"To soothe the theologians, who in his time were pressing so hardly upon Galileo, Descartes was content to say that the operation by which God maintains the world is similar to that by which he created it; so that, if it had pleased him, instead of creating it instantaneously, to allow these laws of evolution to operate, the result would have been what we now see. He began by assuming space to be occupied by perfectly homogeneous and continuous matter. He then supposed this solid substance to be divided into parcels of various shape and size, each of them animated by motion in various directions. These would observe the laws of motion as Descartes defines them:—1. Each would maintain its own condition of rest or motion or magnitude, until altered by contact with another. 2. In such contact the gain or loss of motion to one body would be exactly compensated by the loss or gain to another—the total quantity of motion in the world remaining invariable. 3. Owing to constant contacts, motion would be usually in curved lines, the moving body tending always to follow the tangent to the curve. The result after a period of time would be the differentiation of primitive matter into three kinds. The moving portions of matter, by constant attrition, would be for the most part converted into spheroidal molecules of various sizes. Some larger masses of irregular shape would amalgamate into solid masses; the finer particles rubbed off from the molecules would insert themselves between them, vibrating with far more rapid motion than they. This vibrating ethereal substance would collect towards the centre of a vortex, and form a sun or star: round it would revolve aërial matter, and plunged amidst this, at various distances, the solid masses of the planets. How by degrees yet further differentiation took place... so that the various metals and crystals arose, and finally plant life, and animal life... is described in the Principia and in the Treatise on Man."