At different centres in both North and South America, tribes have become more or less settled and developed permanent phases of early civilization, strongly marked by the later Neolithic cultures. In some exceptional cases, the uses of copper, bronze, and gold are to be noted. Perhaps the most important centres are those of the Incas in Peru, the Mayas, Aztecs, and Terra-humares of Mexico, the cliff-dwellers and Pueblos of southwestern United States, the mound-builders of the Mississippi valley, and the Iroquois nation of northeastern United States and Canada. At the time of the coming of the Europeans to America, the Indian population in general was nomadic, in the hunter-fisher stage of progress; but many of the tribes had tentatively engaged in agriculture, cultivating maize, squashes, and in some cases fruits. Probably the larger supply of food was from animals, birds, fish, and shell-fish, edible roots and grains, such as the wild rice, and fruits from the native trees in the temperate and tropical countries. The social organization was based upon the family and the tribe, and, in a few instances, a federation of tribes like that of the Iroquois nation. _The Incas of Peru_.--When the Spaniards under Pizarro undertook the conquest of the Peruvians, they found the Inca civilization at its highest state of development. However, subsequent investigations discovered other and older seats of civilization of a race in some ways more highly developed than those with whom they came in contact. Among the evidences of this ancient civilization were great temples built of stone, used as public buildings for the administration of religious {188} rights [Transcriber's note: rites?], private buildings of substantial order, and paved roads with numerous bridges. There were likewise ruins of edifices apparently unfinished, and traditions of an ascendent race which had passed away before the development of the Incas of Pizarro's time. In the massive architecture of their buildings there was an attempt to use sculpture on an elaborate scale. They showed some skill in the arts and industries, such as ornamental work in gold, copper, and tin, and the construction of pottery on a large scale. They had learned to weave and spin, and their clothing showed some advancement in artistic design. In agriculture they raised corn and other grains, and developed a state of pastoral life, although the llama was the only domesticated animal of service. Great aqueducts were built and fertilizers were used to increase the productive value of the soil. The dry climate of this territory necessitated the use of water by irrigation, and the limited amount of tillable soil had forced them to use fertilizers to get the largest possible return per acre.