36 STRUGGLE AMONGST ANIMALS 



another and encroaches on its ground " we feel sure 

 that the cause lies as much in one species being 

 favoured as in another being hurt." Natural selec- 

 tion comes about by " the preservation of favoured 

 races " rather than by the extermination of one race 

 by another. 



A curious case of the struggle for existence between 

 species apart in systems of classification, is found 

 in the relation between parasites and their hosts. 

 Protozoa, round worms, thread worms, flat worms 

 and flukes, and many kinds of insects and 

 arachnids, infest the bodies of vertebrate animals 

 as internal or external parasites. We know that 

 serious damage may be done to individuals and 

 species by such parasites, either directly by in- 

 juring the tissues or blocking the blood-vessels, 

 or in a more insidious way by conveying to them 

 the seeds of disease. Here we might expect to 

 find a salient example of the ruthless stress of the 

 struggle for existence between species. None the 

 less there are limits to the severity even of this 

 infliction. My friend Professor Minchin has shown 

 that it can seldom or never be to the advantage of 

 a parasite to kill its host. When a parasite is fatal 

 to its host, as in the case of the trypanosomes that 

 cause one of the forms of sleeping sickness in Africa, 

 he infers that the parasite is a recent intruder. 

 Parasite and host have each to be modified to accom- 

 modate one another, and, unless both are to perish, 

 the result of the struggle for life that each makes, is 

 that in course of time the parasite becomes trans- 

 formed into a relatively or absolutely harmless 

 messmate. 



