In view of the wide range of materials at command, and the limits of the present inquiry, there is need for some definite method of selection, which may secure a careful, though necessarily very general survey of the whole ground. That which seems to give most promise of meeting these requirements is the contemplation in order of the great leading conceptions which have received prominence within recent years in consequence of continued research under strictly scientific methods. These may be said to constitute the scientific revolution of the nineteenth century, giving occasion for reconstructing the popular conceptions of the universe. They claim to mark the truly scientific period, inaugurated by command of instruments never before within reach, allowing an immense advance in the modes of research, and placing the secrets of nature within compass of human observation as they had never been before. The intellectual conditions for observation and inference no doubt remain simply what they have been; the laws of intelligent inquiry are the same, determining sufficiency of evidence, and trustworthiness in reasoning; but the range of observation has been indefinitely multiplied, and things transcending previous conjecture have become matters of certain observation. The telescope and the microscope provided for this revolution. They brought the universe within range as it had never previously been, and thus making an enormous addition to the sum of human knowledge, suggested new modes of contemplating and explaining the facts which had been familiar through all the ages. There can be no reversal of all this--no return on the old methods. Nor can there be reversal in the sense of overturning presently recognized conclusions. There are indeed hosts of theories of which it may be safely predicted that they will be overturned and forgotten; but a veritable knowledge has been acquired, which will certainly be preserved among the treasures of the race. We now know the constituent elements of many forms of existence, and the laws which determine change and continuance, as these were never previously known; and thus there has been vastly extended for us the range of recognized facts.