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produced by sojourn in different species of Glossina. (It is conceivable, further, 
as Bagshawe has suggested, that climate may have some effect upon the 
physiological characters of the parasites; but at present this factor is too 
problematic to be discussed with profit.) As regards the influence exercised 
by the fly, we must note the following points: 
(30) We know that T. gambiense is transmissible by G. morsitans and 
T. brucei by G. palpalis. What we do not know is whether, after a suitable 
number of experimental passages through the normally foreign tsetse species, 
T. gambiense acquires increased and T. brucei diminished virulence to laboratory 
animals; in other words whether the differences between T. gambiense and 
T. brucei are determined by the species of tsetse concerned with the trans¬ 
mission. 
(31) The matter is obviously susceptible of direct experimental proof, 
but before a decision is reached a considerable number of passages must be 
made. The only experimental evidence that I can find consists in {a) some 
experiments by the Nyasaland Commission with a Tanganyika strain of 
T. gambiense and G. morsitans ; (b) the transmission of the Mpumu laboratory 
strain of T. brucei by G. palpalis; and (c) of an East African organism by the 
same tsetse. All these experiments are too limited to justify any definite 
conclusion. 
( а ) In Nyasaland the gambiense strain was cyclically transmitted fiom 
monkey to monkey by laboratory-bred G. morsitans. The particular strain 
employed was inoculated by the syringe into nine other monkeys, of which 
two never became infected, two were alive on the 373rd day, two were alive 
on the 285th day; and, of the three that died, the maximum, minimum, and 
average duration of the disease was 264, 217 and 253 days lespectively. The 
two monkeys infected by laboratory-bred G. morsitans died in 46 and 21/ 
days. Two other Tanganyika gambiense strains were inoculated by the syiinge 
into monkeys. In all, eight animals were employed of which four never became 
infected, two were alive on the 275th day and one on the 373rd day. The 
duration of the disease in the one that died was 31 days. The incubation 
period in this animal, before trypanosomes appeared in the blood, was 23 days, 
suggesting a chronic type of infection, so that the death of the monkey eight 
days later was perhaps due to some other cause. The same objection applies, 
however, to the brief course of the disease in one of the fly-infected monkeys 
(Bruce, 1915). 
(б) The experiments in which T. brucei was transmitted by G. palpalis 
were only three in number, and as far as they go point to no attenuation of 
virulence in the first passage (Fraser, 1912). 
(c) With the brucei- like organism from East Africa, also, no attenuation 
was noticed in the first generation (Duke, 1913 c). 
The German investigators do not state the duration of the disease in the 
monkeys of their experiments, and no attempt was made to transmit beyond 
the first generation. 
