CHAPTER VI 



THE ORIGIN OF THE LAND VERTEBRATES 



No one disputes the truth of the fact that the 

 land Vertebrates, which occupy the most exalted 

 position in the scale of living organisms, must have 

 originated from marine animals possessing the 

 essential characteristics of fishes. All the evidence 

 of palaeontology, of comparative anatomy and em- 

 bryology, converges towards this cardinal tenet. But 

 between the fishes and the lowest forms of land 

 Vertebrates, the Amphibia, there is a serious gap 

 which palaeontology does nothing, and comparative / 

 anatomy not everything, to bridge. 



In the passage from the water to the land that 

 must have occurred, two main systems of organs 

 have been principally affected, the limbs, owing to 

 the change of medium in which progression had to 

 take place, and the respiratory organs, which had to 

 change from the process of obtaining oxygen from 

 the water to that of obtaining it directly from the 

 atmosphere. As a matter of fact comparative ana- 

 tomy enables us to trace the transition in the respira- 

 tory organs with far greater certitude than in the J 

 case of the limbs. 



