You might find something useful in Margaret Sanger's Pivot of Civilization (from this collection), perhaps combining an excerpt from the opening feminist anti-marxism angle with the specific policy directions under the category of the "feeble minded". For 1922 it provides an interesting reflection on postwar "repopulation" agendas as the context in which Social Darwinism might be properly situated, especially given the comparatively benign legacy of birth control in a modern context, and the "celebrity" intro from HG Wells certainly gives it a nice edge. Note: be careful with modern sources on her as she is used as a hand grenade in American political discourse over abortion.

For instance, as a brief example:

To find an answer to this problem which at that point in my experience I could not solve, I determined to study conditions in Europe. Perhaps there I might discover a new approach, a great illumination. Just before the outbreak of the war, I visited France, Spain, Germany and Great Britain. Everywhere I found the same dogmas and prejudices among labor leaders, the same intense but limited vision, the same insistence upon the purely economic phases of human nature, the same belief that if the problem of hunger were solved, the question of the women and children would take care of itself. In this attitude I discovered, then, what seemed to me to be purely masculine reasoning; and because it was purely masculine, it could at best be but half true. Feminine insight must be brought to bear on all questions; and here, it struck me, the fallacy of the masculine, the all-too-masculine, was brutally exposed. I was encouraged and strengthened in this attitude by the support of certain leaders who had studied human nature and who had reached the same conclusion: that civilization could not solve the problem of Hunger until it recognized the titanic strength of the sexual instinct. In Spain, I found that Lorenzo Portet, who was carrying on the work of the martyred Francisco Ferrer, had reached this same conclusion. In Italy, Enrico Malatesta, the valiant leader who was after the war to play so dramatic a role, was likewise combating the current dogma of the orthodox Socialists. In Berlin, Rudolph Rocker was engaged in the thankless task of puncturing the articles of faith of the orthodox Marxian religion. It is quite needless to add that these men who had probed beneath the surface of the problem and had diagnosed so much more completely the complex malady of contemporary society were intensely disliked by the superficial theorists of the neo-Marxian School.

The gospel of Marx had, however, been too long and too thoroughly inculcated into the minds of millions of workers in Europe, to be discarded. It is a flattering doctrine, since it teaches the laborer that all the fault is with someone else, that he is the victim of circumstances, and not even a partner in the creation of his own and his child's misery. Not without significance was the additional discovery that I made. I found that the Marxian influence tended to lead workers to believe that, irrespective of the health of the poor mothers, the earning capacity of the wage-earning fathers, or the upbringing of the children, increase of the proletarian family was a benefit, not a detriment to the revolutionary movement. The greater the number of hungry mouths, the emptier the stomachs, the more quickly would the "Class War" be precipitated. The greater the increase in population among the proletariat, the greater the incentive to revolution. This may not be sound Marxian theory; but it is the manner in which it is popularly accepted. It is the popular belief, wherever the Marxian influence is strong. This I found especially in England and Scotland. In speaking to groups of dockworkers on strike in Glasgow, and before the communist and co-operative guilds throughout England, I discovered a prevailing opposition to the recognition of sex as a factor in the perpetuation of poverty. The leaders and theorists were immovable in their opposition. But when once I succeeded in breaking through the surface opposition of the rank and file of the workers, I found that they were willing to recognize the power of this neglected factor in their lives.

paired up with

There is but one practical and feasible program in handling the great problem of the feeble-minded. That is, as the best authorities are agreed, to prevent the birth of those who would transmit imbecility to their descendants. Feeble-mindedness as investigations and statistics from every country indicate, is invariably associated with an abnormally high rate of fertility. Modern conditions of civilization, as we are continually being reminded, furnish the most favorable breeding-ground for the mental defective, the moron, the imbecile. "We protect the members of a weak strain," says Davenport, "up to the period of reproduction, and then let them free upon the community, and encourage them to leave a large progeny of `feeble-minded': which in turn, protected from mortality and carefully nurtured up to the reproductive period, are again set free to reproduce, and so the stupid work goes on of preserving and increasing our socially unfit strains."

The philosophy of Birth Control points out that as long as civilized communities encourage unrestrained fecundity in the "normal" members of the population--always of course under the cloak of decency and morality--and penalize every attempt to introduce the principle of discrimination and responsibility in parenthood, they will be faced with the ever-increasing problem of feeble-mindedness, that fertile parent of degeneracy, crime, and pauperism. Small as the percentage of the imbecile and half-witted may seem in comparison with the normal members of the community, it should always be remembered that feeble-mindedness is not an unrelated expression of modern civilization. Its roots strike deep into the social fabric. Modern studies indicate that insanity, epilepsy, criminality, prostitution, pauperism, and mental defect, are all organically bound up together and that the least intelligent and the thoroughly degenerate classes in every community are the most prolific. Feeble-mindedness in one generation becomes pauperism or insanity in the next. There is every indication that feeble-mindedness in its protean forms is on the increase, that it has leaped the barriers, and that there is truly, as some of the scientific eugenists have pointed out, a feeble-minded peril to future generations--unless the feeble-minded are prevented from reproducing their kind. To meet this emergency is the immediate and peremptory duty of every State and of all communities. The curious situation has come about that while our statesmen are busy upon their propaganda of "repopulation," and are encouraging the production of large families, they are ignoring the exigent problem of the elimination of the feeble-minded. In this, however, the politicians are at one with the traditions of a civilization which, with its charities and philanthropies, has propped up the defective and degenerate and relieved them of the burdens borne by the healthy sections of the community, thus enabling them more easily and more numerously to propagate their kind. "With the very highest motives," declares Dr. Walter E. Fernald, "modern philanthropic efforts often tend to foster and increase the growth of defect in the community.... The only feeble-minded persons who now receive any official consideration are those who have already become dependent or delinquent, many of whom have already become parents. We lock the barn-door after the horse is stolen. We now have state commissions for controlling the gipsy-moth and the boll weevil, the foot-and-mouth disease, and for protecting the shell-fish and wild game, but we have no commission which even attempts to modify or to control the vast moral and economic forces represented by the feeble-minded persons at large in the community."

Social Darwinism was coined by the Sociologist Herbert Spencer in the 1850s, a good starting point on the topic would be a cursory glance at his writings.