I can hardly doubt that this rule is generally true, though difficult of proof.

Among mammals, we see it strikingly displayed in Bats, and in a lesser degree in the Felidae and Canidae.

We see the same rule in the distribution of butterflies and beetles.

So it is with most of the inhabitants of fresh water, for many of the genera in the most distinct classes range over the world, and many of the species have enormous ranges.

It is not meant that all, but that some of the species have very wide ranges in the genera which range very widely.