THE TAI L 



instances, one of these being Hippopotomus and the other Potoniogale. 

 The size of the tail in the former animal is entirely too small to be of 

 the shghtest use either in swimming or in steering, and yet it is laterally 

 flattened to a phenomenal extent. The only explanation for this condi- 

 tion would seem to be that in spite of its small size it has been wriggled 

 from side to side by the alternate strokes of the hind limbs for a suffi- 

 ciently long time for it to have responded to the same stimulus that 

 would have aflPected it were it sufficiently large to be an aid in pro- 

 pulsion. The case of Potoniogale is more difficult. Its tail is remarkably 

 efficient as a flattened propulsive organ and yet the feet are small and 

 so unmodified that we are justified in assuming that they are not used 

 while the animal is in the water. It seems that the only logical theory 

 is that originally the terrestrial ancestor of this mammal was equipped 

 with small feet while its tail, although round, was unusually heavy, 

 especially at the base, so that in connection with a slim body, it was 

 even then a more efficient swimming organ than the small feet. If it 

 came more natural for this terrestrial ancestor to move the tail from side 

 to side, as a dog does, rather than in the vertical plane, flattening in 

 the horizontal plane would be the inevitable result. 



The insectivores and rodents listed above as having terete or round 

 tails can be dismissed with the statement that they are either not suffi- 

 ciently specialized for there to have been caudal change, or else this 

 member has been unusually conservative. The case of the water opos- 

 sum (Chivonectes) is somewhat diff^erent. The webbing of its hind 

 feet is quite extraordinary in degree, and it is difficult to understand 

 how this was brought about so completely without some flattening of 

 the tail, providing that the animal swims by alternate kicks of the feet- 

 movements which would of necessity involve the wriggling of the tail 

 from side to side and this should be easy of accomplishment because of 

 the robustness of the tail base, as in so many marsupials. There is no 

 authentic statement, I believe, of the exact method of swimming em- 

 ployed by this animal and it does not seem impossible, although it is 

 improbable, that it might propel itself by means of kicking the feet in 

 unison, and if this should ultimately prove to be the case, then the round- 

 ness of its tail would be expected. 



It seems that at least in insectivores and rodents the first change in 

 the tail brought about by their aquatic life is usually the acquisition of 

 a border of hairs, above or below or both, that are longer and stiflfer 

 than those upon other parts of the tail. // there be anything to the 

 theory of the eventual inheritance of acquired characters throughout 



[187] 



