HARMFULNESS OF PROTOZOA n 



insect or tick. Hence arise such forms as Spirochczta 

 duttoni and S. recurrentis, the causative agents of re- 

 lapsing fevers. The grades in the evolutionary scale, 

 then, appear to be free-living organisms, inhabitants 

 of water, invaders of the alimentary canal, parasites 

 of the blood. 



Another interesting point is that probably the 

 more recently a parasite has been introduced into its 

 host, the more virulent it is. In process of time 

 hosts may become so adapted to dangerous parasites 

 that they are unaffected by them, or affected to a less 

 extent. Such is the case with a parasite of rats 

 known as Trypanosoma lewisi. Formerly it is prob- 

 able that the parasite was pathogenic to the rats in 

 whose blood it was found. But the organism has 

 been introduced into rats so long that the latter are 

 now unaffected by the intruder in the majority of 

 cases. An allied parasite, T. gambiense, is the cause 

 of human sleeping sickness. It is possible that in 

 time man may harbour the now deadly organism 

 without fatal effects to himself. In fact, there is 

 already some evidence to show that this happy result 

 is being slowly consummated, for the negroes of West 

 Africa the original home of sleeping sickness are 

 less affected by T. gambiense than are their brethren 

 in East Africa, where the disease has been more 

 recently introduced. 



The life-histories of parasites vary considerably, and 

 the details and structure must be described for each in- 

 dividual. But one point is worthy of note here. In 

 the life of every parasite there is a weak period, and 

 it is then that human ingenuity can apply its resources 



