INSECTS INJURIOUS TO VEGETATION. 387 



researches into insect parasitism, but it is quite unnecessary to go beyond the relations 

 of the human species to its own parasites. 



The parasitic micro-organisms which cause painful and fatal diseases in man are 

 generally considered as thoroughly obnoxious and the natural enemies of the human 

 species in all circumstances. But it is also, and impressively, true that men of different 

 races may be the most inveterate of mutual and natural enemies, and as long as such 

 enmitv characterises the relations between different races of mankind, the parasitic 

 cneniv of one race may easily be the friend of another race, if the two races differ in 

 degree of immunity. This is shown by the efforts of the European race to extend its 

 range and influence into America and Africa respectively. This race was host to 

 numerous species of micro-parasites, many of which were restricted in their 

 geographical distribution to the eastern hemisphere. These parasites were introduced 

 into America by the early colonists, who were to a measurable extent immunized. 

 The native race was less immune and the population was decimated by the diseases 

 which the introduced parasites caused. The defence of the North Americans against 

 the Europeans was correspondingly weakened ; they had to fight against European 

 men, and in addition against a host of the parasites of European man ; the odds were 

 too great, and they were easily conquered and practically exterminated. 



In Africa, however, these conditions are reversed, and it is the native race which is 

 immunized to parasitism by species of micro-organisms to which the European races 

 are susceptible. In consequence the native Africans are not called upon to fight 

 in the defence of their country ; much less to fight against such overwhelming odds 

 as were imposed upon the North Americans. Their parasites are fighting with them 

 against European invasion even more effectively than the parasites of the European 

 invaders fought with them against the aborigines in North America. Until these 

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



In such circumstances it cannot be said that the human parasites which cause such 

 diseases as malaria and yellow fever are the natural enemies of the human species 

 without specifying more particularly which race or variety of the human species is 

 meant, and the conditions and circumstances of the time and place. They are inj urious 

 and often fatal to individual men and women of both European and African races, 

 and might be considered as entirely obnoxious to the African races were all chance 

 of European invasion to be completely eliminated. But they are potentially friends 

 as long as this danger remains even remotely existent, and they are actually and 

 actively friendly to the more immune race whenever it comes into competition with 

 one more susceptible, but otherwise stronger. 



It was just this point about competitors which was overlooked by Dr. Harris, and 

 which has been overlooked by many of his successors, in spite of the fact that it is so 

 strongly emphasized in the Darwinian hypothesis. In the " struggle for existence " 

 Dr. Harris saw a conflict between the evil " noxious tribes " of insects, and the good 

 " beneficial " insect. To us it means something very different, and it is recognised 

 as being waged no less actively between competitors than between predator and prey, 

 or between parasite and host. Competitors are natural, mutual, and often the most 

 inveterate of enemies. The enemy of an enemy is very likely to be a friend. There- 

 fore competitors camiot well possess a mutual enemy, for if it is really an enemy to 

 either it is verv likelv to be a friend to the other. 



