THE EXTERNAL FORM OF WHALES 13 



and of the tail would tend to move the fish down- 

 wards ; in the "hypobatous" tail the movements 

 of the tail would raise it, and thus depress the head ; 

 and in consequence the direction of progression 

 would be away from the air a state of affairs which 

 is precisely what the shark would want. On the 

 other hand, the same movements of the epibatous 

 tail would tend to direct the course of the reptile 

 towards the surface of the water ; so that, after all, 

 the Ichthyosaurus has a tail which is as useful, or 

 nearly so, for enabling its possessor to get quickly 

 to the top of the water, as are the horizontal flukes 

 of the whale. 



DORSAL FIN 



Most whales have a fin on the dorsal side of the 

 body, nearer to the posterior than to the anterior end 

 of the body. The resemblance of this fin to the 

 similarly placed dorsal fin of fishes is obvious. It 

 has even been asserted that there are two dorsal fins 

 in some whales ; but the existence of a second and 

 of a fish-like anal fin seems to be purely mythical. 

 This fin is especially analogous to the fatty fin of the 

 Salmonoid fishes. It is not, however, present in all 

 whales, and, when present, is of very varying size. 

 According to Kiikenthal the fin is not present in 

 the young embryo of those whales which will 

 eventually have a fin. But it is represented by a 

 long dorsal fold reaching back to the flukes. This 

 structure appears to persist in Monodon. The series 

 of low, irregular humps which take the place of the 



