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Working on a species of African butterfly, the Squinting Bush Brown, researchers from the University of Liverpool have discovered a radically different molecular switch that does not require a sex-limited gene or chromosome.

No mental trait has ever been proved to be sex-limited in inheritance, or to exist as a secondary sex character.

The phenomenon is expressed, biologically, in the term, "sex-limited factor"—an incalculable something in the embryo which limits its sex to the sex of one only of its parents.

However, these books stop where the problems of human life begin and should be supplemented by lessons for sex-limited classes.

In other words, the defect is more or less sex-limited.