,9o8.] JENNINGS— HEREDITY IN PROTOZOA, 509 



Evidently, selection is having no effect that can be detected. 

 The fluctuations in the two sets are precisely what would be expected 

 from unavoidable changes in conditions of nutrition ; they show no 

 relation to selection. 



Later another experiment in selection was tried with these same 

 races, g and i. On January 19 I selected from a large culture that 

 had been multiplying freely for a month ( i ) the ten largest speci- 

 mens of g that I could find; (2) the ten smallest specimens of g; 

 (3) the ten largest specimens of i; (4) the ten smallest specimens of i. 



These were allowed to multiply under identical conditions till 

 February 5. Then a sample of fifty of each was measured. The 

 results are as follows : 



Large gf, 114.720 X 33-920 microns.^ 

 Small 5^, 116.912 X 36.070 microns. 

 Large i, 92.000 X 26.960 microns. 

 Small i, 93.583 X 27.500 microns. 



The dift'erence between the two sets of each is slight and without 

 significance, but such as is found is in favor of the progeny of the 

 smaller specimens in each case. 



Evidently, we are not making a start with any eft'ect of selection, 

 and it is useless to continue the experiment. 



Many other attempts were made to break a pure line by selection 

 into several strains ; on this point an immense amount of work was 

 directed. But in most cases the difference between the two sets 

 became equalized almost at once, so that the experiments were not 

 carried farther. As soon as two unequal sets become quite equalized,, 

 there is little opportunity for further selection. In the experiments 

 described above, though their futility seemed evident from the first 

 results, the work was continued for many generations, in order that 

 failure might not be due to lack of perseverance. 



One other set of experiments deserves to be described, because 

 in these the basis for selection was changed. Among the progeny 

 of a certain individual Nf2 conjugation occurred. The conjugants 

 varied in size. This offered an opportunity to make a selection 



° These measurements are found, for another purpose, in Table XXIV., 

 page 491. 



