A PHYSIOLOGICAL CHARACTER. 13 



METHOD OF SELECTION. 

 GENERAL PROCEDURE. 



Propagation of the older lines of D. pulex had progressed from 

 5 to 8 generations before the selection experiments were begun. 

 Selection with lines later used was begun at once upon introduction 

 into the laboratory. 



The culture-bottles were given uniform treatment so far as 

 possible. The plus and minus strains of the same line were kept 

 side by side on the table. The only obvious environmental differ- 

 ences to which the two strains of the same line were subjected arose 

 from the f act that the young from the two strains were transferred 

 to new food, usually on different days. While every effort was made 

 to provide uniform culture-water, only a moderate degree of uni- 

 formity was attainable, but this unavoidable difference in environ- 

 mental treatment was not differential, and in the long run should 

 have affected the two strains equally. 



Unlike the Cladocera material reared under laboratory con- 

 ditions by many workers, the writer's material produced exclusively 

 asexual young, there being with the material subjected to selection 

 no discovered case in which males or sexual eggs were produced. 



The selection tests were conducted in a darkened basement 

 room with dull-black walls. The tank was constructed of plate 

 glass, the bottom being a heavy slab of smooth slate, grooved out 

 to a depth of about 1.5 cm., the grooves being about 2 cm. wide. 

 The plate-glass sides and ends were set in these grooves in an aqua- 

 rium cement. The grooves were filled with the cement so as to leave 

 a smooth surface continuous with the surface of the slab. The 

 lines of contact of the glass sides and ends were cemented together 

 with thick Canada balsam. After the aquarium cement and balsam 

 had been given several days for hardening, the surface of the cement 

 and the inside of the glass sides of the tank were coated with a mix- 

 ture of lampblack, balsam, and turpentine. The result was a tank 

 with uniform dull-black sides and bottom and with transparent ends. 

 The tank measured inside 40 cm. by 26.6 cm. and was 7.2 cm. in 

 depth. An improvement on this tank would be made by substituting 

 thin slabs of smooth slate for the plate-glass sides. 



The source of light was at first a slender, cylindrical, carbon- 

 filament, incandescent lamp. Later similar carbon-filament lamps 

 could not be obtained and a tungsten lamp was used. The lamps 

 used varied from 43.5 to 75.9 candle-power. They were placed at 



