In their religious system the Aztecs, like the other members of the Nahua stock, had reached, in a technical sense, a highly differentiated standard. They had long left behind that stage of the lowest savage life where there is no recognition even of spirits, those substantial and active beings who are made responsible for the changes in the material world that the savage cannot otherwise explain. When the spirit is supposed to be composed, not of flesh and blood, but of some ethereal matter the era of civilization begins. According to savage belief the spirits are made in the image of a man, consisting of flesh and blood like man, and also requiring, like him, nourishment of food and drink. This principle took the widest extension in the Nahuatlacan worship; with the development of tribal life and the organization of confederacies there went hand in hand the regular provision of meat and drink offerings organized on a very large scale to secure the benevolence of the divinities. Various familiar forms of fetich worship were employed. Probably before the fashioning of idols by the hand of man natural objects such as plants, trees, mountains, and animals were worshipped; for example, in Mexico there was a national annual sacrifice to the mountains. In the frequent human sacrifices the victim was slain with a stone knife, on a stone slab, while the neck and limbs were kept in place by a collar and fetters made of stone. From the maize plant developed some of the most important deities in the Mexican religion. There was a long midsummer festival of eight days devoted to this vegetable, one of the prime necessities of life, at which one victim, a slave girl, was offered to the spirit dwelling in the maize. At the end of the eighteenth century the idol before which the sacrificial ceremony was performed was discovered in one of the squares in Mexico, recalling the procession in which it was carried, bound round with skulls, dead snakes, maize leaves, and ears. The toad, as the offspring of water and the symbol of the water spirit, was an object of veneration. The rabbit, as an animal considered totally devoid of sense, was worshipped as a drink god, to whom offerings were made that the worshipper might escape the deleterious effects of an over indulgence in pulque. Like other people in the primitive stage of culture the Mexicans venerated rivers and lakes as manifestations of will.