372 THE GEOGRAPHICAL DISTRIBUTION OF ANIMALS 



As a result of this intense struggle, an animal may be 

 crowded across a barrier into a region entirely new to it, 

 where the conditions are so extremely favorable for its 

 growth and increase that it may become strong enough and 

 numerous enough to actually drive out or kill another 

 closely allied species existing in that region. For example,, 

 the black rat was introduced into America from Europe 

 about 1544. The conditions here were so favorable that 

 it increased in such numbers as to aid very materially in 

 crowding out and annihilating our species of native rats. 

 Later, in 1775, the brown rat was brought over to America. 

 It, in turn, throve prodigiously, and practically exterminated 

 the black rat, but remains as our common pest of barns and 

 outbuildings. Hence one result of the distribution of ani- 

 mals is the extermination of species. 



Another result of the distribution of animals may take 

 place that will be exactly the opposite of the one just de- 

 scribed. Suppose a short-haired, semi-tropical animal 

 living in Mexico should be forced by degrees northward into 

 Canada. One must bear in mind that this forced migration 

 must be very slow and must extend over a long period of 

 time. It is conceivable that, as this species went north- 

 ward, the hair of the individuals of successive generations 

 would gradually grow longer and longer to adapt them to the 

 changed conditions of the climate. By the time the animal 

 reached Canada it might be so changed in regard to its 

 hair and organs of the body as to have become a distinct 

 species. In other words, when an animal is forced across 

 a barrier into a new region, the process of adaptation to sur- 

 roundings and of natural selection may produce an en- 

 tirely new species. Hence a second result of the distribu- 

 tion of animals is the formation of new species. 



