THE TAIL 



or else, more probably, that the tail has always been of such character 

 as to lend itself readily to swimming. This is also indicated by the fact 

 that the feet of the mink and sumpfotter are not in the slightest modified 

 for swimming and the tail, although much less specialized in this direc- 

 tion than that of the otter, plays a very important part in propulsion. In 

 this connection the feet of the otter will be more fully discussed in 

 a future chapter. 



In but one genus of river otter is the tail other than round, at least 

 to an extent that has proven noticeable. In Pteronura this member has 

 a sort of fleshy keel upon either side, and hence, is flattened vertically 

 and expanded horizontally. This is a development which other otters 

 may be expected to follow. 



In the sea otter {Enhydra) the tail is also expanded laterally, evidently 

 to' a more marked degree than in any river otter, but it is relatively 

 shorter and does not reach beyond the tips of the toes when the hind 

 limbs are extended backward. But one cannot discuss the development 

 of the tail of this animal with any degree of assurance until more is 

 known about its swimming habits. Certainly the hind feet of the sea 

 otter are of greater importance in swimming than is the case in the 

 river otter, and therefore, by analogy, the tail is of less importance. But 

 the tail is more specialized, and it is therefore likely that it has experi- 

 enced aquatic influences for a longer period of time. It may once 

 have been relatively shorter and have experienced a stimulus for elonga- 

 tion, or it may always have been approximately of its present length but 

 the use to which the hind feet were put in swimming enabled these 

 members to gain the evolutional ascendency. It appears very likely, 

 however, that the tail of the ancestor of the sea otter when it first took 

 to the water was not equally as long and robust as of the river otter at 

 the same stage of its history, else the tail, being theoretically a more 

 ideal organ for propulsion, would have gained the evolutional ascend- 

 ency, as it has in the Lutrinae, before the hind limbs had gotten well 

 started. 



The future course of enhydrine development is uncertain. If the 

 tail were longer I should predict as a matter of course that this appen- 

 dage would increase in importance and size, gradually supplanting the 

 hind feet as the chief organ of propulsion, and that the latter would 

 dwindle in size. Actually the tail now seems to be at a critical stage. 

 The hind feet are so large and (apparently) efficient that the animal 

 may be unable to alter its present course of development, or the tail may 

 be unable to catch up in the race of aquatic adaption and it may well be 



[195] 



