14 THE president's ADDRESS. 



a " disharmony " in nature. How is the existence of these lethal 

 forms to be explained, and what is their significance ? The best 

 manner for a naturalist to attack this problem is to consider 

 groups in which both harmful and harmless forms occur and are 

 represented by closely allied species. Good instances of such 

 cases are presented by the trypanosomes of mammals, of which 

 two groups may be briefly described and discussed. 



In rodents we find a group of trypanosomes of which Try- 

 panosoma lewisi may be taken as a type, and I shall therefore 

 refer to them for short as the levnsi-gvow^. Tryjxtnosoma leicisi 

 flourishes in the two common species of rats {JIus decumanus and 

 M. rattus), but not in any other animal, not even in the mouse, 

 which has its own peculiar species of trypanosome {T. diUtoni). 

 Similarly T. cunicidi is specific to the rabbit, and T. rabinoivitschi 

 to the hamster. All these four trypanosomes of rodents are, 

 however, very similar in their appearance and morphological 

 characters, and can scarcely be distinguished except by the 

 physiological action and reaction between them and their hosts ; 

 there can be no doubt that they are closely allied species, differ- 

 ing only in that each is adapted constitutionally to parasitism 

 upon a special host. These specific trypanosomes of rodents of the 

 lewisi-grou-p are farther quite harmless, under ordinary circum- 

 stances, to their hosts. When a rat becomes infected with T, 

 lewisi, the trypanosomes at first multiply rapidly and swarm 

 in the blood ; then all multiplication ceases and the parasites 

 gradually die out, after which the rat is immune to the parasite, 

 and cannot be infected with it a second time. 



Constrasting with the lewisi-gvoiv^ is another which we may 

 term the hrucii-gYou^, the members of which are remarkable 

 for their deadly powers, each species being the cause of some 

 fatal disease. Such are 1\ brucii, the cause of nagana, or tsetse- 

 fly disease in horses, cattle and dogs ; T. gamhiense, the cau'^e of 

 sleeping sickness in man ; T. evansi, the cause of surra in horses ; 

 T. equiperdmn, the cause of dourine in horses ; and several other 

 species. In this group also the parasites are practically indis- 

 tinguishable from one another by appearance or structure ; the 

 species can only be identified by their reactions, but this is much 



