A BIT OF COMPARATIVE ANATOMY. 185 



"What!" you exclaim, "'all the outside structure' of an otter, for 

 example?" 



Tes, but not the same appearance. Let me explain to -you how this 

 is. If we could study the outlines of the heads of both, and. compare the 

 skulls, or pictures of the skulls, of the seal and the otter, we should see 

 at once how the bones, and the shape and arrangement of the teeth in 

 one resemble those in the other. And if we had also a picture of the 

 skull of a codfish, we should see how different from it are the skulls of 

 the otter and seal. 



Now look at the limbs. I have heard of a boy who defined a quad- 

 ruped as an animal having a leg at each corner. Perhaps that would fit 

 the otter, but you think that, certainly, it would not describe the seal, 

 " which hasn't legs at all," you say, " but fins or ' flippers.' " 



If I had the time, I could prove to you that the difference between 

 the fin of a fish and the bone-leg of an otter or of a dog, or your own arm, 

 is not so very great ; and it would be easy to show how nearly alike the 

 flipper of the seal and fore-leg of a land mammal really are. On examin- 

 ing diagrams of the bones in a seal's flipper and an otter's fore-leg, you 

 will find that you can match every bone of the one by a similar bone of 

 the other. The shapes of the bones, to be sure, are altered to suit the 

 varied uses of swimming in the water and walking on the land ; but all 

 the parts of the arm and hand (or fore-foot) of the otter, or any other 

 mammal, are seen also in the flipper of our subject — only there they are 

 shortened, thickened, and covered with a membrane which converts them 

 into a paddle instead of a paw. 



The same comparison will hold good for the hind-feet of the otter 

 and the hind-flippers or " tail " (which is not a tail) of the seal ; and it is 

 equally true of the walrus, and of the whale, porpoise, grampus, blackfish 

 and other cetaceans. 



Of course, being mammals, these animals must breathe air. You 

 could drown any of them by forcing it to remain under the water too 

 long. Whales can stay down an hour or more, if necessary, and seals can 

 hold their breath for fifteen or twenty minutes, though they do not like 

 to be under as long as that. Of course, therefore, it is necessary for seals 

 to be able to reach the air, even in spite of the sheet of thick ice which 

 for half the year covers the whole ocean in the arctic seas, where mainly 

 is their home. But in large bodies of ice there always are some holes, no 

 matter how cold the weather may be ; and these holes afford the seals of 

 that region an opportunity to come to the surface to breathe. There are 

 some species-, however, that keep round, smooth-edged air-holes open for 



