648 SCIENTIFIC RECORD FOR 1884. 



inasmuch as it is a mammal adapted for an aquatic life, but it has been 

 converted to its general fish-like form by the peculiar development of 

 its hind limbs into instruments of propulsion through the water, for, 

 though the thighs and legs are small, the feet are large and are the 

 special organs of locomotion in the water, the tail being quite rudi- 

 mentary. In the whales the hind limbs are aborted and the tail devel- 

 oped into a powerful swimming organ. Now, it is very difficult to sup- 

 pose that when the hind limbs had once become so well adapted to a 

 function so essential to the welfare of the animal as that of swimming, 

 they could ever have become reduced and their action transferred to the 

 tail. It is far more reasonable to supi)0se that whales were derived 

 from animals with large tails, which were used in swimming, eventually 

 with such effect that the hind limbs became no longer necessary, and so 

 gradually disappeared. The powerful tail, with lateral cutaneous flanges, 

 of an American species of otter {Pteronura Sandbachii), or the still more 

 familiar tail of the beaver, may give some idea of this member in the 

 primitive cetacea." 



Such are the arguments, in brief, of Professor Flower. But it is not 

 necessary to suppose that the whales have evolved from a specialized 

 form like the Pinnipeds, which he indeed denies, for he rather contends 

 that they show ungulate affinities. It appears to be more reasonable to 

 infer that the cetaceans have descended from quadrupeds with rather 

 weak or ordinary hind limbs. Nevertheless, we find, in the feet of the 

 Otariids, or eared seals, some clew to the possible genesis of the flukes 

 as modifications of the limbs. 



Let us suppose, then, that a terrestrial mammal with even ordinary (but 

 not ungulate) hind limbs should take to the water, and its descendants, 

 following it in habits, should develop processes analogous to the mem- 

 branous extensions of the hind limbs in the eared seals. These maybe em- 

 ployed in swimming, and the osseous parts not assisting, or even being 

 prejudicial to such progress, might become atrophied, and. in the course 

 of atrophy would be overgrown by the integument and muscles, and ap- 

 pear to be pulled forward, while the membranous portions of the feet 

 would become hypertrophied ; this hypertrophy would extend to the mus- 

 cles as well as to the integuments, and, of course, all would become very 

 much modified. In fine, we would in time have a case where the posterior 

 limbs of the ancestral quadruped would be represented in part by the 

 flukes and in part by the included rudimentary bones, the flukes repre- 

 senting the hypertrophied integuments of the primitive members and the 

 bones the atrophied remains of the skeletal portions. The fact that in the 

 specialized aquatic or rather pelagic forms, the tail is always very much 

 reduced or rudimentary, may be considered as entitled to some value as 

 an argument in favor of the view presented, although not much. The 

 transversely expanded tails of the South American otter and beaver 

 are quite exceptional. In most aquatic forms, the tail is more or less 

 compressed. 



