132 CARNEGIE INSTITUTION OF WASHINGTON. 



Experiments with Daphnia. — Dr. A. M. Banta has completed the 

 work of compiling a large number of statistics gained from his experi- 

 ments on the possibility of modifying the reactions of Entomostraca 

 to light by ordinary selection. One of the most extensive of his 

 breeding-hnes is known as line 757. In it, following selection, strains 

 that reacted very dissimilarly appeared. In this line there is little, if 

 any, relation between vigor and reactiveness to light, so that the effect 

 of selection secured in this hne was not due to changes in the relative 

 vigor of the plus or minus strains, that is, of the strains selected for 

 quicker and those selected for slower phototactic response. Dr. 

 Banta says that the divergence of the two strains can not readily be 

 explained as due to mutation, for there is no point in the curve where 

 mutation may be said to have occurred. The other lines, however, 

 generally show no effective selection, and, therefore, some special 

 explanation has to be sought for to account for the divergence of the 

 reaction- time in the plus and minus strains of line 757. It is now 

 proposed to start a test series to see if the effect of selection within 

 line 757 has persisted. So far as results are secured, they seem to 

 show that the effect does indeed persist, although 2| years have elapsed 

 since selection within these strains ceased. 



The principal result of our attempts to accentuate differences by 

 breeding plus and minus strains in flies and Daphnia is to indicate that 

 improvement is effected this way only until such time as a hybrid or 

 complex condition of the genes of the trait in question is reduced to 

 the simplest degree. 



THE SIGNIFICANCE AND CONTROL OF SEX. 

 SEX IN PIGEONS. 



During the absence of Dr. Riddle from Cold Spring Harbor, his 

 assistant. Dr. Ellinor H. Behre, has completed a test of the hypoth- 

 esis that the relative staleness of sperm might be responsible for 

 abnormal sex-ratios. This experiment was suggested by the findings 

 of other investigators, especially among amphibia. It has been found 

 that the length of time during which pigeon sperm-cells retain their 

 capacity to fertilize may vary somewhat, but probably in any case 

 does not exceed 8 days. It is clear, however, from the evidence, 

 that stale sperm does not affect the sex-ratio. This is of some impor- 

 tance, since it shows that the very abnormal sex-ratios obtained in 

 Dr. Riddle's work with the pigeons is neither comphcated nor ex- 

 plained by this fact. 



SEX INTERGRADES IN DAPHNIA. 



As pointed out in earlier reports, Dr. Banta has had the good fortune 

 to find interesting and significant variations from the normal condition 

 of the sexes in the water-fleas, Cladocera, and especially Daphnia 



