2 WATER REPTILES OF THE PAST AND PRESENT 



very long ago. among the rulers of the land, of great size and 

 extraordinary forms. But they have dwindled away, both in size 

 and in numbers, till only a comparatively few of their descendants 

 are left, none of them more than two or three feet in length, and 

 all of them sluggish in disposition and of inoffensive habits. While 

 we may speak of the ami)hil)ians as air-breathing, they are, with 

 few exceptions, water-breathers during the earlier part of their 

 existence. Some may pass their whole lives as water-breathers, 

 while a few begin to breathe air as soon as hatched from the egg; 

 but these are the marked exceptions. 



In many respects the internal structure of the amphibians of 

 the present time is widely different from that of reptiles, though 

 there can be no doubt that the early amphibian ancestors of the 

 modern toads, frogs, and salamanders were also the ancestors of all 

 living and extinct reptiles, and it is a fact that the living amphib- 

 ians differ more from some of the ancient ones than those early 

 am])hibians did from their contemporary reptiles. Discoveries 

 in recent years have bridged over nearly all the essential differ- 

 ences between the two classes so completely that many forms can- 

 not be classified unless one has their nearly complete skeletons. 

 We know that some of the oldest amphibians, belonging to the 

 great division called Stegocephalia, were really water-breathers 

 during a part of their Hves, because distinct impressions of their 

 branchiae, or water-breathing organs, have been discovered in 

 the rocks with their skeletal remains, but we are not at all sure that 

 some of the more highly developed kinds were not air-breathers 

 from the time they left the egg; indeed, we rather suspect that 

 such was the case. 



We are also now quite certain that, from some of the early 

 extinct reptiles — the immediate forbears probably of the great 

 dinosaurs — the class of birds arose, since the structural relation- 

 ships between birds and reptiles are almost as close as those between 

 reptiles and amphibians. 



Huxley believed that the great class of mammals arose directly 

 from the amphibians, and there are some zoologists even yet who 

 think that he was right. But paleontologists are now quite sure 

 that they were evolved from a group of primitive reptiles, known 



