As a river in its seaward course gathers volume and momentum from its tributaries, so does a science, from its inception to its attainment of maturity and beyond, increase its strength, scope and depth by incorporating into itself its historical past, built up by the contributions of those scientists and philosophers whose work marks stages in its progress. A history of science, to fulfil its purpose, must unravel the intricate strands of ideas and facts which form the fabric of modern science and trace them back to their inspiration and source. Such a history would be almost beyond the bounds of human endeavour, and its only practicable substitute tends to degenerate into a mere collection of names of scientists, combined with a review of their individual contributions to the development of science. Science, however, is neither an assemblage of facts nor of ideas. It is a co-ordinated system of facts linked by theories—with the emphasis, generally speaking, on the ideas—and within our limited capacity it can best be interpreted by reconstructing the lives and reliving the thoughts of those men who have done most to fashion it. Such a man was Isaac Newton and such also was James Hutton. It is of little profit to argue as to which was the greater mind. What Newton achieved in the field of astronomy and mathematics Hutton achieved in the field of geology. It was he who provided geology with a theory: and here we must take theory, not as something remote and opposed to fact, but in its original meaning in Greek, namely, comprehension.