Evolution 



while the amphibian breathes to a considerable extent through 

 its thin moist skin, this method of assisting respiration is not 

 available to the reptile. 



Again connected with the improvement of the respiratory 

 process, there is a partial development of a septum dividing the 

 ventricle or pumping chamber of the heart. The value of this 

 division of course lies in the fact that the purified and oxygenated 

 blood from the lungs is prevented from mixing with the venous 

 blood from the body. The course of the blood is from the body 

 to the right auricle, thence to the right ventricle, and thence to 

 the lungs. The pure blood from the lungs returns to the left 

 auricle, passes thence to the ventricle on the same side to be 

 pumped to the general circulation. The disadvantage of a single 

 ventricle, such as occurs in the Amphibia, and the advantage of 

 the regular double circulation, such as that in man, are sufficiently 

 obvious. The division of the ventricle into two chambers is 

 less complete in the lizards and snakes, very nearly perfect in 

 the crocodiles. 



The reptiles, like the Amphibia, are ' cold blooded,' by which 

 is meant, not that their blood is necessarily cold, but that its 

 temperature varies with that of the surroundings, while that of 

 the blood of the mammals and birds is practically constant. 



A very important feature of the reptiles, which they possess 

 in common with the mammals and the birds, is that the embryo 

 produces two membranous outgrowths called respectively the 

 amnion and the allantois, which completely envelop it, and which 

 have important functions in connection with nutrition, respira- 

 tion, and excretion during the period when the young creature is 

 enclosed in the egg. It is, of course, not until we reach the higher 

 mammals that these membranes assume their greatest importance. 



For a considerable time in the world's history the reptiles 

 were the dominant vertebrate class, and in the chalk period 

 especially they were represented by a great variety of forms, 

 and by a number of species of colossal stature, one at least of which 

 was over a hundred feet long. In those times the reptiles were 

 by no means all condemned to crawl on their bellies, for they 



112 



