AQUATIC MAMMALS 



which I then made were that the sea-lion swims by advancing the flippers, 

 not in unison nor yet in alternation but with a sort of galloping move- 

 ment, and then by partial rotation so as to present the broad aspect of 

 the flipper, progressed by means of strong backward sweeps of these 

 members. More recently, however, I have succeeded in obtaining clear 

 views of the whole process. . Undoubtedly the muscles mentioned were 

 developed for backward thrusts at a time when the animals were less 

 specialized and as yet incapable of the speed that they now attain. At 

 present, however, they are too speedy for just this method of propulsion 

 to be efficient, as already discussed at the beginning of this chapter, and 

 for mechanical reasons rather than purely muscular ones, they are 

 obliged when progressing as fast as possible to do so by means of adduc- 

 tive and abductive thrusts of the flippers, with the minimum of ex- 

 tension or flexion. The anterior border rather than the posterior is where 

 the force is applied, while the latter is allowed passively to follow 

 through, so that there is an oblique thrust and both adduction and ab- 

 duction are utilized for propulsion, although the latter undoubtedly is 

 productive of much less power, possibly an insignificant amount. 



Unlike the case of the whales the flipper of a sea-lion is not stiffened 

 by fibrous joints. The elbow does not cut much of a figure in the bend- 

 ing of the external arm for it is close to the body contour, but although 

 possible flexion at the wrist is not great, extension at this point is through 

 an angle of 90 degrees to allow for terrestrial progression, and this 

 must be overcome by mechanical means during swimming so that the 

 wrist will not bend backward during the adductive stroke, and so that 

 muscles will not be wearied by continual effort to prevent such bending. 

 Extension of the arm is no more needed than in whales, the degree of 

 abduction need not be greater, but adduction and flexion must, or at least 

 should be, greater. In relaxed posture the radial or anterior border 

 should be presented straight, forward, for this position is the mean from 

 which pronation and supination are instigated. Accordingly the normal 

 terrestrial position is for the manus to extend from the wrist in an exactly 

 lateral direction rather than somewhat craniad as usually shown in 

 mounted skeletons. 2 



In the seals (Phocidae) the pectoral limb is not used in swimming, as 

 already discussed, save during such acts as turning. It is supposed to be 

 employed for such purposes as scratching holes in the ice, and its strictly 



2 This inaccuracy will be noted in the photograph (fig. 14) of the fur seal 

 herewith depicted, which is the same as that shown in my previous paper re- 

 ferred to. 



[224] 



