This is a fascinating diagram from a zoology text of the 1930s—it’s an illustration of the effects of reproduction rate on the frequency of subsets of the population, and the author was using it to justify eugenics. Up into the 1960s, he was advocating sterilization of the feeble-minded to improve the human race.
Why, this guy must have been one of those evil Darwinists of the kind Michael Egnor, D. James Kennedy and the Discovery Institute deplore, and whose amoral ruthlessness those worthies have blamed on the teachings of evolution! Surprise: these sentiments were expressed by William Tinkle, a creationist, and one of the founding fathers of the organization that preceded the Institute for Creation Research, along with such well-known creationists as Henry Morris and Duane Gish. He completely rejected evolution, natural selection, and the idea that human beings were animals, and published his endorsement of sterilization of “defectives” while Secretary of the Institute for Creation Research. Read more about it at the Panda’s Thumb.