The rule is, that just as the productions of adjacent areas usually resemble each other closely, so do the productions of successive periods in the same area; and as the productions of remote areas generally differ widely, so do the productions of the same area at remote epochs.

We are therefore led irresistibly to the conclusion, that change of species, still more of generic and of family form, is a matter of time.

But time may have led to a change of species in one country, while in another the forms have been more permanent, or the change may have gone on at an equal rate but in a different manner in both.

In either case, the amount of individuality in the productions of a district will be to some extent a measure of the time that a district has been isolated from those that surround it.

Judged by this standard, Celebes must be one of the oldest parts of the Archipelago.