A PHYSIOLOGICAL CHARACTER. 



35 



a better treatment of the data is secured by considering it by unit 

 groups larger than a single brood. Convenient units of comparison 

 are month and two-month periods, and summaries of the data by 

 such periods will be referred to rather than the data by broods. 



Tables 5 and 6 give, in successive columns, the two-month 

 periods; the generations of descent during the periods; the number of 

 broods used in selection; the total number of young tested; the 

 average number of individuals per brood; the average age of the 

 mothers at the time their first broods were produced; the average 

 number of young per day of the mother's age at the time of produc- 

 tion of the first brood — the reproductive index; the total number of 

 individuals moving to the negative end of the tank; the number of 

 individuals failing to reach either end of the tank during the time of 

 the test (15 minutes); the average minimum reaction-time for the 

 different broods; the average maximum reaction-time; the sum of 

 the individual reaction-times (for computation purposes) ; the mean 



tested for use in selection. 



