Erik's posts (here and here) on the seemingly Darwinian politics of modern conservativism aren't wrong about the lilt of these contemporary thinkers, but they do a bit of injustice to the historical ones, because there was no such thing as "Social Darwinism" during the Gilded Age. There was such a thing as William Graham Sumner, and his collected essays bear the title Social Darwinism, but those essays were collected in and published in 1963. The editor of those essays was following the lead established by the historian Richard Hofstadter, whose Social Darwinism in American Thought (1944) identified Sumner as the brains behind the social Darwinist movement in the Progressive Era. The problem is that there wasn't a social Darwinist movement during the Progressive Era. I'm not just kicking against the pricks here—as people writing dissertations are wont to do—as will become clear if you ask yourself a simple question:
When was the Modern Synthesis formulated?
The Modern Synthesis, if you don't know, is the combination of Mendelian genetics with Darwinian evolutionary theory, and represents the moment when the previously theoretical Darwinian model finally found itself a mechanism of transmission. Darwin's theory of natural selection was elegant, but prior to the Modern Synthesis scientists lacked a means of proving that it could exist in nature. When was it formulated? Between 1936 and 1942. Why is that significant?
Because prior to the Modern Synthesis there was little consensus as to the driving force behind the development of species. Russian scientists, for example, were working under Lamarckian assumptions about the heritability of acquired characteristics well into the 1960s. (The had an ideological commitment to keeping the Lamarckian faith after the Modern Synthesis, but eventually even they relented.) Point being, during the Gilded Age and Progressive Era, Darwinian thought wasn't the dominant strain of evolutionary theory. It lacked the evidence required to back up its elegance, and so its status in the scientific community was as tenuous then as its competitors are now. Vernon Kellogg, then president of Stanford (or not?), wrote a book entitled Darwinism Today (1908) that basically argued that there really wasn't any. It devoted itself to explicating "the various new theories of species-forming with ... names, such as heterogenesis, orthogenesis, metakinesis, geographic isolation, biologic isolation, organic selection, or orthoplasty." So why do we associate Darwinism with this period?
Because of the Whigs and their history. The aforementioned Hofstadter wrote Social Darwinism in American Thought in 1944 in order to create a bogeyman whose existence would justify the policies of the New Deal. From what Stephen J. Gould called the "maximal diversity" of evoultionary thought during the Gilded Age and Progressive Era, Hofstadter selected those thinkers whose work contained implications dire enough that politicians in the 1940s could point to them to frighten the masses. Darwinism, as I demonstrated above, wasn't regnant during the period, much less the social application of it, but Hofstadter had handed New Deal liberals their bogeyman and they weren't about to give it up.
Ironically, the scientific community bolstered Hofstadter's claim during the centennial of the Origin in 1959. In a book titled Darwin's Century, Loren Eiseley and his fellow scientists created a teleological narrative of Darwinism's development in which all evolutionary thinkers were groping their way towards the Modern Synthesis. Which is ironic because the key insight of Darwinian thought is that development isn't teleological—that natural selection isn't based on forethought and doesn't working according to a plan. Eiseley and his colleagues transformed the development of Darwinian thought into the stuff of Intelligent Design, and when that narrative was welded onto Hofstadter's, the result was the impression that Darwinism reigned supreme during the Gilded Age and Progressive Era.
It didn't. It only seems to have because people have forgotten all the other evolutionary theories that were in play at the time, the most prominent of which was Lamarckian, not Darwinian, prompting prominent medical thinkers (and popular novelists) like Silas Weir Mitchell to declare:
I have sometimes been led to think that over brain-work tends not only to stunt the body and to contract the pelvis, but, by the law of evolution, to develop bigger headed offspring, or at least offspring with heads relatively disproportioned to the pelvis of the mother.
That's correct. The most prominent neurologist in America opposed educating women because they would become smarter, pass on their larger brains to their children, then die during childbirth. Outside of giraffes, it's difficult to find a more classic formulation of Lamarckian thought. I could go on for ages—or pages, hundreds of them—but I think I've established that "the Social Darwinism movement" is an ahistorical construct designed to justify policies and theories with which I otherwise agree.