When Plato died about 348, his nephew Speusippus became head of the Academy, and Aristotle left Athens. He migrated to Assus, a city on the northwestern coast of Anatolia (in present-day Turkey), where Hermias, a graduate of the Academy, was ruler. Aristotle became a close friend of Hermias and eventually married his ward Pythias. Aristotle helped Hermias to negotiate an alliance with Macedonia, which angered the Persian king, who had Hermias treacherously arrested and put to death about 341. Aristotle saluted Hermias’s memory in “"Ode to Virtue,"” his only surviving poem.
While in Assus and during the subsequent few years when he lived in the city of Mytilene on the island of Lesbos, Aristotle carried out extensive scientific research, particularly in zoology and marine biology. This work was summarized in a book later known, misleadingly, as The History of Animals, to which Aristotle added two short treatises, On the Parts of Animals and On the Generation of Animals. Although Aristotle did not claim to have founded the science of zoology, his detailed observations of a wide variety of organisms were quite without precedent. He—or one of his research assistants—must have been gifted with remarkably acute eyesight, since some of the features of insects that he accurately reports were not again observed until the invention of the microscope in the 17th century.
The scope of Aristotle’s scientific research is astonishing. Much of it is concerned with the classification of animals into genus and species; more than 500 species figure in his treatises, many of them described in detail. The myriad items of information about the anatomy, diet, habitat, modes of copulation, and reproductive systems of mammals, reptiles, fish, and insects are a melange of minute investigation and vestiges of superstition. In some cases his unlikely stories about rare species of fish were proved accurate many centuries later. In other places he states clearly and fairly a biological problem that took millennia to solve, such as the nature of embryonic development.
Despite an admixture of the fabulous, Aristotle’s biological works must be regarded as a stupendous achievement. His inquiries were conducted in a genuinely scientific spirit, and he was always ready to confess ignorance where evidence was insufficient. Whenever there is a conflict between theory and observation, one must trust observation, he insisted, and theories are to be trusted only if their results conform with the observed phenomena.
In 343 or 342 Aristotle was summoned by Philip II to the Macedonian capital at Pella to act as tutor to Philip’s 13-year-old son, the future Alexander the Great. Little is known of the content of Aristotle’s instruction; although the Rhetoric to Alexander was included in the Aristotelian corpus for centuries, it is now commonly regarded as a forgery. By 326 Alexander had made himself master of an empire that stretched from the Danube to the Indus and included Libya and Egypt. Ancient sources report that during his campaigns Alexander arranged for biological specimens to be sent to his tutor from all parts of Greece and Asia Minor.