THE EXTERNAL FORM OF WHALES 19 



mammals. This is precisely what is found in the 

 ichthyopterygium. We may regard, perhaps, the 

 larger cartilages which articulate with the shoulder 

 girdle as corresponding with the humerus, radius, 

 and ulna. The commencing disappearance of marks 

 of distinction between the different elements of the 

 arm is, of course, correlated with the absence of a 

 differentiation of function between its several parts. 

 A broad fin, like that of a fish and of a whale, would 

 be as efficient if there were an absolute similarity 

 between its several cartilages as if there were a 

 differentiation. 



The second point of likeness is not shown in all 

 whales. In Beluga, however, the last finger is 

 divided into two fingers, incompletely it is true, but 

 still the division is plain enough. This is a step in 

 the direction of the polydactylous fin of the fish. 

 In no whale, however, is this feature of resemblance 

 shown to a greater extent. (PI. I., fig. 3, p. 9.) 



Together with these points of likeness, not 

 numerous or strong, it must be admitted, are obvious 

 points of difference. The increased surface of the 

 whale's paddle, desirable in an organ used as a fin, 

 is affected in a different way from the fin of the fish. 



In the whale the area is increased, not much by a 

 multiplication of the fingers, but by their spreading 

 out in a divergent fashion, so as to require a larger 

 skin area, and by the increase of their length caused 

 by the reduplication of the finger bones. The phenom- 

 enon known as " Hyperphalangy " is usual in whales. 

 The typical mammalian foot or hand is composed of 



