30 THE IMPORTANCE OF BIED LIFE 



their host to die. Mother Microgaster did her 

 duty well. Not 2 per cent, of the cabbage-cater- 

 pillars escape her inoculation. 



One might think from the above that the par- 

 asitical and predaceous forms alone could cope 

 with the injurious insects, that the additional 

 drafting of birds for the fray is unnecessary. 

 Such would be the case but for two reasons. In- 

 sects multiply at an enormous rate, and the preda- 

 tory species are greatly inferior in number to 

 the crop-damaging kinds. Just as hawks alone 

 are unable to check the spread of birds, so are 

 insects incapable of holding their kindred within 

 bounds. 



When one stops to consider the terrible insect 

 scourges which from time to time have blighted 

 large areas of the earth, he will realize that Na- 

 ture is not as perfect a guardian as she might be. 

 Many of her laws do not work at all where man 's 

 handiwork is concerned. So slow is she at evolv- 

 ing new creations that there apparently has not 

 been sufficient time to produce a formidable antag- 

 onist to the spread of locusts and army-worms. 

 These insects possibly were active cogs in the 

 machinery for limiting the growth of vegetation 

 before the coming of cultivation, but now they are 

 wholly detrimental to all forms of agriculture. 



It is a matter of historical fact that locust 

 plagues invariably are accompanied by a swarm 

 of birds. Species whose natural food is quite 



