Published in 1872, The Expression of the Emotions in Man and Animals was a book at the very heart of Darwin's research interests – a central pillar of his 'human' series. This book engaged some of the hardest questions in the evolution debate, and it showed the ever-cautious Darwin at his boldest. If Darwin had one goal with Expression, it was to demonstrate the power of his theories for explaining the origin of our most cherished human qualities: morality and intellect. As Darwin explained, "He who admits, on general grounds, that the structure and habits of all animals have been gradually evolved, will look at the whole subject of Expression in a new and interesting light."