In the eighteen-eighties, residents of cities across Britain might have noticed an aged, bald, bewhiskered gentleman sedulously eying every girl he passed on the street while manipulating something in his pocket. What they were seeing was not lechery in action but science. Concealed in the man’s pocket was a device he called a “pricker,” which consisted of a needle mounted on a thimble and a cross-shaped piece of paper. By pricking holes in different parts of the paper, he could surreptitiously record his rating of a female passerby’s appearance, on a scale ranging from attractive to repellent. After many months of wielding his pricker and tallying the results, he drew a “beauty map” of the British Isles. London proved the epicenter of beauty, Aberdeen of its opposite.

Such research was entirely congenial to Francis Galton, a man who took as his motto “Whenever you can, count.” Galton was one of the great Victorian innovators. He explored unknown regions of Africa. He pioneered the fields of weather forecasting and fingerprinting. He discovered statistical rules that revolutionized the methodology of science. Yet today he is most often remembered for an achievement that puts him in a decidedly sinister light: he was the father of eugenics, the science, or pseudoscience, of “improving” the human race by selective breeding.

A new biography, “Extreme Measures: The Dark Visions and Bright Ideas of Francis Galton” (Bloomsbury; $24.95), casts the man’s sinister aspect right in the title. The author, Martin Brookes, is a former evolutionary biologist who worked at University College London’s Galton Laboratory (which, before a sanitizing name change in 1965, was the Galton Laboratory of National Eugenics). Brookes is clearly impressed by the exuberance of Galton’s curiosity and the range of his achievement. Still, he cannot help finding Galton a little dotty, a man gripped by an obsession with counting and measuring that made him “one of the Victorian era’s chief exponents of the scientific folly.” If Brookes is right, Galton was led astray not merely by Victorian prejudice but by a failure to understand the very statistical ideas that he had conceived.

Born in 1822 into a wealthy and distinguished Quaker family—his maternal grandfather was Erasmus Darwin, a revered physician and botanist who wrote poetry about the sex lives of plants—Galton enjoyed a pampered upbringing. As a child, he revelled in his own precocity: “I am four years old and can read any English book. I can say all the Latin Substantives and Adjectives and active verbs besides 52 lines of Latin poetry. I can cast up any sum in addition and multiply by 2, 3, 4, 5, 6, 7, 8, 10. I can also say the pence table. I read French a little and I know the Clock.” When Galton was sixteen, his father decided that he should pursue a medical career, as his grandfather had. He was sent to train in a hospital, but was put off by the screams of unanesthetized patients on the operating table. Seeking guidance from his cousin Charles Darwin, who had just returned from his voyage on the H.M.S. Beagle, Galton was advised to “read Mathematics like a house on fire.” So he enrolled at Cambridge, where, despite his invention of a “gumption-reviver machine” that dripped water on his head, he promptly suffered a breakdown from overwork.

This pattern of frantic intellectual activity followed by nervous collapse continued throughout Galton’s life. His need to earn a living, though, ended when he was twenty-two, with the death of his father. Now in possession of a handsome inheritance, he took up a life of sporting hedonism. In 1845, he went on a hippo-shooting expedition down the Nile, then trekked by camel across the Nubian Desert. He taught himself Arabic and apparently caught a venereal disease from a prostitute—which, his biographer speculates, may account for a noticeable cooling in the young man’s ardor for women.

The world still contained vast uncharted areas, and exploring them seemed an apt vocation to this rich Victorian bachelor. In 1850, Galton sailed to southern Africa and ventured into parts of the interior never before seen by a white man. Before setting out, he purchased a theatrical crown in Drury Lane which he planned to place “on the head of the greatest or most distant potentate I should meet with.” The story of his thousand-mile journey through the bush is grippingly told in this biography. Improvising survival tactics as he went along, he contended with searing heat, scarce water, tribal warfare, marauding lions, shattered axles, dodgy guides, and native helpers whose conflicting dietary superstitions made it impossible to settle on a commonly agreeable meal from the caravan’s mobile larder of sheep and oxen. He became adept in the use of the sextant, at one point using it to measure from afar the curves of an especially buxom native woman—“Venus among Hottentots.” The climax of the journey was his encounter with King Nangoro, a tribal ruler locally reputed to be “the fattest man in the world.” Nangoro was fascinated by the Englishman’s white skin and straight hair, and moderately pleased when the tacky stage crown was placed on his head. But when the King dispatched his niece, smeared in butter and red ochre, to his guest’s tent to serve as a wife for the night, Galton, wearing his one clean suit of white linen, found the naked princess “as capable of leaving a mark on anything she touched as a well-inked printer’s roller . . . so I had her ejected with scant ceremony.”

Galton’s feats made him famous: on his return to England, the thirty-year-old explorer was celebrated in the newspapers and awarded a gold medal by the Royal Geographical Society. After writing a best-selling book on how to survive in the African bush, he decided that he had had enough of the adventurer’s life. He married a rather plain woman from an intellectually illustrious family, with whom he never succeeded in having children, and settled down in South Kensington to a life of scientific dilettantism. His true métier, he had always felt, was measurement. In pursuit of it, he conducted elaborate experiments in the science of tea-making, deriving equations for brewing the perfect cup. Eventually, his interest hit on something that was actually important: the weather. Meteorology could barely be called a science in those days; the forecasting efforts of the British government’s first chief weatherman met with such ridicule that he ended up slitting his throat. Taking the initiative, Galton solicited reports of conditions all over Europe and then created the prototype of the modern weather map. He also discovered a weather pattern that he called the “anti-cyclone”—better known today as the high-pressure system.

Galton might have puttered along for the rest of his life as a minor gentleman scientist had it not been for a dramatic event: the publication of Darwin’s “On the Origin of Species,” in 1859. Reading his cousin’s book, Galton was filled with a sense of clarity and purpose. One thing in it struck him with special force: to illustrate how natural selection shaped species, Darwin cited the breeding of domesticated plants and animals by farmers to produce better strains. Perhaps, Galton concluded, human evolution could be guided in the same way. But where Darwin had thought mainly about the evolution of physical features, like wings and eyes, Galton applied the same hereditary logic to mental attributes, like talent and virtue.“If a twentieth part of the cost and pains were spent in measures for the improvement of the human race that is spent on the improvements of the breed of horses and cattle, what a galaxy of genius might we not create!” he wrote in an 1864 magazine article, his opening eugenics salvo. It was two decades later that he coined the word “eugenics,” from the Greek for “wellborn.”