Evolution — the idea that species are not fixed, but ever changing — was not a simple idea on which all scientists agreed. In his immensely influential 1859 book, On the Origin of Species, British naturalist Charles Darwin argued that all creatures struggle to survive. When individual members of a species are born with random genetic mutations that better suit them for their environment — for example, camouflage coloring for a moth — these characteristics, since they are genetically transmissible, become dominant in future generations. Many scientists rejected this theory of natural selection. They followed a line of thinking laid out by French biologist Jean Baptiste Lamarck, who argued, unlike Darwin, that individual animals or plants could acquire transmittable traits within a single lifetime. A rhinoceros that fought fiercely, in Lamarck’s view, could build up a stronger horn; its offspring would then be born with that trait.

Darwin himself disapproved of the word evolution (which does not appear in his book) because it implied upward progress. In his view, natural selection was blind: environments and species changed randomly. Others were less scrupulous about drawing sweeping conclusions from Darwin’s work. In the 1870s, British philosopher Herbert Spencer spun out an elaborate theory of how human society advanced through “survival of the fittest.” Social Darwinism, as Spencer’s idea became (confusingly) known, found its American champion in William Graham Sumner, a sociology professor at Yale. Competition, said Sumner, was a law of nature, like gravity. Who were the fittest? “Millionaires,” Sumner declared. Their success showed they were “naturally selected.”

Even in the heyday of Social Darwinism, Sumner’s views were controversial (American Voices). Some thinkers objected to the application of biological findings to the realm of society and government. They pointed out that Darwin’s theories applied to finches and tortoises, not human institutions. Social Darwinism, they argued, was simply an excuse for the worst excesses of industrialization. By the early twentieth century, intellectuals revolted against Sumner and his allies.

Meanwhile, though, the most dubious applications of evolutionary ideas were codified into new reproductive laws based on eugenics, a so-called science of human breeding. Eugenicists argued that mentally deficient people should be prevented from reproducing. They proposed sterilizing those deemed “unfit,” especially residents of state asylums for the insane or mentally disabled. In early-twentieth-century America, almost half of the states enacted eugenics laws. By the time eugenics subsided in the 1930s, about twenty thousand people had been sterilized, with California and Virginia taking the lead. Women in Puerto Rico and other U.S. imperial possessions also suffered from eugenics policies. Advocates of eugenics had a broad impact. Because they associated mental unfitness with “lower races” — including people of African, Asian, and Native American descent — their arguments lent support to segregation and racial discrimination. By warning that immigrants from Eastern and Southern Europe would dilute white Americans’ racial purity, eugenicists helped win passage of immigration restriction in the 1920s.