ADAPTATION OF LAND. REPTILES TO LIFE IN WATER 63 



A third t5^e of living aquatic air-breathers is seen in the seals, 

 sea-lions, etc. They are much less highly specialized, however, 

 than the whales or sirenians, since they are still capable of con- 

 siderable freedom upon land, which they recurrently seek for the 

 breeding of their young. They still retain the primitive covering 

 of hair, lost almost entirely by the cetaceans and sirenians and func- 

 tionally replaced for the conservation of heat by a thick layer of 

 blubber. Instead of losing the hind legs and developing the tail 

 as a propelling organ like the whales, the seals encountered pre- 

 cisely the reverse experience. The hind legs have been developed 

 into most efl&cient paddles or sculls, and the tail has been for the 

 most part lost. They are fish-eaters, it is true, but they do not 

 have the long jaws possessed by the porpoises and toothed whales. 



In the sea-otters, beavers, and even the muskrats, we have 

 examples of less complete adaptation of land mammals to water 

 life, the most of them showing the beginnings at least of structural 

 adaptations similar to those of the seals. From an attentive 

 examination of all these animals, living as well as extinct, which 

 have attained partial or complete success as air-breathing water 

 animals, we find certain laws existing, if we may call them such, 

 which we may discuss a little in detail. As we have seen in the 

 comparison of the whale with the seal, the methods of adaptation 

 have not always been the same, and some recent writers have 

 endeavored to classify aquatic animals under many groups, to 

 which they have given learned technical names, most of which will 

 not concern us here in deahng with the reptiles only. 



Beginning with the head, we find that all those reptiles and most 

 of the mammals which have become aquatic fish-eaters have an 

 elongated skull, or rather an elongated face. The jaws are long and 

 slender, and the teeth are not only numerous but also sharp and 

 slender, much like those of the gar-pike, indeed. It is remarkable, 

 too, that in most such animals the external nostrils are situated, 

 not at the extremity of the snout, as in all terrestrial mammals 

 and reptiles, but far back near the eyes. In the whales this position 

 of the nostril enables the animals to breathe without continuous 

 muscular exertion while floating on the surface; that is, the nostrils 

 are at the top of the head. In the sirenians, on the other hand, 

 which live habitually at the bottom of shallow waters, coming to 



