Chapter Eight 



The Trunk 



There are numerous factors affecting the precise conformation of the 

 mammahan thorax, but it is invariably of such shape that only very minor 

 adjustments are necessary in order to bring it to an ideal stream-line form. 

 Perhaps in the majority of quadrupeds the broadest or deepest part of 

 the body contour is just back of the anterior limbs, although this point 

 not infrequently is situated in the posterior thorax, or even through the 

 abdomen, in the more paunchy sorts. But no mammalian thorax is so 

 unsuitable in form that it will offer any real resistance in swimming and 

 in consequence there is no very strong stimulus for a definite change in 

 its shape even when sufficient time has elapsed for there to be very ad- 

 vanced aquatic specialization in other respects. I deem the Pinnipedia, 

 Sirenia and Cetacea to be the only aquatic mammals to have reached this 

 point. It is commonly accepted as fact that the great thoracic diameter 

 of Hippopotamus is a result of aquatic habits. I doubt this and although 

 I may be entirely mistaken I believe that the primary reason that the 

 thorax is larger in this animal than the rhinoceros, for instance, is that 

 the more succulent food of the former necessitates an alimentary tract of 

 prodigious capacity, with stomach more than a dozen feet in length. 



We can be sure, however, that in pinnipeds, sirenians, and cetaceans, 

 the aquatic life has been lived for sufficient time for there to have been 

 modifications in the external form of the trunk — especially in the first 

 and last orders, which are more speedy. The requirements for external 

 conformation are simple and necessitate merely that the greatest girth be 

 approximately at the pivot of motion for swimming. In pinnipeds, 

 sirenians and most cetaceans this is in the region of the anterior thorax, 

 a possible exception being in the case of adult bull sea-lions, where the 

 greatest girth may be at the base of the neck. In balaenid and sperm 

 whales, where it has been argued that because of the huge size of the head 

 the pivot of swimming motion has tended to shift forward, the greatest 

 girth seems to be through the posterior part of the head, as would be 

 expected. For practical purposes, apparently, this is all that is neces- 

 sary, for it would make very little difference whether the cross section 

 of such a body be slightly flattened in the vertical or horizontal plane. 

 We can be sure, however, that if the aquatic influence were uncompli- 



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