VARIATION IN UNICELLULAR ORGANISMS 107 



Also, if certain Protozoa are subjected for long periods to 

 solutions of certain chemicals, their resistance to these chemi- 

 cals may after many generations become increased. Such a 

 result, it may be remarked, is not readily and easily reached; 

 many investigators who have attempted to produce it have 

 failed. But with some Protozoa, under proper conditions, 

 striking results are reached ; the resistance is greatly increased. 

 The animals become acclimatized or immunized to different 

 chemicals, just as higher animals become immunized to 

 certain organic poisons and bacterial products, so that they 

 can later live and thrive under conditions which earlier would 

 have killed them. In these respects the relations are the same 

 in unicellular organisms and in higher animals, save that in 

 the unicellular forms many generations are required for the 

 production of immunity. — 



These matters have been studied most fully in the bacteria 

 and in the parasitic Protozoa.^ In the trypanosomes, flagellate 

 Protozoa that live in the blood of various mammals, including 

 man, the relations found are these : When the vertebrate host 

 — for example, the rat — is infected, the parasites multiply 

 rapidly for a time by fission, till very large numbers are pres- 

 ent in the blood. After a time, almost all of these suddenly 

 die. This is due to the production by the host of an "anti- 

 body," which is poisonous to the trypanosomes. But not all 

 the parasites are killed, for later a large number of them 

 reappear, inducing a "relapse." Investigation shows that this 

 recrudescence of the parasites is not due to the disappearance 

 of the antibody from the blood; it is still present and it is 

 still fatal to parasites of the type that were originally present. 

 What has happened is that a small proportion of the trypan- 

 osomes have acquired resistance to the antibody; they can 

 now live in its presence. These immunized individuals then 

 multiply, so that the host again becomes filled with parasites. 



