THE EXTERNAL FORM OF WHALES 23 



is I. 3, II. 8, III.9, IV. 5, V. 4; of an adult, 2, 8,6, 4, 2 

 are the figures. This looks as if the adaptation to 

 an aquatic life had, as it were, at first overshot the 

 mark, the reduction taking place later ; that the 

 creatures started with too ample a provision for its 

 needs, to be later curtailed. Or, indeed, it seems 

 more likely that the pectoral fin was originally a 

 swimming organ, and is now reduced to a mere 

 balancer. The degenerating muscles argue the same 



o o o 



way. 



The hand muscles of Bal&noptera musculus are in 

 all four. On the extensor side, i.e., the "back of the 

 hand, is a single extensor, the extensor communis 

 digitorum." This has a short muscular head arising 

 between the radius and ulna, it soon passes into 

 tendon, and on the wrist divided into four tendons, 

 one for each finger. On the opposite side of the 

 hand are three muscles ; two of these, the flexor 

 profundis digitorum and the flexor longus pollicis, 

 join together by their tendon, and then split up into 

 four tendons for the four digits. The fourth muscle 

 is the flexor carpi ulnaris ; it runs from the ulna to 

 the pisiform bone in the wrist. We should, therefore, 

 consider the pectoral fin as an organ which has under- 

 gone a change of function. Originally a paddle, large 

 size mainly brought about by hyperphalangy was 

 necessary to it. The assumption of this function by 

 the tail led to a reduction in the hand, which has 

 progressed very much further in some whales than 

 in others. 



