Morgan divides the history of mankind--and this division is also adopted by Engels--into three chief epochs: savagery, barbarism and civilization. Each of the two earlier periods he subdivides into a lower, a medium and a higher stage, because these stages differ in regard to fundamental improvements in the method of obtaining the means of subsistence. Those changes which occur from time to time in the social systems of nations as a result of improved methods of production, Morgan considers one of the chief characteristics in the progress of civilization, which is quite in keeping with the materialistic conception of history as laid down by Karl Marx and Frederick Engels. Thus the lowest stage in the period of savagery represents the childhood of mankind. During this stage men still were tree-dwellers, and fruit and roots constituted their chief nourishment; but even then articulated language began to take form. The medium stage of savagery begins with the consumption of small animals such as fish, crabs, etc., for food, and with the discovery of fire. Men begin to manufacture weapons, clubs and spears made of wood and stone, and this means the inception of the hunt and probably also of war among neighboring hordes, who contended with one another for the sources of nourishment and the most desirable dwelling places and hunting grounds. At this stage also cannibalism appears, which is still met with among some tribes in Africa, Australia and Polynesia. The higher stage of savagery is characterized by the invention of the bow and arrow; the invention of the art of weaving; the making of mats and baskets from bast and reeds, and the manufacture of stone implements. As the beginning of the lowest stage of barbarism, Morgan denotes the invention of pottery. Man learns the domestication of wild animals with the resultant production of meat and milk, and thereby obtains the use of hides, horns and furs for the most varied purposes. Hand in hand with the domestication of animals, agriculture begins to develop. In the western part of the world corn is cultivated; in the eastern part, almost all kinds of grain, with the exception of corn, is grown. During the medium stage of barbarism we find an increasing domestication of useful animals in the East, and in the West we find an improved cultivation of nourishing plants with the aid of artificial irrigation. The use of stones and sun-dried bricks for building purposes is also originated at this time. Domestication and breeding favor the formation of herds and flocks and lead to a pastoral life, and the necessity of producing larger quantities of nourishment for both men and animals leads to increased agriculture. The result is a more sedentary mode of life with an accompanying increase in provisions and greater diversity of same, and gradually cannibalism disappears.