GEOGRAPHICAL DISTRIBUTION OF ANIMALS 993 
creases in abundance, injuring fruit trees and grain ficlds. 
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 Dlinois 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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