150 A. FRANKLIN SHULL 



are trustworthy, as I am convinced they are, we have here the 

 remarkable result that the pure line from the cross yields not 

 only a greater percentage of male-producers than does either 

 parent line alone, but decidedly greater than the percentages of 

 both parent lines combined. 



This result is so remarkable that it became of interest to breed 

 an individual of the pure line from the cross back to a member of 

 one of the parent lines. This was done in the following experiment. 



Experiment XXXVI. Several females of the pure line derived 

 from the cross between a New York female and a Baltimore 

 male, in the last experiment, were mated with males of the New 

 York line on June 13 and 14. Of the winter eggs laid by these 

 females June 14 and following days, one hatched June 22. The 

 female from this egg gave rise to the line shown in the middle 

 column of table 38. By an extension of meaning, I shall speak of 

 the whole line derived from the cross between the New York 

 female and Baltimore male as Fi, and of the whole line derived 

 from the cross between an Fi female and a New York male as F2. 

 With F2 are compared those parts of the parent (Fi and New 

 York) lines which occurred simultaneously with it. 



A comparison of table 37 with table 38 is most interesting. In 

 the former, where two rotifers in no way related to one another 

 were crossed, the Fi gave rise to a line having a much higher pro- 

 portion of male-producers than either parent line; in the latter 

 table, where two related rotifers were crossed, the pure line result- 

 ing from the F2 has a proportion of male-producers intermediate 

 between those of its two parent lines. It may also be remarked 

 that in the former experiment Fi contained larger families and pro- 

 duced more generations in a given time than did either parent line ; 

 whereas, in the latter experiment, the size of family in F2 and the 

 number of generations in a given time is intermediate between 

 those of the two parent lines. If this is not an isolated case, it is 

 important. 



