And the definition of life as self-movement seems to become almost comical, for on that definition surely the whole physical universe, the only perpetual motion machine we know of, is itself alive. A discussion of this question can at the utmost only be suggestive. Very few positive assertions have been made, nor can their number be added to, in reference to a question which is bound to be asked: How long has life existed on the earth? The study of radium and its presence in the earth’s crust alone suffices to abolish altogether the old estimates, and new ones cannot yet be substituted. Only it is certain that the past history of planetary life may be far longer than any previous estimate has indicated. It now seems that the earth is not only not self-cooling, but actually self-heating, and if on the older assumption Lord Kelvin could talk of a hundred million years since, so to speak, water first became wet, and life, as we know it, possible, who shall say of how long periods we may speculate now? Meanwhile, the glass-eyed stare vacantly around them and declare that the progress of science means the destruction of the spirit of wonder and reverence. To them we reply in the words of the Earth Spirit in Goethe’s “Faust”: “At the whirring loom of Time unawed, I weave the living garment of God.” C. W. SALEEBY [Illustration: THE MASTERY OF THE EARTH AND HOW MAN OBTAINED IT BY DR. ARCHDALL REID] All the world--at any rate, all that part of the world which is acquainted with the facts--is now agreed that man is a product of evolution, and that his remote ancestors were of different bodily make and shape, and of different mental type and calibre, from their late descendants. No study of human kind can be comprehensive that does not include a survey of the mode by which the faculties that have given man the mastery of the earth were evolved. [Sidenote: We Know the Present by the Past] A history of his evolution, based, like a political history, on episodes, cannot, of course, be written. But man is a bundle of parts and capabilities. By comparing the civilised being with the savage and the savage with lower animals, we are able to trace, in many important particulars at least, his natural history with a degree of certainty to which, I think, no political history can aspire. As our comprehension of adult man is helped by a knowledge of the development of the child, so our understanding of our species is aided by a study of its past. Armed with some clear conceptions of what man was, and is, we shall be the better fitted to investigate social and political change, and to perceive how it happens that while some nations have inherited the earth and the fruits thereof, others have stagnated or fallen into decay. [Sidenote: How Man Learns by Experience]