Disease, from time to time, will ease the pressure. Diseases are parasites, and it must not be forgotten that just as there are drifts in the world of man, so are there drifts in the world of micro-organisms-- hunger-quests for food. Little is known of the micro-organic world, but that little is appalling; and no census of it will ever be taken, for there is the true, literal "abysmal fecundity." Multitudinous as man is, all his totality of individuals is as nothing in comparison with the inconceivable vastness of numbers of the micro-organisms. In your body, or in mine, right now, are swarming more individual entities than there are human beings in the world to-day. It is to us an invisible world. We only guess its nearest confines. With our powerful microscopes and ultramicroscopes, enlarging diameters twenty thousand times, we catch but the slightest glimpses of that profundity of infinitesimal life. Little is known of that world, save in a general way. We know that out of it arise diseases, new to us, that afflict and destroy man. We do not know whether these diseases are merely the drifts, in a fresh direction, of already-existing breeds of micro-organisms, or whether they are new, absolutely new, breeds themselves just spontaneously generated. The latter hypothesis is tenable, for we theorise that if spontaneous generation still occurs on the earth, it is far more likely to occur in the form of simple organisms than of complicated organisms. Another thing we know, and that is that it is in crowded populations that new diseases arise. They have done so in the past. They do so to-day. And no matter how wise are our physicians and bacteriologists, no matter how successfully they cope with these invaders, new invaders continue to arise--new drifts of hungry life seeking to devour us. And so we are justified in believing that in the saturated populations of the future, when life is suffocating in the pressure against subsistence, that new, and ever new, hosts of destroying micro-organisms will continue to arise and fling themselves upon earth-crowded man to give him room. There may even be plagues of unprecedented ferocity that will depopulate great areas before the wit of man can overcome them. And this we know: that no matter how often these invisible hosts may be overcome by man's becoming immune to them through a cruel and terrible selection, new hosts will ever arise of these micro-organisms that were in the world before he came and that will be here after he is gone.