The first effect of climate, food, and soil upon man that may be noted is that they lead man to accumulate wealth. These accumulations permit that degree of leisure from “making a living” which enables some members of society to acquire knowledge. Upon these acquisitions of knowledge, particularly of socialized knowledge, civilization depends. This progress in the early stages of civilization rests on two circumstances: “First, on the energy and regularity with which labor is conducted, and second, on the returns made to that labor by the bounty of nature.”[XV-4] Both these causes are the results of physical antecedents. The returns which are made to labor are regulated by the fertility of the soil. Moreover, Buckle asserted, the energy and regularity with which labor is conducted will be entirely dependent on the influence of climate.[XV-5] When heat is intense, men will be indisposed and partly unfitted for active industry. Climate also affects the regularity of the habits of laborers. In very cold climates, the weather interferes with regular habits and produces desultoriness. In southern countries regular labor is likewise prevented--this time by the heat. Thus, in the early stages of civilization the fundamental law may be stated: the soil regulates the returns made to any given amount of labor; the climate regulates the energy and constancy of labor itself.[XV-6] Of the two primary causes of primitive societary growth, the fertility of the soil is more important than the climatic influences. It is only where soil fertility exists that civilization can arise at all.[XV-7] But in Europe, climate has been more effective than soil fertility. In Europe a climate has existed which has stimulated human activities. Since the mental powers of man are unlimited they are more important, once they get started, than the powers of nature, which are limited and stationary. Man has endless capacity, through his dynamic mental tendencies to develop the physical resources of the earth. The birth rate depends on food supply. In hot countries, where less food per capita is required than in cold countries, and where an abundance of food exists, the birth rate is very high. In cold countries highly carbonized food is necessary, but this food is largely animal in origin and great risk is involved in procuring it. Hence the people of cold countries become adventuresome.[XV-8] By the study of physical laws it is possible to determine what the national food of a country will be. In India, for example, the physical conditions are decidedly favorable to the growth of rice, which is the most nutritive of all cereals, and which, consequently, is a causal factor in a high birth rate.