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not only refrains from injuring and weakening its host, but wliicli in 

 some manner serves to benefit it. One species of organism is injurious 

 or beneficial in its relation to another species only in accordance with 

 the circumstances and conditions prevailing at the time and place 

 which brings them into contact. Thus, the parasitic micro-organisms 

 which cause disease in man are generally considered to be obnoxious 

 and the natural enemies of the human species in all circumstances, but 

 so long as enmity characterises the relations between difierent races 

 of mankind, the parasitic enemy of one race may easily be the friend 

 of another race, if the two races differ in degree of immunity. In 

 America, for example, where many European parasites were introduced 

 with the early colonists, the native race, being less resistant than the 

 partially immunised Europeans, fell ready victims to the diseases 

 introduced by the parasites ; this weakened their defence and they 

 were easily conquered and practically exterminated by the combined 

 forces of parasites and men. In Africa, on the other hand, conditions 

 are reversed, and it is the native race which is immunised to parasites 

 to which the European races are susceptible ; until these parasites 

 have been conquered Africa will remain in possession of the Africans. 



In both these instances, and there are many more, the parasites 

 are actually and actively friendly to the more immune race whenever 

 it comes into competition with one more susceptible but otherwise 

 stronger. It is this question of competition which was overlooked by 

 Dr. Harris and has been overlooked by so many modern entomologists, 

 although it is so strongly emphasised in the Darwinian hypothesis. 

 The struggle, which Dr. Harris looked upon as a conflict between 

 noxious insects causing injury and beneficial insects working for good, 

 is now recognised as a no less active struggle between competitors 

 than between predator and prey or between parasite and host, and 

 when natural enmity of a parasite towards its competing hosts is 

 unequally displayed, the parasite may well be the real enemy of the 

 one and the real friend of the other. 



This principle applies to the inter-relations between all plant and 

 animal species and therefore to the relations between insects and their 

 plant hosts on the one hand and their parasites and predatory enemies 

 on the other. It explains also such obscure phenomena as the 

 disappearance of the American cabbage butterfly throughout large 

 areas following the introduction of a European competitor. Other 

 examples were noticed in connection with attempts to introduce into 

 America the parasites and " natural enemies " of gipsy and brown-tail 

 moths. To quote an example among trees, the gipsy moth 

 [Lymantria dis]jar] attacks both oak and pine, but the former more 

 severely than the latter. Wliere the two trees are competitors for 

 space, as in New England, the pine trees, though injured to some 

 extent, have benefited through the oaks having been injured in a 

 proportionately greater degree. Vegetation, in fact, has not been 

 injured by the gipsy moth ; certain plants and trees only have been 

 injured to the benefit of their competitors. Phytophagous insects as 

 a class cannot therefore be considered as generally injurious rather 

 than beneficial to green plants as a class, or to vegetation. Such an 

 assumption accords neither with fact nor with the theory of natural 

 evolution and natural control of species. With a few exceptions, such 

 as the migratory locust and other very indiscriminate feeders, insects 



