LIFE CYCLE OF HYDATINA SENTA 309 
rotifers more than two years before, and to which none had been 
added since. This culture had been examined many times for 
rotifers, but none were seen until the single specimen which pro- 
duced the line recorded as the New York line was found. I 
am inclined to think, therefore, that this female had recently 
hatched from a fertilized egg, and that the New York line was 
accordingly about a month older than the Baltimore line. 
Whether this difference inage may account forthe difference in the 
proportion of male-producers between the two lines is uncertain. 
That differences in the proportion of male-producers not depend- 
ent on differences in age may exist between two lines is shown, 
however, by another experiment, in which the F, line was crossed 
back to the New York line, and in several other cases not recorded 
in that paper. In the cases to which I refer, the older line pro- 
duced more male-producers than the younger line. 
Uncertainty as to the age of the original lines, therefore, can 
not invalidate the conclusion that differences dependent on an 
internal agent do exist between parthenogenetic lines; it merely 
modifies our conception of the nature of those differences, a sub- 
ject that is discussed elsewhere. 
Some of the long parthenogenetic lines recorded in table 1, it 
is to be noted, do not show an evident decrease in the proportion 
of male-producers; nor do they show an increase. Each of these 
lines began with a low percentage of male-producers, and could 
not have decreased much. These lines showed considerable 
fluctuations in the proportion of male-producers, periods of few 
male-producers being followed by periods of many. If such a 
fluctuating line began with a long period of few male-producers, 
a decrease in the proportion of male-producers.could only be 
discovered by breeding it through a large number of generations, 
including several waves of male-producers, and finding that suc- 
cessive waves were less marked. Forty-six generations are hardly 
enough for this. The progressive decrease in the proportion of 
male-producers is so marked in some cases, and its absence in 
some lines so easily explained, that I am inclined to regard it as 
a general phenomenon. 
THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 12, NO. 2 
