G. H. F. Nuttall 
277 
as of importance in the spread of infection, the general trend of opinion 
to-day is to regard such transference as possible but of slight significance 
in the epidemiology of trypanosomiasis. 
Owing to the widespread interest evoked by recent advances in 
tropical medicine, a large number of observers in all parts of the world 
have devoted much attention to the study of microscopic parasites 
occurring in the blood of vertebrates. The result has been that we 
now know of a vast number of hosts which harbour trypanosomes 
in their blood, and the literature relating to new species of these 
haematozoa has grown to be one of considerable magnitude. The general 
tendency has been to consider each species of animal as the carrier 
of a species of trypanosome peculiar to itself, this being doubtless 
due in part to the supposedly specific character of the adaptation shown 
by the parasite to its host. For a long time, one of the best known 
trypanosomes, T. lewisi, was regarded as peculiar to rats, and it is only 
recently that it has been shown to be also capable of living parasitically 
in other rodents. We know now that certain species of trypanosomes 
possess a wide range of pathogenicity, such forms as T. brucei, 
T. gamhiense, T. evansi, and others being pathogenic for many different 
species of mammals. In some cases, morphological characters suffi¬ 
ciently differentiate the species, but our confidence in some of these 
characters has been shaken since we know that a species of trypanosome 
may alter its appearance in changing its host. Whereas the immunity 
reactions have been used to differentiate species of trypanosomes, 
a means of distinguishing species to which no zoologist will agree, we 
now know that this means of differentiation cannot be relied upon 
since the virulence (an obscure character) of the parasites can be 
considerably modified experimentally. 
Of the discoveries which have been made of recent years with 
regard to trypanosomiasis, those which concern the mode of infection 
by blood-sucking ectoparasites unquestionably spring into prominence 
both because of their great scientific interest and their great practical 
bearing upon preventive measures designed especially to protect men 
and the more valuable domesticated animals against trypanosome 
infections. Sleeping sickness and Nagana in Africa are amongst the 
most deadly diseases known, and I shall commence by outlining what 
we know to-day regarding the way in which they are spread. 
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