THE PRESIDENT'S ADDRESS. 443 



is an indirect consequence of the occupation of the African con- 

 tinent by European Powers. Formerly the native tribes were 

 constantly at war with one another, and a negro never travelled 

 any great distance from his own village. Now caravans move in 

 everv direction, and doubtless in this way the disease has been 

 spread by porters and other natives already infected with the 

 trypanosome coming into regions where tsetse-flies abound, and 

 there infecting the flies, which in their turn have disseminated 

 the infection amongst the previously healthy population. 



Although it was proved experimentally that the disease is 

 propagated by tsetse-flies, the exact method by which this is 

 effected has remained hitherto somewhat mysterious. It was 

 proved that the infection could be conveyed by what may be 

 termed the direct mechanical method ; that is to say, that if a 

 fly has sucked recently the blood of an infected person, its pro- 

 boscis may contain living trypanosomes, and if it inserts its 

 proboscis, immediately or in a short time afterwards, into the 

 skin of a healthy person, it may convey the infection simply by 

 means of its contaminated proboscis. Experiments showed that 

 infection in this direct manner only took place up to forty-eight 

 hours after the fly had fed on the infected subject, and all 

 attempts to obtain infection with flies at a longer interval than 

 forty-eight hours gave negative results. Experimental evidence 

 was therefore lacking for the existence of a developmental cycle 

 of the parasite in the fly, although it was argued by many writers 

 that for various reasons such a cycle must exist. Quite recently, 

 however, a positive result has been obtained by Prof. Kleine, 

 Director of the German Sleeping Sickness Commission in German 

 East Africa. Experimenting with nagana by feeding a batch of 

 flies first on an infected animal and then on a long succession of 

 healthy animals, he has made the most interesting and important 

 discovery that the flies are not infectious at all until some three 

 weeks after their first feed, and that then they infect every 

 animal upon which they are fed. This result indicates that the 

 incubation-period — that is to say, the time occupied by the parasite 

 in its cycle of development in the fly — is far longer than any one 

 had suspected, and that the negative results of former investi- 

 gators are to be explained by their experiments not having been 

 extended over a sufficiently long period. It must be borne in 

 mind that to those working in Tropical Africa it is often difficult 



