Agriculture had reached its highest level in Peru among the native tribes. The soil was artificially enriched with manure and guano brought from the islands; extensive systems of irrigation were carried out, and implements of bronze, as spades and hoes, took the place of the ruder tools of stone or wood. The crops were maize, potatoes both white and sweet, yucca, peppers, tobacco and cotton. Of domestic animals the llama and paco were bred for their hair, for sacrifices and as beasts of burden, but not for draft, for riding nor for milking.[298] The herds often numbered many thousands. The Inca dog was a descendant of the wolf,[299] and monkeys, birds and guinea pigs were common pets. Cotton and hair of the various species of the llama were spun and woven into a large variety of fabrics, often ornamented with geometric designs in color. The pottery was exceedingly varied in forms. Natural objects were imitated in clay with fidelity and expression, and when a desirable model was not at hand, the potter was an adept in moulding curious trick-jars that would not empty their contents in the expected direction, or would emit a strange note from the gurgling fluid, or such as could be used as whistles, or he could turn out terra-cotta flutes and the like. Not less adroit were the artists in metal, especially in bronze and in gold and silver. The early writers are filled with expressions of astonishment at the amount, variety and beauty of the Incarian gold work. Its amount we may well credit when we are told that the value of the precious metals shipped to Spain within twenty-five years after the conquest was four hundred million ducats of gold. There are specimens enough remaining to judge of its artistic designs. They are quite ingenious and show dexterous manipulation, but rarely hint at a sense of the beautiful. Peruvian architecture was peculiar and imposing. It showed no trace of an inspiration from Yucatan or Mexico. Its special features were cyclopean walls of huge stones fitted together without mortar; structures of several stories in height, not erected upon tumuli or pyramids; the doors narrowing in breadth toward the top; the absence of pillars or arches; the avoidance of exterior and mural decoration; the artistic disposition of niches in the walls; and the extreme solidity of the foundations. These points show that Inca architecture was not derived from that north of the isthmus of Panama. In the decorative effects of the art they were deficient; neither their sculpture in stone nor their mural paintings at all equalled those of Yucatan. The only plan they had devised to record or to recall ideas was by means of knotted strings of various colors and sizes, called quipus. These could have been nothing more than mere mnemonic aids, highly artificial and limited in their application.