524 JENNINGS— HEREDITY IN PROTOZOA. [April 24, 



admitted that the experimental results go strongly against the effect- 

 iveness of selection among slight fluctuating variations in producing 

 new inherited characteristics. 



How, then, do the different pure lines rise? This is after all the 

 main problem. Toward its solution further investigations of this 

 series will be directed. It is proposed to study in detail (i) the 

 effects of conjugation on variation, heredity and the production of 

 new races; (2) the effects of long-continued differences in environ- 

 mental action on different divisions of the same line; (3) the ques- 

 tion whether the different lines arise from something like mutations. 

 Further, (4) additional different way of exercising selection within 

 a single line will be tested. The question may be raised whether the 

 production " by mutation " of such slight differences in size as we 

 are here dealing with would not be essentially the same as their 

 production by the inheritance of slight variations — since the extent 

 of the " mutations " would not be greater than what we should call 

 slight variations in size. The difference between the two conceptions 

 almost or quite vanishes when we come to deal with such minute 

 changes in characteristics as those we find in the different lines of 

 Paramecium. The " mutation " would be merely a rare, heritable, 

 variation, and it is now clear that heritable variations in size are 

 much rarer than had been supposed ; their number is so small that 

 in Paramecium they are not statistically detectible among the many 

 non-heritable fluctuations due to the environment. 



Raquette Lake, New York, 

 August 22, 1908. 



