I have, perhaps, said enough to make it clear that an impulse toward caste is found in human nature itself. Whether it spreads through and dominates the system of life, as in India, or remains subordinate, as with us, depends upon the strength or weakness of other impulses which limit its operation. As certain types of vegetation, like the ferns, which at one time were dominant in the forests, are now overshadowed by plants of higher organization, so caste, which we must, on the whole, reckon to be an inferior principle, tends to be supplanted by something freer and more rational. FOOTNOTES: [106] See the memoir on the subject in his Inquiries into Human Faculty. CHAPTER XIX CONDITIONS FAVORING OR OPPOSING THE GROWTH OF CASTE THREE CONDITIONS AFFECTING THE INCREASE OR DIMINUTION OF CASTE—RACE CASTE—IMMIGRATION AND CONQUEST—GRADUAL DIFFERENTIATION OF FUNCTIONS; MEDIÆVAL CASTE; INDIA—INFLUENCE OF SETTLED CONDITIONS—INFLUENCE OF THE STATE OF COMMUNICATION AND ENLIGHTENMENT—CONCLUSION. There seem to be three conditions which, chiefly, make for the increase or diminution of the caste principle. These are, first, likeness or unlikeness in the constituents of the population; second, the rate of social change (whether we have to do with a settled or a shifting system), and, finally, the state of communication and enlightenment. Unlikeness in the constituents, a settled system and a low state of communication and enlightenment favor the growth of caste, and _vice versa_. The first provides natural lines of cleavage and so makes it easier to split into hereditary groups; the second gives inheritance time to consolidate its power, while the third means the absence of those conscious and rational forces which are its chief rivals. The most important sorts of unlikeness in the constituents of the population are perhaps three: differences in race; differences, apart from race, due to immigration or conquest, and unlikeness due to the gradual differentiation of social functions within a population originally homogeneous. Two races of different temperament and capacity, distinct to the eye and living side by side in the same community, tend strongly to become castes, no matter how equal the social system may otherwise be. The difference, as being hereditary, answers in its nature to the idea of caste, and the external sign serves to make it conscious and definite. The race caste existing in the Southern United States illustrates the impotence of democratic traditions to overcome the caste spirit when fostered by obvious physical and psychical differences. This spirit is immeasurably strong on the part of the whites, and there is no apparent prospect of its diminution.