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. 
o i f. —~—-Male abnormal x Female normal 
OS jp \ vseeeMale normal x Female abnormal 
— 7 i \ 
6 f \ ” 
Ge \ eA 
& 4 i ° \ mn = so / x “ig 
[eee - x eater 
Qe i. mS : aad y 4 
ae; New Ne hal 
FE ppt $A or 5 ede OP i ae ees nee Se 
oes is 2.5 os 45 2°55. 65... 75 “Bs "Ss 
Percentage of abnormal individuals 
Fria. 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 keptup. 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 wing's 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 
