310 Succession of the Chap. xi. 



most skilful 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 embryos of recent 

 animals belonging to the same classes ; and that the geological suc- 

 cession of extinct forms is nearly parallel 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 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 ivithin 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- 



