INHERITANCE IN TRYPANOSOMES 697 



After an acquired character has remained constant during 

 several thousand generations, extending over a period of more 

 than a year, one might easily assume that the character was 

 fixed. Whilst in the blood of the rat, however, the trypanosomes 

 only multiply asexually and therefore the character is not 

 eliminated. After these trypanosomes have been taken into the 

 gut of the intermediate host, in which the sexual cycle pre- 

 sumably takes place, the acquired character completely dis- 

 appears and the parasites recover their normal behaviour 

 towards the drug. As the acquired power of resisting a drug 

 is eliminated by passing through the intermediate host, there 

 seems little danger of arsenic-resistant races of T. gambiense, 

 the cause of sleeping sickness, being spread by the tsetse-fly, 

 its intermediate host. 



