14 EXPERIMENTS WITH DROSOPHILA AMPELOPHILA. 



The building up of an abnormal strain from a long-inbred normal one 

 was also nearly completed when it was stopped by accident. I did not 

 think it worth while to start it anew, as its accomplishment would prove 

 little, since it might be said that the normal strain was a "mixed general 

 population" due to normality (inhibiting) units masking all sorts of 

 latent abnormality units. 



In nature a small percentage of flies have the fifth longitudinal vein 

 somewhat shortened (see fig. 47) . This variation also appears in the 

 experimental strains. Rather as a matter of curiosity, I selected for 

 shortened veins during a few generations and very quickly obtained such 

 specimens as are illustrated in figs. 48 to 50. One can not go further 

 in this direction without some special technique, because the wings, 

 lacking the support of the veins, droop and catch in the fly's food. 

 Probably breeding could be continued by cutting off the parent wings 

 when matings are made. I did not try it, as it was already very evident 

 that selection was just as effective in the negative as in the positive 

 direction. 



On the other hand, all attempts to fix, by selection, some particular 

 type of abnormality utterly failed. It was thought possible that the 

 great variety of forms which the extra veins showed was due to a mix- 

 ture of a number of simple forms and that selection might isolate these 

 simple types. The most hopeful was a simple forking of the second 

 longitudinal vein (see fig. 10). Selection for this type was started sev- 

 eral times, but never went beyond the fifth generation, because, although 

 there were plenty of abnormal flies in each generation, there was no 

 increase in the number showing this particular type, and sooner or later 

 a generation would contain none of them from which to breed. The 

 same was true in the experiments aimed to fix the abnormality on, for 

 example, the third longitudinal vein, but to keep it off of the second. 

 It is easy to have all the abnormal flies abnormal only on the second 

 longitudinal vein, providing one be content with small abnormalities. 

 However, as soon as one increases greatly, by selection, the abnormality 

 on the second vein, the other veins begin to be abnormal. 



These are the facts: Starting with slight extra veins, either in wild 

 material or in material selected and inbred for normal venation, we 

 can quickly get by selection 100 per cent abnormal offspring. In future 

 generations this strain can be quickly brought back again to its normal 

 condition by selection. Selection also quickly shortens the veins and 

 would probably largely do away with them, provided some technique 

 were adopted to keep the results of selection alive. But selection, 

 accompanied by the strictest inbreeding (brother X sister and parent X 

 child) failed to isolate any unit characterized by a given form or extent 

 of abnormality. 



The interpretation of these facts would doubtless vary with varying 

 opinions as to unit-characters. 



