It is to be observed that ‘Heraclides of Pontus’ (at one time a pupil of Plato’s) discovered the movement of the two inner planets round the sun. It is possible (as contended by Sciaparelli) that he believed all the planets to move round the sun, and the sun round the earth, in fact anticipated Tycho Brahe. Further, there is a statement that he anticipated Aristarchus as to the movement of the earth; but Sir T. Heath, who examines the evidence very fully, concludes that the evidence has been misread. Aristarchus certainly contended for the diurnal rotation of the earth, but this was rejected by Hipparchus and passed out of account for many centuries. The history of the emergence of the heliocentric theory has a curiously close counterpart in that of the circulation of the blood. Harvey communicated his discovery to the College of Physicians on April 17, 1616, but he had kept it back for twelve years out of deference to the great and deserved authority of Galen, which it was dangerous to dispute, as Copernicus held back his ‘Treatise of Revolutions’ for thirty years, because it was very dangerous, even for the nephew of a Bishop, himself the Canon of a cathedral far north of the Alps, to question the findings of Ptolemy. ‘Yet for years the profession had been in latent possession of a knowledge of the circulation. Indeed a good case has been made out for Hippocrates, in whose works occur some remarkably suggestive sentences’ (see _The Growth of Truth_, the Harveian Oration of 1906, by Sir William Osler, M.D., F.R.S.). Bacon, who ‘writes philosophy like a Lord Chancellor’—i.e. seeks to eliminate error from facts stated, and then to apply the law (see De Morgan, _Bundle of Paradoxes_, p. 50)—, would have none of the Copernican hypothesis. Nor would Sir Thomas Browne, though he preferred Dr. Harvey’s discovery ‘to that of America’. But truth will out, at her own time and through the ministers of her choice. Behind the horseplay of the Stoics and Academics, on the subject of the centre of the universe and the laws which light and heavy bodies obey, there seems to lie some real groping after a general cosmic law, such as gravitation. Thus the earth and the moon draw bodies, each from its own surface to its own centre, and if the earth draws the moon, it is as a part of herself, once ejected and now reclaimed.