16 SELECTION IN CLADOCERA ON THE BASIS OF 



to the negative end of the tank does not in most cases really indicate 

 a negative phototropism. Such individuals, if tested a second time, 

 do not ordinarily repeat the behavior. 1 The best interpretation 

 seemed to be that individuals which reacted negatively were stimu- 

 lated by the light as their sisters were, but that a " negative" re- 

 sponse was ordinarily called forth by some accessory influence. 

 Occasionally this additional influence obviously was accidental 

 mechanical stimulation; but usually negative responses could not be 

 thus accounted for, and often they were certainly not due to this 

 cause. 



The data for negatively reacting individuals were treated as 

 though the individuals had reacted positively, except that the number 

 of such occurrences was tabulated and is frequently referred to in the 

 treatment of the data. It is questionable if, in making selections 

 in the minus strains, one was justified in selecting individuals (where 

 such a selection was possible) which went to the negative end of the 

 tank. Cladocera are so generally positive that negative reactions 

 are at once open to a question as to significance, but it is believed 

 that the possibility of an actual change or mutation producing a 

 really negatively reacting individual is sufficient justification for 

 selecting these individuals in the minus strains. 



In case a daphnid remained unaccounted for at the end of the 

 test of a brood, the tank was emptied, twice carefully rinsed, and 

 filled to the proper depth with fresh water. At all times in the 

 experiments every conceived precaution was taken to exclude 

 extraneous light, to guard against reflection from any object without 

 or small foreign body within the tank, to eliminate all mechanical 

 stimulation, and particularly to guard against the possible mis- 

 placing of an individual daphnid in the handling of the broods. 2 



In brief, in the selection experiments the entire first brood, soon 

 after its release from the mother's brood-pouch, was removed from 

 the culture-bottle, placed in a small vial, taken to a darkened 

 room, and subjected to known and always uniform directive light- 

 stimulation under carefully controlled conditions, the object being 

 to attempt to procure (by selection through many generations) a 

 strain more responsive to light than a second strain from the same 



1 In the plus strain of Line 757 minus individuals occurred in 17 broods out of 172 broods 

 from which selections were made. In 4 of these cases there were more than a single minus indi- 

 vidual (19 individuals in 4 broods). In the minus strain of Line 757 minus individuals occurred 

 in 13 of 175 actual selection tests, and in one of these cases there were 3 minus individuals 

 (table 42). 



* The pipette in which the daphnids were handled was carefully rinsed after handling each 

 brood. In conducting the selections the vials to receive the selected animals were arranged so 

 that the one nearest at hand was the one in which the next individual removed from the tank 

 was to be placed. A small opening was made in the cardboard screen in order to permit a small 

 amount of light to fall upon the observer's watch and note-book. The pipette could readily be 

 examined in this light and the animal seen within it, in case there was any doubt about the 

 daphnid having been drawn into the pipette when its removal from the tank was attempted. 

 The table on which the experiments were conducted was closed off from the remainder of the 

 room by a black curtain. 



