PROBLEM OF SEXUAL SELECTION IN DROSOPHILA 365 



It appears that sexual selection is not involved in any of 

 these cases. The impression gained from observation of over 

 1,000 " contests " of this sort is that the outcome is not a matter 

 of choice. A female, in the great majority of cases, seems to 

 allow the first active amorous male that comes along to pair 

 with her; or, if she is disinclined to mate, resists all males appar- 

 ently indifferently. When a male is sexually excited he pairs 

 with the first female he finds which will allow him to. As a 

 result, the more active and vigorous males are likely to win 

 their contests, and the greater the difference in vigor, the greater 

 the proportion of times the better male wins. This is just what 

 the results show to happen. So far as the small numbers show, 

 the vermilion male is at no disadvantage and the curved male 

 is not quite able to hold his own, while it is certain that the 

 yellow and white are far behind their normal opponents. 



In the converse case, a less vigorous female will be less likely 

 to resist or escape from the male successfully if she be disin- 

 clined to mate. But this influence should have less effect on 

 the result than the one discussed above, since both females 

 will often be willing to mate and will not try to escape. This 

 again agrees exactly with the facts as given above. 



The two results of unequal vigor discussed above seem to 

 me quite adequate to explain all the results obtained. There 

 is no evidence of any ' choice ' on the part either of males 

 or of females. Unlike the abnormal venation studied by Lutz, 

 these mutants probably have no significance from the point of 

 view of sexual selection, in the narrower sense of that term. 



SUMMARY 



Experiments indicate that sight is not essential in sex recog- 

 nition in Drosophlia. The olfactory and tactile senses are prob- 

 ably both concerned, as in most other insects. 



The wings of the male play a conspicuous part in normal 

 courtship. Experiments with males from which the wings had 

 been removed indicated that the function of these organs in 

 courtship is the production of sexual excitement in the female. 



Experiments were carried out with four mutants (white eyes, 

 vermilion eyes, yellow body color, and curved wings), involving 

 the observation of 839 contested matings. 



As a result of these experiments it seems probable that the 



