I realize the difficulty of applying this rule in many high schools where the foundations of sex-education are well laid on the biological basis. There is no reason why the biological studies should not be coeducational through nature-study and biology as far as the development of frogs and birds and, in a general way, of mammals. In fact, both of my textbooks, the "Applied Biology" and the "Introduction to Biology," which emphasize reproduction of organisms more than other high-school books, have been used throughout in coeducational classes. However, these books stop where the problems of human life begin and should be supplemented by lessons for sex-limited classes. There are writers who suggest that segregation of the sexes for teaching concerning human life may be at present a necessity because complete frankness on sexual questions is certainly obstructed by tradition; but we must not ignore the deep social reasons why, in general, there must be maintained a certain amount of reserve between the sexes in the consideration of some important problems of life. No educational theory or practice can possibly alter the fundamental physical or psychical relations of the sexes which nature seems to have fixed immutably. [Sidenote: Married women as teachers.] One other point that deserves attention in this connection is the common statement that only married women, preferably mothers, can be competent instructors of young women. This strikes me as more than absurd. Personal experience is not always necessary for teaching in any line. The greatest medical teachers have not had the diseases they describe so clearly. The best elementary teachers and specialists on the care of children are not always mothers; on the contrary, some of these are men. The fact is that these teachers have learned, not from personal experience, but from the great accumulations of scientific knowledge of medicine, hygiene, and education. There is an abundance of such knowledge relating to sex that may be clearly understood by bright women who have no bi-personal knowledge of sex. Therefore, I believe that it is nonsense to insist that only women with complete sexual experiences can be efficient guides for other women. Sec. 19. _The Child's First Teachers of Sex-knowledge_ [Sidenote: Mothers and other first teachers.] The first instruction which may begin to lay the foundation for the individual's sex-education should be given in early childhood by parents, or by other adults, who happen to be on the most intimate personal terms with the child. Usually this means the mother; but there are numerous cases of teachers, governesses, grandmothers, and even fathers who have greater personal influence with certain children than their mothers have. The essential point is that the child should be instructed only by an adult who can exert the greatest personal influence. [Sidenote: Mothers and adolescent boys.]