844 SUCCESSION OF THE 



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 

 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 super- 

 vened at a not early age, and having been inherited at a 

 corresponding age. This process, while it leaves the em- 

 bryo 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 pic- 

 ture, preserved by nature, of the former and less modified 

 condition of the species. This view may be true, a,nu ^et 

 may never be capable of proof Seeing, for instance, that 

 the oldest known mammals, reptiles, and fishes strictly be- 

 long 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 commoik 

 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 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 armor, 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 thes? facts that I strongly in- 

 sisted, in 1839 and 1845, on this " law of the succession of 

 types," — on "this wonderful relationship in the same con- 

 tinent between the dead and the living." Professor Owen 

 has subsequently extended the same generalization to the 



