Anaximander’s pupil, Anaximenes, departed radically from the teachings of Thales. He thought that air, not water, was the cause of all things, yet he held that in the beginning all creatures were formed from a primordial slime of earth and water. Another pupil of Anaximander, Xenophanes (576-480), made himself famous by discovering the true nature of fossils. Before his time, and indeed, for thousands of years afterward, fossils were held to be accidents, or natural growths, or creations of a devil, or of a god who delighted in puzzling his earthly children. Xenophanes rightly interpreted them to be the remains of animals, and from this concluded that seas formerly covered what is now dry land. Empedocles, (495-435) taught what is probably the first clearly formulated theory of evolution. He supposed that many parts of animals, such as heads, legs, necks, eyes, ears, and so on, were formed separately, and were kept apart by the mysterious forces of hate. But love of part for part finally overcame the baser passion, and the various sections came together to form bodies. The combinations, unfortunately, were entirely accidental, and did not always result in satisfactory creatures. One body, for example, might possess several heads and no legs; another might have an abundance of arms and legs, but be without a head. These monstrosities were unable to keep themselves alive, and so perished, leaving the world to the bodies that had come together in proper combinations. Thus Empedocles, more than two thousand years before the first zoologist framed and taught a theory of organic evolution that seemed to offer anything worth while, conceived one of the most important of evolutionary principles--that of natural selection.