When the The Origin of Species was published, Darwin carefully sent copies to all of the scientific leaders of his day, both those he thought would hate the book and those he hoped would love it. In all of his accompanying letters he was extremely self-effacing. Darwin's old mentor, Adam Sedgwick, for instance, wrote a savage review of Darwin's book in the Spectator.
His few supporters turned out to be those he had been preparing all along: Hooker, Lyell, and, most importantly, Huxley. Huxley, who up until this point had been supportive but never yet entirely convinced, was finally firmly on Darwin's side after seeing the abundant evidence and clear argumentation. Huxley was renowned for his temper, and he soon became known as "Darwin's bulldog" for his willingness to tenaciously defend the theory of evolution by natural selection.
Huxley had a chance to test his teeth in a debate that took place in 1860. Darwin's most vocal critics, aside from Sedgwick, were the primate anatomist Richard Owen and the Bishop of Oxford, Samuel Wilberforce. During the annual meeting of the British Association at Oxford, after a talk that made some reference to Darwin's ideas, Wilberforce gave a half-hour tirade against Darwinism. Huxley, who was also in the audience, gave an impassioned reply, accusing the Bishop of "prejudice and falsehood." Tensions were high. At the time it was unclear who won the debate, but it has gone down in history as victory for Darwin. In any case it was clear that Darwinism could not be ignored.
Events like these made Darwin a household name, at least among the well-educated and the well-off. He started receiving visits from the literary and scientific stars of the day. But he was still weak and prone to bouts of illness after too much socializing, so during the next ten years he did his best to maintain his quiet, isolated life at the Down House. He focused on some of the problems that had fascinated him as a naturalist, turning away from the theorizing that dominated his Origin of Species and back to the close observation that had always satisfied him best. Research for his books, On the Various Contrivances by which Orchids are Fertilized by Insects and The Movements and Habits of Climbing Plants, once again turned his workshop and now his garden into a place of constant activity. Unlike during his pigeon period, there was little carnage, except when his interest briefly turned to insect-eating plants.
His most ambitious book during the 1860's was The Variation of Animals and Plants under Domestication. Here he tried to understand the dynamics of inheritance, the actual mechanisms by which a pigeon with a slightly longer beak or oddly-colored feather might pass its traits down to succeeding generations. He correctly discovered that some traits tended to overpower others; this corresponds to the modern concept of dominant and recessive genes. But his "pan- genesis" theory of how traits were actually passed from generation to generation was flawed. He thought that each cell in the body contributed to the sperm and egg cells that created new life. According to this theory, small particles called gemules were supposed to travel from every cell of the body, through the blood stream, to the sperm and eggs, thereby conveying information about the current state of the organism. Part of the reason that this theory appealed to Darwin was that it allowed changes that occurred during an organism's lifetime to be inherited by the organism's children: if the skin remained tan for long enough, the gemules in those tanned skin cells would be transported to the egg or sperm cells and thereby affect the skin color of future generations. Unfortunately there was little evidence to support this theory, even in the 1860's, and many of Darwin's peers, while admiring his observations on variation, dismissed his theoretical explanation of it.
After the Variations, Darwin's next big project was to write a book on three topics: the evolution of the human species, sexual selection, and the expression of emotions in animals and humans. As usual, he had more to say on all three of these topics than he originally thought, so the first two topics were published in 1871's The Descent of Man, and Selection in relation to Sex and the third topic a year later in The Expression of Emotions in Animals and Man. Darwin had never wanted to tackle the controversial issue of whether humans were evolved just like other animals, hoping that some other well- respected evolutionist would bite the bullet and write a definitive defense of the idea. But when he realized that such a book was not going to happen without him, he poured all of his enthusiasm and obsession for detail into the project. The Expression of Emotions itself, which has had an enormous impact on the study of the physiology and psychology of emotion, can also be read as a long argument on the unity of the human species with its animal ancestors. Darwin expected outrage after publishing The Descent of Man , but his expectations were disappointed. All of the outrage had come twelve years earlier, when the Origin had been published. Despite the care taken by Darwin in that earlier work to avoid any mention of the human species, it was obvious to many of the Origin's readers that all of the arguments applied to pigeons could as well be applied to humans. So by the time the Descent came out, when the Origin had already been translated into German, Russian, Dutch, Italian, and Swedish, the argument over the origin of the human species was well-worn. Nevertheless the Descent provided a powerful piece of ammunition in the continuing war between creationists and evolutionists.