DEPARTMENT OF EXPERIMENTAL EVOLUTION. 131 



induces a third or a chain of events which are the true causes of the 

 high mortahty. Consequently it becomes of importance to know what 

 other characters are correlated with weight of the seed that has been 

 planted. Dr. Harris has shown that in beans there is a sensible rela- 

 tionship between the weight of seed planted and (1) the number of pods 

 on the plant that develops out of that seed; also (2) the number of 

 ovules and the number of seeds in the pod produced by the plant 

 developing from it. 



Factors Influencing the Weight of Seeds and their Number in a Pod, 



J. A. Harris. 



In beans, the weight of seeds decreases as the number of seeds per 

 pod increases; also, the seeds are the heavier the farther they are from 

 the attached end of the pod. In fact, the free end of the pod seems 

 more favorable for the fertilization and development of the ovules in 

 the pod. But, remarkably enough, the number of ovules per pod 

 seems not to influence the weight of the seeds eventually developing 

 in the pod in beans. In another legume, Cercis canadensis, there is a 

 marked positive correlation between the number of ovules and of ripe 

 seeds in a pod. As a corollary about the same proportion of the 

 ovules are fertilized and develop, whether the number of ovules be small 

 or great; if anything, the pods with the larger number of ovules pro- 

 duce relatively fewer seeds. Dr. Harris has summed up results of his 

 recent work on selection in a paper, ''Current Progress in the Study of 

 Natural Selection," published in the Popular Science Monthly. 



Selection of Strains of Daphnia with reference to Reaction to 

 Light, A. M. Banta. 



The selection in a parthenogenetic species for greatest and least 

 sensitiveness to light has been continued now for 110 generations in 

 some cases and the number of individuals measured approaches 20,000. 

 The results continue to show an effect of selection in some lines, but not 

 in all, and the differences are in most cases statistically significant. 

 The differences produced by selection have become largest in lines of 

 the species (Simocephalus) which for a long time seemed the least 

 responsive to selection. 



Incidentally to the foregoing investigation, the result has been gained 

 that 100 generations of Daphnia pulex have been reared parthenoge- 

 netically without sexual forms appearing at any time. There is no 

 evidence of decreased vigor or loss of vitality in the lines. Hence, it 

 appears that there is not a necessary sexual cycle in the reproduction of 

 this daphnid. These observations lend additional evidence to the con- 

 clusion that the sexual cycle in Daphnia is not an inherent, necessary 

 thing, but that it is determined by external conditions. 



