As the nineteenth century drew to its end, the mechanism and pattern of Nature seemed to have been revealed to the scientist in broad outline; and his researches appeared to some degree, especially in the physical sciences, to have assumed the form of investigations into a structure that was more or less known and established by the work of those who had gone before him. Scientific thought had already undergone three great changes that are properly described as revolutions, since they were no mere changes of emphasis but fundamental changes in outlook. They had all been effected in modern times and in western Europe. The seventeenth century had seen the revolution in mechanics and the foundation of modern physics, begun by Galileo and completed by Newton and marked particularly by the publication of Newton's Principia in 1687; in the eighteenth century there came the revolution in chemistry, brought about by Lavoisier's classic experiments and associated, so far as such events may be dated, with the publication of his Traité élémentaire de chimie in 1789, a date which still conveniently marks the foundation of modern chemistry; the revolution in biology was more recent, introduced by the publication of Darwin's Origin of Species in 1859. Biology had not kept pace with the physical sciences, but it too now seemed at last to have set out on its modern road; and the scientific mind appeared to be concerned at this period with what may be described in general terms as an increasingly refined anatomy of nature.