The mouse is an excellent example of Gesner's own observation through dissection. Earlier literature had claimed that a mouse's liver waxed and waned with the phases of the moon. He decided to test this theory by dissecting mice at different times of the month and found there to be no difference in the size of its liver. Pictured here is the rat. 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.