The first editions of the work were issued with no little anxiety on the part of both author and publishers as to how it would be received by the reading public. It was anticipated that no little adverse criticism, and perhaps severe condemnation, would be pronounced by many whose education and general mode of thought had been such as to unfit them to appreciate it; but it was hoped that persons of more thoughtful and unbiased minds would receive the work kindly, and would readily co-operate with the publishers in its circulation. This anticipation has been more than realized. Wherever the book has been introduced, it has met with a warm reception; and of the several thousand persons into whose hands the work has been placed, hundreds have gratefully acknowledged the benefit which they have received from its perusal, and it is hoped that a large proportion have been greatly benefited. The cordial reception which the work has met from the press everywhere has undoubtedly contributed in great measure to its popularity. The demand for the work has exhausted several editions in rapid succession, and has seemed to require its preparation in the greatly enlarged and in every way improved form in which it now appears. The addition of two whole chapters for the purpose of bringing the subject directly before the minds of boys and girls in a proper manner, adds greatly to the interest and value of the work, as there seemed to be a slight deficiency in this particular in the former editions. J. H. K. BATTLE CREEK, MICH., _October, 1879_. CONTENTS. PAGE. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 _SEX IN LIVING FORMS_.