396 
Mammalian Trypanosomes of Africa 
(32) From the foregoing considerations it may therefore be concluded that 
there is at present no certain evidence to prove that the invertebrate host 
determines—or influences—the characters of the trypanosomes which it is 
capable of transmitting; and the evidence supplied by the discovery of a 
virulent strain of polymorphic trypanosomes on Damba is against such a 
supposition. Whether the species of tsetse is a crucial factor in the production 
of the brucei or gambiense type can, however, be put to the test of experiment. 
(33) The arguments briefly developed in the preceding paragraphs have, 
no doubt, presented themselves to other investigators of trypanosomiasis in 
Africa. They are not advanced here as incontrovertible truths, but rather as 
a legitimate working hypothesis which may serve as an incentive to further 
useful research. But the point that I wish to emphasise is that, in any strain 
of trypanosomes, the physiological characters—including ability to infect 
man—are largely determined by the environment. If the arguments here 
advanced are sound, then we are justified in concluding that the polymorphic 
mammalian trypanosomes of Africa all belong to a single species , and not to a 
multitude of species. This species is characterised by its polymorphism in its 
vertebrate hosts, and its anterior station in the salivary glands of its Glossina 
intermediary. Included in this species there are many different varieties or 
strains, distinguishable from one another by characters of minor systematic im¬ 
portance. These strains are not immutable, but variable, and are determined by 
the environment in which the species lives: they are not constant, under varying 
external conditions, but each different strain is dependent upon, and is produced 
as a response to, a particular environment. If we call this species Trypanosoma 
brucei, then “T. gambiense ,” “T. rliodesiensef and “T. nigeriense ” are to be 
regarded as particular strains of T. brucei which have become, after sojourn 
in other hosts, more or less adapted to life in the blood of man. 
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Blacklock, B. (1912). On the Presence of Posterior-nucleated Parasites in a strain of 
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Bruce, D. and others (1910 a). The Development of Trypanosomes in Tsetse Flies. Pro¬ 
ceedings of Royal Society, Ser. B, lxxxii. 369. 
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- (1914 a). The Trypanosomes causing the disease in Man in Nyassaland. Partin. 
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- (1914 6). Infectivity of Glossina morsitans in Nyasaland during 1912 and 1913. 
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