706 



THE AMERICAN NATURALIST 



[Vol. L 



duces the normal distribution of the two sexes. To-day it 

 is well known to every one how this insight was reached 

 practically simultaneously by the study of Mendelian in- 

 heritance and the cytological investigation of the chromo- 

 somes and how the two solutions are in most wonderful 

 harmony. The outstanding facts which I regard as one 

 of the corner-stones of modern biology are familiar even 

 to the beginner in biology. Cytologically one of the 

 sexes, the heterogametic sex, contains only one X- 

 chromosome, the other, the homogametic sex, two of 

 them. Every one knows how the maturation division 

 separates entire chromosomes and therefore the hetero- 

 gametic sex produces two kinds of sex-cells, i. e., with 

 and without X-chromosomes, but the homogametic only 

 one kind, all with X-chromosomes; and how chance- 

 fertilization produces again the two parental combina- 

 tions, that is, the two sexes. You are furthermore famil- 

 iar with the fact that the Mendelian experiments yield 

 practically the same result. If we substitute the term 

 " sex-factor" for "X-chromosome" and "hetero-homo- 

 zygous" for " hetero-homogametic, " the facts are iden- 

 tical. One sex is heterozygous for the sex-factors, say 

 Ff ; the other homozygous, say FF ; the .first one produces 

 two kinds of gametes, the second only one, and chance- 

 fertilization results in equal numbers of the parental com- 

 binations. The results of experiments on sex-limited 

 inheritance have shown, finally, that both sets of facts are 

 the same thing, only expressed in different language, in 

 other words, that the X-chromosomes are the vehicles for 

 the distribution of the sex-factors. If we state further 

 that there are animals in which the heterozygous sex is 

 the female, and others where it is the male, we know the 

 elementary facts from which any further study of the 

 sex-problem has to start. 



Now that we know the elementary mechanism of sex- 

 distribution, can we regard the sex-problem as solved! 

 I do not think so. What are the sex-factors and how do 

 they determine sex? Are the two sexes clean-cut alter- 

 natives and is it therefore impossible to transform one 



