GEOGRAPHICAL DISTRIBUTION 315 



As examples of the first class we may take the absence of 

 kingbirds or meadow larks or coyotes in Europe, the absence 

 of the lion and tiger in South America, the absence of the civet 

 cat in New York, and that of the bobolink or the Chinese fly- 

 ing fox in California. In each of these cases there is no evident 

 reason why the species in question should not maintain itself 

 if once introduced. The fact that it does not exist is, in general, 

 an evidence that it has never passed the barriers which separate 

 the region in question from its original home. 



Local illustrations of the same kind may be found in moun- 

 tainous regions. In the Yosemite Valley in California, for ex- 

 ample, the trout ascend the Merced River to the base of the 

 Vernal fall. They cannot rise above this and so the streams 

 and lakes above this fall are destitute of fish. 



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 adaptation to the climate, 

 perhaps from severity of competition with other birds, most 

 likely because the few individuals become so widely scattered 

 that they do not find one another at mating time. In other 

 cases the introduced species has been better fitted for the con- 

 ditions of life than the native forms themselves, and so has 

 gradually crowded out the latter. Both these cases are il- 

 lustrated among the rats. The black rat (Mus rattus), first 

 introduced into America from Europe about 1544, tended to 

 crowd out the native wild rats (Sigmodon) , while the brown rat 

 (Mus decumanus), brought in still later, about 1775, in turn 

 practically exterminated the black rat, its fitness for the con- 

 ditions of life here being greater than that of the other European 

 species. 



Of the third class, or species altered in a new environment, 

 examples are numerous, but in most cases the causes involved 

 can only be inferred from their effects. One class of illustra- 

 tions may be taken from island faunas. 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 may maintain 

 themselves, increasing in numbers so as to occupy the territory, 

 but in so doing only those kinds will survive that can jit them- 

 selves to the new conditions. Through this process new species 



