264 THE DOCTRINE OF DESCENT. 



some genera, in course of preparation, so that in the 

 full-grown animal its origin may still be recognized ; 

 in others, as the genus Campsognathus, it is accom- 

 plished. We allude to the peculiarity already discussed 

 in p. 10, that the upper portion of the tarsus is anchy- 

 losed with the tibia, the lower with the metatarsus, and 

 that the ankle-joint is hence inserted into the tarsus. 



All existing reptiles are sharply distinguished from 

 the Amphibians and Fishes by several phenomena ac- 

 companying their development. They possess two 

 organs enveloping the embryo ; the amnion, which is 

 essentially a protecting sheath, and the allantois, by 

 which the fcetal circulation, nutrition, and respiration is 

 regulated and carried on. In the Batrachians we find 

 indications at least of the allantois, and must suppose 

 that the greater part of the fossil reptiles had already 

 adopted this advance in general organization. It implies 

 an advance, inasmuch as animals developed by the aid 

 of the amnion and allantois make further progress during 

 the embryonic phase than is Uie case with the inferior 

 Vertebrata, and that they hence leave the egg with greater 

 powers of resistance. We must ascribe the adoption of 

 the amnion and allantois to remote periods of amphibian 

 and reptile development, for the additional reason that 

 the possession of their embryonic sheaths and organs 

 is shared by the birds which are descended from true 

 reptiles, and by the mammals which cannot be descended 

 from true reptiles. 



The birds are, anatomically, so closely allied to the 

 reptiles, that Huxley, who has carried out the com- 

 parison most rigorously, has joined the two classes into a 

 greater systematic unit, under the name of Sauropsida, 



