Using his observations and theories, Aristotle was the first to attempt a system of animal classification, in which he contrasted animals containing blood with those that were bloodless. The animals with blood included those now grouped as mammals (except the whales, which he placed in a separate group), birds, amphibians, reptiles, and fishes.

Aristotle was the first to attempt to classify all the kinds of animals in his History of Animals (Historia Animalium in Latin). 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 classification of animals grouped together animals with similar characters into genera (used in a much broader sense than present-day biologists use the term) and then distinguished the species within the genera. He divided the animals into two types: those with blood, and those without blood (or at least without red blood).