LAW OF PROGRESS AND APPEARANCE OF GROUPS 245 



Philosophique to the demonstration of this progress, 

 both in the general organization of beings and in 

 the details of each function. It might almost 

 be said that this notion had become commonplace 

 through being constantly reproduced in so many 

 works. 



We cannot, in fact, fail to recognize that, on the 

 whole, the most perfect groups, that is to say, those 

 highest in the zoological hierarchy, have appeared 

 at relatively recent epochs. We do not, as yet, 

 know of any Vertebrates in the Cambrian or in the 

 pre-Cambrian. Primary times are characterized by 

 the reign of several inferior groups, the Tetracorals 

 the tessellated Crinoids, the Cystoidea, the Blas- 

 toi'ds, and especially the Brachiopods. Among Verte- 

 brates, the cold-blooded and lower types alone are 

 represented in these periods by the Fishes, Amphi- 

 bians, and Reptiles. Up till then no bird or mammal 

 appears to have arrived on the palaeozoic continents. 

 In Secondary times the Invertebrates have hardly 

 any further progress left to achieve ; but in the world 

 of Vertebrates the marine and terrestrial Reptiles 

 occupy easily the first rank. The Birds are rare, 

 and among Mammals those orders with lower 

 organizations, marsupial or monotreme, alone ap- 

 pear to be represented. Finally, in the Tertiary 

 era, the orders of Mammals belonging to the highest 

 or the most differentiated types, like the Probos- 

 cidians, Equidae, Ruminants with antlers or with 

 horns, and Apes only appear in Neogenic times, and 

 Man, who represents, in point of cerebral develop- 

 ment at least, the highest point of the organized 

 world, seems, as far as our knowledge goes, to have 



