THE MODES OF LIFE OF THE PROTOZOA 25 



and one interest of the parasite may, so to speak, clash with another. 

 But in all cases alike it is perfectly clear that the death of the host 

 before the parasite has matured its propagative phases leads simply 

 to the extirpation of the parasite, and is a suicidal policy on its 

 part, a glaring disharmony in Nature. This conclusion is borne out 

 by a general survey of the facts of parasitism in the Protozoa, since 

 the vast majority of these parasites are quite harmless to their 

 hosts, and lethal parasites, greatly in the minority when compared 

 with harmless forms, must be considered as exceptional and aberrant 

 types of parasites, from a general point of view. 



The parasitic Protozoa of lethal properties present a problem 

 which can be best attacked by considering and comparing two 

 cases of closely allied parasites, the one harmless, the other lethal, 

 to their hosts. Very instructive cases of this kind are furnished 

 by trypanosomes (vide infra, p. 285). The common parasite of the 

 rat, Trypanosoma lewisi, is perfectly harmless as a rule to its host, 

 and the infection runs a very definite course. When the parasite is 

 introduced into the blood of a healthy and susceptible rat, it enters 

 at once upon a period of rapid multiplication, which lasts about 

 twelve days. At the end of that time the parasite swarms in the 

 rat's blood, without perceptibly affecting its general health. After 

 about twelve or thirteen days the multiplication of the parasite 

 ceases entirely ; the swarming period lasts generally about a month, 

 and after that the parasites begin steadily to diminish and dis- 

 appear, until after a variable length of time, usually three to five 

 months, the blood is quite free from them, and the rat, cured from 

 the attack, is now quite immune to the parasite, and cannot be 

 infected by it a second time. 



The behaviour and effects of a pathogenic trypanosome, such as 

 T. brucii, when introduced into a rat's blood, contrast sharply 

 with that just described. Not only do the trypanosomes begin 

 to multiply at once, but they never cease to do so while the host 

 remains alive. By the fifth or sixth day there are practically more 

 trypanosomes in the blood than blood-corpuscles, and the death 

 of the host soon follows when this stage has been reached. 



Trypanosoma lewisi is a type of a well-marked group of try- 

 panosomes, which may be conveniently denoted the lewisi-group 

 (Fig. 11). Such are T. cuniculi of the rabbit ; T. duttoni of the 

 mouse ; T. rabinowitschi of the hamster ; T. blanchardi of the dor- 

 mouse ; T. microti of Microtus arvalis ; and T. elyomis of the lerot 

 (Eliomys quercinus). All these species of trypanosomes are ex- 

 ceedingly similar in their appearance and structure ; each species, 

 however, appears to be perfectly specific to its particular species 

 of host. The trypanosome of the rat, for instance, will not flourish 

 in any other host, not even in a mouse, under normal circumstances. 



