284 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, and 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 itself 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 

 lakes 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 w^ll stocked. The reason for this is ap- 



