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 foetal 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 the 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, 
