io A BOOK OF WHALES 



discovered to have possessed a dorsal fin not unlike 

 that of the whales, and a caudal fork which, unlike 

 that of the whales, was vertical in direction. Now 

 the Ichthyosaurus had undoubted hind limbs, so that 

 there can be no question of any correspondence here. 

 The fact, therefore, that the whale's tail, unlike that 

 of the fish, is at right angles to the axis of the body, 

 and so far resembles the complex "tail" of the seal 

 is no argument, even from analogy, in favour of its 

 having a limb-like character. The Ichthyosaurus has 

 no more right to a tail than the whale, save by virtue 

 of its being an aquatic creature ; the tail is in both 

 a secondary adaptation to the needs of their existence. 



We must look, as Dr. Kukenthal remarks, to the 

 broad tail of the beaver for an analogy to the flukes 

 of the whale. 



It is, however, somewhat astonishing to find that 

 the whale, unlike the Ichthyosaurus, which is with 

 equal certainty derived from a terrestrial ancestor, has 

 transverse tail fins ; astonishing, since the universality 

 of a vertical fin in fish seems to argue its greater use 

 as a swimming organ. The only conclusion to which 

 this question seems to lead is that reptiles, that are 

 not so thoroughly modified for an aquatic life as the 

 Ichthyosaurus, and are yet largely or entirely aquatic, 

 such as crocodiles and sea-snakes, have a vertically 

 compressed tail, while among mammals it is generally 

 flattened from above downwards in such forms, 

 instances of this being the beaver and the platypus. 

 But this is not universal, only prevalent, for in the 

 West African insectivore otter (JPotamogale) we have 



