618 
GENETICS: A. R. MIDDLETON 
hundred and thirty days ranges from 178 to 187, while for the slow 
lines the range is but from 116 to 128. The slowest fast-selected line 
produced 50 more generations than the fastest slow-selected line. 
To determine whether the difference in fission rate thus produced is 
heritable, parts of the two sets were removed at intervals and sub- 
jected to culture without selection (by ^balanced selection,' in which 
unavoidable selections in one direction were always compensated by 
equal numbers of selections in the other direction). By long continued 
culture without selection, it was found that the difference was herit- 
able. For example, after the two halves of the clone had been sub- 
jected to continuous opposite selection for 80 days it was found that 
the average difference per line per day had increased from 0.267 gener- 
ation for the first thirty days to 0.415 generation for the last thirty 
days of that period. To test the permanence of this result these two 
sets of thirty lines were now subjected to ninety days of balanced selec- 
tion, or ten days longer than the lines had been subjected to opposite 
selection. It was found that the average difference per line per day in 
favor of the progeny of the fast-selected set was for the three con- 
secutive thirty-day periods of this experiment: 0.213, 0.256 and 0.284 
generation. 
Also, representatives of the two sets after 80 days of selection and 40 
days of no selection ('balanced selection'), were subjected to mass cul- 
ture for twelve days. Further balanced selection of these for fifty days 
showed that the inherited difference of fission rate still persisted. Thus 
the inherited difference produced by 80 days of selection had lasted for 
102 days without selection. 
Experiments with reversed selection showed that the inherited differ- 
ence could be reversed in the same way that it is produced ; the originally 
fast set was thus caused to become the slower one, and vice versa. 
Continuation of these two sets without selection showed again that the 
difference so produced was heritable. 
Thus in this case the selection of small individual variations in fission 
rate has split the single clone (derived vegetatively from a single parent) 
into two heritably diverse divisions and the effect of selection was 
cumulative. 
This experiment was now twice repeated. The first repetition was 
with the progeny of a single individual taken from one of the fast lines 
of the first set of experiments. Here again selection produced from the 
progeny of a single individual two sets differing hereditarily in average 
fission rate. Again, a new wild individual was obtained from a new 
mass culture. From this, two sets of thirty individuals each, all belong- 
