INHERITANCE OF ABNORMAL VENATION. 13 



reasonably supposed to have established as " pure " a line as exists in a 

 given case, the following facts may be of interest. 



The abnormalities obtained, both in the direction of veins added and 

 of veins lacking, far surpass those found in nature in this or any other 

 insect with which I am familiar. Furthermore, they do not even remotely 

 suggest the venation of any of this fly's relatives. Something new has 

 been produced. In the strain whose early history has just been described 

 there was, at the start, no definite effort made to build up an abnormal 

 race as quickly as possible. Later I tried to do this from wild material 

 obtained from other localities. 



Starting with an abnormal male and a normal female from Boston and 

 an abnormal male and female from Bloomsburg, Pennsylvania, I rigidly 

 selected for additional veins. The record for each successive set of two 

 generations was 8.8, 5.5, 11.5, 14.3, 30.3, 45.8, 85.9, and 100 per cent 

 abnormal. Thereafter mass-breeding was practiced and the abnormal 

 strain preserved for about a year by merely starting a fresh jar every 

 couple of weeks with the most abnormal individuals found at that time. 



The abnormalities in this strain were of the same nature and extent 

 as in the one started from the Long Island material. It would seem 

 that this increase in the percentage of abnormal individuals up to 100 

 per cent and the subsequent increase of the intensity of the abnormalities 

 can not be due to the gradual weeding out of all units but the one or 

 several desired, because one quickly gets things which one can safely 

 say did not exist in the population with which we started, or, to be more 

 exact, which we do not see. Some can probably imagine that the ' 'units' ' 

 for each successive grade of abnormality existed in the parents with 

 which we started, but that they were held in check by an equal number 

 of inhibiting " units " of corresponding powers, so that the result could 

 be explained by saying that in the selection we cut out step by step suc- 

 cessively stronger inhibiting units, thus allowing successively greater 

 abnormality-producing units to manifest themselves. On any other 

 hypothesis, it seems to me, we must admit the cumulative effect of 

 selection upon a "unit," i. e., within a pure line. 



But, upon this hypothesis, how can we account for the occasional nor- 

 mal flies? Why do not the inhibiting units stay cut out after we have 

 once gotten rid of them so thoroughly that all the flies of several suc- 

 cessive generations show strong added veins? Perhaps they do stay 

 cut out and these occasional normals are merely fluctuating variations 

 in the abnormal unit. If so, and if selection does not have a cumula- 

 tive effect within a unit, it would be impossible to return to normality 

 from a series of inbred generations of abnormality. But it is possible. 

 Starting with a family which had one normal offspring in a total of 133 

 (99.2 per cent abnormal) and selecting to reduce the extra veins, the 

 percentage of abnormal offspring in successive generations was 81.8, 

 66.2, 32.9, 12.5, 17.0, 0.0, 0.0, 0.0, and so on, as a typical normal strain. 



