42 SELECTION IN CLADOCERA ON THE BASIS OF 



to bear on the general vigor of the stock were tabulated. In con- 

 sidering this point three criteria of the general vigor of the material 

 were used, the age of the mother at the time her first brood was 

 produced, the number of young in her first brood, and the interval 

 between the first and second broods. From a general knowledge of 

 the material and use in other connections of these measures of the 

 vigor of the material, the writer can state that these are satisfactory 

 criteria. Figure IA shows graphically the average number of young 

 in the first brood for Line 695 by the two-month periods of the 

 experiment. Figure IB shows the average age of mother for the 

 same periods. 



1913 1913 1914 1915 1916 



4-5 6-9 12-1 4-5 8-9 12-1 4-5 8-9 12-1 4-5 8-9 12-1 4-5 8-9 



00 



5 



I 



10 



B 



!' $5 S* 8 P SJ ^ 8 s** 



4-5 8-9 12-1 4-5 8-9 12-1 4-5 8-9 12-1 4-5 8-9 ^ 12-1 



I 



1912 1913 1914 1915 T 1916 



C . , * T 5 U 



FIGURE 1. Line 695. Reproductive vigor. 



A. Average number in the first brood by two-month periods. The vertical scaling above 

 the base-line indicates the number of young; the horizontal scaling, the two-month periods of 

 the experiment. Solid circles show positions for the plus strain; open circles for the minus strain. 



B. Average age of mothers at time first brood was produced. Vertical scaling below the 

 base-line indicates the number of days. 



C. Actual values of reproductive indices (average number of young in first brood divided 

 by average age of mother at time first brood was produced). Vertical scaling indicates the 

 numerical values of the reproductive index. Each numerical value of the reproductive index 

 (in C) is obtained by dividing the corresponding numerical value in A by the corresponding 

 numerical value in B. 



