About a year ago the author was honored by an invitation to addressthe New-England Women's Club in Boston. He accepted the invitation,and selected for his subject the relation of sex to the education ofwomen. The essay excited an unexpected amount of discussion. Briefreports of it found their way into the public journals. Teachers andothers interested in the education of girls, in different parts of thecountry, who read these reports, or heard of them, made inquiry, byletter or otherwise, respecting it. Various and conflicting criticismswere passed upon it. This manifestation of interest in a brief andunstudied lecture to a small club appeared to the author to indicate ageneral appreciation of the importance of the theme he had chosen,compelled him to review carefully the statements he had made, and hasemboldened him to think that their publication in a more comprehensiveform, with added physiological details and clinical illustrations,might contribute something, however little, to the cause of soundeducation. Moreover, his own conviction, not only of the importance ofthe subject, but of the soundness of the conclusions he has reached,and of the necessity of bringing physiological facts and lawsprominently to the notice of all who are interested in education,conspires with the interest excited by the theme of his lecture tojustify him in presenting these pages to the public. The leisure ofhis last professional vacation has been devoted to their preparation.The original address, with the exception of a few verbal alterations,is incorporated into them.