268 THE DOCTRINE OF DESCENT. 



of their docility, to be the highest birds. But all this 

 is arbitrary, and can only accidentally correspond with 

 the true and unknown ramification of the ornithic branch 

 in the pedigree of the Vertebrata. 



The most ancient known remains of the Mammalia 

 are found in the Trias. They occur somewhat more 

 frequently in the central Mesozoic strata, and they all 

 belong to Marsupial animals. Now as Marsupials, in 

 comparison with the inferior classes of vertebrate ani- 

 mals from which they must be derived, are very highly 

 developed, and as in the Monotremata (Duck-mole, 

 Ornithorhyncus, and Porcupine ant-eater, Echidna,) we 

 possess mammals which are manifestly far beneath the 

 Marsupials, we are referred entirely to conjecture and 

 inference for the origin of the mammals. These point 

 to amphibian-like beings, in which certain peculiarities of 

 the mammalian skull, as the double condyle of the occi- 

 put, were prefigured, and which by the formation of 

 the amnios and allantois approached the true reptiles. 

 These progenitors of the Mammaha are not, however, 

 represented in any order of reptiles or amphibians now 

 extant. The pedigree (p. 269) in which we have grouped 

 the more accurately known fossil Mammalia with those 

 now living, contains considerable gaps, and rests in a 

 great measure on hypothesis, but it gives, never- 

 theless, with approximate probability a correct rep- 

 resentation of the consanguinity of the orders, and in 

 comparison with the system as it was constructed in 

 the school-books prior to the revival of the doctrine 

 of Descent, it must be esteemed a great and suggestive 

 advance. 



As regards the structure of their skull, the constitu- 



