Aristotle (384 BCE-322 BCE)
Aristotle collected the courses he gave to his students in hundreds of papyrus rolls, which later became his books History of Animals, Generation of Animals and Parts of Animals. These books are the first empirical and systematic attempt to understand nature. Aristotle´s world view was entirely different from that of his teacher Plato. According to him, our world consisted only of observable facts; it was the world of “down here”. For him, the world had not been created by any god. He perceived all species as a continuum, where the differences among them were only minor changes and adaptations.
He conceived all species as if they were eternal. He claimed that there was a continuum of the non-living to the living, from plants to animals. He never imagined something equivalent to evolution or natural selection. He classified correctly the approximately 500 known species of animals at the time, into animals with blood (vertebrates) and animals without blood (invertebrates). Animals with blood were either those born from eggs (birds and fish) or those born alive (mammals). Bloodless animals were insects, crustaceans, and mollusks.