36 



SELECTION IN CLADOCERA ON THE BASIS OF 



individual reaction-time; the standard deviation of this mean; and 

 its probable error. 



For the minus strain, table 6 gives three additional columns: the 

 difference between the means for the plus and the minus strains ; the 

 probable error of this difference; and the quotient of this difference 

 divided by the probable error. 



These tables are divided into sections, each containing the data 

 for a period of a year, except the data obtained previous to August 



TABLE 6. Selection data summarized by two-month periods for Line 695 minus. 



Including all the data available, some of which is additional to that for the broods tested. 



b All available data is here used, including that for No. 2 mothers whose broods were not ordinarily tested for use in selection. 



1, 1912, and subsequent to July 31, 1916. The data obtained previ- 

 ous to August 1, 1912, deserve less serious consideration than may 

 be given the data obtained later, since previous to that time improved 

 methods of handling the material and additional experimental pre- 

 cautions were occasionally being devised and utilized. From August 

 1, 1912, the methods of handling the Cladocera material were pretty 

 well standardized and there seems every reason to believe that the 

 data are such as would be obtained by repetition of the experiments. 



