Page - 388 - in The Origin of Species

388 ORIGIN OF SPECIES naturalist, from an examination of the species of the two countries, could not have foreseen this result. Agassiz and several other highly competent judges insist that ancient animals resemble to a certain extent the em- bryos of recent animals belonging to the same classes ; and that the geological succession of extinct forms is nearly par- allel with the embryological development of existing forms. This view accords admirably well with our theory. In a future chapter I shall attempt to show that the adult differs from its embryo, owing to variations having supervened at a not early age, and having been inherited at a corresponding age. This process, whilst it leaves the embryo almost unal- tered, continually adds, in the course of successive genera- tions, 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 de- gree less distinct from each other than are the typical mem- bers of the same groups at the present day, it would be vain to look for animals having the common embryological char- acter of the Vertebrata, until beds rich in fossils are discov- ered 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