984 ANIMAL LIFE 
But the reverse of this is often true. There are many cases 
in which a species introduced in a new territory, through 
the voluntary or involuntary influence of man, has shown a 
marvelous adaptation and power of persistence. The rapid 
spread of rabbits and pigs as wild animals in Australia, of 
horses and cattle in South America, avd of the English 
sparrow in North America, of bumble-bees and house- 
flies in New Zealand, are illustrations of this. Not one 
of these animals has maintained itsclf in the wild state 
in its native land as successfully as in these new countries 
to which it has been introduced. The work of, introduc- 
tion of useful animals illustrates the same fact. The shad, 
striped bass, and cat-fish from the Potomac River, intro- 
duced into the Sacramento River and its tributaries by the 
United States Fish Commission, are examples in point. 
These valued food-fishes are nowhere more at home than in 
the new waters where no species of their types had ever 
existed before. The carp, originally brought to Europe 
from China, and thence to the United States as a food- 
fish, becomes in California a nuisance, which can not be 
eradicated, destroying. the eggs and the foodstuff of far 
better fish. 
In all mountain regions waterfalls are likely to occur, 
and these serve as barriers, preventing the ascent of trout 
and other fishes. On this account in the mountains of Cali- 
fornia, Colorado, Wyoming, and other States, hundreds of 
Jakes and streams suitable for trout are found in which no 
fishes ever exist. In the Yellowstone Park this fact is es- 
pecially noticeable. This region is a high volcanic plateau, 
formed by the filling of an ancient granite basin with a vast 
deposit of lava. The streams of the park are very cold and 
clear, in every way favorable for the growth of trout; yet, 
with the exception of a single stream, the Yellowstone 
River, none of the streams was found to contain any fish 
in that part of it lying on the plateau. Below the plateau 
all of them are well stocked. The reason for this is ap- 
