AQUATIC MAMMALS 



in this light or as having been caused by decreased terrestrial activity. 

 But such alteration has been too slight in degree to make discussion 

 profitable. 



As the anterior limb of aquatic mammals has always been a subject 

 for the liveliest speculation there will undoubtedly be expected of me 

 some statement regarding my conviction on this question. Unfortunately 

 the facts do not warrant any very strong convictions, for each case is 

 different and seems to constitute a law unto itself. In the first place it 

 is likely that the paths followed by aquatic reptiles are throughout most 

 of their course so different, because of a different equipment to begin 

 with, that they can certainly not be compared with any intelligence, at 

 least during their earlier stages. At long last, after a staggering length 

 of time, the paddle of an ichthyosaur and that of a cetacean, if used 

 for the same purpose throughout ages, may show a convergence of char- 

 acters to the point where they have essentially similar details. The 

 Cetacea already show promise of this, but no other aquatic mammal is 

 sufficiently specialized for this to be apparent. Diverse sorts pursue in- 

 dividual paths and even though these trend in the same general direc- 

 tion they are often far apart and have become deflected from the straight 

 line by numerous obstacles. All that I feel convinced of at present is 

 that in aquatic mammals there is a stimulus for the shortening of the 

 length of the part of the arm situated between the scapula and the manus. 

 It further seems likely that the control of the arm as a swimming paddle 

 or equilibrator is facilitated by a humerus shortened to some degree and 

 with the insertions of some of the critical muscles shifted farther distad. 

 In addition it appears logical that after the disappearance of a synovial 

 elbow joint and marked atrophy of the musculature of the lower arm 

 the shortening of the antibrachium should be enabled to progress at an 

 accelerated rate. But any opinion as to whether the brachium or anti- 

 brachium should be shorter in some hypothetical aquatic mammal would 

 be merely fanciful. 



Regarding the course of future development, we are justified in con- 

 sidering that eventually a flipper of the character of that found in Plat- 

 amsta (fig. 36) will likely assume the essential characters of that of an 

 ichthyosaur (fig. 34), in which the humerus, radius and ulna have been 

 reduced to flattened ossicles of the same appearance as the carpal bones. 

 This may or may not be the final goal of the pectoral limbs of mysticetes, 

 sirenians and non-phocid pinnipeds. It is not improbable but is surely 

 not a certainty. I will even go so far as to say that it seems to me un- 

 likely that the true seals will ever assume this type of flipper char- 



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