This theory of matter and the evolution of the world marks a notable advance over any previous scientific theory. It was well developed by numerous teachers of the Milesian philosophical school and has played a great rôle in intellectual history. The daring nature of some of Anaximander's explanations of earthly organisms may be realized from a sketch of his views on the evolution of animals. He taught that living creatures arose from the moist element as it was evaporated by the sun. Man at first resembled a fish. All animals were developed in the moisture wrapped in a protecting cover or bark. As they advanced in age, they came out into a drier atmosphere and discarded their protective coats. Man was not an original creation, but resulted from the fusion of other species. Anaximander's reason for this statement was that the period of infancy of the human being is so long that had he been born that way originally he could not have survived. There must have been a slow development from ancient ancestors. This may be regarded as an anticipation of the Darwinian theory. Thus man's thoughts in succeeding ages have a rhythmic swing. Anaximenes rejected some of Anaximander's ideas and furnished new ones to take their places. He was not so daring a thinker as his predecessor, and his theory of the world was not as interesting as Anaximander's. Many of his teachings, however, are accepted as sound to-day. Anaximenes contended that the basic element was not boundless, but determinate. Innumerable substances are derivable from it and, just as our soul, like an atmosphere, holds us together, so do breath and air encompass the whole world. Air is always in motion, otherwise so many changes could not be made by it. It differs in various substances in virtue of its rarefaction and condensation. The perpetual changes taking place in the world owing to the instability of matter were emphasized by Heraclitus. He taught that there is nothing immutable in the world process excepting the law or principle which governs it. Cosmological speculations were not the only ones attracting the attention of the Greek scientists. Pythagoras, for example, founded a philosophical college devoted to mathematical studies which resulted in the development of arithmetic to points beyond the requirements of commerce. He made arithmetic the basis of a profound philosophical system.