478 AUSTIN RALPH MIDDLETON 



0.267 generation more per day than each slow hne. During the 

 fourth and fifth ten-day periods each fast line produced, on the 

 average 0.300 generation more per day than each slow line. 

 During the sixth, seventh and eighth ten-day periods this daily 

 average difference per line was 0.415 generation. Table 8 shows 

 that during the ninth, tenth, eleventh, twelfth and thirteenth 

 ten-day periods it was 0.305 generation, which is considerably 

 smaller than the 0.415 generation difference of the sixth, seventh 

 and eighth ten-day periods of Experiment 1. But, as in a pre- 

 vious case, this is due merely to the fact that the average fission 

 rate for all lines has decreased; relative to this average fission 

 rate the difference between fast and slow lines has not decreased, 

 but on the contrary has increased. For part 1 of Experiment 1, 

 the difference between the fast and the slow lines in number of 

 generations produced was 6.9 per cent of the total number of 

 generations produced by all; for part 2 it was 12,8 per cent; for 

 part 3, 19.3 per cent, and for part 4 it was 21.2 per cent. Conse- 

 quently, throughout the entire period of selection (thirteen ten- 

 day periods), the proportional difference between fast and slow 

 lines has steadily increased. 



Figure 10 further emphasizes the genuineness of the differ- 

 ence of fission rate between these two sets of lines for Experi- 

 ment 1, part 4. It shows the curves of variation of these two 

 sets of lines of that experiment. 



Experiment 1-C. Mass culture and balanced selection, March 

 17 to May 4, 1914. 



In order to demonstrate as conclusively as possible whether 

 the apparent average difference of fission rate between the two 

 sets of lines was hereditary or not, it was decided to subject 

 them to a period of mass culture and then to balanced selection. 

 This was to determine whether the average difference of fission 

 rate had survived the mass culture treatment. On March 4, 

 1914, all the animals remaining in the 'fast' concavities after 

 the transfer of the chosen individuals had been made to fresh 

 slides were placed, unwashed in a single circular glass dish 3f 

 inches in diameter and 2 inches deep in 50 cc. of ^2 per cent 



