2i6 PROBLEMS OF GENETICS 



the novel conditions applied, and as I understand the evidence, 

 the appearance of the mutational forms does not with any 

 regularity follow upon the application of the changed conditions. 



Researches into the variation of these lower forms will, no 

 doubt, be continued on a comprehensive scale. So long as the 

 instances recorded are each isolated examples it is impossible to 

 know what value they possess. If they could be coordinated in 

 such a way as to provide some general conception of the types of 

 variation in properties to which bacteria, or any considerable 

 group of them, are habitually liable, the knowledge might 

 greatly advance the elucidation of genetic problems. 



Of mutational changes directly produced with regularity in 

 micro-organisms by treatment, the experiments with trypano- 

 somes provide some of the clearest examples. A summary of 

 the evidence was lately published by Dobell, 4 from which the 

 present account is taken. The most definite fact of this kind 

 established is that certain dyes introduced into the blood of the 

 host have the effect of destroying the small organ known as the 

 "kineto-nucleus" in the trypanosomes. The trypanosomes thus 

 altered continue to breed, and give rise to races destitute of 

 kinetonuclei. This observation was originally made by Wer- 

 bitzki and has been confirmed by several observers. The exact 

 way in which this alteration is effected in the trypanosomes is not 

 quite definitely made out, but there is good reason for supposing 

 that the dyes have a direct and specific action upon the kineto- 

 nucleus itself, and circumstances make it improbable that in 

 some division a daughter-organism without that body is pro- 

 duced, or that any selection of a pre-existing defective variety 

 occurs. 



Ehrlich has suggested with great probability that the dyes 

 which possess this action owe it to the fact that they have the 

 particular chemical linkage which he calls "ortho-quinoid." In 

 outward respects, such as motility and general appearance, the 

 modified organisms are unchanged, but their virulence is di- 

 minished. As regards the possibility of the defective strain re- 



4 C. C. Dobell, Jour. Genetics, 1912, II, p. 201, where full references are given. 

 Still more recently the same author has contributed an excellent summary of 

 the evidence relating to bacteria (ibid., II, 1913, p. 325). 



