THYPANOSOMES 139 



It is doubtful whether an effect of this kind in uni- 

 cellular forms can be compared to the inheritance of 

 characters in multicellular forms. In no bacilli is there 

 any differentiation between somatic and germinal tissue, nor 

 do we know anything with regard to their multiplication 

 beyond the fact that they reproduce by simple division and 

 budding. According to Biitschli and Minot's theory 1 a 

 swarm of unicellular forms between the periods of con- 

 jugation may be regarded as corresponding to the soma 

 of a multicellular organism. Looked at in this light, 

 the continuation of an acquired character during several 

 generations of bacilli in which no conjugation has been 

 demonstrated affords no evidence one way or the other 

 that any effect has been produced upon the germplasm. 

 On the other hand, with regard to some unicellular animals, 

 certain cases which are cited as examples of the inheritance 

 of acquired characters seem to afford evidence of quite a 

 contrary kind. 



Several tropical diseases, among them sleeping sickness, 

 are caused by minute unicellular animals called trypano- 

 somes. An arsenic compound, atoxyl, has been found to 

 destroy these parasites when injected into the animal in 

 which they are living, and to cause their disappearance, 

 temporarily at any rate, from the blood. It has been found 

 that strains of trypanosomes obtained from animals that 

 have been treated with atoxyl and have subsequently re- 

 lapsed, are highly resistant to it. In fact, in some cases, a 

 strain of trypanosomes entirely resistant to atoxyl has been 

 produced. The explanation has been offered that this resist- 

 ance to atoxyl is due to the transmission of an acquired 

 character. The trypanosomes have gradually become used 

 to the atoxyl and have developed resistance to it, which 

 they have handed on to succeeding generations. 



Some animals are much more resistant than others to 

 trypanosomes, and if a strain be taken from an animal 

 which is comparatively resistant, it is found to have in- 



1 See p. 5. 



