320 Aaron Franklin Shull 



The data show that, besides the considerable fluctuation in the 

 percentage of male-producers which appears between one genera- 

 tion and the next, there may also be long-continued periods in 

 which few male-producers appear, followed by equally long periods 

 in which they are abundant. The generations between one winter 

 egg and the next do not behave as a strain having a fairly con- 

 stant proportion of male-producers. 



Variability of the Percentage of Male-producers in Related Strains 

 under Like Conditions 



Experiment II. To determine what difference in the percent- 

 age of male-producers must be obtained to give positive indica- 

 tions of the influence of a given agent, two series of generations 

 were reared under like conditions. Both series were reared 

 in the same water, at the same temperature, and were fed approxi- 

 mately equal amounts of the same food cultures. The experiment 

 was performed twice, A and B, Table II. In A, the parents of 

 the two series were first cousins once removed; m B, fourth cousins. 



The difference in the first experiment is about six per cent, 

 in the second less than 2 per cent. In the former case, where the 

 difference between the two percentages of male-producers is 

 greatest, the ratioof the higher to the lower percentage is about i.i 

 to i.o. In experiments designed to test the influence of a given 

 agent, unless the ratio of the higher to the lower percentage of 

 male-producers is greater than i.i to i.o, it is not safe, therefore, 

 to infer from a single experiment that the agent in question has 

 any influence; and the greater this ratio, the stronger is the evi- 

 dence of such influence. In case of an agent having but slight 

 eflPect, this effect should be shown by numerous experiments 

 giving small differences of practically uniform sign. 



Influence of Quantity of Food Culture on Percentage of Male- 

 producers. 



Experiment III. On July 22 two sister individuals, respec- 

 tively the first and second of their family, from the 17th generation 



