ZOOLOGY AND HUMAN WELFARE 



organization of government men is arrayed against it. 

 Heavy poisoning gets some of the adults, and thorough 

 cultivation of the soil may reduce the number of living 

 grubs; but these measures are not very successful, since 

 strong poison injures many plants and since a good deal 

 of ground is not open to cultivation. In this case, again, 

 stringent quarantine is being attempted, to prevent the 

 spread of the pest, and great efforts are being made to 

 discover or invent some improved means of control. 



The reason why introduced species often become so 

 important as destroyers of w^ealth is because they leave 

 most of their natural enemies behind w^hen they emigrate. 

 Whether the animal is taken to a new region by human 

 agency (on purpose or accidentally), or whether it migrates 

 of its own accord, it may find the new region suitable in 

 climate and other living conditions, while being devoid 

 of such parasites, predators, and diseases as serve to keep 

 its numbers down in its original area of distribution. 

 Native species often multiply and become economically 

 important because a suitable food plant is extensively 

 cultivated by man, and thus life is made easy; but in such 

 cases a more or less equivalent increase in natural enemies 

 will usually occur. This is not so likely with introduced 

 forms. 



Realization of these ecological factors has led to a new 

 method of control, now in active development. If the 

 parasites have been left behind, why not search them out 

 and bring them again into contact with their prey in the 

 new region? It is at once evident that a good deal of 

 strictly scientific zoological work is called for in carrying 

 out this idea. The efforts of experts in classification, field 

 work, breeding and rearing, embryology, and behavior, 

 may be required in combination, and over long periods of 

 time, before a foreign species can be discovered, introduced, 

 reared in numbers, and established as an efficient enemy 



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