within comparatively recent times. For this doctrine of Agassiz accords
well with the theory of natural selection. In a future chapter I shall
attempt to show that the adult differs from its embryo, owing to
variations supervening 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;
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