We may observe, in passing, that the same causes which have contributed to this ethnological complexity, have, at the same time, aided in the development of the cosmical idea--the idea of the unity of the universe. At first, tribes had little communication with each other, and knew nothing of geography beyond the limits of their own hunting grounds. They knew as little of the vastness of the earth outside of their domain as of that of the universe. This could only be conjectured from the vantage ground of some degree of intellectual culture, and the idea must remain vague and indefinite till after long ages of real experience and intellectual unfolding. It was not till after Alexander's conquests in the East, the extension of the Roman Empire, the invention of the mariner's compass, the discovery of America, and the circumnavigation of the globe, together with the perfection of optical instruments by the use of which the true character of the celestial bodies was demonstrated, that the cosmical idea became truly a scientific one. (Humboldt.) Thus were the partial and fragmentary notions of early peoples at length corrected, enlarged, unitized. Closely akin to this is the development of the god-idea. Fetich-worship is that of the rudest people. They see a god in every individual object, in every stream, in every tree, in every stone. All they see is, however, shrouded in mystery, and they have a blind veneration for every object. A step farther, and the developing mind generalizes these objects. The individual trees, for example, are taken collectively, and their divine representative worshipped as the god of the groves. There are, at the same time, other unitizing conceptions of the god-idea. There is a god of the hills, a god of the streams, of the seas, and so on. New classes of divinities may be evolved in the mythological system; the strong and salient passions of our nature may come to have their deities--to be unitized, at length, with all other gods. Meantime, mankind are forming into states, with some degree of regular government; and apparently in accordance with this fact, the gods are subjected to the partial control of one who is greater than all the rest, and who is their father and king, but himself subject to the decrees of Fate. Another grand step, and seemingly in correspondence with the more centralized government of a vast and powerful empire, we hear of one God only, who is all-powerful, and master of Fate itself, with a hierarchy of angels, powers, and principalities, reaching from God to man, and subordinate to the Central Will, which rules all things, whether 'in the armies of heaven or among the inhabitants of the earth.' Thus did the idea of one God eventually swallow up all the others; and the god-idea was completely unitized. We now come to consider the political and social evolution of mankind, as it appears to be revealed by the comparison of various stages of national growth.