ANCIENT AND LIVING FORMS. 307 



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 tlie 

 lowest Cambrian strata — a discovery of which the chance 

 is small. 



ON" THE SUCCESSION" OF THE SAME TYPES WITHIN THE 

 THE SAME AKEAS, DURIN^G THE LATEll 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 arma- 

 dillo, 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 re- 

 lated 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 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 

 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 see it also in the birds of the caves of 

 Brazil. Mr. Woodward has showm that the same law holds 

 good with sea-shells, but, from the wide distribution of 

 most molluscs, it is not well displayed by them. Other 

 cases could be added, as the relation between the extinct 

 and living land-shells of Madeira; and between the extinct 

 and living brackish water-shells of the Aralo-Caspian Sea. 



Now, what does this remarkable law of the sncoession of 

 the same types within the same areas mean? He would be a 



