THE PELVIC LIMB 



the epidermis, and were entirely comparable, except in size, with the 

 hind limb buds of other mammals. Apparently their lesser definition 

 in the 17 mm. Phocaena, and still less in a 27 mm. Lagenorhynchus, in- 

 dicates that a reduction in absolute size begins as the external cetacean 

 characters (chiefly the relative increase in tail size) are initiated. In 

 another Phocaena fetus, of 18 mm., the limb was represented merely by 

 a papilla, which is the penultimate step in external disappearance. 



Before summarizing the discussion of the cetacean pelvic limb it may 

 be well to consider briefly the case of some of the extinct aquatic rep- 

 tiles. "-In almost all the larger sorts known the pes was practically as 

 large as the manus, even though adaptation had progressed to the point 

 where hyperphalangy was already far advanced. It has previously been 

 argued herein that this was probably attributable largely to a difference in 

 the methods of swimming employed by these reptiles compared with 

 the case in mammals, partly because of difference in bodily conforma- 

 tion as well as equipment and inherent tendencies. It was only when 

 aquatic adaptation had become very far advanced indeed, to the point 

 comparable with that now attained by the Cetacea, that the hind limbs 

 had begun really to shrink in size, as illustrated by Ichthyosaurus. In 

 bodily form these reptiles seem to have been as highly modified for an 

 active aquatic life as cetaceans and it is not likely that the former 

 had need for retaining the pedes as balancers. Hence it ap- 

 pears necessary to believe either that the external hind limbs of aquatic 

 reptiles were much slower to disappear, because employed for a much 

 longer time for active swimming in a four-limbed manner, or else that 

 the Cetacea were phenomenally amenable to influences which resulted 

 in elimination of the pedes, for which there is not the slightest evi- 

 dence. 



To complete this chapter it is only necessary to offer a brief summary. 

 If a mammal be well modified for an aquatic life but without present 

 evidence that its hind limbs have ever been used to an important ex- 

 tent for either swimming or steering, these may remain relatively un- 

 altered in most respects, or become peculiarly specialized in an unpre- 

 dictable manner (as in the gluteus maximus of Potomogale) . If these 

 members are used as the chief method of swimming or steering and 

 are highly altered accordingly, one may expect with a considerable de- 

 gree of confidence that there will be a shortening of the femur, fre- 

 quently accompanied by shortening of the ilium. If the latter does not 

 occur the lesser gluteal mass should be very robust, and if it does, the 

 gluteus maximus may be very powerful. The crotch will tend to mi- 



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