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But the debt is not all on one side. When the ''economic" ento- 

 mologist has discovered the chief death-factors of an insect, and, 

 by introducing them into another region produces the same con- 

 dition as exists in the original habitat, the evolutionist must 

 take these facts into consideration, and not place the whole bur- 

 den of the struggle for existence upon some more conspicuous, 

 but less-important, factors. 



The investigation of the various death factors which make 

 up the struggle for existence of a species, and the transportation 

 of certain of them to a new locality, iiaturally leads one to con- 

 sider what part in natural selection they play. It is in the 

 hope of turning attention towards certain aspects of these com- 

 plex problems that I bring before you the following notes on : 



The Effect of Parasitism on the Struggle for Existence and 

 Natural Selection. 



Darwin laid great stress upon the severe competition among 

 closely allied organisms. These animals, living under the same 

 conditions, and upon the same food, are brought into closer com- 

 petition than those having diiferent habitats and food. Among 

 the higher animals an active, physical struggle is presumed to 

 take place, while among the lower animals this struggle is pre- 

 sumed to be passive. 



Among phytophagous insects it is difficult to follow all the 

 stages of this competition, for there is never a direct struggle, 

 and only on rare occasions, and as an abnormal phenomenon, is 

 there a shortage of food which causes a direct competition. 



The phenomenon familiar to every field entomologist, of 

 two or more closely allied species of equal fertility, and living 

 under similar conditions, standing in vastly different numerical 

 ratio to one another, is bound up with this question of competi- 

 tion. If we study these allied species separately it is very diffi- 

 cult to find a reason for this numerical difference ; but if they 

 be studied as a group then the cumulative effect of the various 

 death factors acting upon them, some of which, taken separately, 

 may appear insigiiificant, may appear as a sufficient reason. 



An allied phenomenon, of an introduced insect supplanting 

 a native allied species, of which Pieris rapae in Canada is a 

 good example, is also connected with the same question of the 

 struggle for existence. In some such cases it is possible that 

 the intruder upsets the balance of parasitism, and thus brings 

 about the reduction or extermination of the native species. 



