THE president's ADDRESS, 15 



more difficult in the 6r?(m-group, since, unlike the lewisi-grouip, 

 the species are not limited to one particular type of host. Every 

 member of the hrucii-group can flourish in a great many different 

 hosts, though there are limits even to their powers of adaptability. 

 Thus T. britcii can flourish in ruminants, dogs, rats, and many 

 other mammals, but apparently not in man. T. gambiense can 

 infect man, monkeys, dogs, rats, etc., but not ruminants. When 

 a rat is infected by T. brucii, the infection runs a course quite 

 diflferent from that of T. lewisi. The parasites in the former case 

 multiply indefinitely until the blood contains more trypanosomes 

 than blood-corpuscles, and the multiplication of the parasite is 

 only ended by the death of the host and with it of the parasites. 



Comparing and contrasting these two groups of trypanosomes, 

 it is impossible to avoid in the first place the conviction that in each 

 group the species are closely allied, and descended from a common 

 ancestor. The ancestor of the lewisi-grouip had certain structural 

 characters distinguishing it from that of the brucii-grou-p, but 

 each of these two ancestral species has given rise to a number 

 of species which are practically indistinguishable by structural 

 characters, and can only be diflerentiated by their physiological 

 reactions. We have here a beautiful instance of species in the 

 act of arising, and caught, so to speak, at a stage in which 

 divergent physiological adaptation has not yet brought about 

 structural differentiation. It is seen further that the species of 

 the lewisi-group are quite specific in their reactions, and are each 

 limited to certain definite hosts, while those of the brucii-group 

 are far less specific, and are limited only to a comparatively wide 

 range of hosts. We may reasonably regard the lewisi-group 

 as the further advanced in the evolution of specific characters, 

 and the brucii-gronp as being in a more incipient stage of specific 

 differentiation. Finally, it is to be noted that the leivisi-gvoup 

 are typical harmless parasites, while the b7'iccii- group are 

 characteristically of the lethal type. 



Further light is thrown on this question by a very interesting 

 discovery made by Bruce with regard to T. brucii. He found 

 that in districts where tsetse-fly disease was rife, T. britcii was 

 to be found as a natural parasite of wild game, to which it is 



