EXTERNAL FEATURES 



direction, or else by inherent limitations or influence of environment, 

 have been or are being prevented from doing so. But all existing aqua- 

 tic mammals, excepting, if one prefer, the hippopotamus, are of stream- 

 line form to a greater or lesser degree dependent upon the stage of aqua- 

 tic perfection which they approach in other respects. The corpulent 

 bulls of the Steller sea-lion and the walrus are more ponderous than we 

 should judge best for their aquatic efficiency, but the bulk of the bulls 

 of these two animals is doubtless a secondary sexual character the de- 

 velopment of which evidently proved to be no critical handicap. Simi- 

 larly with the female walrus, only slightly less corpulent. The latter 

 pinniped feeds upon inactive prey such as clams. Therefore speed, and 

 the more slender proportions which are requisite to its attainment, is not 

 a necessity in the securing of food, and it is evident that there was not a 

 strong stimulus for it to develop great speed in order to escape from its 

 enemies, else it would have become extinct or more speedy long since. 

 This shows that the adoption of an aquatic habitat will not develop a 

 slender body form, but that the stimulus for this is the necessity to travel 

 through the water speedily either for the purpose of securing fast-mov- 

 ing food or to escape from rapid enemies. 



Although it has evidently been aquatic for a very long geologic time 

 the form of the hippopotamus is far from suited to traveling with rapid- 

 ity through the water, although the celerity of its movements is said to 

 be quite surprising. It has, however, been under the necessity of lit- 

 tle besides sinking sluggishly beneath the surface of its pool or moving 

 languidly about. A hippopotamus that could dash about a small river 

 at the rate of thirty miles an hour would in all probability promptly 

 flatten itself against a rock. In other words, there has been no stimulus 

 for this mammal to develop a more perfectly streamline form, or if 

 some slight influence of this sort has been experienced, it would doubt- 

 less overborne by the greater importance of its ability to seek enormous 

 quantities of green fodder on shore. 



The long-tailed zeuglodonts did not conform to the aquatic ideal of 

 bodily shape as they were anguilhform, and this relative inefficiency may 

 have contributed to their disappearance ; but the short-tailed genera also 

 became extinct and the critical factor doubtless lay elsewhere. The 

 shape of modern whales is quite variable. Some of the small porpoises 

 are very speedy and with ease encircle a fast boat, the inference being 

 that they can travel considerably in excess of thirty miles per hour. The 

 larger, pelagic sorts of toothed whales are slower, however. The sperm 

 whale is decelerated by its enormous head and the mass of its sperma- 



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