196 THE ANIMALS OF LAKES AND RIVERS 



life-history as well as lungs, illustrate the transition 

 from the fresh-water to the terrestrial hfe, some forms 

 showing a tendency to revert to the aquatic life ; 

 (2) such forms as otter and desman among mammals, 

 water-tortoise and crocodile among reptiles, show land 

 animals in the process of reacquiring aquatic charac- 

 ters, either because food is easier to get in water than 

 on land, or because Mfe is safer there for relatively 

 helpless animals ; (3) the fresh-water dolphin of the 

 Ganges, the manatee, &c., exempHfy the acquisition 

 of the fresh- water habit by animals adapted to Hfe in 

 the sea, but driven from the sea by the competition of 

 more advanced forms, or quitting it volvmtarily in 

 search of food. The seals found in Lake Baikal and the 

 Caspian Sea are truly marine forms, more or less 

 accidentally cut off from their natural habitat. 



The remaining animals of lakes and rivers, includ- 

 ing fishes and invertebrates, are of more ancient origin, 

 and have had more time to become fundamentally 

 modified. . The manatee is still a marine animal, the 

 fresh-water dolphin of the Ganges became fiuviatile at 

 a period which is geologically but of yesterday, but the 

 fresh-water hydra, the fresh-water crayfish, the fresh- 

 water mussels have had time to become greatly modified. 



The fauna of swift rivers must in the general case 

 consist only of powerful swimmers hke fish and fresh- 

 water crayfish, or of animals which can so fix them- 

 selves as to avoid the force of the currents. In lakes 

 and ponds, on the other hand, there is a greater wealth 

 of hfe, and it is possible, as in the sea, to divide this 

 life into littoral, pelagic, and abyssal groups. But of 

 these groups the Mttoral is by far the largest, because 

 it is enriched by many forms, like the larvae of insects, 

 which retain some dependence upon the land. The 



