A PHYSIOLOGICAL CHARACTER. 163 



SUMMARY. 



1. In experiments undertaken to get additional data on the 

 effects of selection within the pure line, Cladocera material was 

 chosen because of certain advantages: (a) it reproduces rapidly, (6) 

 is readily handled in the laboratory, (c) under controlled conditions 

 it is exclusively parthenogenetic, and (d) in its parthenogenetic 

 reproduction there is a single maturation division without reduction, 

 so that presumably it presents none of the complications present in 

 material in which segregation occurs during maturation and in 

 which fertilization occurs. 



2. Selection was based upon a physiological character: reactive- 

 ness to directive light stimulation. 



3. The material consisted of three species: Daphnia pulex, D. 

 longispina, and Simocephalus exspinosus, collected at different times 

 from three ponds at Cold Spring Harbor. 



4. The animals were reared in 200-c. c. wide-mouthed bottles 

 with about 100 c. c. of culture-water which was ordinarily unchanged 

 from the time a young female was placed in the bottle until she 

 produced young. 



5. Selections were made during the first day of life of the young 

 daphnids. The plus and minus strains of each line came from a 

 single progenitor. 



6. The selections were conducted with light intensities of ap- 

 proximately 120 candle-meters. 



7. Environmental factors modified reaction-time; but, presum- 

 ably, these were horizontal influences equally operative in both strains 

 and with all the individuals tested in making each selection, and 

 hence did not influence the selections per se. 



The relation between environmental conditions and both re- 

 action-time and vigor is considered in some detail. 



(a) There are shown to be low correlations between the tempera- 

 ture of the water in the experimental tank and reaction-time of 

 young daphnids. These correlations are generally positive, meaning 

 that reactiveness is decreased with increase in temperature. This 

 result was not anticipated. It is suggested that where this relation 

 holds it may be due to greater oxygen-content in the water of the 

 experimental tank with lower temperatures. 



(6) Some substance exhaled from the observer's breath, pre- 

 sumably CO 2 , influences reactiveness. Frequent changes of the 

 water in the experimental tank reduced this influence to a minimum. 



(c) Relatively temporary chemical (?) differences in the water 

 used in the experimental tank sometimes influences reaction-time to 

 a marked degree. 



