The facts observed and recorded by others assisted me to advance so far on the trodden path that I made an effort to snatch a secret from Nature. What I succeeded in obtaining, though small, induced me to set forth in the following pages the perhaps not unimportant results. The labor was long, and engaged my attention for years. And yet, amidst my continuous labors in the province of Embryology, it remained all the time a matter of secondary importance, my principal attention being engaged by far more extensive studies. My desire is to stimulate others to wider observation. May the facts which I here discuss prove of utility, and encourage further studies in this direction with the assistance of modern science. If we are not in a position to control the processes of Nature, we can nevertheless exercise over them a more or less effective influence, so as to obtain such results as are possible. Whatsoever the question may be that we propose to discuss, it is sometimes very difficult to reach any answer. And yet, when experience and diligence have helped us over the difficulties, we succeed at last in reaching the answer desired. The difficulties assume much less formidable shapes when an individual is satisfied with shaking his head and regarding the whole affair with mistrust. In that way the inexperienced and lazy are at once able to launch their views without further trouble. They believe or they disbelieve; and they like to have their say. Any one can in this way easily win himself a place amongst those who have written on a topic. The man who desires to obtain a lasting place takes on his shoulders heavier responsibilities. This book contains but a portion of the vast and wide-reaching literature dealing with the subject in hand. That literature extends back to the date of man’s earliest intellectual labors. The observations that have been recorded by others are here followed by methods of investigation, and by considerations which may serve to elucidate the facts. In conclusion, a section has been dedicated to the methods which I recommend for the artificial influencing of sex. Some particular experiments are subjoined. May my little book, then, go out into the world and make known my views, which are founded exclusively upon facts.