20 EXPERIMENTS WITH DROSOPHILA AMPELOPHILA. 



of the work. The results of the seven crosses between the abnormal 

 and normal strains in the fifty-ninth generation— all that were made at 

 that time— are of interest in this connection and are shown in table 23. 

 Any theory applied to these data must accord with the following facts: 

 (1) Abnormalities occasionally appear in the venation of the wings 

 of wild Drosophila ampelophila. These are usually added veins. Since 

 evolution in the Diptera has been accompanied by a reduction in the 

 number of veins, these abnormalties are of the nature of " reversions." 

 The tendency to produce extra veins is inherited and has been increased 

 by selection. This is also true of the tendency to shortening of veins. 



m 



A Male abnormal x Female normal 



^ 8 / . V --Male normal x. Female abnormal 



I 7 ' \ 



£ 6 / \ 



% * / ... \ .-•• \ ,••' / \ v- 



fc 3 // \ \ / \ ,-• / \ 



XJ o V '. V v^. »• / V 



§ I .7 V \ K N. / N N 



Z I — ^ 



5 15 25 35 45 55 65 75 85 95 



Percentage of abnormal individuals 

 Fig. 53. 



An examination of more than 50,000 abnormal wings has revealed an 

 immense diversity of forms which the abnormality assumes. Not only 

 are new forms being constantly discovered, but the intensity of the ab- 

 normality has constantly increased as long as selection for that end has 

 been kept up. The limit of the increase was apparently not reached, but 

 the extra veins have always been very crude, only rarely assuming a 

 form and position comparable to ordinary veins. 



(2) A greater percentage of females than of males is abnormal. The 

 formula 



Percentage of abnormal sisters = 1.5 X percentage of abnormal brothers 



approximately describes the average condition in the various families. 

 Attempts to change significantly this relation have failed, and seem 

 destined to fail, for change of sex in the ancestry does not weaken 

 inheritance. 



(3) The lower range in the variation of abnormality certainly includes 

 barely discernible deviations from normality and presumably just indis- 

 cernible deviations also. The latter would be considered normal. 



(4) Frequently one wing of a fly is abnormal, the other not visibly so. 

 There is a direct relation between the percentage of abnormal offspring 

 of a given mating which are abnormal in both wings and the total per- 

 centage of abnormal offspring. Furthermore, on the average, parents 

 which are abnormal in both wings give a larger percentage of abnormal 



