22 SELECTION IN CLADOCERA ON THE BASIS OF 



have been the most vigorous individuals of their respective broods; 

 but on the whole there seems no good reason for believing this to 

 have been the case. If the differences were merely differences in 

 quickness of movement or swimming ability, it would be quite as 

 interesting to develop ''hare" and "tortoise" strains of Cladocera as 

 to develop strains more reactive and less reactive to light; and the 

 result would have the same fundamental bearing on the problem of 

 selection within the pure line. That the reaction-times of the vast 

 majority of individuals were not influenced by differences in vigor is 

 attested by the fact that there is no consistent relation between 

 vigor (as measured by the mean reproductive index) and mean re- 

 action-time. This point is discussed more fully in connection with 

 the detailed analysis of the data for the various lines. 



(/) The strains subjected to selection did not show greater 

 mortality nor less vigorous growth and reproductive ability than 

 those lines not used in the selection experiments. The only difference 

 in conditions through which those selected (as compared with those 

 strains not subjected to selection) passed previous to being placed 

 in the experimental tank was being placed for a time in a relatively 

 small amount of the culture-water. There is no reason to think that 

 this profoundly affected their light-reactions, or if it had, that the 

 effect would not have been a horizontal rather than a differential one. 



(1) and (la) On numerous occasions the individuals of a brood 

 were caused to react in the experimental tank a number of times. 

 Of course, on second and subsequent trials only a single individual 

 could be handled at one time in the tank because of the danger of 

 mistaking the identity of the individuals. 



Of 10 such experiments, conducted within a few days and includ- 

 ing all such experiments made wilhin those days, there was com- 

 plete agreement in reaction order in 6 experiments; in 3 there was 

 fair agreement, and in 1 there was very poor agreement in reaction 

 order. Even in the last-mentioned case, however, the individuals 

 which reacted more quickly during the first test in general reacted 

 more quickly on the average of all the trials, but the reaction order 

 was greatly changed. The objection that the selections in general 

 may have been made on the basis of vigor of individuals has been, 

 we think, effectively discounted. In view of this fact, it is believed 

 that such consistent results as those just indicated show conclusively 

 that neither physiological states nor any other factor operated to 

 obviate or hamper the selection on the basis of fundamental indi- 

 vidual differences in reactiveness to light. It is believed that in the 

 vast majority of cases the two selected in the plus strains were more 

 reactive to the illumination to which they were subjected than their 

 fellows, and that those selected in the minus strain were less positive 

 than their fellows. 



