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 



