20 SELECTION IN CLADOCERA ON THE BASIS OF 



selection (if an effect be obtained) than a summary of the entire 

 selection data. The difficulty is that the amount of this data is so 

 limited. For example, there are only 41 of these same-day broods 

 for Line 695, out of a total of 192 broods of the plus strain and 180 

 broods of the minus strain for which data were secured. 



Possible Criticisms of Methods of Selection. 



Certain possible criticisms of the methods used in the selection 

 experiments occurred to the writer and doubtless may occur to others. 



1. Was there assurance that the individual daphnids selected 

 were really the most and the least positively reacting individuals in 

 their respective broods? (a) Was there reason to think that the 

 individual daphnid first reaching the positive end of the tank was the 

 one most influenced by the light? (6) May not this individual have 

 been affected by a slight swirl in the water in being released from 

 the cylinder or in some other manner subjected to mechanical stimu- 

 lation? (c) Or may it have been affected differently from its fellows, 

 due to some other condition external to itself? (d) Or may it have 

 been influenced by some internal factor or factors and consequently 

 its reaction determined by some "physiological state?" (e) Or may 

 its reaction-time not have been influenced by its general vigor or by 

 its ability as a swimmer? (/) Was the method of handhng such as 

 in any manner to unfavorably affect the animals and thus render 

 their activities untrustworthy? 



2. If the individual selected was really the most positively photo- 

 tactic at the time of the selection, was it also the most positively 

 phototactic an hour or a day later? 



3. Was the selection method used calculated to produce a more 

 vigorous race as compared with a less vigorous race? 



4. Assuming real and lasting differences in light-reaction be- 

 tween sibs, are these differences heritable anyway? 



If one accepted these possible criticisms in their fullest impli- 

 cation it would at once be seen that the results here set forth were 

 impossible of attainment, and the result with Line 757 is perhaps 

 sufficient answer to these questions. The writer does not believe 

 that these possible criticisms are at all damaging to or can be right- 

 fully applied to these experiments; however, they will be discussed 

 individually. (1) and (a) under it will be discussed after its sub- 

 sidiary questions, (6) to (/), have been considered. 



(6) Observations directed upon the effect of mechanical stimu- 

 lation show that a daphnid when mechanically stimulated usually 

 moves promptly and rapidly; frequently the movement is from the 

 source of mechanical stimulation if that is directive; but if non- 

 directive, the movement is equally prompt and vigorous, though 

 usually not different in direction from that in which the animal was 



