But where there is a cheap national food, the increase in population becomes very great. As a result, there are multitudes of people who are able to keep just above the subsistence level. A few individuals who understand the operation of these physical laws are able to manipulate the multitudes in such a way as to make themselves immensely wealthy. Since wealth, after intellect, is the most permanent source of power, a great inequality of wealth has been accompanied by a corresponding inequality of social and political power.[XV-9] It produces classes and even castes. Poverty provokes contempt. Class conflict results. The poor are ground low, murmur, and are again subjected to ignominy. Under such conditions democracy has a hard struggle. When physical conditions favor one class, that class will constitute itself the government and bitterly oppose the extension of government to all other classes. In Europe there was no cheap national food, no blind multiplication of population, and hence no such disparity between classes as in India. In Europe it has been easier for democratic movements to spread. Early civilization developed in the Euphrates valley, the Nile valley, and in the exceedingly fertile regions of Peru, Central America, and Mexico. Modern civilization is found largely in fertile river valleys, such as the Thames, Seine, Rhine, Po, Danube, Hudson, Mississippi. But in the Amazon valley, the fertility of soil has not invited the growth of a large population. The trade winds have brought in a superabundance of moisture, producing torrential rains, and a luxuriance of plant life and a complexity of virile animal life which thus far have defied the skill of man to overcome. The fourth physical factor which Buckle presents is the general aspects of nature. Of these the first class excites the imagination and the second stimulates the rational operations of the intellect.[XV-10] In regard to natural phenomena it may be said that whatever inspires feelings of terror, of the vague and uncontrollable, and of great wonder tends to inflame the imagination and to cause it to dominate the intellectual processes. Where nature is continually exhibiting its power, man feels his inferiority. He assumes a helpless attitude. He ceases to inquire or to think. His imagination, rather than his reason, reigns. On the other hand, where nature works smoothly and quietly, man begins to assert his individuality. He even essays to dominate nature and other men. His cognition develops and his volition expresses itself vigorously.