The more ancient any form is, the more, as a general rule, it differs from living forms.

But, as Buckland long ago remarked, extinct species can all be classed either in still existing groups, or between them.

That the extinct forms of life help to fill up the intervals between existing genera, families, and orders, is certainly true; but as this statement has often been ignored or even denied, it may be well to make some remarks on this subject, and to give some instances.