Chapter Three 



External Features 



JroR GREATEST efficiency, which simply means the attainment of con- 

 siderable speed with the least expenditure of effort, certain requisites of a 

 self-evident nature are necessary to an aquatic mammal of high specializa- 

 tion. The body must be of streamline form without excrescences that 

 would offer resistance to the water. The propulsive force should be 

 from the rear, as has been learned from a study of shipbuilding, and 

 there must be some sort of apparatus for steering and equilibration. The 

 limitations of mechanical adaptation in a vertebrate necessitates that 

 separate means for steering be situated where this will receive no great 

 disturbance from motions of the body concomitant to propulsion. Hence 

 the rudder must be elsewhere than near the activators for progression. 

 But there may be accessory equilibrators, as a dorsal fin. An additional 

 necessity is that in an active mammal that is thoroughly aquatic there 

 must be an alteration by means of which respiration may be carried on 

 while the animal is traveling at full speed. The above qualifications 

 are mechanically essential to ideal efficiency, a thesis that may be accepted 

 without argument, and is a goal toward which every mammal heads as 

 soon as it takes to the water. Whether every such mammal will attain this 

 goal eventually is a different matter, and is dependent upon its inherent 

 capabilities and inhibitions for such evolution, and environmental factors, 

 which at times deflect it at a tangent, so that later it may be incapable of 

 regaining the straight course leading to ideal aquatic specialization. 



A streamline body form is not a fixed shape that must fit into a parti- 

 cular mold, but it can be long or short, narrow or relatively broad, shal- 

 low or deep. It must taper at both ends, after a fashion, however. Some 

 terrestrial mammals already have a streamline form, while to attain it 

 others must pass through successive steps. As a standard of perfection 

 in this regard it is safe to take the swiftest of the cetaceans, and we can 

 rest assured that this form is close to the ideal, for it is approached to al- 

 most identical degree, with but minor variations, by three diverse phyla 

 of vertebrates — the porpoise, ichthyosaur, and certain sharks — as already 

 mentioned. Other aquatic mammals that differ from this body form 

 either have had insufficient time to attain it, although headed in this 



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