The microbes and other selective agencies have been improving the race, or at any rate have in the past been preventing its deterioration, but it by no means follows that this action is to be permitted to them in the future. We have studied them and have followed out their life histories; we know on what they thrive, and also that which is injurious to them; we can exterminate them; and human affection, that emotion beyond all others that we have to trust to for race perfection, demands complete control over “their reckless inroads.” But if the selecting microbe is to disappear, we have to replace it by something else. If the individuals of to-day are to have the advantage of better surface drainage, and an absence of their microscopic foes, the children of the future must not be the sufferers; and we must replace the selective influence of the microbe by the selective action of man’s forethought, which shall provide that these children shall alone be produced by healthy parents. We need have no fear of the removal of the selective influences that at present surround us, provided a selection is still carried on; but if we remove selective influences without replacing them by others, then racial decay is certainly and inevitably upon us. At the present time people with strong strains of insanity or phthisis marry freely. The dangers are to a certain extent realised, but these are generally overcome by the power either of personal attraction or dowry. A man may be summoned for neglecting to send his son to school, but at present there is no strong public feeling against the knowingly begetting a son who all his life may suffer from weak lungs or brain, and hence obvious disease is no bar in the marriage market. _How it is that the Production of Children by Diseased Parents is tolerated._ We cannot wonder at this state of things when we recall the fact that in less advanced times than the present a rapidly recruited population was often the determining cause of a nation’s continued existence. The depopulation produced by war and zymotic disease was often so dreadful that nations with great fertility alone survived; and thus it came about that all minor questions were sunk in the one great necessity, namely, that of keeping the population large enough to resist extinction or to effect foreign conquest.