508 Universihj of California Puhlications in Zoology [Vol. 13 



4. Genetic Differences. — The fact that the flies differ genetically 

 from one anotlier is shown hy the results ohtainod in mating different 

 brothers and sisters of the same family. The results of these matings 

 are shown in table XIX. 



To find out if we were dealing with a genetic factor or with one 

 purely the result of environment, two different counts were made of 

 the hatchings of the same flies, in matings between extras. The flies 

 were put in a bottle and left there for six days, then removed and jfut 

 in another bottle with fresh food, left there for the same lengtli of 

 time. The results of the two hatchings are shown in table XX, from 

 w^hich it appears that there was no significant difference in the number 

 of extras in the first and second hatchings, although the number 

 differed significantly in different families, being consistently higher 

 in the Y strain than in the X. 



Family 



Y Fog 

 Yg 



YF,a 



XjF.c c?5 $5 40 5 10 first 



C c? 5 $ 5 45 4 8 second 



Y Fscr c? 5 5-5 60 15 20 first 



d-'' J'5 ?-5 43 10 18.8 second 



The normal flies were more viable than the extra-bristled fl.ies, and 

 this may have had a disturbing influence on some of the ratios. In all 

 cases, larger cultures were obtained when two normals were mated 

 together than when two extras were mated together. Often in the 

 latter case there was partial and in some cases complete sterility. 

 Three of the nine extra-bristled flies from Y F^ generation died, one 

 was sterile, and two mated together were partially sterile, producing 

 only twenty-five flies. 



In the F^ generation of normals in the Y strain, thirteen extra- 

 bristled flies were found. Of these, two when mated together were 

 sterile and soon died, two mated to normals were partially sterile, 

 producing twenty-nine flies. Three others died. In Y® Fo from nine 

 extra-bristled flies five died, two mated together were sterile and one 

 mated with a normal was partially sterile. 



T^ 



LBLE 



XX 







AND Second 



Hatchings 



FROM THE 



Same Par 



Bristles N 



ormals 



Extras Per cent 



Hatching 



c^5 ?5 



10 



5 



21.8 



first 



c?5 $5 



25 



4 



17.5 



second 



c^5 ?5 



75 



10 



12.9 



first 



c^5 25 



37 



7 



15.8 



second 



S5 $5 



25 



3 



10.3 



first 



c?5 $5 



30 



2 



6.2 



second 



