Adaptation to Environment 333 



a mouse with a certain strain of trypanosomes, and 

 treating it with a certain arsenic compound, which 

 inhibited somewhat the propagation of the parasites 

 but did not kill them all. Four or five days later 

 trypanosomes from this mouse were transmitted to 

 another mouse and after twenty-four hours this mouse 

 was treated with a stronger dose of the same arsenic 

 compound; and this process was repeated. After the 

 third transmission or later, the trypanosomes can resist 

 considerably higher doses of the same poison than at 

 first and this resistance is retained for years. Ehrlich 

 seems to have taken it for granted that he had succeeded 

 in transforming the surviving trypanosomes into a 

 type which is permanently more resistant to the arsenic 

 compound than was the original strain. 



The writer is not entirely convinced that in these 

 experiments a possibility was sufficiently considered 

 which is suggested by Johannsen's experiments on the 

 importance of pure lines in work on heredity. Ac- 

 cording to this author a strain of trypanosomes taken 

 at random should, in all likelihood, contain a population 

 consisting of strains with different degrees of resistance. 

 If a high but not the maximal concentration of an 

 arsenic compound is repeatedly injected into the in- 

 fected mice the weaker populations of trypanosomes 

 are killed and only the more resistant survive. These 

 of course continue to retain their resistance if trans- 

 planted to hosts of the same species. According to this 



