THE TAIL 



as are too short to be of any possible use for propulsion, and they thus 

 comprise all aquatic mammals except the capybara, which is entirely de- 

 void of a tail. 



1. Tails narrow in the horizontal and broadened in the vertical plane. 

 In the first group below are included all those genera in which the tail 

 has been flattened in the horizontal plane, or else is provided with a 

 ventral keel of stiff hairs. In the second table are those genera in which 

 the tail is still terete but which may be expected eventually to develop a 

 narrow tail rather than a flat one. There is some doubt in this regard re- 

 specting Chironectes, as explained later. 



Tail narrow Tail round 



Desmana Chironectes 



Galemys Neosorex 



Neomys Atophyrax 



Chimarrogale Myocastor 



Crossogale Dasymys 



Nectogale Nilopegamys 



Limnogale Arvicola 



Potomogale Neofiber 



Crossomys Rheomys 



Hydromys Anotomys 

 Parahydromys 

 Ondatra 

 Ichthyomys 

 Hippopotamus 



It will be noted that an the above mammals having horizontally flat- 

 tened tails are either insectivores or rodents with the single exception of 

 the hippopotamus, and in none except Potomogale, and possibly Limno- 

 gale, does the tail constitute the principal organ of propulsion. Also that 

 none, except Potomogale perhaps, is really very highly specialized in an 

 aquatic direction. 



From a study of the question I am led to believe that in order that 

 an aquatic mammal shall finally acquire a tail that is horizontally flat- 

 tened its terrestrial ancestor must have had the following characteristics : 

 a normally cylindrical body not particularly elongated ; a tail, preferably 

 of considerable length, which was not much enlarged at the base, thus 

 showing an abrupt transition in size between the hinder end of the 

 body proper and the base of the tail (of the character occurring in 

 typical rodents, as the rat) ; feet of the normal rodent or insectivore 



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