How, now, have these various worlds been formed out of the formless, indefinite, indeterminate matter of {26} Anaximander? On this question Anaximander is vague and has nothing very definite to put forward. Indeterminate matter by a vaguely conceived process separates itself into "the hot" and "the cold." The cold is moist or damp. This cold and moist matter becomes the earth, in the centre of the universe. The hot matter collects into a sphere of fire surrounding the earth. The earth in the centre was originally fluid. The heat of the surrounding sphere caused the waters of the earth progressively to evaporate giving rise to the envelope of air which surrounds the earth. For the early Greeks regarded the air and vapour as the same thing. As this air or vapour expanded under the action of heat it burst the outside hot sphere of fire into a series of enormous "wheel-shaped husks," resembling cart wheels, which encircle the earth. You may naturally ask how it is that if these are composed of fire we do not see them continually glowing. Anaximander's answer was that these wheel-shaped husks are encrusted with thick, opaque vapour, which conceals the inner fire from our view. But there are apertures, or pipe-like holes in the vapour-crust, and through these the fire gleams, causing the appearance of the sun, stars, and moon. You will note that the moon was, on this theory, considered to be fiery, and not, as we now know it to be, a cold surface reflecting the sun's light. There were three of these "cart wheels"; the first was that of the sun, furthest away from the earth, nearer to us was that of the moon, and closest of all was that of the fixed stars. The "wheel-shaped husks" containing the heavenly bodies are revolved round the earth by means of currents of air. The earth in the centre was believed by {27} Anaximander to be not spherical but cylindrical. Men live on the top end of this pillar or cylinder. Anaximander also developed a striking theory about the origin and evolution of living beings. In the beginning the earth was fluid and in the gradual drying up by evaporation of this fluid, living beings were produced from the heat and moisture. In the first instance these beings were of a low order. They gradually evolved into successively higher and higher organisms by means of adaptation to their environment. Man was in the first instance a fish living in the water. The gradual drying up left parts of the earth high and dry, and marine animals migrated to the land, and their fins by adaptation became members fitted for movement on land. The resemblance of this primitive theory to modern theories of evolution is remarkable. It is easy to exaggerate its importance, but it is at any rate clear that Anaximander had, by a happy guess, hit upon the central idea of adaptation of species to their environment.