424 ELEMENTARY BIOLOGY 



respect to any species, he must proceed cautiously and on the 

 basis of a thorough knowledge of all the relations of the species 

 concerned, and not merely on the basis of a superficial answer 

 to the question, Is that species useful or harmful to me ? It 

 is practically impossible to reduce the numbers of one species 

 without producing far-reaching effects upon other species. It is 

 not a matter of interfering with nature's plans, as some sup- 

 pose. It is a matter of disturbing a certain balance that it has 

 taken a long time to establish, with possibilities of unknown 

 consequences. 



When rabbits were introduced into Australia, they multiplied so 

 rapidly that before many years they became a real pest, and the 

 government offered bounties for their extermination. Here was a 

 region admirably fitted for the life of these animals, and until man 

 interfered there had been no such animals there. The same kind of 

 thing happened with the introduction of the water cress into New 

 Zealand, and with the introduction of the English sparrow into the 

 United States. Probably these organisms throve better in the new 

 surroundings because they did not here meet their old enemies. 

 These facts should help us to realize how closely dependent upon 

 one another the various species of plants and animals are. 



