Civilizations have perished. We do not know, of course, how many civilizations have disappeared without leaving a trace. But we do know that races have existed, even in our own islands, who were capable of transporting and building up great pieces of rock, and of arranging them in an astronomically significant manner. Did these races perish under the attacks of an enemy? Or did they die off owing to the exhaustion of their own vitality? We may suppose that it was the former rather than the latter alternative. For in view of the great and increasing power of the human race we hardly need fear any enemies from without. Our destruction, if ever it comes, will come from within. It will come if and when we develop a Will to Die. Such a Will to Die has been observed on a small scale in the wave of suicide sweeping over Central Europe after the Great War. But it was only a symptom of readjustment. It was partial, and practically confined to the class bound up with social and governmental stability, and incapable of adaptation to radically changed conditions. A more ominous phenomenon is the fall in the birth-rate of most of the insular tribes of the Pacific, which in some cases has led to their total extinction. The causes of this fall have never been fully cleared up. If it is due to the introduction of diseases by Indo-European germ carriers, or the adoption of modern clothing, or the destruction of native standards of morality by the introduction of Asiatic religions, there is no cause for alarm concerning the future of the human race as a whole. But if the mere discouragement of the tribe when faced with new conditions can act directly upon the instinct of procreation, and affect, so to speak, the _élan vital_ of the germ-plasm, then we are faced with the gravest danger to which the human race can be exposed. For such an effect would be too insidious to be dealt with by public measures. It would be a psychological disease of the most fatal and virulent kind. It might affect the whole human race during some crisis in its fortunes, and might bring about its destruction by the failure of the sex instinct. Although such a failure must always be a menacing possibility, it is a remote one, and at no time has it threatened the human race as a whole. Hunger and Love have kept our race going up to now, and, barring unforeseen developments, they will do so _in sœcula sœculorum_. But there are other dangers. A new germ might be evolved which, like the gonococcus, might attack the germ plasm, and produce general sterility. This is one of the dangers that _can_ be fought by hygienic and sanitary measures, and the wiles and intricacies of bacteriological novelties may be safely left to the ever-increasing resources of bacteriology itself.