Just as the writer of a song may not be its best performer, Charles Darwin's genius lay more in developing the theory of natural selection than in forcefully promoting it in the world. For that, there was Thomas Henry Huxley, such an aggressive defender of evolution that he was known as "Darwin's bulldog."
The early life of Huxley (1825-1895) was much different from the privileged existence of Darwin. He was one of eight children, growing up outside of London without much money. He left school early, trained to be a doctor, and educated himself in science, history, and philosophy, becoming one of the most broadly informed and influential figures in Victorian science.
Like Darwin, Huxley studied natural history while traveling on a naval ship. Initially, Huxley did not accept evolution at all. But Darwin converted him with the On the Origin of Species, and Huxley mused afterwards, "How extremely stupid not to have thought of that!"
Although he was much more than a defender of Darwin -- he led the movement toward the professionalization of science, for example -- Huxley is best known for his public exchange in 1860 with Bishop Samuel Wilberforce. The bishop, a clever, witty debater, opened himself to attack by making a gentle joke about Huxley's ancestry. Huxley, furious, replied famously to the effect that he would rather be descended from an ape than a bishop.
Huxley, the first of a family of important scientists, is also known for coining the term "agnostic" to distinguish skeptics like himself from atheists, who denied the existence of a god. To Huxley, agnosticism was an acceptance of the idea that the human mind could not go beyond certain limits, namely the ability to analyze actual experience. To the confusion of his religious opponents, however, Huxley was very much a moralist and intolerant of sin.
This was a man who today would be called a "workaholic," who pushed himself so hard that his life was shortened. When he died in 1895, his funeral was attended by "the greatest constellation of Victorian scientists ever to gather on one spot," says his biographer, Adrian Desmond.