ening at a not early age, and being inherited at
a corresponding 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 ancient and less modified condition of each animal. This
view may be true, and yet it may never be capable of full proof. Seeing,
for instance, that the oldest known mammals, reptiles, and fish strictly
belong to their own 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 far beneath the lowest Silurian strata are discovered--a
discovery of which the chance is very 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 genera of 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 t