16 NATURAL HISTORY OF SLEEPING SICKNESS 



the house fly, contain in the alimentary canal Flagellates 

 of a type closely allied to Trypanosomes, which live a 

 natural parasitic life in the fly only. 



When the ancestors of the present blood-suckers became 

 addicted to this habit, probably a development of the 

 habit of feeding on the fluid exuding from sores, it may 

 well have happened that some of these internal parasites 

 were inoculated into the blood of the animal. Those 

 that survived may well have caused disease as a result 

 of their vital activities ; if so, by the process of natural 

 selection, those which soonest became adapted to the 

 new conditions and no longer brought about a condition 

 unfavourable to the host, on whose life they would now 

 depend, would have most chance of surviving. 



This theory is supported by some very interesting 

 work done by Fantham and Porter, who showed that 

 Herpetomonas jaculum, a normal inhabitant of the gut 

 of the " Water scorpion " {Nepa cinerea), when introduced 

 into the peritoneal cavity of mice or even taken by them 

 in food eventually finds its way into the blood and causes 

 symptoms like those of " Kala-azar " in man, from which 

 the mouse dies.^ But if the mouse had not died so 

 quickly there might have been developed such immunity 

 to the new substances circulating in its blood as the result 

 of the vital activities of the Herpetomonas that friendly 

 relations between the two would result and mammal 

 and parasite would become"'mutually immune to each 

 other, as appears to be the case with wild game and 

 T. brucei. ^^ 



When man with his non-immune domestic animals 

 comes into relation with this equilibrised system the 

 equilibrium is disturbed ; the Trypanosome finds itself 

 introduced into the circulation of hosts to which it is 

 a novelty and *' disease " results. 



1 Fantham and Porter, Proc. Camb. Phil Soc, 1915, vol. 18, pp. 39-50, 

 137-48. 



