8 



EXPERIMENTS WITH DROSOPHILA AMPELOPHILA. 



equaled in the classes where the two wings markedly differ, indicates a 

 definite positive correlation. 



Furthermore, when one wing is abnormal the chances that the other 

 one will be abnormal also are 62 in 100 in the case of the males and 74 

 in 100 in the case of the females. This is an estimate based upon 4,000 

 pedigreed individuals. It will probably not hold for wild flies, since a 

 large part of the 4,000 were from the abnormal strain; hence the esti- 

 mated chances are larger than they would be in nature, because, as will 

 be shown shortly, there is a close relation between the percentage of 

 abnormal offspring in a family and the likelihood that an abnormal fly 

 will be abnormal in both wings. It does, however, give an idea of the 

 correlation which exists between the two wings with respect to the 

 presence or absence of abnormal venation when such abnormalities are 

 well fixed, and it brings out the further point that there is a sexual dif- 

 ference to be considered. 



SEXUAL DIMORPHISM. 



The females show a greater tendency to be abnormal than do the 

 males, and, when abnormal, their abnormalities are, on the average, 

 more intense than those of the males. The first of these points is illus- 

 trated in table 4 and fig. 51. Table 4 shows the percentage of abnor- 

 mal males and females in 200 families. It will be noted that as the 

 percentage of abnormal males increases the percentage of their sisters 

 which are abnormal increases until the latter have become practically 

 100 per cent abnormal. Then, since they can go no further, their 

 brothers gain on them in abnormality until we get families in which 

 100 per cent of both males and females are abnormal. In fig. 51 the 

 crosses show the position of the mean percentage of abnormal sisters for 

 each 10 per cent grade of abnormal brothers. A line is drawn to show 

 the condition when for each per cent of male abnormality the female 

 abnormality is 1.5 per cent. Thus, when 40 per cent of the males are 

 abnormal, 60 per cent of their sisters are abnormal. Corresponding to 

 60 per cent male abnormality, we get 90 per cent female abnormality. 

 Beyond that the females can go little further, hence the line becomes 



