A PHYSIOLOGICAL CHARACTER. 15 



to as its reaction-time.^ The test was continued for 15 minutes, 

 unless all individuals had reached an end of the tank earlier; any- 

 remaining at the end of 15 minutes were removed and arbitrarily 

 assigned a reaction-time of 15 minutes.^ Another brood was then 

 tested in a like manner. 



In testing a brood of the plus strain, the individual first reaching 

 the positive end of the tank was at once placed in a separate vial 

 and later transferred to the No. 1 bottle of the new generation. The 

 second one to reach the positive end was likewise placed in a separate 

 vial, to be later transferred to the No. 2 bottle. The others were 

 returned to the vial in which they had been conveyed to the experi- 

 mental room. Three or more of these were later transferred to the 

 "* prinie ^'' bottle of the new generation. With the broods of the minus 

 strains the procedure was the same, except that the two quickest to 

 reach the negative end, the two which moved farthest toward the 

 negative end, the two moving least toward the positive end of the 

 tank, or the two slowest in reaching the positive end were selected 

 for the No. 1 and No. 2 bottles of the new generation. Negatively 

 reacting individuals did not occur in most of the broods, nor were 

 there usually individuals which showed any tendency to react nega- 

 tively to light. With D. pulex and D. longispina few individuals 

 failed to move to the positive end within the limits of the experiment. 

 Hence very frequently in the minus strains the No. 1 and No. 2 

 individuals selected were respectively the one slowest and the one 

 next slowest in reaching the positive end of the tank. 



As with the data for the over-time individuals, the data for the 

 negatively reacting individuals presented some difficulty. The 

 occurrence of negatively reacting individuals was irregular and more 

 or less spasmodic. Table 2 presents some data illustrating this 

 point. Reference to any of the tables presenting the data by broods 

 wiU show that when negatively reacting individuals occur they are 

 frequently relatively numerous. The writer believes negatively re- 

 acting individuals are (usually, at any rate) influenced by some 

 unusual environmental factor and that an individual's swimming 



^The "reaction-time" as recorded in the notes and used in this paper indicates, in seconds, 

 the interval between the release of the young daphnid from the glass cylinder in the middle of 

 the experimental tank and the time at which it reached the end of the tank. Strictly speaking, 

 this is the real reaction-time — the time consumed in the beginning of movement and estab- 

 lishing orientation with reference to the light — plus the time consumed by the animal in swimming 

 to the end of the tank. 



*The arbitrary assumption of 15 minutes as the reaction-time of individuals which as a 

 matter of fact did not react within that length of time is open to criticism, but no better method 

 of utilizing this significant portion of the data was ascertained. The " over-time " individuals 

 can not be disregarded. The data for them are obviously very significant. In a slightly reactive 

 strain, such as the minus strain of Line 757, they constitute a large portion of the individuals 

 tested and represent a striking manifestation of the low responsiveness to light in this as in other 

 strains. As pointed out in another connection, the arbitrary assumption of 900 seconds for 

 their reaction-times greatly minimizes the rightful influence upon mean reaction-time of 

 their slight or non-reactiveness to light. 



