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.

If we confine our attention either to the living or to the extinct species of the same class, the series is far less perfect than if we combine both into one general system.

In the writings of Professor Owen we continually meet with the expression of generalised forms, as applied to extinct animals; and in the writings of Agassiz, of prophetic or synthetic types; and these terms imply that such forms are, in fact, intermediate or connecting links.

Another distinguished palaeontologist, M.