Aristoteles de Animalibus.

Written in the fourth century B.C.E., Aristotle's History of Animals had a powerful influence on zoology for some two thousand years. Generally seen as a pioneering work of zoology, Aristotle frames his text by explaining that he is investigating the what (the existing facts about animals) prior to establishing the why (the causes of these characteristics). The book is thus an attempt to apply philosophy to part of the natural world. Throughout the work, Aristotle seeks to identify differences, both between individuals and between groups. The work contains many accurate eye-witness observations, in particular of the marine biology around the island of Lesbos, such as that the octopus had color-changing abilities and a sperm-transferring tentacle, that the young of a dogfish grow inside their mother's body, and that the male river catfish guards the eggs after the female has left. Some of these were long considered fanciful before being rediscovered in the nineteenth century. It continued to be a primary source of knowledge until in the sixteenth century zoologists including Conrad Gessner, all influenced by Aristotle, wrote their own studies of the subject.