io8 GENETIC VARIATIONS 



now resistant to the antibody. This has been found to occur 

 even when the original infection was by means of a single 

 trypanosome, so that it is clear that resistance has been 

 acquired by individuals of the same strain that were before 

 non-resistant. In a similar way, resistance has been found to 

 be induced toward drugs introduced into the body for the 

 purpose of killing the parasites. 



The resistance once acquired lasts for a long time, for 

 hundreds or thousands of generations. Ehrlich found that 

 resistance thus induced by the arsenious drug atoxyl con- 

 tinued for three years, even though the parasites had been 

 transferred to animals in which the drug was not present. 



Great numbers of cases of this type are known in the para- 

 sitic Protozoa. This acquisition of new and adaptive features, 

 inherited in reproduction by fission, is a characteristic feature 

 in the life of these parasitic organisms; it is a matter of great 

 practical importance in the treatment of the disorders caused 

 by the parasites. 



Knowledge of these matters in the free-living Protozoa is 

 less extensive; there is much need here for further work. But 

 a number of the features that are of importance for genetics 

 have been brought to light, principally by the work of Jollos, 

 and of Neuschloss. By long subjection of the free-living proto- 

 zoan Paramecium caudatum to certain chemicals, resistance 

 to these chemicals has been greatly increased. This has been 

 accomplished with certain compounds of arsenic and anti- 

 mony, and with certain organic compounds, notably quinine, 

 methylene blue, trypan blue, and fuchsin. 



It was shown by Neuschloss that the acquirement of re- 

 sistance to such materials is due to an induced change in the 

 intimate chemical processes occurring in the animals; a 

 change in their metabolism. All the substances named above 

 are injurious to the organisms. The Paramecia acquire 



