Life as an unmarried daughter doesn’t confine Alma, who spends years marking the boundaries of mosses competing for territory in a boulder field, marauding in slow motion. Over decades she measures their conquests in millimeters, and is thrilled.

A scientifically minded reader might want a few more details about what Alma sees through her microscope as she classifies her discoveries. Admittedly, a reader otherwise inclined might prefer the Victorian equivalent of a car chase; you can’t please everyone. The novel is frontloaded with its most hair-raising exploits as back story on Alma’s father, a plant thief whose boyhood punishment was to be packed off on the madcap voyages of Captain Cook. Real events provide ample substrate for a novel that entwines the historic and the imagined so subtly as to read like good nonfiction for most of its first half. It crosses over to page turner after the introduction of the author’s most beguiling invention, the deliciously named Ambrose Pike. (Dickensian names abound here: earthy Hanneke de Groot, fluttering Retta Snow, the Reverend Welles of unplumbed depths, a Polynesian messiah named Tomorrow Morning.) By this point we are ready to see some action in the love department for Alma, who is “like a book that had opened to the same page every single day” for most of her 48 years.

True to his name, Ambrose Pike is both ambrosia and piercing sword, and their mutual captivation proceeds too quickly into what may be the most thrillingly bizarre marriage proposal in literature. But wretched misunderstandings ensue, leaving Alma depressed and without enthusiasm for life or even mosses. She is plunged into her own “Eat, Pray, Love” adventure, though there is to be little praying and less eating in the seaside hut where she finally washes up in Tahiti.

The book’s locales are captured in glittering portraits: in Tahiti the palm trees rustle like silk, the prostitutes are all business, and feral boys tear around in manic enterprise until they drop to sleep on the sand. Alma has labored all her years to know the world, but only outside her protected domain can she glimpse the real mechanics of creation, and they aren’t pretty. The planet offers limited resources; competition is harsh and constant. “Anything less than a fight for endurance,” Alma realizes, “is a refusal of the great covenant of life.” In a manner that is somehow both predictable and breathtaking, the novel tips its hand and we see all roads leading to the greatest unifying theory ever conceived by natural science. Alma’s mosses on the boulder field are, of course, analogues to Darwin’s ­finches on the Galápagos Islands. Alma hasn’t read the book Darwin will eventually publish — but Gilbert most certainly has, and she maintains an admirable mastery of the big ideas she paraphrases as Alma’s “Theory of Competitive Alteration.”