H. L. Duke 
355 
the two organisms are, from the systematic standpoint, “races,” “strains,” 
or “varieties” of a single species. Consequently it appears evident that the 
polymorphic mammalian trypanosomes of Africa should properly be regarded 
as constituting a single species, which is divisible into a number of more or 
less distinct strains or varieties. These strains cannot properly be termed 
“species,” for they do not display those constant morphological differences 
from one another on which the zoological conception of a “species’ is based. 
Now the artificial propagation of trypanosomes has demonstrated that, 
under laboratory conditions, the species or strains tend to split up into sub¬ 
sidiary strains; and these may, if the conditions are kept constant, acquire 
a certain fixity in the course of time. Nevertheless their physiological peculi¬ 
arities, acquired in this manner, obviously cannot be regarded as characters 
which are distinctive of the species from which they were originally derived. 
We are not justified, for example, in using the immunity reactions of laboratory- 
bred trypanosomes as criteria for their specific determination. We cannot 
maintain that the immunological characters of a strain of “ T. brucei ” which 
has been kept for ten years in laboratory animals, under artificial conditions, 
are the standard to which all strains of the species T. brucei must conform. 
We know, indeed, that many of our freshly isolated strains of T . brucei — 
using the name in its historic and valid zoological senses—would not pass 
such a test of “specificity.” 
But if the splitting of a species of trypanosome into strains or races can 
and does occur in the laboratory, then we have some justification for supposing 
that similar splitting may have occurred, and may still occur, in nature— 
especially if unusual conditions, comparable with those of the laboratory, 
are brought into operation. We should expect, from our laboratory experience, 
that if a trypanosome, which normally inhabits one host, were transplanted 
into a strange host for many successive generations, then it would undergo 
modification in some of its physiological characters. Or again, if, in some 
way, direct transmission by the fly were substituted for cyclical transmission, 
we should expect to find the trypanosomes undergoing changes similar to 
those which attend their artificial propagation by the syringe. There is already 
some evidence that such a natural evolution of races has occurred and is still 
occurring in the case of the polymorphic mammalian trypanosomes of Africa. 
The evidence, both direct and indirect, appears to point clearly to the con¬ 
clusion that all these trypanosomes form a single species, divisible into a 
number of physiological races—some of which have arisen under natural con¬ 
ditions, others under the artificial conditions of the laboratory: and there 
appears, on the other hand, to be no good reason for supposing that the 
so-called “species” into which these trypanosomes have been subdivided by 
various laboratory workers are true species in the zoological sense. This, very 
briefly, is the thesis in support of which the following lines are written. 
I shall now submit further evidence bearing upon this problem. The 
evidence presented is all indirect, and includes (1) an examination of the 
