As some few of the old and intermediate forms having transmitted to the present day descendants but little modified, these constitute our so-called osculant or aberrant groups.

The more aberrant any form is, the greater must be the number of connecting forms which have been exterminated and utterly lost.

And we have evidence of aberrant groups having suffered severely from extinction, for they are almost always represented by extremely few species; and such species as do occur are generally very distinct from each other, which again implies extinction.

The genera Ornithorhynchus and Lepidosiren, for example, would not have been less aberrant had each been represented by a dozen species, instead of as at present by a single one, or by two or three.