264 THE DOCTRINE OF DESCENT. 



course of preparation, so that in the full-grown animal its 

 origin may still be recognized; in others, as the genus 

 Campsogriathus, it is accomplished. We allude to the 

 peculiarity already discussed in p. 10, that the upper 

 portion of the tarsus is anchylosed 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 or- 

 gans enveloping the embryo; the amnion, which is es- 

 sentially a protecting sheath, and the allantois, by which 

 the foetal circulation, nutrition, and respiration is regu- 

 lated and carried on. In the Batrachians we find indi- 

 cations 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 ad- 

 vance, inasmuch as animals developed by the aid of the 

 amnion and allantois make further progress during the 

 embryonic phase than is the case with the inferior Ver- 

 tebrata, 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, 



