Mammals to Aquatic Life. 157 



the tail supersedes the posterior extremities as a motive organ. 

 This new method of locomotion, which possesses as great an 

 advantage over the movement bj means of the extremities 

 as does the screw-steamer over the row-boat, brought into 

 existence two very noticeable changes in the form of the bodj 

 — in the first place the loss of the hind limbs, and then the 

 transformation of the tail by expansion into a caudal fin. 



In the case also of many temporarily aquatic animals the 

 tail has already begun to take part in the function of 

 swimming, and in consequence thereof has undergone an 

 expansion, which is in most cases horizontal, as in the 

 platypus or the beaver, the musk-rat and the desman alone 

 possessing a laterally compressed tail. Nevertheless the 

 functional importance of the caudal extremity cannot outweigh 

 that of the hind limbs, since the latter are essential to the 

 power of locomotion upon dry land which these animals 

 possess. 



The transition from the one principle to the other can be 

 beautifully traced in the case of the seals; for the eared seals 

 pass a relatively large proportion of their time on land, and 

 consequently their hinder extremities still possess the power 

 of locomotion, their position with relation to the body is similar 

 to what obtains in the case of other land-animals, and in the 

 water fore and hind limbs are equally utilized in the action of 

 swimming. The true seals, on the other hand, live much 

 more exclusively in the water, and therefore the function of 

 swimming predominates and is transferred to the hind limbs, 

 which, projecting from the body posterioidy, have assumed a 

 tail-like shape and perform similar movements to those of, let 

 us say, the tail of the whale, while the fore limbs relinquish 

 the functions of mere oars, and are employed more for the 

 purposes of balancing and turning. 



The new method of motion attains its highest development 

 in the Sirenians and the whales ; the powerfully expanded 

 caudal fin has become the sole motile organ, the anterior 

 extremities functioning henceforth as rudders. We therefore 

 see that in the practice of swimming by the series of aquatic 

 mammals a new principle is gradually evolved, in that the 

 motive power is transferred to the hinder end of the body, the 

 consequence of which is the assumption of a more and more 

 fish-like form. 



Tiie lines which we must consider the phylogcnctic deve- 

 lopment of these processes to have followed have already been 

 traced in a masterly manner by Koux *. According to this 

 * W. lioux, " Beitriige zur Morpliologie der functionellen Anpassung. 

 — I. Structur eines hoch din'erouzirten bindegewebigeii Organes (der 

 SchwanzHosse des Delpliin)/' Arcliiv f. Anat. u. Physiol. 1883, p. 70. 



