CASTLE. — INBREEDING, CROSS-BREEDING, AND SELECTION. 775 



The considerable proportion of individuals which pupated on the front 

 side of the several chambers is doubtless made up largely of individuals 

 which pupated at night, when the directive effect of light would be elim- 

 inated, or nearly so. White light evidently has the strongest directive 

 effect, while blue and red are about equally effective, orange and green 

 being less effective still. The observed effects are perhaps due to differ- 

 ences in the intensity rather than in the quality of the illumination in the 

 various chambers. 



It is probable that the larvae avoid light of every kiud at pupation, 

 for in each of the five cultures more pupae are found on the back of the 

 chamber than on its front. No emphasis is laid on the qualitative result 

 obtained because the lights employed were not entirely pure and the 

 intensities were not uniform, so that from this experiment alone it is 

 impossible to say whether or not any one sort of ray is more influential 

 than another in determining the result. 



Age at which Sexual Maturity is attained. 



Certain experiments were made to determine the earliest time after 

 emergence of the imagoes at which fruitful matings could occur between 

 the sexes. In four cases males and females were left together for 

 twenty-four hours or less, after hatching, and then separated. None 

 of these females laid fertile eggs. Two females which were left with 

 males for thirty-nine hours after hatching and then separated both laid 

 fertile eggs. It follows that the flies become sexually mature at some 

 time between twenty-four and thirty-nine hours after hatching. The 

 time was not more precisely determined. 



7s a Male capable of Copulating more than Once ? 



Yes, as the following experiment shows. 



Four unfertilized females were placed in a jar with a single male, 

 which had not yet copulated, and left for twenty-seven hours. Each 

 female was then placed in a separate jar and the male allowed to escape. 

 All the females produced offspring. 



As a control on this experiment, two virgin females were placed in a 

 jar with food. After a few days several eggs were observed, but none of 

 these hatched. [This experiment has since been several times repeated, 

 always with the same result. Apparently Drosophila is incapable of 

 parthenogenetic reproduction. W. E. C] 



