84 BIRDS IN THEIR RELATIONS TO MAN. 



would otherwise perish if not eaten, interested, therefore, in 

 establishing a rate of reproduction for itself which will not 

 unduly lessen its food supply. Its interest in the numbers of 

 each species of the group it eats will evidently be the same 

 as its interest in the group as a whole, since the group as a 

 whole can be kept at the highest number possible only by 

 keeping each species at the highest number possible. 11 



Professor Forbes goes on to show that when the rate of 

 reproduction of a parasite is relatively too great it causes 

 fluctuations in numbers which are injurious both to the 

 parasite and its host, and concludes that in a state of nature 

 " the annihilation of all the established enemies of a species 

 would, as a rule, have no effect to increase its final average 

 numbers. 11 



Such being the case where man has not interfered with 

 nature, we have next to inquire to what extent these princi- 

 ples hold good, under the conditions of modern agriculture, 

 for those insects which feed upon cultivated crops. Evidently 

 a chief element of disturbance of the natural order here lies 

 in the enormously increased food supply, an increase so great 

 and so subject to multiplication by man that it is a rare event 

 for an insect to reach its limit. If a crop in a given locality 

 is destroyed by insects, seed from another region is usually 

 planted the following season, so that, while under natural 

 conditions the insect would have been starved out, it is instead 

 given an increased opportunity to develop. In consequence 

 of this, the law that no animal can multiply beyond the limits 

 of its food supply becomes practically inoperative. 



But while this is true of the plant-feeding species, it is not 

 true of the parasite that preys upon it. The latter is still 

 under the operation of the primal food law : when it has 

 reduced the numbers of its host to a point where it must 

 cease to multiply because there are no caterpillars in which 

 to deposit its eggs, man does not step in and furnish a supply 

 of caterpillars to keep up the activities of the parasite. Con- 



