174 ZOOLOGICAL PHILOSOPHY 



I noticed that this animal is able to unite its hind feet as we join 

 cm hands, and that on then separating the digits between which there 

 are membranes, it forms a fairly large paddle, which it uses for travelUng 

 about in the water in the same way as fishes use their tail as a fin. 



This seal drags itself about on the ground with some speed by means 

 of an imdulatory movement of the body, and without any help from 

 its hind legs, which remain inactive and are stretched out. In thus 

 dragging itself about, it derives help from its fore-legs only by support- 

 ing itself on the arms up to the wrists, without making any special 

 use of the hands. It seizes its prey either with its hind feet or with 

 its mouth, and although it sometimes uses its hands to rend the prey 

 that it holds in its mouth, these hands appear to be used principally 

 for swimming or locomotion in the water. Finally, as this animal often 

 remains imder water for a longish time and even feeds there in comfort, 

 I have noticed that it easily and completely closes its nostrils just as 

 we close our eyes ; this is very useful to it when immersed in the hquid 

 that it inhabits. 



As this seal is well known, I shall give no description of it. My 

 purpose here is simply to remark that the amphibians have their 

 hind legs set on in the same direction as the axis of their body, for the 

 simple reason that these animals are compelled to use them habitually 

 as a caudal fin by uniting them and by separating the digits so as to 

 form a large paddle. With this artificial fin they are then able to 

 strike the water either to the right or left, and thus move rapidly in 

 various directions. 



The two hind legs of seals are so often uxiited and used as a fin that 

 they would not simply have this backward direction in continuance 

 of the body but would be permanently united as in the walruses, 

 were it not for the fact that the animals in question also use them very 

 frequently for seizing and carrying off their prey. Now the special 

 movements required by these actions prevents the hind legs of seals 

 from becoming permanently united, and only allow them to be joined 

 together momentarily. 



Walruses, on the contrary, which are accustomed to feeding on grass, 

 which they come and browse on the shore, only use their hind legs 

 as a caudal fin ; so that in most of them these legs are permanently 

 united with one another and with the tail, and cannot be separated. 



We find here a new proof of the power of habit over the form and 

 state of the organs, a proof that I may add to all those already set 

 forth in Chapter VII. 



I might add still another very striking proof drawn from mammals. 

 The faculty of flight would seem to be quite foreign to them ; yet I 

 can show how nature has gradually produced extensions of the animal's 



