26 



THE PROTOZOA 



Roudsky suggests that all this group of trypanosomes constitutes 

 in reality a single species ; in any case, it is reasonable to regard 

 them as forms recently evolved from a common ancestor, incipient 

 species which have not advanced beyond the stage of physiological 

 differentiation. 



In like manner, T. brucii is a type of a group of trypanosomes 

 which may be termed the brucii-group (Fig. 12) ; other members 

 of it are T. gambiense, the parasite of human sleeping sickness ; 

 T. evansi, causing surra in horses ; T. equiperdum, x)f dourine in 

 horses ; and several other species. These forms also are exceedingly 

 similar in appearance and structure, though easily distinguishable 

 from members of the lewisi -group. They are all of them very 

 lethal, as a rule, to their hosts ; and they differ further from the try- 



FIG. 11. Trypanosomes of the lewsi- group. A, T. lewisi (rat) ; B, T. duttoni 

 (mouse) ; C, T. cuniculi (rabbit) ; D, T. microti (Microtus arvalis) ; E, T. 

 dyomis (Eliomys quercinus) ; A and C, from preparations ; B, after Thiroux ; 

 D, after Laveran and Pettit; E, after Franga. All figures magnified 2,000 

 diameters. 



panosomes of the lewisi-group in the fact that a given member of 

 the brucii-group is not specific to a particular host, but can flourish 

 and exert its lethal powers in a great variety of vertebrate hosts 

 a fact which, coupled with their very similar morphology, renders 

 ithe exact determination of the species of this group very difficult, 

 and often a matter of controversy. All these facts point to the 

 briLcii-group being also descended from a common ancestral form ; 

 they may be regarded as incipient species in which the process of 

 evolution has not yet the degree of physiological specialization 

 reached in the lewisi-group. This view receives support from .the 

 fact that a new race or species of the brucii-group has been made 

 known this year (1911) namely, T. rhodesiense, a trypanosome 

 pathogenic to human beings which appears to have come into 

 existence as a species very recently. 



