cultures The ancient cultures of Mesopotamia, the Indian subcontinent, and China, among others, produced renowned surgeons and students of the natural sciences such as Susruta and Zhang Zhongjing, reflecting independent sophisticated systems of natural philosophy. Taoist philosophers, such as Zhuangzi in the 4th century BC, expressed ideas related to evolution, such as denying the fixity of biological species and speculating that species had developed differing attributes in response to differing environments. The ancient Indian Ayurveda tradition independently developed the concept of three humours, resembling that of the four humours of ancient Greek medicine, though the Ayurvedic system included further complications, such as the body being composed of five elements and seven basic tissues. Ayurvedic writers also classified living things into four categories based on the method of birth (from the womb, eggs, heat & moisture, and seeds) and explained the conception of a fetus in detail. They also made considerable advances in the field of surgery, often without the use of human dissection or animal vivisection. One of the earliest Ayurvedic treatises was the Sushruta Samhita, attributed to Sushruta in the 6th century BC. It was also an early materia medica, describing 700 medicinal plants, 64 preparations from mineral sources, and 57 preparations based on animal sources. However, the roots of modern zoology are usually traced back to the secular tradition of ancient Greek philosophy. Ancient Greek traditions The pre-Socratic philosophers asked many questions about life but produced little systematic knowledge of specifically zoological interest—though the attempts of the atomists to explain life in purely physical terms would recur periodically through the history of zoology. However, the medical theories of Hippocrates and his followers, especially humorism, had a lasting impact. Aristotelian zoology Aristotle The philosopher Aristotle created the science of biology, basing its theory on both his metaphysical principles and on observation. He proposed theories for the processes of metabolism, temperature regulation, information processing, embryonic development and inheritance. He made detailed observations of nature, especially the habits and attributes of animals in the sea at Lesbos. He classified 540 animal species, and dissected at least 50. Aristotle, and nearly all Western scholars after him until the 18th century, believed that creatures were arranged in a graded scale of perfection rising from plants on up to humans: the scala naturae or Great Chain of Being. Hellenistic zoology A few scholars in the Hellenistic period under the Ptolemies—particularly Herophilus of Chalcedon and