g age. This process, whilst it
leaves the embryo almost unaltered, continually adds, in the course of
successive generations, more and more difference to the adult. Thus the
embryo comes to be left as a sort of picture, preserved by nature, of
the former and less modified condition of the species. This view may be
true, and yet may never be capable of proof. 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. We see the same law in this author's
restorations of the extinct and gigantic birds of New Zealand. We see
it also in the birds of the caves of Brazil. Mr. Woodward has shown that
the same law holds good with sea-shells, but, from the wide distribution
of most molluscs, it is not well displayed by them. Other cases could
be added, as the relation between the extinct and living land-shells of
Madeira; and between the extinct and living brackish water-shells of the
Aralo-Caspian Sea.

Now, what does this remarkable law of the succession of the same types
within the same a