In 1680, Tyson studied a porpoise and established that it is a mammal. He noted that the convoluted structures of the brains were closer to those of land quadrupeds than those of fish. In 1698, he dissected a chimpanzee on display at the London docks, and as a result wrote a book, Orang-Outang, sive Homo Sylvestris: or, the Anatomy of a Pygmie Compared with that of a Monkey, an Ape, and a Man. In the book he came to the conclusion that the chimpanzee has more in common with man than with monkeys, particularly with respect to its brain. This work was republished in 1894 with an introduction by Bertram C. A. Windle and a short biography of Tyson.
