Even the writings of such extravagant geniuses as BRUNO and CAEDAN gave indications of the progress of the human mind; and had RAMUS not shaken the authority of the _Organon_ of Aristotle we might not have had the _Novum Organon_ of BACON. Men slide into their degree in the scale of genius often by the exercise of a single quality which their predecessors did not possess, or by completing what at first was left imperfect. Truth is a single point in knowledge, as beauty is in art: ages revolve till a NEWTON and a LOCKE accomplish what an ARISTOTLE and a DESCARTES began. The old theory of animal spirits, observes Professor Dugald Stewart, was applied by DESCARTES to explain the mental phenomena which led NEWTON into that train of thinking, which served as the groundwork of HARTLEY'S theory of vibrations. The learning of one man makes others learned, and the influence of genius is in nothing more remarkable than in its effects on its brothers. SELDEN'S treatise on the Syrian and Arabian Deities enabled MILTON to comprise, in one hundred and thirty beautiful lines, the two large and learned syntagma which Selden had composed on that abstract subject. LELAND, the father of British antiquities, impelled STOWE to work on his "Survey of London;" and Stowe's "London" inspired CAMDEN'S stupendous "Britannia." Herodotus produced Thucydides, and Thucydides Xenophon. With us HUME, ROBERTSON, and GIBBON rose almost simultaneously by mutual inspiration. There exists a perpetual action and reaction in the history of the human mind. It has frequently been inquired why certain periods seem to have been more favourable to a particular class of genius than another; or, in other words, why men of genius appear in clusters. We have theories respecting barren periods, which are only satisfactorily accounted for by moral causes. Genius generates enthusiasm and rivalry; but, having reached the meridian of its class, we find that there can be no progress in the limited perfection of human nature. All excellence in art, if it cannot advance, must decline. Important discoveries are often obtained by accident; but the single work of a man of genius, which has at length changed the character of a people, and even of an age, is slowly matured in meditation. Even the mechanical inventions of genius must first become perfect in its own solitary abode ere the world can possess them. Men of genius then produce their usefulness in privacy; but it may not be of immediate application, and is often undervalued by their own generation.