GENETICS: A. R. MIDDLETON 
619 
ing to the seventh fiHal generation, were obtained; and subjected, in the 
manner previously described, one to 'fast' the other to 'slow' selection 
for thirty days. The excess of generations produced by the fast-selected 
lines, expressed as a percentage of the total number of generations pro- 
duced by both sets, was 1.99% for the first ten days; 4.36% for the 
second ten days, and 7.10% for the third ten days; thus show'ng a 
gradual increase and indicating a cumulative effect of selection. 
To test the permanence of this result these two sets of lines were 
now subjected to balanced selection for 21 days. On every day but 
one the lines that had been subjected to fast-selection averaged higher 
than the others, and the percentage that the difference in favor of the 
fast set is of the total number of generations produced by both together 
was practically constant; it was 3.97% for the first ten days and 5.01% 
for the last eleven days. The results are thus the same as in our first 
set of experiments. 
It was found that if conjugation occurred in the fast set, and like- 
wise in the slow set, the difference produced by selection continued to 
exist after conjugation. 
This third series of experiments has entirely corroborated the results 
of the first and second series. In a second clone, unrelated to that used 
for the first and second series of experiments, opposite selection for thirty 
days produced a heritable difference of average fission rate, a differ- 
ence that gradually increased as selection progressed, indicating again 
that the effect of selection on this physiological character is cumulative. 
This average difference persisted through twenty-one days of balanced 
selection, twenty-nine days of mass culture followed by conjugation, and 
then fifteen days of further balanced selection. 
All the experiments thus give concordant results; through selection 
of individual differences in fission rate it is possible to divide a clone 
into two divisions differing hereditarily in rate of multiplication. The 
effects of selection are cumulative; the hereditary differences between 
the two divisions become greater the longer selection continues. By 
reversing the direction of selection the hereditary differences between 
the sets are reversed. The hereditary differences between the sets 
persist through the ordeal of conjugation. 
Hence, in reference to the physiological character studied, the selec- 
tion of small individual variations such as appear within the pure strain 
or clone is an effective evolutionary procedure. 
How are we to account for the difference between the results here 
set forth and those of older investigators? In Stylonychia we are deal- 
ing with an organism which is large enough to be easily handled and 
