But what if Darwin’s evidence had led to conclusions that did not support his belief in the unitary origins of mankind? Would he have fudged the data? Desmond and Moore don’t really address the question. One is left with the impression that Darwin was amazingly lucky that his benevolent preconceptions turned out to fit the facts.

In his lively and wide-ranging “Angels and Ages,” Adam Gopnik suggests that when facts and values clash we might live in accordance with our beliefs anyway. “It might be true — there is absolutely no such evidence, but it might be true — that different ethnic groups, or sexes, have on average different innate aptitudes for math or science,” he muses. “We might decide to even things out, give some people extra help toward that end, or we might decide just to live with the disparity.”

Gopnik’s short book takes its impetus from a striking historical coincidence: “On Feb. 12, 1809, two baby boys were born within a few hours of each other on either side of the Atlantic.” Those babies, one rich and one poor, as in a plot of Mark Twain’s, were Darwin and Abraham Lincoln. Though he makes some dubious claims about parallels — “lives lived in one time have similar shapes” — Gopnik’s real comparison is between two writers. “They matter most,” he claims, “because they wrote so well.” More specifically, Darwin and Lincoln, drawing on their seemingly unpoetic backgrounds in legal argument and natural history, invented “a new kind of eloquence” that we still use for “the way we live now and the way we talk at home and in public.”

Both of Gopnik’s interwoven essays, originally published in different form as articles in The New Yorker, where he is a staff writer, involve mysteries to be solved. The first mystery is whether the secretary of war Edwin Stanton said by Lincoln’s deathbed, “Now he belongs to the ages” or “Now he belongs to the angels” — whether, in other words, he invoked the consolations of historical memory or religion. Witnesses reported both. For Darwin, the mystery is why he delayed the publication of his theory of natural selection, with all essentials in place in 1838, for 20 years. Gopnik dutifully offers solutions to both mysteries, but his real interests lie elsewhere.

Gopnik is as convinced as Desmond and Moore that Darwin was no kind of racist. “The one thing that you could not read into Darwin’s writings was racism,” he writes. And yet, in another sense the books seem directly opposed. What Gopnik finds in Darwin’s early career is not some overarching moral principle but rather “pure plain looking.” What set Darwin apart was that “he liked to look at things the way an artist likes to draw, the way a composer likes to play the piano, the way a cook likes to chop onions.” Desmond and Moore think the key to Darwin was the lowly slave; Gopnik thinks the key to Darwin was the lowly earthworm, the subject of his last book. Darwin’s emphasis on “the homely, the overlooked, the undervalued” made him, in Gopnik’s view, both a great scientist and a great writer.

If Darwin’s eloquence rests on “the slow crawl of fact . . . building toward a big blade of point,” Gopnik finds a related eloquence in Lincoln’s preference for legal minutiae — “the close crawl across the facts of a case” — over grandiose oratory. In this regard, Lincoln and Darwin were “nearsighted visionaries.” Some of Gopnik’s best pages are extended analyses of passages in which Lincoln, in his great speeches, pursues “the drill of monosyllabic summation — the urge, natural to a lawyer, to say something hard one last time in short, flat words,” or in which Darwin finds evidence that the sexual passion of earthworms “is strong enough to overcome for a time their dread of light.”

For Gopnik, who has published popular books about the joys of parenting in Paris and New York, Lincoln and Darwin were down-to-earth because they were devoted “family men.” His attempt to find coziness in their family life sometimes verges on the sentimental, as he himself acknowledges. “We want the Darwins (like the Lincolns) to be loving and indulgent and attentive parents because then they will be like us.” Gopnik may be right that the early deaths of their children, Annie Darwin at age 10 and Willie Lincoln at 11, introduced a fatalistic note of tragedy — beyond what Lincoln called the “awful arithmetic” of the Civil War or Darwin called the relentless “struggle for life” — into their prose styles. But it is his astute analysis of those styles that shows us why these thinkers and writers, who maintained “a tragic consciousness without robbing it of a hopeful view,” have so robustly survived to our own time.