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Mammalian Trypanosomes of Africa 
from the other polymorphic mammalian trypanosomes chiefly by its power 
to survive in man. The speculations which arose concerning this trypanosome 
were mainly directed, however, towards elucidating the factors determining 
the spread of trypanosomiasis through native communities—rather than 
towards the explanation of the origin of the trypanosome itself and its rela¬ 
tions to allied “species.” With the appearance on the scene of T. rhodesiense, 
two suggestions regarding its origin were put forward. Bruce and his co¬ 
workers in Nyasaland, and Kinghorn and Yorke, held that the newly recog¬ 
nised human trypanosome was merely a race or variety of the widely dis¬ 
tributed T. brucei —a view to which I have always subscribed. The German 
investigators, on the other hand, held that T. brucei and T. rhodesiense were 
distinct species; and lately Taute has published interesting inoculation experi¬ 
ments on man which, he considers, support the German view. 
In nature the great majority of mammalian trypanosomes are transmitted 
by the agency of insects, of which the most important are the Glossinae. The 
fly can transmit the parasite from mammal to mammal by two methods, 
namely (a) the direct, and (b) the indirect or cyclical. Direct transmission 
consists in the mechanical transference of essentially unaltered trypanosomes 
from host to host; and it may occur when a fly is disturbed in the act of feeding, 
and completes its meal—within a sufficiently short time—on an adjacent 
animal. Cyclical transmission, on the other hand, involves complicated de¬ 
velopmental changes of the trypanosome inside the fly; and during the earlier 
stages of this development the insect is not capable of transmitting the 
parasite to another mammal— i.e., it is not infective. 
There is now experimental evidence to show that cyclical development 
in the fly exerts a steadying influence on the trypanosome, checking tendencies 
towards variation and keeping it true to type. Moreover, there is every reason 
to believe that, under undisturbed natural conditions, the cyclical is by far 
the commoner and more normal method of transmission, in the case of the 
great majority of the mammalian trypanosomes of Africa. 
It thus seems clear that the continued maintenance of a strain of try¬ 
panosomes by direct inoculation with the syringe, under unnatural climatic 
conditions and in hosts which are very different from those of their natural 
environment, tends to encourage the development, in course of time, of 
varieties or races differing from the original type. Such artificially propagated 
strains may, and probably do, lose their power to survive in their normal 
insect intermediary—the very power, that is to say, which is necessary for 
their survival and perpetuation in nature. Moreover, it is clear that these 
artificially produced “ strains ” cannot be regarded as “ species ” in the orthodox 
zoological sense of the term. Zoologists are agreed that “ good " species are 
characterised by their morphological—not merely physiological—peculiarities. 
If two organisms are structurally identical, at all stages in development, they 
belong to the same species. But if they display—in addition to such structural 
identity—differences which are confined to certain physiological features, then 
