128 ' THE ORIGIN OF SPECIES 



owing to variations having supervened at a not early 

 age, and having been inherited at a corresponding age. 

 This process, while it leaves the eini)ryo almost un- 

 altered, 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 mod- 

 ified 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 Yertebrata, 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 mam- 

 mals 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 Pro- 

 fessor Owen has shown in the most striking manner that 

 most of the fossil mammals, buried there in such num- 

 bers, are related to South American types. This rela- 

 tionship is even more clearly seen in the wonderful col- 

 lection of fossil bones made by MM. Lund and Clausen 



