206 ROSCOE R. HYDE 



animals seem to be better somatically than either parent. To be 

 sm'e, their fertility inter se is much lower but it should be remem- 

 bered that while the individuals belong to one generation the egg 

 and sperm produced belong to the next. 



The following suggestion is offered as a tentative explanation: 

 It is probable that some factor or factors, let us say, regulate egg 

 production in the normal wild fly.^ Let us designate these as AB. 



Let us assume that the truncates have dropped out some factor 

 from this mechanism, then their formula would be expressed by 

 Ab. The other stock used had been inbreeding in mass cultures 

 in the laboratory and had probably lost some factor, let us say 

 A. Then their formulae would be expressed by aB. When the 

 two races are crossed, the two factors are brought together again, 

 thus ABab, and the normal egg-laying function is restored. The 

 explanation holds at least tentatively for other improvements to 

 be seen in the hybrid. The number of factors concerned is not to 

 be looked upon to be as simple as the formula would seem to indi- 

 cate. Instead of two factors, as the formula shows, there may be 

 many units concerned, but the principle is the same. The things 

 lost out of the geiTu-plasm of one are made good by those present 

 in the other and thus normal conditions are restored. If this is so 

 we should expect to find members of this species in nature that live 

 well on to one hundred days and some that have high egg produc- 

 tion and high fertility combined. They should produce well on 

 to two thousand flies. 



FERTILITY OF THE LONG-WINGED BROTHERS AND SISTERS OF 

 THE TRUNCATES 



When the long-winged flies thrown by the truncates are bred 

 together their productivity is greater than their truncate brothers 

 and sisters. This fact is brought out in table 23 where 120 is 

 recorded as the average per pair. Their truncate brothers and 

 sisters used for control produced an average of 77 offspring per 

 pair. The flies were of the same ancestry and subject to the same 

 environmental conditions. That the long-winged forms produce 



* Whether the actual formation of germ cells, in this case eggs, or the regulatory 

 mechanism that governs their actual maturation and discharge is involved is a 

 problem more difficult of solution. 



