A PHYSIOLOGICAL CHARACTER. 21 



previously oriented. In either case the effect is ordinarily very 

 temporary. 



In the vast majority of cases, in making the selections, when the 

 cylinder was lifted to release the animals the daphnids at first failed 

 to move at all or moved very slowly and only later began swimming 

 or increased the rate of movement. In spite of all precautions, 

 doubtless some individuals were mechanically stimulated and thus 

 influenced in their reactions; occasionally such was obviously the 

 case. But while such stimulation might influence the rate of move- 

 ment and occasionally the direction of movement, these effects were 

 temporary and the occurrences of such influences were to all appear- 

 ances infrequent, so that it is believed the aggregate effect of such 

 influences upon the selections is of little or no significance. 



(c) Unquestionably it is true that even when every precaution 

 is taken to secure uniformity of conditions and to eliminate all 

 extraneous light, to eliminate disturbances of the direction of the 

 incident rays of light, to prevent mechanical or other accessory 

 stimulation, and to provide uniformity in handling the animals, etc., 

 perfect conditions are not attainable. Imperfect experimental con- 

 trol of environmental conditions is obvious enough and is referred to 

 elsewhere in this paper (see pages 15, 17-20, and 128-149). But there 

 are no grounds for assuming that these imperfectly controlled environ- 

 mental conditions were differential or in any appreciable way affected the 

 selections. To assume that in any considerable number of the selec- 

 tions differential stimulation occurred would, we believe, go beyond 

 the facts and be quite unwarranted. 



(d) It is obvious that conditions internal to the animals them- 

 selves are not subject to complete control. The writer does not 

 doubt that " physiological states'' were occasionally a factor, and 

 sometimes a determining factor, in individual selections, but he does 

 not believe that internal conditions were frequently a determining 

 factor. In cases of repeated tests of the reaction-times of a number 

 of individual daphnids, sometimes several hours or even 2 or 3 days 

 apart, the later results usually agreed with the earlier results i. e., 

 the animals which at the first test were most responsive to the light 

 on subsequent tests were the most responsive or among the most 

 responsive individuals. Hence there is excellent reason for believing 

 that the individuals selected were actually the most responsive and 

 the least responsive individuals of their respective broods. But 

 assuming that temporary physiological states did sometimes influence 

 the selections (as must occasionally have been the case), it would 

 merely operate to render the selections less effective, and unless the 

 internal temporary physiological factors were factors in a prevailing 

 number of the selections, they could not neutralize the selections. 



(e) The individuals selected in the plus strains may have been 

 merely the best swimmers, regardless of general vigor, or they may 



