From The Task of Social Hygiene by Havelock Ellis
The first result of reform at this point was that procreation became a deliberate act. Up till then the method of propagating the race was the same as that which savages have carried on during thousands of years, the chief difference being that whereas savages have frequently sought to compensate their recklessness by destroying their inferior offspring, we had accepted all the offspring, good, bad, and indifferent, produced by our indiscriminate recklessness, shielding ourselves by a false theology. Children “came,” and their parents disclaimed all responsibility for their coming. The children were “sent by God,” and if they all turned out to be idiots, the responsibility was God’s. But when it became generally realized that it was possible to limit offspring without interfering with conjugal life a step of immense importance was achieved. It became clear to all that the Divine force works through us, and that we are not entitled to cast the burden of our evil actions on any Higher Power. Marriage no longer fatally involved an endless procession of children who, in so far as they survived at all, were in a large number of cases doomed to disease, neglect, misery, and ignorance. The new Social Hygiene was for the first time rendered possible.
And updating with a few more quotes:
When at the end of the seventeenth century, Muralt, a highly intelligent Swiss gentleman, visited England, and wrote his by no means unsympathetic Lettres sur les Anglais, he was struck by a curious contradiction in the English character. They are a good-natured people, he observed, very rich, so well-nourished that sometimes they die of obesity, and they detest cruelty so much that by royal proclamation it is ordained that the fish and the ducks of the ponds should be duly and properly fed. Yet he found that this good-natured, rich, cruelty-hating nation systematically allowed the prisoners in their gaols to die of starvation. “The great cruelty of the English,” Muralt remarks, “lies in permitting evil rather than in doing it.”  The root of the apparent contradiction lay clearly in a somewhat excessive independence and devotion to liberty. We give a man full liberty, they seem to have said, to work, to become rich, to grow fat. But if he will not work, let him starve. In that point of view there were involved certain fallacies, which became clearer during the course of social evolution.
Whenever human beings breed in reckless and unrestrained profusion—as is the case under some conditions before a free and self-conscious civilization is attained—there is an immense infantile mortality. It is claimed, on the one hand, that this is beneficial, and need not be interfered with. The weak are killed off, it is said, and the strong survive; there is a process of natural survival of the fittest. That is true. But it is equally true, as has also been clearly seen on the other hand, that though the relatively strongest survive, their relative strength has been impaired by the very influences which have proved altogether fatal to their weaker brethren.
The duty of purifying, ordering, and consolidating the banks of the stream must still remain.  But when we are able to control the stream at its source we are able to some extent to prevent the contamination of that stream by filth, and ensure that its muddy floods shall not sweep away the results of our laborious work on the banks. Our sense of social responsibility is developing into a sense of racial responsibility, and that development is expressed in the nature of the tasks of Social Hygiene which now lie before us.
“Increase and multiply” was the legendary injunction uttered on the threshold of an empty world. It is singularly out of place in an age in which the earth and the sea, if not indeed the very air, swarm with countless myriads of undistinguished and indistinguishable human creatures, until the beauty of the world is befouled and the glory of the Heavens bedimmed. To stem back that tide is the task now imposed on our heroism, to elevate and purify and refine the race, to introduce the ideal of quality in place of the ideal of quantity which has run riot so long, with the results we see. “As the Northern Saga tells that Odin must sacrifice his eye to attain the higher wisdom,” concludes Fahlbeck, “so Man also, in order to win the treasures of culture and refinement, must give not only his eye but his life, if not his own life that of his posterity.”
The compulsory presentation of certificates of health and good breeding as a preliminary to marriage forms no part of Eugenics, nor is compulsory sterilization a demand made by any reasonable eugenist. Certainly the custom of securing certificates of health and ability is excellent, not only as a preliminary to marriage, but as a general custom. Certainly, also, there are cases in which sterilization is desirable, if voluntarily accepted.  But neither certification nor sterilization should be compulsory. They only have their value if they are intelligent and deliberate, springing out of a widened and enlightened sense of personal responsibility to society and to the race.
A problem which is often and justly cited as one to be settled by Eugenics is that presented by the existence among us of the large class of the feeble-minded. No doubt there are some who would regret the disappearance of the feeble-minded from our midst. The philosophies of the Bergsonian type, which to-day prevail so widely, place intuition above reason, and the “pure fool” has sometimes been enshrined and idolized. But we may remember that Eugenics can never prevent absolutely the occurrence of feeble-minded persons, even in the extreme degree of the imbecile and the idiot.  They come within the range of variation, by the same right as genius so comes. We cannot, it may be, prevent the occurrence of such persons, but we can prevent them from being the founders of families tending to resemble themselves.
It is not only in themselves that the feeble-minded are a burden on the present generation and a menace to future generations. In large measure they form the reservoir from which the predatory classes are recruited. This is, for instance, the case as regards prostitutes. Feeble-minded girls, of fairly high grade, may often be said to be predestined to prostitution if left to themselves, not because they are vicious, but because they are weak and have little power of resistance. They cannot properly weigh their actions against the results of their actions, and even if they are intelligent enough to do that, they are still too weak to regulate their actions accordingly. Moreover, even when, as often happens among the high-grade feeble-minded, they are quite able and willing to work, after they have lost their “respectability” by having a child, the opportunities for work become more restricted, and they drift into prostitution. It has been found that of nearly 15,000 women who passed through Magdalen Homes in England, over 2500, or more than sixteen per cent—and this is probably an under-estimate—were definitely feeble-minded. The women belonging to this feeble-minded group were known to have added 1000 illegitimate children to the population. In Germany Bonhoeffer found among 190 prostitutes who passed through a prison that 102 were hereditarily degenerate and 53 feeble-minded. This would be an over-estimate as regards average prostitutes, though the offences were no doubt usually trivial, but in any case the association between prostitution and feeble-mindedness is intimate. Everywhere, there can be no doubt, the ranks of prostitution contain a considerable proportion of women who were, at the very outset, in some slight degree feeble-minded, mentally and morally a little blunted through some taint of inheritance.
These classes, with their tendency to weak-mindedness, their inborn laziness, lack of vitality, and unfitness for organized activity, contain the people who complain that they are starving for want of work, though they will never perform any work that is given them. Feeble-mindedness is an absolute dead-weight on the race. It is an evil that is unmitigated. The heavy and complicated social burdens and injuries it inflicts on the present generation are without compensation, while the unquestionable fact that in any degree it is highly inheritable renders it a deteriorating poison to the race; it depreciates the quality of a people. The task of Social Hygiene which lies before us cannot be attempted by this feeble folk. Not only can they not share it, but they impede it; their clumsy hands are for ever becoming entangled in the delicate mechanism of our modern civilization. Their very existence is itself an impediment. Apart altogether from the gross and obvious burden in money and social machinery which the protection they need, and the protection we need against them, casts upon the community,  they dilute the spiritual quality of the community to a degree which makes it an inapt medium for any high achievement.