(30) 



The further idea, that within one of these "pure lines" 

 no variation in genetic constitution is possible, except, 

 perhaps, by a sudden large mutation, is not direct obser- 

 vation tut hypothesis. It is an hypothesis, to be sure, 

 which was based on all the experimental evidence available 

 at the time it was formulated, but it is still an hypothesis 

 subject to revision at any time when new experimental re- 

 sults demand it. 



For some years, the great majority of the experimental 

 results tended to confirm this hypothesis, and it gained 

 such a firm footing that all the contradictory cases were 

 dismissed as aberrant types, results of a failure to pro- 

 perly control the environment, observational errors, etc. 

 But of late the tide seems to be turning somewhat. Stout 

 (1915), in Coleus, Stocking (1915) in Paramecium, Middle- 

 ton (1915) in Stylonychia, and Jennings (1916) in Difflugia 

 have all shown that minute heritable variations do occur 

 within clones derived from a single ancestor by vegetative 

 multiplication. In the present work on Centropyxis, the 

 same conclusion was reached. 



We must admit, it seems to me, that in these plants 

 and protozoans reproducing asexually, small variations in 

 genetic constitution do continually occur. How does this 

 affect the "pure line" hypothesis ? The issue can be met 

 in two ways. 



In the first place, it is still possible to reaffirm 

 the entire validity of the original pure line hypothesis. 



