THE PELVIC LIMB 



tail) in the vertical plane. It is not probable that an actively predaceous 

 mammal of the sort that it is likely the whale has always been would 

 evolve a tail expanded in the horizontal plane for the purpose of keep- 

 ing it near the bottom, as seems to be the case in the platypus, but 

 rather that it developed along the same lines that the tail of the otter 

 is now following. The speed of development of such a tail should be 

 materially assisted by the presence of short, and relatively weak, rather 

 than long and powerful, pedes, and such small feet would, presumably, 

 disappear more rapidly than would those which had become large and 

 highly specialized. Hence there seems to be considerable evidence that 

 the Cetacea could most readily and quickly have evolved from some sort 

 of terrestrial ancestor having many of the chief bodily characteristics now 

 occurring in the common river otter. 



[309] 



