12 THE MAMMALIA. 



leave of the sea. On the other hand, the American 

 lamantin is still undergoing the transition, and 

 feels as comfortable in the sea as it does in the 

 lowest currents of large streams. The dugong of 

 Eastern Africa has, however, remained perfectly 

 faithful to its old, habitual element. An interest- 

 ing example of the occurrence of a mammal in an 

 inland lake is the dog-fish of the Caspian Sea. It 

 was, in fact, simply left there. That this large 

 sheet of water was at one time connected with the 

 sea is a long since established fact. 



By this remark, and a return to the geological 

 past (even though it be to a most recent past), we 

 have again entered the only path which leads to 

 the understanding of all the geographical configu- 

 rations of the present, more particularly to the 

 distribution of the organisms, and above all to that 

 of the land animals. The difficulty of meeting 

 with closely related species, orders, and larger 

 groups, in regions lying far apart and separated 

 by high chains of mountains or impassable oceans, 

 has, since Buffon's days, been quietly settled with 

 the word ' vicariate,' which proves anything but a 

 true understanding of the matter. When it is said 

 that the Marsupials ' vicariate ' in Australia for the 

 other groups distributed on the other continents, 



