FISSION RATE OF STYLONYCHIA PUSTULATA 501 



Cladocera. It appears not at all inconceivable that in these 

 organisms an equal number of selections, covering as great a 

 number of generations as were made in Stylonychia, would have 

 given similar heritable effects. What all the work shows (and 

 here ni}^ own is not in positive disagreement) is that heritable 

 variations of considerable extent do not occur so frequently as 

 was at one time supposed, so that a few selections are not suffi- 

 cient for establishing a definite positive effect. But negative 

 results from a few selections are not sufficient for disproving the 

 occurrence of heritable small variations which may be gradually 

 accumulated. This indeed has been admitted by many of those 

 that have obtained negative results ; thus Johannsen remarked 

 that ' ' there is the possibility that a selection of fluctuating vari- 

 ants, during very many generations, might divert the type of 

 a line" ('03, p. 62) ; Jennings says ''what the pure line work shows 

 (agreeing in this with other lines of evidence) is that the changes 

 on which selection may act are few and far between instead of 

 abundant . ." ('10, p. 144), and East states that "as 



a result of these experiments I would not go so far as to say that 

 variations in power of resisting physiological or fungus diseases 

 do not occur in asexual reproduction, but I do believe that the 

 relative possibility that the commercial grower will obtain dis- 

 ease-resisting varieties in this way is negligible" ('10, p. 134). 



As a result of this work upon Stylonychia it is possible to 

 substitute for such indefinite remarks, precise data as to the 

 occurrence of heritable variations and their accumulation through 

 selection, when sufficiently long continued. And this can hardly 

 fail to have influence on the conception of the hereditary con- 

 stitution or genotype as a fixed thing, changing only discontinu- 

 ously by marked steps or mutations, that do not intergrade. 



SUMMARY 



In Stylonychia pustulata, by the opposite selection through 

 more than 150 generations of small individual variations occur- 

 ring among the progeny of a single individual, it was possible to 

 produce two sets differing hereditarily in rate of fission. During 

 selection there was a gradual increase in the average heritable 



