Periodicity characterizes the female organization, and developes feminine force. Persistence characterizes the male organization, and develops masculine force. Education will draw the best out of each by adjusting its methods to the periodicity of one and the persistence of the other. Before going farther, it is essential to acquire a definite notion of what is meant, or, at least, of what we mean in this discussion, by the term co-education. Following its etymology, _con-educare_, it signifies to draw out together, or to unite in education; and this union refers to the time and place, rather than to the methods and kinds of education. In this sense any school or college may utilize its buildings, apparatus, and instructors to give appropriate education to the two sexes as well as to different ages of the same sex. This is juxtaposition in education. When the Massachusetts Institute of Technology teaches one class of young men chemistry, and another class engineering, in the same building and at the same time, it co-educates those two classes. In this sense it is possible that many advantages might be obtained from the co-education of the sexes, that would more than counterbalance the evils of crowding large numbers of them together. This sort of co-education does not exclude appropriate classification, nor compel the two sexes to follow the same methods or the same regimen. Another signification of co-education, and, as we apprehend, the one in which it is commonly used, includes time, place, government, methods, studies, and regimen. This is identical co-education. This means, that boys and girls shall be taught the same things, at the same time, in the same place, by the same faculty, with the same methods, and under the same regimen. This admits age and proficiency, but not sex, as a factor in classification. It is against the co-education of the sexes, in this sense of identical co-education, that physiology protests; and it is this identity of education, the prominent characteristic of our American school-system, that has produced the evils described in the clinical part of this essay, and that threatens to push the degeneration of the female sex still farther on. In these pages, co-education of the sexes is used in its common acceptation of identical co-education.