xiv] CHROMOSOMESASBEARERSOFFACTORS 237 



greater or less amount of a sex-determining substance borne 

 by the sex-chromosomes. In the groups in which the female 

 has the constitution XX and the male X or XT ^ femaleness 

 is supposed to be due to the "double dose" of ^-chromatin, 

 while in the Lepidoptera and Birds males are produced by 

 a double dose of a (presumably) different substance in the 

 chromosome that is equally paired in the male and not in the 

 female. This idea that sex is due to a quantitative rather 

 than a qualitative difference in the chromosome complement 

 is not only simpler than the other, but is supported by those 

 cases (for example, several moths) in which the crossing of 

 two species produces either all males, or males and gynan- 

 dromorphs, when the cross is made one way, but both males 

 and females in the converse cross. This suggests that the 

 spermatozoa of one species introduce so much of the sex- 

 determining substance that all the eggs, whether they con- 

 tain a sex-chromosome or not, are partially or completely 

 turned into males, while in the converse cross the less potent 

 spermatozoa of the second species allow the sex to be deter- 

 mined by the presence or absence of a sex-chromosome in 

 the egg. 



The most important evidence in this direction has been 

 furnished byGoLDSCHMiDT's (1916) remarkable experiments 

 with races of the Moth Lymantria dispar. He concludes that 

 a male-determining factor Af, of varying potency, is borne 

 by a sex-chromosome, and is opposed to a female-deter- 

 mining factor F borne by the egg-cytoplasm. When there 

 are two sex-chromosomes, as there are in the male of Lepido- 

 ptera, MM overpowers F, and the individual is a male, 

 while when there is only one M chromosome, as in the 

 female, F overpowers M. Both F and M vary in potency in 

 different races, so that by crossing selected races it is possible 

 to get individuals in which the potency of F equals that of 



