LAW OF PROGRESS AND APPEARANCE OF GROUPS 249 



whose existing descendants dwell in the fresh 

 waters of the Southern Hemisphere. As we know, 

 on the other hand, that marine Turtles of the 

 group of Turtles with dermic breastplates, or 

 Athecae, already lived in the Triassic and infra- 

 Liassic seas, we may predict the future discovery 

 of Chelonians of the Permian epoch and probably 

 even much earlier. 



The warm-blooded Vertebrates are likewise much 

 older than we suppose at the present time. The 

 discovery of the Archceopteryx of the upper Jurassic 

 of Bavaria with its separate fingers armed with 

 claws, its beak furnished with conical teeth, and its 

 long lizard's tail, if it has demonstrated beyond 

 dispute the reptilian connections of the class of 

 Birds, in no way enlightens us as to the precise 

 period when the divergence of these two organic 

 types occurred. The Archceopteryx is already, in its 

 structure taken as a whole, a true Bird furnished, 

 without any doubt, with a very long ancestral 

 genealogy, which for the present escapes our ob- 

 servation. 



Mammals, if we take the class as a whole, appear 

 for the first time in the higher Trias and in the 

 Rhsetian. The Dromatherium sylvestre of the Trias 

 of Carolina, so far as may be judged from one single 

 semi-mandible, seems to be connected with the 

 group of insectivorous Marsupials, but has less 

 complicated molars with a single point. As to the 

 Microlestes antiquus of the Rhsetian of Wurtemburg 

 and England, it seems logical to connect it with 

 the Plagiaulacidae, that is to say, with the Multi- 

 tuberculata having a marsupial or perhaps a mono- 



