Intersexes in moths; Bird intersexes; Higher metabolism of males; Quantitative difference between sex factors; Old ideas of intersexuality; Modern surgery and human intersexes; Quantitative theory a Mendelian explanation; Peculiar complication in the case of man; Chemical life cycles of the sexes; Functional-reproductive period and the sex problem; Relative significance of physiological sex differences.
Crossing European and Japanese gypsy moths, Goldschmidt [[1, ][2, ][3, ][4]] noticed that the sex types secured were not pure—i.e., that certain crosses produced females which bore a distinctly greater resemblance to the male type than others, and vice versa. One of these hybrids of "intersexes," as he calls them, would always possess some female and some male sexual characters. He found that he could separate the males and females, respectively, into seven distinct grades with respect to their modification toward the opposite-sex type, and could produce any one of these grades at will by breeding.
For example, the seven grades of females were roughly as follows: (1) Pure females; (2) Females with feathered antennæ like males and producing fewer than the normal number of eggs; (3) Appearance of the brown (male) patches on the white female wings; ripe eggs in abdomen, but only hairs in the egg-sponge laid; instincts still female; (4) Instincts less female; whole sections of wings with male colouration, interspersed with cuneiform female sectors; abdomen smaller, males less attracted; reproduction impossible; (5) Male colouration over almost the entire wing; abdomen almost male, with few ripe eggs; instincts intermediate between male and female; (6) Like males, but with rudimentary ovaries and show female traits in some other organs; (7) Males with a few traces of female origin, notably wing-shape.
The males showed the same graded approach to the female type. Their instincts likewise became more and more female as the type was modified in that direction. That is, a moth would be 12% or 35% female, and so on.
Goldschmidt watched the crosses which produced seven different grades of maleness in his females. The moth material, like the birds and mammals, suggested a dual basis for sex in each individual. The grades of maleness and femaleness made it seem probable that the factor which determines sex must be stronger in some instances than in others, i.e., that the difference between two of these grades of female is originally quantitative, not qualitative—in amount rather than in kind.
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.
Wild pigeons begin the season by throwing a predominance of males, and the first eggs of the clutches also tend to produce males all along. In both cases, the male-producing eggs were found to be the ones with the smaller yolks. Family crosses also produce small yolks, which hatch out nearly all males. Some pairs of birds, however, have nearly all female offspring. Riddle investigated a large number of these cases and found the amount of yolk material to be large. In other words, there seems to be a definite relation between the amount of yolk and sex.
A great number of clever experiments were carried out to find out if eggs originally predisposed to one sex were actually used to produce the other. Selective fertilization with different kinds of sperm was impossible, since in these birds there is only one type of sperm—two of eggs—as to the sex chromosome. For instance, by overworking females at egg-production, the same birds which had been producing more males than females were made to reverse that relation.