THE PELVIC LIMB 



along the borders or webbing between the toes, or a combination of the 

 two, and this speciahzation increases, theoretically, until the time when 

 the tail shall have become sufficiently modified so that it is capable of 

 furnishing greater propulsive power than the feet are capable of pro- 

 ducing, when the latter will rapidly fall into disuse and tend to disap- 

 pear.. 



If a mammal use its hind feet for swimming by any such method 

 as that employed by the seal, with lateral motions in a sole-to-sole posi- 

 tion, the action will be entirely symmetrical and the tendency will be 

 for the feet to become symmetrical, with fifth and first toes of equal 

 length and longer than the third, giving the foot a shape comparable 

 to the tail of a fish. Any oar-like use of the hind feet will tend to pro- 

 duce, or rather to accentuate, an asymmetrical foot, with either the first 

 or the fifth toe the longest, according to the precise method in which 

 the foot is used. Usually this manner of swimming consists in alternate 

 strokes of the feet, these reaching either down or out so as to act upon 

 relatively undisturbed water. There is no mechanical reason why an 

 aquatic mammal should not swim by placing the broad part of the feet 

 horizontally upon either side of the tail and oscillating the entire hinder 

 end in the vertical plane, but no mammal is surely known to do this, 

 although it is not unlikely that the sea otter may employ this motion. 

 It is also not improbable that asymmetrical development of the feet, com- 

 parable to that in the sea otter, might follow the phocid method of 

 swimming provided there were also a tail of moderate length ; but then 

 the feet would not be the sole means of propulsion, for the tail would 

 help. 



The potentialities of the mammalian foot for furnishing alternating 

 propulsive strokes of the character that is employed by most water birds 

 seems to be limited, and the two can hardly be compared. If a jerboa 

 (Dzpus), with its elongated, fused metatarsals now took to the water 

 it might eventually evolve a propelling foot very similar to that of a 

 duck or grebe, but so far as the evidence goes no aquatic mammal ever 

 had a terrestrial ancestor with pedal equipment of this type. Nor is it 

 probable that any highly aquatic mammal has evolved from the cursorial 

 ungulate type with fused metatarsals, and there is probably no stimulus 

 connected with a life in the water that would produce such fusion. 



In an aquatic bird such as a duck the propelling mechanism gains 

 proper leverage by being upon the end of a tarsus of considerable length. 

 The latter is relatively narrow and ofl^ers but slight resistance during 

 recovery, while at the same time the foot collapses almost like a closed 



[271] 



