Their dispersal may, however, be mainly attributed to changes in the level of the land within the recent period, causing rivers to flow into each other.

Instances, also, could be given of this having occurred during floods, without any change of level.

The wide differences of the fish on the opposite sides of most mountain-ranges, which are continuous and consequently must, from an early period, have completely prevented the inosculation of the river systems on the two sides, leads to the same conclusion.

Some fresh-water fish belong to very ancient forms, and in such cases there will have been ample time for great geographical changes, and consequently time and means for much migration.

Moreover, Dr.