On the other hand, the vast open plains of Russia have allowed the Slavs of the districts which lie round Novgorod, Moscow, and Kiev to spread out among and Russify the Lithuanian and Finnish, to some extent also the Tartar, races, who originally held by far the larger part of that area. So, too, the Ural range, which, though long, is neither high nor difficult to pass, has opposed no serious obstacle to the overflow of population from Russia into Siberia. That in North America the Alleghanies have had a comparatively slight effect upon political history, although they did for a time arrest the march of colonisation, is due partly to the fact that they are a mass of comparatively low parallel ranges, with fertile valleys between, partly to the already advanced civilisation of the Anglo-Americans of the Atlantic seaboard, who found no great difficulty in making their way across, against the uncertain resistance of small and non-cohesive Indian tribes. A far more formidable natural barrier is formed between the Mississippi Valley and the Pacific slope by the Rocky Mountains, with the deserts of Arizona, Utah, Nevada, and Idaho. But the discovery of steam power has so much reduced the importance of this barrier that it does not seriously threaten the maintenance of a united American republic. In one respect the New World presents a remarkable contrast to the Old. The earliest civilisations of the latter seem to have sprung up in fertile river valleys. Those of the former are found not on the banks of streams like the Nile or Euphrates, but on elevated plateaux, where the heat of a tropical sun is mitigated by height above sea level. It was in the lofty lake basin of Tezcuco and Mexico, and on the comparatively level ground which lies between the parallel ranges of the Peruvian and Bolivian Andes, that American races had reached their finest intellectual development, not in the far richer, but also hotter and less healthy river valleys of Brazil, or (unless we are to except Yucatan) on the scorching shores of the Caribbean Sea. Nature was in those regions too strong for man, and held him down in savagery. [Sidenote: How Nature fixes Sites of Cities]