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the same will probably obtain elsewhere, there is more than one 
species of Trypanosoma, and though the effects towards some 
animals—cattle—are the same, the action on others -goats and sheep 
—is different. May this, too, not account for many of the 
discrepancies in travellers’ reports? For example, the opinions 
regarding the susceptibility of the donkey are very diverse ; we found 
that this animal took one of the Rhodesian trypanosomes, whilst a 
second donkey did not become affected with the other form of the 
germ which occurs there. 
‘ In many countries, and districts of countries too, where trypano¬ 
somiasis is rife, the amount of game is negligible. From whence 
does the virus, the causa causans, then arise ? When the conditions 
are examined it is found that there is always at least one species of 
domestic animal which is more resistant than another : it may never 
be sick; or if it does take the disease it may live for two or three 
years like the camel with the Indian fly disease, and it may even 
recover. In South America, fly disease is seen chiefly in the horse, 
and cattle rarely suffer, but they, in common with the camel of India, 
take the place of the non-existent game, and act as a reservoir which 
under the required condition can issue supplies of trypanosome to 
all comers ; causing death in those susceptible and converting the 
others into additional reservoirs. 
‘ But what is the required condition without which the presence 
of a reservoir is not greatly to be feared ? Bruce showed it to be a 
biting fly. In Africa travellers blamed the tsetse ; in India natives 
incriminate the "horse fly" {Tabanidae), known in Rhodesia 
as the “ hippo fly,” and a smaller variety locally called " blind fly ” 
(genus Haematopota ) ; in South America and parts of Asia the 
“ stinging house fly ” (genus Stomoxys) is blamed. 
‘ With few exceptions no one in South or Central Africa has even 
suggested the possibility of any fly other than tsetse causing the 
disease; and since the time when Bruce proved the connection 
scientifically, what was at first a mere suspicion influenced by native 
tales has become a conviction, to the exclusion of all other flies. It 
is not necessary to look far for the reason. Stock owners knew they 
could keep their animals around the farmstead and in certain areas 
with impunity, even though horse flies and the stinging house flies 
were present. Compared with the tsetse these other flies are 
