H. L. Duke 
853 
of far-seeing research into what was then an unopened chapter in Zoology. 
In this work the trypanosome was studied on the spot, its relations to its 
insect and mammalian hosts were thoroughly investigated, and the principle 
of a “natural reservoir” was clearly established. Unfortunately, however, 
the study of mammalian trypanosomes under natural conditions offers con¬ 
siderable difficulties, and it became necessary to transport the numerous 
strains, isolated from time to time in various parts of Africa, to European 
laboratories, where they could be kept up by animal passage and subjected 
to examination by various methods. 
In the course of years many strains of trypanosomes have been collected 
in this manner, and have been perpetuated by direct inoculation from animal 
to animal with the syringe in various European laboratories. To these strains 
various names have been applied by different observers. Numerous tests have 
been devised to aid in the differentiation of the so-called “species” into which 
they have been subdivided, and a great deal of controversy has arisen as to 
the identity of this or that strain. A great stimulus has been given to research 
on this subject owing to the circumstance that the polymorphic mammalian 
trypanosomes have a special importance as producers of disease in man 
himself, as well as in his domestic stock; and also because the susceptibility 
of laboratory animals to infection with these trypanosomes has enabled them 
to be studied under convenient laboratory conditions. Consequently, a great 
mass of literature, dealing with the various “species” of trypanosomes be¬ 
longing to this group, has now been accumulated. 
One of the most striking results of the European work on the polymorphic 
mammalian trypanosomes of Africa, has been the revelation that, in the 
course of years, the laboratory strains of these organisms may undergo great 
changes in morphology, in virulence, and in their immunity reactions. A 
strain of trypanosomes may thus become, after some years of laboratory 
upkeep, very different from what it was originally, when freshly isolated 
from its natural host: and this has led to the splitting up of “species” and 
consequent confusion in classification. 
A further impulse in the direction of splitting up the polymorphic group 
was supplied by the recognition in recent years of the human trypanosomiasis 
of Nyasaland. The parasite associated with this disease was said to be dis¬ 
tinguishable from all other trypanosomes by the possession of “posterior- 
nuclear” forms, and as Trypanosoma rhodesiense it was duly added to the 
list of “species.” Soon afterwards, however, investigators reported the occur¬ 
rence of “posterior-nuclear” forms—and incidentally of “anterior-nuclear” 
forms and other distortions—in other “species” of polymorphic trypanosomes, 
thus adding to the general confusion. 
Until the recognition of the human trypanosome of Nyasaland, the position 
of man in relation to these protozoa offered no special difficulty. Man was 
held to be susceptible to, or capable of infection by, one species of trypanosome 
only—namely, T. gambiense : and, accordingly, T. gambiense was distinguished 
