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 pe- 

 culiarities of the mammalian skull, such as the double 

 condyle of the occiput, were prefigured, and which 

 by the formation of the amnios and allantois ap- 

 proached the true reptiles. These progenitors of the 

 Mammalia 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, nevertheless, with approximate 

 probability a correct representation 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. 



