438 ANIMAL STUDIES 



346. Species debarred by inability to maintain their ground. 



Examples of the second class are seen in animals that 

 man has introduced from one country to another. The 

 nightingale, the starling, and the skylark of Europe have 

 been repeatedly set free in the United States. But none of 

 these colonies has long endured, perhaps from lack of adap- 

 tation to the climate, more likely from severity of competi- 

 tion with other birds. In other cases the introduced species 

 has been better fitted for the conditions of life than the 

 native forms themselves, and so has gradually crowded out 

 the latter. Both these cases are illustrated among the rats. 

 The black rat, first introduced into America from Europe 

 about 1544, helped crowd out the native rats, while the 

 brown rat, brought in still later, about 1775, in turn practi- 

 cally exterminated the black rat, its fitness for the condi- 

 tions of life here being still greater than that of the other 

 European species. 



347. Species altered by adaptation to new conditions. 

 Of the third class or species altered in a new environment 

 examples are numerous, but in most cases the causes in- 

 volved can only be inferred from their effects. One class 

 of illustrations maybe taken from island faunae. An island 

 is set off from the mainland by barriers which species of 

 land animals can very rarely cross. On an island a few waifs 

 of wave and storm may maintain themselves, increasing in 

 numbers so as to occupy the territory; but in so doing 

 only those will survive that can fit themselves to the new 

 conditions. Through this process a new species will be 

 formed, like the parent species in general structure, but 

 having gained new traits adjusted to the new environ- 

 ment. 



To processes of this kind, on a larger or smaller scale, 

 the variety in the animal life of the globe is very largely 

 due. Isolation and adaptation give the clew to the forma- 

 tion of a very large proportion of the " new species " in 

 any group. 



