C. DOBELL 215 



In two cases, where mechanical transmission by the louse was 

 believed to have occurred — that is, where no development of the 

 Trypanosomes in the louse intervened — it was found that the 

 transmitted Trypanosomes had retained their power of resisting 

 arsenic. The incubation period in the louse in these cases was only 

 5 days. 



By daily injecting emulsions made from the bodies of lice — in which 

 Trypanosomes were developing — into uninfected rats, Gonder was able 

 to determine how long the arsenic-resistance of the Trypanosomes 

 persists. He found that it persists for 12 days in the body of the 

 louse. After this period, the Trypanosomes lose their resistance to 

 arsenic, and become normal. 



Cultures of normal and arsenic-resistant races of T. lewisi were 

 made in artificial media'. Both races behaved exactly alike. The 

 non-resistant races, when reinoculated into rats, were still non-re- 

 sistant: the arsenic-resistant races remained arsenic-resistant. Both 

 races underwent similar structural changes in the cultures — being 

 gradually converted into Crithidia-Vike forms in the course of some 

 3 months. These forms, when injected back into rats, assumed the 

 normal Trypanosome form once more — the incubation period being 

 3 — 11 days. Multiplication occurred in the artificial cultures. 



Ehrlich (1911) and Gonder (1911) have interpreted the foregoing 

 facts in the following way. They suppose that the development which 

 T. leiuisi xmdergoes in the louse constitutes a sexual cycle in the life- 

 history of this species. They suppose that the resistance to arsenic, 

 which the Trypanosomes have been made to acquire, persists only so 

 long as the asexual cycle endures — that is, during the period when the 

 Trypanosomes are in the blood of the rat, or in artificial culture media. 

 When the sexual cycle takes place in the body of the louse, the acquired 

 resistance of the race is lost, and the individuals revert to their original 

 non-resistant condition. The "acquired character" is thus "inherited" 

 in asexual reproduction only-. 



This extremely interesting and suggestive idea cannot be regarded 

 at present as anything more than a hypothesis. For in the first place. 



' T. leioisi was first successfully cultivated in an artificial medium by Novy and 

 MacNeal, in 1903. Since then, many other workers have succeeded in cultivating a 

 number of other species. 



° Far-reaching conclusions regarding " the inheritance of acquired characters " can be 

 di'awn from these experiments only by those who are content with words and unable 

 or unwilling to analyse the facts. 



