196 THE ANIMALS OF LAKES AND RIVERS 
life-history as well as lungs, illustrate the transition 
from the fresh-water to the terrestrial life, 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 life is safer there for relatively 
helpless animals; (3) the fresh-water dolphin of the 
Ganges, the manatee, &c., exemplify the acquisition 
of the fresh-water habit by animals adapted to life in 
the sea, but driven from the sea by the competition of 
more advanced forms, or quitting it voluntarily 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 fluviatile 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 like 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 life, and it is possible, as in the sea, to divide this 
life into littoral, pelagic, and abyssal groups. But of 
these groups the littoral 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 
