Aristotle was the first to attempt to classify all the kinds of animals in his History of Animals. He grouped the types of creatures according to their similarities: animals with blood and animals without blood, animals that live on water and animals that live on land. Aristotle's view of life was hierarchical. He assumed that creatures could be grouped in order from lowest to highest, with the human species being the highest. Subsequent commentators on Aristotle interpreted this as a "ladder of nature" (scale nature) or a "Great Chain of Being," but these were not Aristotle's terms. His system of classification was not evolutionary, and the various species on the ladder had no specific genetic relationship to each other. Aristotle regarded the essence of species as fixed and unchanging, and this view persisted for the next two thousand years.