The story behind Darwin’s difficult decision

This year marks the 150th anniversary of the publication of what was probably the most momentous book of the 19th century, Charles Darwin’s “On the Origin of Species.’’ In it, the great naturalist set out his theory of natural selection, from which we derive our understanding of human evolution. “Darwin’s Darkest Hour’’ looks at how a pivotal moment in the scientist’s life led him to publish the book.

The program begins with Darwin receiving a letter from a fellow naturalist, Alfred Russel Wallace, with a draft of a paper wherein Wallace had arrived at the idea of natural selection independent of Darwin’s research. Darwin had spent two decades developing the theory, but shrank from publishing because of the controversy the theory was sure to elicit. (If that seems odd, consider today’s fights over creationism.) “Twenty years’ work, and I’m beaten to the post!’’ he laments.

The remainder of the broadcast shows how he came to the decision to publish. Henry Ian Cusick is very appealing as Darwin, though it’s a bit of aural whiplash hearing him speak with an English accent instead of the Scottish burr he has on “Lost.’’ Frances O’Connor (“Mansfield Park’’) plays Darwin’s wife, Emma, and makes the best of a thankless part. Her main job, when not fretting, is to offer loving prompts to get Darwin to explain his theory. She actually says at one point, “You loved the Galápagos Islands, didn’t you?’’

Most of the action takes place at Down House, the Darwin home in idyllic Kent. It’s usually seen in honeyed light, with many Steadicam shots. The latter don’t exactly feel period. When not clueing in Emma, Darwin spends much of his time playing “Professor Papa’’ to their many children. You’d hardly know that the real man was a demon for work or how painfully he struggled with ill health. There are a few references to his medical history, and we see him (discreetly, of course) vomit once. But that’s about it. Some real drama, a child’s medical crisis, provides a jolt of wrenching emotion amid the scenes of domestic bliss.

The script makes clever use of flashbacks, though after a while things become a bit didactic. We glimpse Darwin’s days as a medical student, conflicts with his father, and, of course, his epochal voyage on HMS Beagle. This allows for a fair amount of nature footage from, yes, the Galápagos. Any confused viewers should finally regain their bearings. Tortoises and penguins rarely appear on “Masterpiece Classic.’’

Mark Feeney can be reached at mfeeney@globe.com.