All that we want to show is, that where many species of a genus have been formed, on an average many are still forming; and this certainly holds good.

There are other relations between the species of large genera and their recorded varieties which deserve notice.

We have seen that there is no infallible criterion by which to distinguish species and well-marked varieties; and when intermediate links have not been found between doubtful forms, naturalists are compelled to come to a determination by the amount of difference between them, judging by analogy whether or not the amount suffices to raise one or both to the rank of species.