A PHYSIOLOGICAL CHARACTER. 117 



single brood in the first longer period failed to contain individuals 

 which did not respond during the 15 minutes to the test. In the 

 final longer period nearly half the broods tested contained no over- 

 time individuals. Of the last 10 broods tested, only 3 contained 

 over-time individuals and the last 5 broods contained none. The 

 change in reactiveness in this strain is also strikingly indicated by 

 again referring to the average minimum reaction-times in the first 

 and last periods. For the first nine-month period it was 468 seconds; 

 in the last period 94 seconds. These figures, it will be recalled, refer 

 only to the most reactive individuals of each brood, but when these 

 have become 5 times as reactive as during the early period of the 

 experiment, the fact can not fail to have considerable significance. 



The change in responsiveness in the plus strain of Line 757 was 

 in no other way as striking to an observer as the change in the general 

 behavior of the animals in the experimental tank. Early in the 

 history of this strain the young daphnids, on being released in the 

 experimental tank, ordinarily behaved as follows: first they settled 

 to the bottom in a rather close group; after 3 to 5 minutes a few 

 (usually less than 30 per cent) moved a little — ordinarily toward the 

 source of light, though many movements were indifferent to the 

 light. (In about a third of the earlier selection tests of entire broods, 

 there was no movement at all after the animals settled to the bottom 

 of the tank, though these were vigorous individuals.) Of the small 

 number swimming toward the light, about two-thirds, in spite of 

 many pauses, made their way to the end of the tank within the 

 period (15 minutes) of the test. Toward the close of the selection 

 experiment with Line 757, the behavior of the 757 plus strain was 

 essentially as follows : the animals, on being released, ordinarily settled 

 to the bottom as previously, but within a few seconds some of 

 them began to swim toward the light and within a minute or two all 

 were usually under way toward the positive end of the tank, which 

 the majority reached with few interruptions of their swimming 

 movements. In the earlier part of the experiment the animals 

 frequently did not move a second time after once swimming for 

 a moment and then settling to the bottom or holding fast to the 

 surface film or the sides of the tank. At the close of the experiment 

 the movements in the plus strain were not only more prompt and 

 general, but were less often interrupted, and when interrupted were 

 generally resumed rather promptly. 



There was to the eye of the observer possibly some increase in 

 reactiveness of the 757 minus strain as exhibited in its general be- 

 havior, but it was slight as compared with the change which occurred 

 in the 757 plus strain. 



The lowered mean reaction-times for the strains of 757 during 

 the course of the selection experiment are in partial accord with 

 the general lowering of the mean which occurred for all of the strains 



