24 THE PROTOZOA THE DAWN OF LIFE 



up the newly opened trade-roads of the Congo basin and spread 

 through Uganda into British East Africa. 



" In the west of Uganda, since the disease was first noticed, 

 in 1901, more than 200,000 people have died of it. Out of 300,000 

 living on the shores and islands of the great Lake Victoria Nyanza 

 less than 100,000 remain, the rest having perished from sleeping 

 sickness or human trypanosomiasis. In every case the disease 

 has been transmitted from one victim to another by certain species 

 of true flies." l 



The organism which by its presence and multiplication in the 

 blood is the cause of this terrible disease is the Trypanosoma gam- 

 biense. Other Trypanosomes are the cause of various more or less 

 fatal diseases in domestic animals, such as surra, a disease which 

 affects horses in India; nagana or tsetse-fly disease among cattle 

 and horses in South Africa; dourine, which attacks horses and 

 mules in Northern Africa; and the mat de caderas of horses in 

 South America. 



Members of all the chief classes of vertebrates, with the excep- 

 tion of the Cyclostomes 2 (which may prove on further investiga- 

 tion not to be immune), harbour these parasites, mammals, birds, 

 and fishes furnishing the greater number of hosts, though neither 

 amphibians nor reptiles are exempt. 



In the consideration of the occurrence of these Trypanosomes, 

 it is necessary to draw a careful distinction between true or 

 natural hosts and strange or casual hosts, for in the case of the 

 natural host a condition of mutual toleration has been reached, 

 owing to the long-existing association between parasite and host. 

 The entry of a Trypanosome into a host which has never been 

 previously liable to its invasion, however, usually produces serious, 

 if not fatal, results, owing to the casual host being unaccustomed 

 and unadapted to its presence. 



Recent research all points to the probability that every patho- 

 genic or lethal Trypanosome has some tolerant indigenous wild 

 animal host which serves as a latent source of supply from which 

 strange animals entering the district may become affected ; the 



c* ? ron } r my lec T tu re on " Industrial Entomology : The Economic Importance of 

 a Study of Insect Life," delivered before the Royal Society of Arts, May aoth, 1908, 

 and published in the Society's Journal, Vol. Ivi., p. 688. 



The Cyclostomata (literally " round mouths ") are a class of vertebrates separ- 

 ated from the true fishes by certain fundamental divergences. The hag-fish and 

 lamprey are types of the two orders into which the Cyclostomes are divided 



