A PHYSIOLOGICAL CHARACTER. 19 



test series were planned. These test series were conducted under a 

 plan by which large numbers of broods of the plus and the minus 

 strains of the same line were experimented upon 072 the same day 

 and as nearly as was experimentally possible under precisely the same 

 conditions. The plan was as follows: First, it was necessary to 

 wait until such time as the plus and minus strains of the same line 

 reproduced on the same day. A number of these young, from 12 

 to 20, of each of the strains were then transferred to individual 

 bottles, given the same food, kept grouped together on the same 

 table, and in every way treated alike. When their first broods 

 appeared, four broods of equal or nearly equal size two from the plus 

 and two from the minus strain were chosen, the choice being limited 

 to broods released from the mother's brood-pouch within 2 or 3 hours 

 of the same time. Thus we had broods containing equal or nearly 

 equal numbers of individuals, of the same age (within 2 or 3 hours), 

 and from mothers of the same age which had themselves received 

 identical treatment from birth; other " quartettes " were selected in 

 like manner. Further, the four broods, constituting a quartette, 

 were handled in a definite order; a plus brood was experimented with 

 first, then a minus brood, then the second minus brood of the quar- 

 tette, and finally the second plus brood was tested. The tank was 

 then emptied and replenished with fresh water. The second quar- 

 tette was handled in reverse order: first a minus brood, then (in 

 order) the two plus broods, and the second minus brood. The next 

 quartette was then handled in the order indicated for the first quar- 

 tette. Still other quartettes of broods were chosen from the second 

 and later broods of the test-series mothers until the numbers of in- 

 dividuals tested from each strain were quite large in most cases 

 larger than 600 and in several cases in excess of 1,000. 



The test-series, it was hoped, might serve as a means of eliminat- 

 ing most of the disturbing factors unquestionably present during the 

 reaction-tests, due to the fact that the selections in the plus strain and 

 the minus strain of the same line were usually, through necessity, 

 made on different days. 



SAME-DAY BROODS. 



The two strains of the same line only occasionally reproduced 

 on the same day, so that consecutive reaction-tests of a brood of 

 each of these two strains were not ordinarily possible; however, 

 during the whole course of the selection experiments, there were a 

 number of selections in the plus and minus strains of the same line 

 on the same days. In addition to the complete tabulation, the data 

 for these "same-day" broods are tabulated separately. Such a 

 tabulation for the two strains of Line 695 is given in table 8. Such 

 a comparison of the mean reaction-times of the same-day broods 

 ought perhaps to afford a safer criterion of the effectiveness of 



