120 



"). General Discussion. 



The following is olTered by way of explanation of the foregoing facts. 

 Let it be assumed that the complex upon which the length of life of the 

 wild fly depends is expressed by the formula AB. The Truncate stock 

 arose as a mutation from the wild stock and possibly some factor has 

 changed to a. Consequently its formula would be aB. The inbred stock 

 had been in captivity for some time, and it is possible that the B had 

 changed to b. Its formula would be Ab. On crossing these two stocks a 

 hybrid would result, the formula for which is abAB. Consequently in the 

 hybrid, normal conditions are restored, and a fly results that lives longer 

 than either parent. The same explanation holds in the case of the 

 increased egg production to be seen in the hybrid. If this is true we 

 should expect to find wild stocks that live about fifty days and with high 

 egg production and high fertilizing power of the gametes combined. They 

 should be very high producers. The number of factors, however, is not 

 looked upon to be as simple as the formula would seem to indicate. In- 

 stead of two factors as the formula shows, there may be many hundreds, 

 but the principle is the same. The things lost or changed in the germ 

 plasm of one stock are compensated in the hybrid by the factors trans- 

 mitted liy the other stock, and thus normal conditions are restored. 



6. Conclusions. 



1. Hybrids between the Truncate stock and the Inbred stock are 

 more vigorous than either parent as shown by the fact that the hybrid 

 lives 47 days while the parents live 21.4 and 37.4 days respectively. 



2. The flies from the Truncate stock live 21.4 days. The females 

 live 18.4 while the males live 26.4 days. 



3. The flies from the Inbred stock live 37.4 days. The females live 

 34.5 days while the males live 40.5 days. 



4. The shortened length of life of the Truncate stock reappears 

 among the grandchildren after skipping a generation when crossed to 

 the Inbred stock. The grandchildren lived an average of 29.5 days. 

 Those descended from Ihc Trnucate grandmother lived 2!)..^ days. The 

 males lived 32.8 days and the females lived 25.9 days. The flies de- 

 scende<l from the Truncate grandfather lived 29.3 days. The males lived 

 .".1.1 (l;i.\s, wliilc I lie females lived 27. .'l da vs. 



