While the physical man was adapting himself to all manner of climates, the mental man dragged himself up from the hunting life of the Old Stone Age to the invention of writing and the perfecting of an accurate calendar. On the way—and as slow, necessary steps in his progress—he developed agriculture, and invented or perfected the arts and crafts of pottery, weaving, dyeing, metallurgy, sculpture, poetry, painting, architecture, city planning. In his agriculture he utilized irrigation, discovered fertilizers, and developed a great many exclusively American plants which now supply more than half of the world’s provender. He was probably the first to write numbers effectively through the use of zero and numerical position. He practiced trepanning—the removal of a piece of skull bone to relieve pressure on the brain—and he discovered how to use certain medicines and narcotics. In his textiles he employed all the weaves known to us today. He contrived efficient methods of government. He proliferated into 368 major tribal groups, and developed fifteen culture centers of distinct individuality.[9] In the United States he left 100,000 mounds as the product of one of his cultures; in Middle America, 4,000 ceremonial cities of stone. He practiced most types of religion except atheism. Certain things that he made resemble things of the Old World; most are peculiar to him and his life. Of Indian culture traits Clark Wissler remarks that “the range in variety and individuality seems even greater in aboriginal America than in the primitive Old World.”[10] Progress is slow in the stone age. It seems to have been particularly slow in the Old Stone Age, or paleolithic period, when man spent half a million to a million years learning to chip stone and hunt and gather food efficiently. Things went much faster in the New Stone Age, or neolithic period, when he was learning to cultivate plants and make pottery and polish stone tools. To move from the beginnings of agriculture to the beginnings of metallurgy, which superseded the neolithic, may have taken as little as 700 years in the Old World and certainly not much more than 4,000 or 5,000.