Seeing, for instance, that the oldest known mammals, reptiles, and fishes strictly belong to their proper classes, though some of these old forms are in a slight degree less distinct from each other than are the typical members of the same groups at the present day, it would be vain to look for animals having the common embryological character of the Vertebrata, until beds rich in fossils are discovered far beneath the lowest Cambrian strata--a discovery of which the chance is small.
ON THE SUCCESSION OF THE SAME TYPES WITHIN THE SAME AREAS, DURING THE LATER TERTIARY PERIODS.
Mr.Clift many years ago showed that the fossil mammals from the Australian caves were closely allied to the living marsupials of that continent.

In South America, a similar relationship is manifest, even to an uneducated eye, in the gigantic pieces of armour, like those of the armadillo, found in several parts of La Plata; and Professor Owen has shown in the most striking manner that most of the fossil mammals, buried there in such numbers, are related to South American types.

This relationship is even more clearly seen in the wonderful collection of fossil bones made by MM.

Lund and Clausen in the caves of Brazil.

I was so much impressed with these facts that I strongly insisted, in 1839 and 1845, on this "law of the succession of types,"-- on "this wonderful relationship in the same continent between the dead and the living." Professor Owen has subsequently extended the same generalisation to the mammals of the Old World.