288 A. FRANKLIN SHULL 
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Number of Generation 
Fig. 3 
great that I have taken the generations in groups of five. Despite 
the enormous differences among the groups of five generations, 
there is seen to be a general diminution in the proportion of male- 
producers through the series. In like manner, line 5 (hears) 
shows such a decrease in a most unmistakable manner. 
The remaining five lines, including line 8 which would show a 
considerable decrease in the early generations, are combined and 
plotted in fig. 4. Here it would perhaps not be justifiable to 
assume a decrease in the proportion of male-producers in the 
lines as a whole. A possible explanation of these differences 
between different lines, and the importance of the phenomenon 
of decrease in the proportion of male-producers where it occurs, 
are discussed elsewhere. 
Decrease in size of family with long-continued parthenogenesis 
It became apparent in several lines of rotifers that the families 
became gradually smaller, and that it was increasingly difficult 
to maintain the animals in a healthy and vigorous condition. 
To determine whether there were any possible connection between 
the diminution in the size of family (reduction of vigor) and the 
decrease in the number of male-producers, I have plotted the size 
of family in those lines that showed the decrease in the propor- 
tion of male-producers. 
For the sake of comparison, the generations are again taken in 
groups of five. Fig. 5 represents lines 1 and 2 of table 1, and may 
