In 1902 Dutton described a trypanosome, _T. gambiense_, which he and Forde had found the year before in the blood of a patient suffering from a peculiar type of fever in Gambia. In 1902-1903 Castellani found the same parasite in the cerebro-spinal fluid of sleeping-sickness patients and definitely reported it as the causative organism of the disease. His work soon found abundant confirmation, and it was discovered that the sleeping sickness was but the ultimate phase of the fever discovered by Dutton and Forde. When Castellani made known his discovery of the trypanosome of sleeping sickness, Brumpt, in France, and Sambon, in England, independently advanced the theory that the disease was transmitted by the tsetse-fly, _Glossina palpalis_. This theory was based upon the geographical distribution and epidemiology of the disease. Since then it has been abundantly verified by experimental evidence. Fortunately for the elucidation of problems relating to the methods of transfer of sleeping sickness, _Trypanosoma gambiense_ is pathogenic for many species of animals. In monkeys it produces symptoms very similar to those caused in man. Bruce early showed that _Glossina palpalis_ "fed on healthy monkeys eight, twelve, twenty-four and forty-eight hours after having fed on a native suffering from trypanosomiasis, invariably transmitted the disease. After three days the flies failed to transmit it." In his summary in Osler's Modern Medicine, he continues "But this is not the only proof that these flies can carry the infective agent. On the lake shore there was a large native population among whom we had found about one-third to be harboring trypanosomes in their blood. The tsetse-flies caught on this lake shore, brought to the laboratory in cages, and placed straightway on healthy monkeys, gave them the disease in every instance, and furnished a startling proof of the danger of loitering along the lake shore among those infected flies." As in the case of nagana, Bruce and most of the earlier investigators supposed the transmission of the sleeping sickness trypanosome by _Glossina palpalis_ to be purely mechanical. The work of Kleine (1909) clearly showed that for _Trypanosoma gambiense_ as well as for _Trypanosoma brucei_ the fly served as an essential host. Indeed, Kleine and many subsequent investigators are inclined to think that there is practically no mechanical transmission of trypanosomes from animal to animal by _Glossina_ in nature, and that the many successful experiments of the earlier investigators were due to the fact that they used wild flies which already harbored the transformed parasite rather than directly inoculated it from the blood of the diseased experimental animals. While the criticism is applicable to some of the work, this extreme view is not fully justified by the evidence at hand.