His second test inquiry consisted in ascertaining whether the tsetze flies of Uganda are actually found, experimentally, to be capable of carrying the trypanosome from one infected person to another. For this purpose it was necessary to make use of monkeys, certain species of which were ascertained to be liable to the infection of the sleeping sickness trypanosome when this was introduced by means of injection through a syringe. Such monkeys were found to develop the chief symptoms of sleeping sickness, and ultimately died of the disease, their cerebro-spinal fluid being invaded by the parasites. Accordingly it was possible to use monkeys as test animals. It was found by Colonel Bruce that tsetze flies (_Glossina palpalis_) which had been made to bite infected negroes could carry the infection to the monkeys; and it was also found that even when a number of tsetze flies, not specially prepared, were allowed to bite a monkey, the latter eventually developed the trypanosome in its blood and cerebro-spinal fluid, thus showing that the tsetze flies, as naturally occurring in the country around Entebbe, contain many of them, the trypanosome ready to pass from the fly to a human or simian victim, when casually bitten by the fly. Experiments such as these of infection by the fly, and the use of monkeys in the research, require very great care; and it is quite reasonable to ask that they shall be repeated and most carefully checked before they are considered as demonstrative and absolutely certain. It may, however, be considered as practically certain that the sleeping sickness is due to the presence in the cerebro-spinal fluid of quantities of a minute parasite, the _Trypanosoma Gambiense_, which is carried from man to man by the _palpalis_ tsetze fly, which sucks it up from the blood of an infected individual and conveys it to previously uninfected individuals. The natives in Uganda lie about and sleep under the shade of trees where the tsetze flies are especially abundant; and they are quite indifferent to the bites of flies of one kind and another. It is the dislike to the mere touch of a fly, still more to its bite, which has protected Europeans almost entirely from the sleeping sickness. Unfortunately there is no immunity for Europeans in the matter; and the existence of half a dozen or more cases of white people infected with the trypanosome, who have ultimately died in England or elsewhere in Europe from sleeping-sickness contracted through the bite of a fly in Africa, is abundant proof that there is not, as has been supposed, any special freedom from the disease for white people.[24]