ical root-crops is confined to strictly tropical climates, so that increase of latitude and altitude would bring to starch-eating peoples the necessity of a change of food plants. Indeed, altitude seems to have played a larger part than latitude in this transformation which brought about the adoption by primitive American peoples of Indian corn, 'Irish' potato, arracacha, oca and other crops of the temperate plateaus of South America. Without reasonable doubt, maize is the oldest of cereals, and the large soft kernels which distinguish it from all other food-grasses are exactly the character which would render it easily available to the meal-eating aborigines of America, though it is not to be supposed that the wild progenitor of the Indian corn had any very close similarity to our cultivated plant. Moreover, everywhere in tropical America maize is still prepared by methods adapted to root-crops instead of as a cereal. The rough stone slab (metate) against which they had rubbed their cassava or other starch-producing roots was well suited to making paste from maize, softened by soaking in water with lime or ashes, and throughout tropical America it has remained in use to the present day. Among the tribes of the arid temperate regions where the tropical root-crops were excluded the metate was deepened into the mortar in which seeds too small to be collected or handled singly are also bruised into meal.
It is also not impossible that maize was the first plant to be grown by man from seed, a cultural method permitting much easier and more rapid distribution than had been practicable with the root-crops grown from cuttings and tubers. Like other species cultivated in the highlands of tropical America most varieties of maize do not thrive in moist equatorial regions of low elevations, so that it did not seriously supplant the root-crops, though having a far wider distribution than any other plant cultivated by the aborigines in pre-Spanish America. Nor did the utilization of maize mark the limit of cereal cultures in America, though no other small foodseed of the new world compares in popularity with rice, wheat, barley, rye and oats. Even in Mexico, the supposed home of maize, the seeds of Amaranthus and Salvia (Chia) attained considerable economic importance. In addition to their use as food the latter were made to furnish a demulcent drink and an edible oil valued as an unguent and in applying pigments, a series of functions closely parallel to those of sesame, perhaps the most ancient of old world herbaceous seed-crops. Wild seeds of many kinds were collected by the Indians of the United States and Mexico, including wild rice (Zizania) and Uniola, another rice-like, aquatic grass of the shallow shore-water of the Gulf of California. In Chili there existed also several incipient cultures of small-seeded plants, such as Madia, while the people of the bleak plateaus of Peru and Bolivia had developed a unique
