Conrad Gesner's 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.