Mating European moths with European, or Japanese with Japanese, produced pure, uniform sex-types, male and female. But a cross of European with Japanese strains resulted in intersexes. Goldschmidt concluded that (1) all individuals carried the genetic basis for both sexes; and (2) that these basic factors were two chemicals of enzyme nature. One of these he called Andrase, enzyme producing maleness, the other Gynase, enzyme producing femaleness. Further, (3) since each chemical sex determiner is present in both individuals in every cross, there must be two chemical "doses" of maleness and two of femaleness struggling for mastery in each fertilized egg. (4) If the total dose of maleness exceeds the total dose of femaleness, the sex will be male, and _vice versa_. (5) These quantities get fixed by natural selection in a single race which always lives in the same environment, i.e., the doses of maleness and femaleness in a given sex always bear practically the same relation to each other. Hence the types are fixed and uniform. (6) But different races are likely to have a different strength of chemical sex-doses, so that when they are crossed, the ratios of maleness to femaleness are upset. Often they are almost exactly equal, which produces a type half male and half female--or 2/3, or 1/3, etc. The proof of this theory is that it solved the problems. Goldschmidt was able to work out the strengths of the doses of each sex in his various individuals, and thereby to predict the exact grade of intersexuality which would result from a given cross. Riddle's work on pigeons [5,6] brings us much nearer to man, and suggests the results noted by both Goldschmidt and Lillie. As in the Free-Martin cattle, there is an apparent reversal of the sex predisposition of the fertilized egg. As in the gypsy moths, different grades of intersexes were observed. In the pigeons, it was found that more yolk material tended to produce a larger proportion of females. The most minute quantitative measurements were made of this factor, to eliminate any possibility of error. The chromosome mechanisms practically force us to suppose that about half the eggs are originally predisposed to maleness, half to femaleness. A pigeon's clutch normally consists of two eggs, one with a large yolk and one with a small yolk. But the half-and-half numerical relation of males to females varies considerably--i.e., not all the large-yolked eggs produce females or all the small-yolked ones males.