Page - 427 - in The Origin of Species

CHAPTER XIII Geographical Distribution—contimied Distribution of fresh-water productions—On the inhabitants of oceanic islands—Absence of Batrachians and of terrestrial Mam- mals—On the relation of the inhabitants of islands to those of the nearest mainland—On colonisation from the nearest source with subsequent modification—Summary of the last and present chapter. FRESH-WATER PRODUCTIONS AS lakes and river-systems are separated from each l\ 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 re- verse. 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, &c., and at the dissimilarity of the surroimd- ing 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 be- come 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 neces- sary 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. Giinther has lately shown that the Galaxias 427