Gesner cites Vesalius' De Fabrica in his belief that monkeys are far more different from human beings anatomically than the noted ancient physician Galen had supposed. Galen often dissected monkeys to gain knowledge of human anatomy and physiology. Historiae Animalium (Studies on Animals) is considered to be the first modern zoological work. This first attempt to describe many of the animals accurately is illustrated with hand-colored woodcuts drawn from personal observations by Gesner and his colleagues. Conrad Gesner (March 26, 1516 - December 13, 1565) was a Swiss naturalist and bibliographer. To his contemporaries he was best known as a botanist, but in 1551 he was the first to describe brown adipose tissue; and in 1565 the first to document the pencil. He died of the plague, at the age of 49, the year after his ennoblement.