We see the same principle in the character of most of the blind animals inhabiting the caves of America and of Europe.

Other analogous facts could be given.
It will, I believe, be found universally true, that wherever in two regions, let them be ever so distant, many closely allied or representative species occur, there will likewise be found some identical species; and wherever many closely-allied species occur, there will be found many forms which some naturalists rank as distinct species, and others as mere varieties; these doubtful forms showing us the steps in the process of modification.
The relation between the power and extent of migration in certain species, either at the present or at some former period, and the existence at remote points of the world of closely allied species, is shown in another and more general way.

Mr.Gould remarked to me long ago, that in those genera of birds which range over the world, many of the species have very wide ranges.

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.