Aristotle presents a systematic survey of animal life, arranging species by external form and mode of life and comparing their habits. He describes anatomy and physiology across groups—humans, quadrupeds, birds, fish, cephalopods, and insects—distinguishing oviparous and viviparous types, and discusses reproduction, development, sensory and locomotive functions, and ecological relations. Observations are gathered into ten books that combine empirical detail with attempts at natural classification and explanations linking structure to function. The work interweaves cited authorities and firsthand observation while providing identifications and indices to support further study.