CHAPTER 12.  GEOGRAPHICAL DISTRIBUTION—­continued.

Distribution of fresh-water productions. 
On the inhabitants of oceanic islands. 
Absence of Batrachians and of terrestrial Mammals. 
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 chapters.

As lakes and river-systems are separated from each 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 impassable barrier, that they never would have extended to distant countries.  But the case is exactly the reverse.  Not only have many fresh-water species, belonging to quite different classes, an enormous range, but allied species prevail in a remarkable manner throughout the world.  I well remember, when first collecting in the fresh waters of Brazil, feeling much surprise at the similarity of the fresh-water insects, shells, etc., and at the dissimilarity of the surrounding terrestrial beings, compared with those of Britain.

But this power in fresh-water productions of ranging widely, though so unexpected, can, I think, in most cases be explained by their having become fitted, in a manner highly useful to them, for short and frequent migrations from pond to pond, or from stream to stream; and liability to wide dispersal would follow from this capacity as an almost necessary consequence.  We can here consider only a few cases.  In regard to fish, I believe that the same species never occur in the fresh waters of distant continents.  But on the same continent the species often range widely and almost capriciously; for two river-systems will have some fish in common and some different.  A few facts seem to favour the possibility of their occasional transport by accidental means; like that of the live fish not rarely dropped by whirlwinds in India, and the vitality of their ova when removed from the water.  But I am inclined to attribute the dispersal of fresh-water fish mainly to slight changes within the recent period in the level of the land, having caused rivers to flow into each other.  Instances, also, could be given of this having occurred during floods, without any change of level.  We have evidence in the loess of the Rhine of considerable changes of level in the land within a very recent geological period, and when the surface was peopled by existing land and fresh-water shells.  The wide difference of the fish on opposite sides of continuous 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: