Eugenics is not, for instance, a sub-section of applied mathematics. It is at once a science, and a religion, based upon the laws of life, and recognising in them the foundation of society. We shall some day have a eugenic sociology, to which the first part of this volume seeks to contribute: and the sociology and politics which have not yet discovered that man is mortal will go to their own place. Only when we begin to think and work continuously at eugenics is its range revealed. The present volume is a mere introduction to the principles of the subject: the full elucidation of its practice is a problem for generations to come. Nor is it easy to set logical limits to our inquiry. We may say that eugenics deals with conceptions: and that the care of the expectant mother is outside its scope: but of what use is it to have a eugenic conception if its product is thereafter to be ruined by, for instance, the introduction of lead into the mother's organism? Again, the care of the individual is, in part, a eugenic concern: for if we desire his offspring we desire that he shall not contract transmissible disease nor vitiate his tissues with such a racial poison as alcohol. Plainly, everything that affects every possible parent is a matter of eugenic concern: and not only those factors which affect the choice for parenthood. It follows that the second portion of this volume, which deals with the practice of eugenics, cannot be more than merely indicative. In the available space it has been attempted to define certain constituents of practical eugenics, but in any case the entire ground has not been surveyed. The concept of the _racial poisons_ may be commended to special consideration. Whether a poison be so-called “chemical,” as lead, or made by a living organism, as the poison of syphilis, is of great practical importance, because of the infection involved in the second case: but, in principle, both cases belong to the same category. Sooner or later, eugenists must face the transmissible infections, and repudiate as hideous and devilish the so-called morality which discountenances any attempt to save unborn innocence from a nameless fate. He or she who would rather leave this matter is placing “religion” or “morality” or “politics” above the welfare of the life to come, and therein continuing the daily prostitution of those great names.