1 6 THE GEOLOGICAL EVIDENCES 



properly constituted -into a distinct class of their 

 own. They are provided, in addition to gills, with 

 true lungs, by means of which they respire the 

 oxygen of the air directly, and with which there 

 stands in immediate relation a pulmonary circula- 

 tion, operated by a heart with three chambers. 



Having thus established the relationship existing 

 between fishes and amphibians, it will be well to 

 consider in how far this relationship also extends 

 to the third group of cold-blooded animals, the 

 reptiles. Manifestly, a reptile is most closely related 

 to the amphibians, from which it differs primarily 

 in never breathing by means of gills, and in having 

 but a single articulation to the base of the skull, 

 instead of the two seen in an amphibian. It may 

 also be added that the amphibian has a naked 

 skin, whereas nearly all reptiles are provided with 

 scales or plates developed in the integument. In 

 other important points of structure such as the 

 lungs, heart, and circulation a reptile agrees essen- 

 tially with an adult amphibian, and indeed more so 

 than certain reptiles agree with one another. The 



