AQUATIC MAMMALS 



terior border really supplies the force, and precedes the posterior bor- 

 der, the motion of which is more passive. By this method of swimming 

 the possible speed is very great indeed, and in practice depends directly 

 only upon the power with which the flipper is adducted. Another ad- 

 vantage is that this need not entail any lost motion, for abduction, al- 

 though necessarily much weaker because of muscle conformation, con- 

 sists of the same motions as adduction but in the opposite direction, and 

 can contribute at least some propulsive force. Thus in swimming by 

 adduction and abduction in the transverse or largely vertical plane all 

 movements can be utilized for forward propulsion and the only resis- 

 tance is offered by the anterior borders of the flippers. It is undoubt- 

 edly by just this method, or one substantially the same, that aquatic 

 birds, including penguins, which habitually pursue speedy prey by "fly- 

 ing" under water utilize their wings; but the case of the marine turtles 

 is different. 



I have had but limited opportunity for observing the actions of pen- 

 guins, but so far as my experience goes the motions of their pectoral ap- 

 pendages differ in no important respect from those of sea-lions. Move- 

 ment is almost entirely in the abductive-adductive (transverse) plane, the 

 wings are "feathered" during the stroke so that the force applied is ob- 

 lique, and although the abductive or upward stroke is too fast to follow 

 satisfactorily it seems likely that it is so performed that it furnishes at 

 least a slight amount of forward propulsion. There is the difference, 

 however, that in the penguin the static posture of the paddles is al- 

 most horizontal, while in the sea-lion it is more adducted, and the arc 

 of movement varies accordingly. 



The swimming movements of the marine turtles are rather hard to 

 describe. The elbow protrudes from the body contour and the enormous 

 humerus is worked chiefly up and down. The forearm segment, how- 

 ever, is to all intents a mechanical part of the paddle and the whole is 

 extended (see figure 34) with respect to the humerus in a manner never 

 encountered in the Mammalia. By virtue of this alteration at the elbow 

 the turtle's flipper is given a definitely caudal inclination, so that al- 

 though the humerus is chiefly worked dorsad and ventrad from a trans- 

 verse position, the flipper, with its axis almost parallel to that of the 

 body, operates by thrusting the water chiefly backward by means of the 

 palm. Perhaps this situation has some bearing upon the fact that hyper- 

 phalangy is not met with among the turtles. 



By the above it is not meant to imply that no other swimming motions 

 are indulged in by either penguins or turtles. Those described are 



[212] 



