63 



\\VM.\N KKKI) C.RKEX. 



prevent excessive evaporation. In each of these experiments 

 four containers were used, of about one pint capacity, two thirds 

 lull of water at room temperature when the Daphnia were in- 

 troduced. Each vessel contained a sufficient quantity of green 

 alga:' for 25 females. These containers were placed in an electric 

 parafin bath and the temperature maintained at 28 C. The 

 first one or two broods produced under high temperatures were 

 found to contain the usual proportions of males and the two kinds 

 of females (see tabulated summary of experiment 3). These 

 first broods were discarded. Thus all of the offspring in the 

 tabulated results of one of these experiments given below, passed 

 their entire ontogeny at a temperature of 28 C. 



Total number of offspring . . . I 288 



The most important point to be noted in these results is that 

 all kinds of offspring continued to appear to the last. Although 

 the percentages vary somewhat, they are within the limit of 

 normal variation. Other experiments were performed to de- 

 termine the effect of lower temperatures. All of the kinds of 

 offspring were secured at a temperature of 14 C., but the general 

 metabolism is so much lowered at this temperature that the 

 individuals were too few to justify any conclusions as to the ratio 

 of the sexes. Both low and high temperatures, since they lower 

 the rate of metabolism, induce a decrease in the average size of 

 the broods, in the number of broods, viability of the offspring, 

 and shorten I he lives o! the females used in experimenting. In 

 this connection it is of interest to note that females which were 

 reproducing parthenogenetically have been obtained every 



