146 THE KINGDOM OF MAN 



fly and the disease was as beautiful a piece of scientific 

 investigation as biologists have ever seen. A curious and 

 very important fact was discovered by Bruce namely, 

 that the native big game (zebras, antelopes, and probably 

 buffaloes), are tolerant of the parasite. The Trypanosoma 

 grows and multiplies in their blood, but does not kill 

 them or even injure them. It is only the unaccustomed 

 introduced animals from Europe which are poisoned by 

 the chemical excreta of the Trypanosomes and die in 

 consequence. Hence the wild creatures brought into 

 a condition of tolerance by natural selection and the 

 dying out of those susceptible to the poison form a sort 

 of ' reservoir ' of deadly Trypanosomes for the Tsetze 

 flies to carry into the blood of new-comers. The same 

 phenomenon of ' reservoir-hosts ' (as I have elsewhere 

 called them) has since been observed in the case of 

 malaria ; the children of the native blacks in Africa 

 and in other malarious regions are tolerant of the malarial 

 parasite, as many as 80 per cent, of children under ten 

 being found to be infected, and yet not suffering from 

 the poison. This is not the same thing as the im- 

 munity which consists in repiihion or destruction of the 

 parasite. 



The Trypanosomes have acquired a terrible notoriety 

 within the last four years, since another species, also 

 carried by a Tsetze fly of another species, has been dis- 

 covered by Castellani in cases of Sleeping Sickness in 

 Uganda, and demonstrated by Colonel Bruce to be the 

 cause of that awful disease. 1 Over 200,000 natives of 

 Uganda have died from it within the last five years. It 

 is incurable, and, sad to relate, not only a certain number 

 of European employes have succumbed to it in tropical 

 Africa, but a brave young officer of the Army Medical 

 1 See the next chapter, devoted to this subject. 



