No. 519] THE EFFECTIVENESS OF SELECTION 145 



of Paramecium I found to show constant hereditary dif- 

 ferences of one two-hundredth of a millimeter in length. 

 Hanel found the genotypes of Hydra to differ in the 

 average number of tentacles merely by the fraction of 

 a tentacle. That even smaller hereditary differences are 

 not described is certainly due only to the impossibility 

 of more accurate measurements; the observed differ- 

 ences go straight down to the limits set by the probable 

 error of our measures. Genotypes so differing have not 

 risen from one another by large mutations. The geno- 

 typic work lends no support to the idea that evolution 

 occurs by large steps, for it reveals a continuous series 

 of the minutest differences between great numbers of 

 existing races. 



All together, I think we may say that the pure line or 

 genotype concept presents an instrument of analysis 

 which is worthy, on the basis of what it has thus far 

 done, of a thorough tryout for future work, and no one 

 interested in these questions can afford to neglect it. 

 This conclusion is quite independent of the concrete re- 

 sults reached; the efficacy of selection in modifying geno- 

 types may be demonstrated to-morrow, but the demon- 

 strators will need to show precisely the relation of their 

 results to the pure line concept. 



