widely separatedcontinents.Butwithinonecontinent,individualspeciesrange widely and almost capriciously: two river systems can have some fish in common and some different. A few observations favor the possibility that they occasionally spread by “accidental” means; for example, live fish are sometimes dropped bywhirlwindsinIndia,and fish eggsremain viable after their removal from water. However, I attribute the dispersal of freshwater fish mainly to minor changes in land level within the recent period that caused rivers to flow into one another. Examples could also be given of this result brought about by floods without any changes THE GEOGRAPHICAL DISTRIBUTION OF LIFE, CONTINUED 12 The Geographical Distribution of Life, Continued 239 in level. The loess of the Rhine reveals considerable changes in level within a very recent geological period when the surface was inhabited by still-extant terrestrial and freshwater shelled mollusks. The remarkable difference between fish on opposite sides of continuous mountain ranges, which must have parted river systems and prevented them from intertwining since an early period, suggests the same conclusion. The many cases of related freshwater fish occurring in widely separated parts of the world are currently inexplicable. But some freshwater fish belong to very ancient forms, and in these cases there has been ample time for great geographical changes, and as a consequence, for much migration. Also, saltwater fish can slowly become accustomed to living in freshwater, and according to Valenciennes, there are very few fish groups confined exclusively to freshwater. A marine member of a freshwater group could conceivably travel far along the shores of the sea and subsequently become modified and adapted to the freshwaters of a distant land. Some freshwater shelled mollusks have a very wide range, and species related to them–which, according to my theory, descended from a commonancestor andmust haveproceededfromasinglelocation–prevail throughout the world. Their distribution perplexed me at first because their eggs are unlikely to be transported by birds, and they perish in seawater, as do the adults. I could not even understand how some naturalized species have spread rapidly within a region. But I have made two observations that shed light on this problem (no doubt many others remain to be made). I have twice seen a duck suddenly emerging from a duckweed-covered pond with the little plants adhering to its back. And in moving a little duckweed from one aquarium to another, I unintentionally stocked the second with freshwater shelled mollusks from the first. But another agency is perhaps more effective: I suspended a duck’s feet–which may represent those of a bird sleeping in a pond–in anaquarium.Theaquariumcontainedmanyhatchingeggsoffreshwater shelled mollusks, and I found hatched mollusks crawling on the duck’s feet, clinging so firmly that when they were taken out of the water, they couldnotbejarredoff.(Theyvoluntarilydroppedoffatamoreadvanced age.) These just-hatched mollusks survived on the duck’s feet, in damp air, from twelve to twenty hours. In this amount of time a duck or heron 240 Darwin’s On the Origin of Species could fly at least six or seven hundred miles and would surely alight on a pond or rivulet if blown across the sea to an oceanic island or any other distantpoint.SirCharlesLyellinformsmethataDyticus beetlehas been caught with a freshwater mollusk similar to a limpet firmly adhering to it, and a water beetle of the same family once landed aboard the Beagle when she was forty-five miles from the nearest land. How much farther it might have flown...