To return now to the question of mental homogeneity of a population as a condition of national character and collective mental life. Purity of race is the most obvious condition of such homogeneity; but few, if any, nations that have attained any high level of civilisation have been racially homogeneous; probably for the simple reason that the civilisation of such a nation would crystallise at an early stage into rigid forms which would render further progress impossible. This has been the fate of most civilisations of the past; as Walter Bagehot put it, their cake of custom has so hardened as to become brittle, incapable of partial modification and growth, so that, like a crystal, it must either resist completely every modifying influence or be shattered irretrievably[62]. Certainly none of the European nations are racially homogeneous. Nevertheless, some of them approach homogeneity of innate qualities, or, rather, the degree of heterogeneity is much less in some than in others. Consider the case of England. Before the Anglo-Saxon invasion the population consisted in all probability of a mixture of the northern fair race with a darker race, probably that of _H. Mediterraneus_, in some proportion that we cannot determine, with small islands of _H. Alpinus_ or of stocks formed by an earlier blending of this with the Nordic race. The Anglo-Saxon invasion brought great numbers of the pure representatives of the Northern race of closely allied stocks; and these did not confine themselves to any one region, but, entering at many points of the south and east coasts, diffused themselves throughout almost all England, imposing themselves as masters upon those Britons whom they did not drive out. Ever since that time a crossing of the stocks has been going on freely, little hindered by differences of area, language, law, or custom. And, with the exception of small numbers of the Northern stock, Danes and Normans, the population has not received any considerable additions since the Saxon invasion.