In the year 1885, with no special object in view other than a desire for information, I began a systematized investigation of the facts which at that time had been established by naturalists relative to the development of mankind from lower orders of life. It was not, however, until the year 1886, after a careful reading of _The Descent of Man_, by Mr. Darwin, that I first became impressed with the belief that the theory of evolution, as enunciated by scientists, furnishes much evidence going to show that the female among all the orders of life, man included, represents a higher stage of development than the male. Although at the time indicated, the belief that man has descended from lower orders in the scale of being had been accepted by the leading minds both in Europe and America, for reasons which have not been explained, scientists, generally, seemed inclined to ignore certain facts connected with this theory which tend to prove the superiority of the female organism. Scarcely considering at the outset whether my task would eventually take the form of a magazine article, or whether it would be extended to the dimensions of a book, I set myself to work to show that some of the conclusions of the savants regarding the subject of sex-development are not in accord with their premises. While writing the first part of this volume, and while reasoning on the facts established by scientists in connection with the observations which have been made in these later years relative to the growth of human society and the development of human institutions, it seemed clear to me that the history of life on the earth presents an unbroken chain of evidence going to prove the importance of the female; and, so struck was I by the manner in which the facts of science and those of history harmonize, that I decided to embrace within my work some of the results of my former research. I therefore set about the task of tracing, in a brief manner, the growth of the primary characters observed in the two diverging sex-columns, according to the facts and principles enunciated in the theory of natural development. It is not perhaps singular, during an age dominated by theological dogmatism, and in which no definite knowledge relative to the development of life on the earth had been gained, that man should have regarded himself as an infinitely superior being. Neither is it remarkable that woman, who was supposed to have appeared later on the scene of action than did her male mate, and who owed her existence to a surgical operation performed upon him, should have been regarded simply as an appendage, a creature brought forth in response to the requirements of the masculine nature.