14 SELECTION IN CLADOCERA ON THE BASIS OF 



the appropriate distances from the tank to obtain an illumination 

 of 120 candle-meters, calculated to the middle of the tank.^ 



The heat-screen, 4.5 cm. thick, with parallel glass sides and 

 filled with distilled water, was placed at the end of the tank through 

 which the illumination was received. An upright black cardboard 

 screen was placed between the heat-screen and the end of the tank, 

 so as to permit rays of light to enter the end of the tank only in the 

 area between the bottom of the tank and the level of the surface 

 of the water. The water within the tank was filtered pond-water 

 and was maintained at a depth of 1.8 cm. Fresh water was placed 

 in the tank each day and a change of water was made after testing 

 and selecting from each four broods. 



The young animals to be tested, consisting of entire first 

 broods, were removed from the mothers' bottles by means of a small 

 pipette, and taken to the experimental room in 10 mm. cylindrical 

 vials. The vials were placed in a wire basket and immersed to 

 about 3 cm. depth in a dish of water of the same temperature as 

 the water in the experimental tank and left about an hour before 

 the tests were begun. The temperature of the experimental room 

 varied from 14° C. during the coldest weather in winter to sometimes 

 as high as 22° C. in summer. Very rarely the temperature was as 

 low as 10° C. or as high as 24° C. 



In preparation for the testing of each brood, the entire brood 

 was (by means of the small pipette) placed in the center of the 

 experimental tank within an upright cylinder of glass 1 cm. in diam- 

 eter. The experimental light was turned on and all extraneous 

 light was eliminated. The animals were then left undisturbed 

 within this cylinder for 2 minutes, when the cylinder was lifted care- 

 fully from the water, thus releasing the animals in the center of the 

 tank; the temperature of water and time of release of the animals 

 were recorded just before the release. The illumination within the 

 water was sufficient to make the young daphnids just visible to the 

 eye of one accustomed to working with them. As soon as a daphnid 

 reached either end of the tank it was removed and the time of its 

 arrival recorded, together with the proper sign to indicate to which 

 end of the tank it had gone (see page 12). The interval between the 

 release of the animal and its arrival at the end of the tank is referred 



' No allowance was made for the diminution in amount of light due to its passage through 

 the heat-scrcon, the plate-glass end of the tank, and the water within, nor was allowance made 

 for the somewhat different results in illumination due to using lamps of different candle- 

 power. While the distances at which the lamps were used were such as to give a calculated 

 illumination of 120 candle-meters at the middle of the tank, the illumination at the ends of the 

 tank would of course be somewhat different with the lamps differing in candle-power. This 

 fact was not taken into consideration in originally substituting a 60-watt Mazda lamp (about 

 75.3 candle-power) for the carbon-filament lamp of 43.5 candle-power. Having for a time used 

 the lamp with the higher candle-power, it was considered wise to continue its use. Any variation 

 in absolute intensity of the light at the different parts of the tank is regrettable, but it should 

 of course be equally effective with the plus and minus strains and presumably could not disturb 

 the course of the experiments. 



