232 DISCOVERY en. 



seized in the course of a few days with fever and wast- 

 ing, and they almost invariably die. Books of African 

 travel are full of records of horses, teams of oxen, and 

 herds of native cattle having been destroyed by the 

 tsetse-fly disease ; and on one occasion a native army, 

 proceeding to the attack of an enemy, was effectually 

 routed by having incautiously crossed fly-country. 



Scientific investigations carried on chiefly by Sir 

 David and Lady Bruce have shown that sleeping-sickness, 

 which has destroyed millions of human beings in Central 

 Africa, is probably spread by the bite of a tsetse-fly 

 closely related to that which causes the fly-disease in 

 cattle. Though the suffering caused by sleeping-sickness 

 has been known for many years, it was not until toward 

 the end of the nineteenth century that a systematic 

 study of its cause was undertaken. It was soon found 

 that the tsetse-fly does not possess a venom of its own, 

 but is the carrier of poison matter. When the fly 

 bites a sick person or animal, it sucks up some of the 

 parasites of the disease. These multiply and persist 

 within the body of the insect, and may be transmitted 

 to every person on whom it feeds during several weeks, 

 and perhaps months. Sleeping-sickness was long sup- 

 posed to be fatal to black races only, but the immunity 

 of the white man from it was disproved by the death 

 in 1907 of Lieutenant Tulloch, who contracted the 

 disease while engaged in investigating it in Uganda, 

 and whose name must be added to the same honourable 

 roll as that upon which the names of Lazear and Myers 

 are inscribed. 



In the case of sleeping-sickness, then, we have a par- 

 ticular insect as the agent for the spread of a particular 

 disease. People who devote attention to the study of 

 insects are usually considered to be concerning them- 



