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.

Hence the amount of difference is one very important criterion in settling whether two forms should be ranked as species or varieties.