EVOLUTION AND EXTERNAL APPEARANCE 



6l 



extent (Fig. 21), and all the bones which are present in, say, our own 

 arms, can also be found in the flipper of the whale or the dolphin. True, 

 the hiunerus is short, and the forearm bones (radius and ulna) are short 

 and flattened, but the flippers of all Cetaceans (with the exception of 

 Rorquals which lack a thumb) have five digits. The digits - although 

 sepai'ately distinguishable internally, are enclosed in a common integu- 

 ment, thus giving the flipper its characteristic flat shape. The fin as a 

 whole is a stiff but elastic paddle with the shoulder joint as its only true 

 movable part. 



In normal maminals, man included, every finger has three phalanges, 

 except the thumb, which has two. In Cetaceans, however, the central 

 digits have a larger number of phalangeal elements. Hyperphalangy is 

 most pronounced in the long flippers of the Pilot Whale, whose second and 

 third digits have 14 and 11 phalanges respectively (Fig. 25). 



But like the pelvic limb, the flipper of the very young embryo started as 

 a normal limb, just like our own arm, which began as a flat, leaf-shaped 

 outgrowth of the trunk. Subsequently, it becomes a kind of stalk, flattened 

 at the end. In man, the wrist develops from the notch just below the 

 thickened end which gives rise to the five fingers, but in whales the notch 

 disappears again and the 'arm' assumes its characteristic fin shape 

 (Fig. 26). 



From the fact that whales have vestiges of pelvic limbs which arose in 

 the same way as those of normal mammals, we may infer that whales did 

 not originally have their present form but that, in the dim and distant 



Figure 25. Skeleton of a Pilot Whalers right flipper. [Flower.) 



