GEOGRAPHICAL DISTRIBUTION AND DESCENT. 



25 



type was accordingly well established from 

 the time of the Miocene epoch, and the local 

 isolation of the present species is no doubt to 

 be explained by this wider distribution in 

 former times. 



The sea-cows approach the whales in the 

 characters due to special adaptation, and if 

 we take into account only these characters, 

 to which, as already mentioned, belong the 

 form of the body, the presence of a horizontal 

 tail fin, and the absence of visible hind-limbs, 

 we may rank them as a sub-order of the 

 Cetacea. But all the rest of their organiza- 

 tion proves clearly enough that the sea-cows 

 are derived from a different stock from the 

 whales, and that by their thick fleshy lips set 



with tactile hairs, their dentition, their small 

 distinct head capable of being moved at the 

 neck, the position of their nostrils, the structure 

 of their skull, their brain, and their organs 

 of reproduction, and by a number of other 

 characters which we cannot fully explain 

 here, they are associated with the ungulates. 

 Since the ungulates are already met with in 

 the oldest Eocene strata, while the sea-cows 

 first appear in the Miocene, there is nothing 

 to prevent us from regarding them as a 

 retrograde branch of the former, as repre- 

 sentatives of a type which, by a process 

 similar to that which we have demonstrated 

 in the case of the seals and the carnivores, 

 has adapted itself to an aquatic life. 



Vol. II. 



36 



