s RevisionDarwin published The Descent of Man in 1871. It contained the following seemingly common-sense sentence, so characteristic of Darwin's anecdotal style, his roundabout way of making his point:In the agony of death a dog has been known to caress his master, and every one has heard of the dog suffering under vivisection, who licked the hand of the operator; this man, unless he had a heart of stone, must have felt remorse to the last hour of his life. 1 Had everyone really heard of this in 1871? Darwin gives no clue of the identity of this man, whether it were real or imagined, a trope of scientific cruelty that was somehow already in the air or a repetition of an anecdote on the lips of the chattering classes. As Darwin was writing, there were the first glimpses of antivivisectionist sentiment in public discourse, but nothing like an organized movement. This aside appeared in a long discussion on the evolution of sympathy, its limits and the possibilities for its extension. Darwin's sentimental note was published without an inkling that it might have political traction. Events soon overtook him.By the time Darwin came to do the revisions for a second edition of Descent, the ethical landscape had changed. Darwin's common-sense aside had become politically sensitive. There was a risk, and not a small one, of it being used against him, to align him with his enemies and to alienate him from his friends and allies in the worlds of science and medicine. Darwin did not like pain, or cruelty or the idea that pain was intrinsic to scientific investigation, but Darwin did believe in the virtues of experiment, and he knew that physiology was at the centre of medical and scientific progress. It was the crucible of new knowledge about the human and other species, and it was the hope for new remedies against disease and suffering. He also knew,