GEOGRAPHICAL DISTRIBUTION OF ANIMALS 293 



creases in abundance, injuring fruit trees and grain fields. 

 It is then necessary to pay for its destruction also. 



To destroy hawks or owls because they catch chickens 

 may increase the numbers and destructiveness of field-mice 

 on which they also prey. To shoot robins, linnets, and 

 other birds that destroy small fruits is likely to increase 

 greatly the insect pests on which these birds also feed. 

 The inter-relations of species and species are so close that 

 none should be exterminated by man unless its habits and 

 relations have been subjected to careful scientific study. 

 Still less should any new ones -be introduced without the 

 fullest consideration of the possible results. For example, 

 the mongoose, a weasel-like creature, was introduced from 

 India into Jamaica to kill rats and mice. It killed also the 

 lizards, and thus produced a plague of fleas, an insect which 

 the lizards kept in check. The English sparrow, intro- 

 duced that it might feed on insects inhabiting shade-trees, 

 has become a nuisance, crowding out better birds and not 

 accomplishing the purpose for which it was brought to the 

 United States. 



To most kinds of animals a mountain range must act as 

 a barrier to distribution. In a region having high moun- 

 tains a species will become in time split up into several, 

 because the individuals in one valley will be isolated from 

 those of another. The fauna of California furnishes many 

 illustrations of this, as among its mountain chains are 

 many deep valleys shut off from each other and having 

 different peculiarities of temperature. For this reason two 

 counties of California differ much more widely in their 

 fauna than do two counties in Illinois. But Illinois as a 

 whole has more different kinds of animals than California, 

 because no barrier anywhere prevents their entrance. The 

 State has, we may say, its doors wide open to immigrants 

 from all quarters. The same is true of Iowa or of Kansas 

 or Kentucky. Illinois has a richer fauna than Iowa, be- 

 cause its extension is north and south, and it therefore 

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