But the wide ranging power of fresh-water productions 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, within their own countries; and liability to wide dispersal would follow from this capacity as an almost necessary 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.