80 WILD NEIGHBORS CHAP. 
work, and the salmon to climb or leap up water- 
falls, the ascent of which excites our amazement. 
Alligators, crocodiles, and aquatic lizards, such as 
those of India and Egypt, have little other means 
of. progress under water, yet they are powerful 
swimmers; the Nile monitor, in fact, can swim 
much faster than young crocodiles of its own size, 
of which it captures and devours large numbers, 
by reason of the vertical flattening of its tail. 
The profound diving of a whale, the follow-my- 
leader bounding play of the porpoise and dolphin, 
and the impetus for soaring gained by the flying- 
fish, are all due to the propulsion of the tail, the 
principle of which is embodied in the two-bladed 
propellers of our swift steamships. Even some of 
the diving-birds make their way under the surface 
by closing their wings and sculling the short and 
stiff feathers of the tail, though other diving-birds 
paddle with their wings under the water just as 
they fly in the air. 
In all these flying and swimming creatures, not 
only birds and fishes, but the marine mammals and 
the flying quadrupeds, the tail is a rudder, as well 
as a propeller and balance. This is easily observa- 
ble not only in the flight of any bird, but in that of 
the flying and leaping squirrels; and no doubt it 
is an essential part of the apparatus for flight pos- 
sessed by these animals, — including the checking 
and controlling of the speed, as observation of a 
bird passing or alighting will quickly show; while 
