"I will leave my mark somewhere, and it shall be clear and distinct," wrote Thomas Henry Huxley to his sister in 1850, shortly after the twenty-five-year-old scientist had returned from four years of travels as assistant surgeon on H.M.S. Rattlesnake in its mission of surveying the passage between the Great Barrier Reef and the Australian coast. A few years later, following the 1859 appearance of Charles Darwin's On the Origin of Species, Huxley's "mark" was made in his self-designated role as "Darwin's Bulldog," the foremost advocate of Darwinian evolution.

Tenacious and articulate, Huxley became the Victorian era's clear and distinct voice in the most fiercely debated issue of his generation. But his role as Darwin's spokesman should not eclipse his own contributions as a leading Victorian scientist, educator, and essayist with remarkable literary gifts for combining rationalism and imagination. In fact, so clearly and forcefully did Huxley make...