Alison Gopnik in The Atlantic:
For 2,000 years, there was an intuitive, elegant, compelling picture of how the world worked. It was called “the ladder of nature.” In the canonical version, God was at the top, followed by angels, who were followed by humans. Then came the animals, starting with noble wild beasts and descending to domestic animals and insects. Human animals followed the scheme, too. Women ranked lower than men, and children were beneath them. The ladder of nature was a scientific picture, but it was also a moral and political one. It was only natural that creatures higher up would have dominion over those lower down.
Darwin’s theory of evolution by natural selection delivered a serious blow to this conception. Natural selection is a blind historical process, stripped of moral hierarchy. A cockroach is just as well adapted to its environment as I am to mine. In fact, the bug may be better adapted—cockroaches have been around a lot longer than humans have, and may well survive after we are gone. But the very word evolution can imply a progression—New Agers talk about becoming “more evolved”—and in the 19th century, it was still common to translate evolutionary ideas into ladder-of-nature terms.
Modern biological science has in principle rejected the ladder of nature. But the intuitive picture is still powerful. In particular, the idea that children and nonhuman animals are lesser beings has been surprisingly persistent.