THE MODES OF LIFE OF THE PROTOZOA 



27 



A further point of great interest in this connection is that 

 T. brucii in Africa appears, from the observations of Bruce, to 

 occur as a natural parasite of wild game, and to be as harmless to 

 these its natural hosts as T. lewisi is to rats. The physiological 

 difference between these two species is that T. lewisi is perfectly 

 specific to its natural host, whereas T. brucii is capable of flourish- 

 ing in others, with most deadly effects. Hence the pathogenic 

 properties of T. brucii would appear to be exerted on hosts to which 



FIG. 12. Trypanosomes of the brucii-gmup. A, B, C, T. brucii of "nagana," three 

 forms slender, intermediate, and stumpy ; D, E, F, T. gambiense of sleeping 

 sickness, the three corresponding forms ; G, H, T. evansi of " surra," two forms 

 I, T. vivax ; J, T. nanum. A to C, I, and J, after Bruce, Hamerton, Bateman, 

 and Mackie (411); G and H, after Bruce (404); D to F, from preparations. 

 All figures magnified 2,000 diameters. 



it is a new parasite, and not on those to which, like T. lewisi, it 

 has established harmonic relations in the course of evolution. 

 The pathogenic properties of T. brucii, and doubtless of other 

 similar forms, may from this point of view be characterized* as a 

 disharmony associated with the first steps in the origin of species. 



The problem of the origin of diseases caused by parasites is 

 essentially a problem of the same nature as the origin of species. 

 The first step in the formation of new species is a process of varia- 

 tion in an established species. Similarly, in the process of forma- 



