442 ELEMENTARY ZOOLOGY 



that "habitat" is not necessarily coincident with "only 

 fit region." Shad, striped bass, and catfish from the 

 Potomac River have been introduced into and now thrive 

 in the Sacramento River in California. In fact the whole 

 work of the introduction and diffusion of valuable food- 

 animals in territory not naturally included in the habitat 

 of the species is based on our knowledge that the habitat 

 of a species is often determined by physical barriers rather 

 than by exclusive fitness of environment. Within the 

 natural habitat the environment is fit for the species' 

 existence, outside of it the environment may be fit. 



But there occur numerous instances where a species 

 successful in leaving its orignal habitat is unsuccessful in 

 attempting to maintain itself on new ground. Man has 

 introduced various animals from one country to another. 

 The English sparrow (naturally debarred from this country 

 by the ocean barrier), brought to America from Europe, 

 has covered its new territory rapidly and maintains itself 

 with brilliant success. But the nightingale, the starling 

 and skylark which have been repeatedly introduced and 

 set free are unable to maintain themselves here. 



Species-extinguishing and species-forming. Accom- 

 panying the constant slow migrating of species into new 

 habitats and the constant slow changing of environment 

 and conditions everywhere is to be seen a constant modi- 

 fication of the fauna of any region due to the inability of 

 some species to maintain their ground, the predominating 

 increase of others, and the modifying or adaptive chang- 

 ing of others into new forms. In 1544 the black rat of 

 Europe was introduced into America and it soon crowded 

 out the native rats, being in its turn crowded out by the 

 European brown rat (the present common rat in buildings), 

 introduced about 1775. Here we have the original native 

 species unable to maintain itself in competition with in- 

 troduced forms. 



