H. L. Duke 
393 
mammalian blood. The occurrence in brucei regions of gambiense- like trypano¬ 
somes such as T. multiforme will also be intelligible when we bear in mind the 
opportunities which occur for the introduction of the gambiense type of 
organism in its human host into morsitans country, and the time which must 
elapse before the change of vertebrate environment can make itself felt on 
the trypanosome. The evidence from morsitans areas seems to be that, until 
recent years, man has been immune to the locally widespread T. brucei , and 
is still so in some areas. Similarly, negative evidence in Uganda points to 
the conclusion that the exposure of natives, at the present day, to the palpalis 
of the lake-shore prohibited area, does not result in man becoming infected 
every time that he is bitten by a cyclically-infected fly. The introduction of 
a number of human beings into the fly area might well lead, however, to the 
gradual acquisition by the trypanosome of powers to survive in these hosts. 
This seems to have happened in Rhodesia and in Nyasaland, and the same 
thing may happen on Sesse unless special precautions are taken to prevent it. 
(26) There is another line of speculation which, however grotesque it may 
appear to those who believe in a multiplicity of species among the poly¬ 
morphic trypanosomes, at any rate suggests a line of practical investigation 
to which the literature at my disposal makes no reference. The polymorphic 
mammalian trypanosomes are parasites for whose propagation in nature a 
long sojourn in the blood-host (mammal), with consequent maximum exposure 
to the insect intermediary, is desirable—if not essential. Trypanosomes tend 
to become adapted to their blood-hosts in such a way that they assume the 
character of harmless guests rather than pathogenic parasites. An acute and 
fatal disease of the host is detrimental to the parasite. Those mammals, 
therefore, which display true tolerance to a particular trypanosome may be 
regarded as its natural hosts. 
For the main groups of pathogenic African trypanosomes the tolerant 
hosts are as follows: 
(1) T. vivax group ... ... game, and possibly stock. 
(2) T. nanum group ... ... game, and possibly stock (cf. below). 
(3) Polymorphic group: 
(a) capable of survival in 
man— T. gambiense . .. man (in endemic areas) stock and game. 
(b) incapable of survival in 
man— T. brucei ... game, and possibly stock. 
Man and game are antagonistic to some extent, for big-game country is not 
suited to extensive human occupation; that is to say, large numbers of game 
animals and large native populations are, under primitive conditions, mutually 
exclusive. 
(27) The two principal game tsetses, G. morsitans and G. pallidipes, in¬ 
habit as a rule country where man would play but a minor part as a food 
animal. On the other hand, man, in primitive Africa, was probably often 
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