“Evolution’s Captain” (HarperCollins, $24.95) is Peter Nichols’ fourth book with a maritime theme and is further evidence of his skill as a historian, researcher and elegant writer.

The book has two protagonists: Naval Lt. Robert FitzRoy and naturalist Charles Darwin. Both were from the upper crust of English society. Their lives would be closely linked for five years, in which they would become fast friends, then bitter opponents in their ideology.

FitzRoy, initially the more prominent, was a highly intelligent and capable naval officer. His fame has been surpassed by that of Darwin, who, as a young man, had shown little inclination toward study or self-improvement.

FitzRoy was in Rio de Janeiro, Brazil, in 1828 when he was appointed to command the brig HMS Beagle, whose captain had committed suicide during a surveying mission.

The following January, the Beagle and two other ships sailed from Rio to survey the southern shores of Tierra del Fuego at the tip of South America. About a year later, FitzRoy decided to take four Fuegians — three males and a female — to England. He reasoned that if they learned English and the rudiments of civilized life and were returned to Tierra del Fuego after a few years, it would help future relations with the natives.

FitzRoy and the four Fuegians arrived in England in October 1830. One of the native men soon died of smallpox, but at least one of the other males had sex with the female.

This prompted FitzRoy to get them out of England and back to Tierra del Fuego without delay. He arranged to be commissioned to further survey the Tierra del Fuego area and again was given command of the Beagle.

FitzRoy, then 26, requested that “some well-educated and scientific person” be assigned as a naturalist for the expedition. This is when Darwin, 22, entered FitzRoy’s life.

The Beagle sailed in late 1831 on the trip during which Darwin would develop his theories on the origin and evolution of species. The Beagle surveyed the southeast coasts of Brazil and Argentina before continuing to Tierra del Fuego, where the Fuegians aboard were restored to their people. Darwin spent much time ashore observing nature while FitzRoy performed his surveys.

FitzRoy, a fundamentalist Christian, and Darwin engaged in lengthy disagreements about the Creation, which caused FitzRoy bouts of depression during the ensuing year of the survey. FitzRoy took the Beagle west across the Pacific Ocean on its way back to England, where it dropped anchor in October 1836.

Nichols then describes Darwin’s ascendancy and the success of his writings on evolution that earned him prestige in the scientific community. In contrast, FitzRoy’s account of the expedition was not successful, and his views were often ridiculed. He suffered from depression and remorse; he saw himself as a failure and as the agent who had involuntarily helped Darwin develop views that FitzRoy considered heresy. In 1865, FitzRoy committed suicide.

The author shows fairness in dealing with the two men: Darwin’s success is treated with the same moderation as is FitzRoy’s backslide into despair.

Mountains in Patagonia were named for FitzRoy and Darwin after their deaths. But today, Darwin’s accomplishments are widely known, while FitzRoy has been largely forgotten.

There is plenty of fascinating material in “Evolution’s Captain” as well as a sense of drama that never lets up.