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, 
