THE PECTORAL LIMB 



in helping to introduce food into the mouth or in prying up stones upon 

 the bottom. Third, the most efficient method of applying the propulsive 

 force in swimming is from the hinder end, and it is likely that the ani- 

 mal very quickly discovers this fact. Especially if there be present a 

 tail of respectable proportions and length I regard it almost as a cer- 

 tainty that in mammals the fore limbs will not constitute the primary 

 means of propulsion. It seems likely, then, that unless there be some 

 special feature of body conformation (as a short tail) or special feeding 

 habits that might introduce disturbing elements, the final, primary, pro- 

 pulsive organ of an aquatic mammal will not be the pair of pectoral 

 limbs. 



If a highly specialized aquatic mammal swim by means of oscilla- 

 tions of its hinder end — either the tail or hind feet — it should have 

 some anterior provision for steering and equilibration. A slow baleen 

 whale, with its enormous head, might possibly get along very well with- 

 out a rudder, but for an odontocete or pinniped, pursuing individual 

 food items, inability to steer (i.e. make sharp turns) would spell speedy 

 death by starvation. If the head be of sufficiently small size so that a 

 neck of considerable length be possible (see chapter 7), then this part 

 of the animal can take over most of the function of rudder. It can 

 swim along in a straight line with neck retracted until such time as it 

 desires to turn abruptly, when the head can be thrust sharply to the 

 side and the body will follow. But at times the head has other duties 

 to perform. Perhaps a speedy fish is being pursued and the endeavor is 

 to seize it as quickly as possible. Then is it advisable that there be a 

 separate apparatus for steering, and this is supplied by the fore feet. 

 Thus in the true seals (Phocidae) there is such nice interaction between 

 the head and the neck on the one hand, and the anterior limbs on the oth- 

 er, that it is impossible to tell which is of the most importance in steering. 

 But it is important to note that in this animal it is likely that if the fore 

 limbs were held immovable against the sides steering would be accom- 

 plished with just as much effectiveness, at least for a short while, al- 

 though it is probable that exhaustion would follow more speedily. 



In the Cetacea matters are somewhat different. The whole body may 

 be curved moderately in any direction and thus effect turning, but the 

 neck is so short that it is capable of little more curvature than the thorax 

 and it is likely that when the animal is progressing at speed the entire 

 vertebral column is so occupied with^oscillations concomitant to swim- 

 ming that other motions of the body concerned purely with vertical steer- 

 ing are not often attempted, this preferably being accomplished by sim- 



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