CHAPTER XXXIX 



THE EVOLUTION OF THE REPTILES 



That the Reptiles were evolved from the amphibia there is no 

 doubt whatever, and indeed, in some cases it is difficult to 

 decide whether a fossil is an amphibian or a reptile. The 

 most important distinctive features are the fact that the centra 

 of the vertebral column are formed from the pleurocentral 

 elements while the hypocentral elements are very much reduced, 

 and the absence of grooves for lateral-line canals on the skull. 

 This latter point shows that the reptiles had become definitely 

 terrestrial. They emancipated themselves from the water by 

 overcoming the three obstacles which checked the amphibia ; 

 viz. the necessity of water for breathing, for copulating, and 

 for the embryo to develop in. 



The first of these was countered by a better development of 

 the lungs and the adoption of the method of expanding the 

 thoracic cavity by means of the ribs, for replenishing their 

 content of air. The skin was thus enabled to become dry and 

 horny, and to be of greater efficiency in protection. 



The next difficulty was surmounted by the development of 

 copulatory organs with which the sperm could be inserted 

 straight into the oviducts of the female, and fertilisation was 

 internal. The sperm swam to the egg in the mucous fluid of 

 the oviduct instead of in the pond water. 



The last obstacle was overcome by a group of adaptations. 

 The egg was laid on land, and the albumen surrounding it was 

 itself surrounded by a shell composed of lime salts secreted by 

 the oviducts. In this way the egg was protected from drying 

 up, and from becoming flattened and collapsed like a " poached 

 egg," which it would otherwise be. The embryo became 



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