I would not have it supposed, however (because I dwell thus upon opportunity), that I agree with those whose zeal for education and training leads them to depreciate natural differences. I do not know how to talk with men who believe in native equality: it seems to me that they lack common sense and observation. How can they fail to see that children in the same family, even twins, as Mr. Galton has shown,[106] are often widely divergent in ability, one destined to leadership and another to obscurity? The two variables of personality, “nature and nurture,” are without doubt of equal diversity and importance, and they must work together to bring about any notable achievement. Natural ability is essential; but, no matter how great, it cannot know or develop its power without opportunity. Indeed, great natural faculty is often more dependent on circumstance than is mediocrity—because of some trait, like extreme sensitiveness, that unfits it for miscellaneous competition. Opportunity, moreover, means different things in different cases, and is not to be identified with wealth or facile circumstances of any sort. Some degrees and kinds of difficulty are helpful, others not. And yet, leaving out, on the one hand, unusual talent or energy, and, on the other, decided weakness or dulness, the mass of men are guided chiefly by early surroundings and training, which determine for them, in a general way, what sort of life they will take up, and contribute much to their success or failure in it. Society, even in a comparatively free country, is thus vaguely divided into hereditary strata or sections, from which the majority do not depart. If the transmission of function from father to son has become established, a caste spirit, a sentiment in favor of such transmission and opposed to the passage from one class into another, may arise and be shared even by the unprivileged classes. The individual then thinks of himself and his family as identified with his caste, and sympathizes with others who have the same feeling. The caste thus becomes a psychical organism, consolidated by community of sentiment and tradition. In some measure the ruling class in England, for example, has hung together in this way, and the same is partly true of the lower orders. No doubt there is generally some protest against a hereditary system on the part of restless members of the lower castes—certainly this was always the case in Europe—but it may be practically insignificant. And out of caste sentiment arise institutions, social, political and economic—like the mediæval system in Europe, much of which still survives—whose tendency is to define and perpetuate hereditary distinctions.