216 Miitation in Mlcro-Organisms 



it lias not been proved that the louse is the normal intermediate host 

 of T. lewisi. There is much evidence to show that it is the flea, and 

 not the louse, which is the normal carrier of the Trypanosome from rat 

 to rat: though the louse may occasionally be the means of infection. 

 Secondly, it has never yet been proved that the development which the 

 Trypanosomes undergo in the gut of the louse is a sexual development. 

 A sexual cycle in the louse was first described by Prowazek, and has 

 since been alleged to occur by Baldrey, Rodeuwaldt, and others, whose 

 results Gonder says he can confirm "almost entirely." To prove the 

 existence of a sexual cycle in the louse, however, something more than 

 the arbitrary seriation of certain stained specimens is requisite. Until 

 the publication of more convincing evidence — derived from a study 

 of the living organisms, and from careful cytological research — it is 

 not justifiable to conclude that conjugation of the Trypanosomes occurs 

 in the body of the louse. It is, moreover, obvious that Gonder's own 

 results cannot be held to prove that conjugation occurs in the louse: 

 his interpretation is based on the supposition that conjugation does 

 occur. And there is really no reason why the development — which 

 the Trypanosomes appear undoubtedly to undergo — in the louse, should 

 be regarded as necessarily of a sexual nature. 



Since it has been found (Mesnil and Brimont [1908], Breinl and 

 Nierenstein [1908]) that the resistance of a race of Trypanosomes to 

 arsenic is manifested only so long as the race remains in a given host, 

 it is not impossible that Gonder's results are explicable on the same 

 principle. T. leivisi niay remain arsenic-resistant so long as it con- 

 tinues in the blood of the rat, or in an artificial medium : but a change 

 of host {i.e. from rat to louse) may abolish the resistance — just as T. 

 brucei, arsenic-resistant in donkeys, becomes non-resistant when trans- 

 planted into rats. (Of. p. 213.) If one substance can bring about 

 arsenic-resistance, it is at least conceivable that another substance 

 can remove it. And it is possible that the body of the louse may 

 furnish such a substance. At all events, there is no need to assume 

 the existence of sexual phenomena to account for the results of the 

 experiments. 



Ehrlich and his followers regard resistance to di'ugs or sera as a 

 direct consequence of the action of the substances in question upon 

 tlie living protoplasm. That is to say, they suppose that when a 

 Trypanosome is treated with a minute quantity of arsenic, its proto- 

 plasm becomes changed in such a way as to make it resist the drug 

 when applied subsequently. New races of Trypanosomes are thus 



