No. 581] SHORTER CONTRIBUTIONS AND DISCUSSION 319 



males and all red females — the normal dominant color. This 

 proved then that the new fly did not contain the factor for cherry. 



It was then suggested that this was a second case of the appear- 

 ance of vermilion in mutant stock. Since the fly was a male 

 several matings were possible, and it was therefore crossed to a 

 vermilion female. As in the previous case if vermilion were 

 common to both stocks, the offspring should be all vermilion. 

 This condition was actually found in all the F, offspring of this 

 second cross, and the F 2 's gave vermilion and the orange-eyed 

 fly — now shown to be vermilion purple— in approximately 3:1 

 classes. This demonstrated that a mutation to vermilion had 

 taken place in a fly already having purple eyes, for as the cherry 

 cross indicated, the pure mutant stock contains the normal 

 dominant allelomorph to every factor except the one it shows. 



In order to demonstrate that the purple factor was still pres- 

 ent unchanged in the germ cells of the double recessive (ver- 

 milion purple) fly, the original male was also mated to pure 

 purple-oyed stock. As in the other cases if both parents contain 

 the factor for purple all the offspring should be purple. This 

 was the actual result obtained, and it proves the original mutant 

 to have been the double recessive vermilion purple. 



In this case also it is not possible that the result could have 

 been due to contamination. This would indeed be highly im- 

 probable when the original parents were a single isolated pair, 

 the female of which was a virgin when first mated. But even 

 had a vermilion male been able to mate with the heterozygous 

 female only red-eyed flies could have been produced. The ver- 

 milion purple combination could not occur because the germ 

 cells of each animal carry the normal dominant allelomorph of 

 the mutation in the other. 



Our results in these two cases show that mutations within other 

 mutant stocks occur, and they also indicate that in the case of 

 vermilion we have a mutation which has recently reappeared 

 twice. T. H. Morgan, 



Columbia University Harold PLOUGH 



THE EVENING PRIMROSE VARIETIES OP DE VRIES 

 No explanation of the "variation" of heterozygous plants had 

 presented itself until Mendel went back to the haploid generation, 

 and referred the differences in the progeny of hetero/.ygotes to the 

 segregation of differences in the pollen-grains and embryo-sacs 

 from which the plants had arisen. He dealt, however, with plants 



