224 climate and geography: Asia and the Pacific India This was possible because of the vast stretches of desert and grassland in inner Asia that lacked settled inhabitants and so could be crossed without local resistance or having to pay local taxes or tariffs The main items exported from China were silk and later porcelain But the West did not produce anything that was compact enough to transport by caravan and was desired in China, so the return traffic was mostly in gold Technology also flowed along the Silk Road: not only the secret of silkworm cultivation (which reached the Byzantine Empire in the mid-sixth century) but also gunpowder and the spinning wheel, which were Chinese inventions On the other hand, the bubonic plague, or Black Death, was also endemic in inner Asia and probably traveled to the West along the Silk Road to spark the pandemic in western Europe (1348–50) that killed one-third of the population The religions of Buddhism and Manichaeism also traveled along the Silk Road and became dominant in inner Asia before the arrival of Islam Conflict between the Muslim states of inner Asia and China disrupted trade on the Silk Road after the eighth cen- tury, but it was restored in the later Middle Ages when the Mongol conquest of the whole area between China and Baghdad in the 13th century unified the area and again ended barriers to trade The famous visit to China by the Venetian merchant Marco Polo was along the Silk Road and took place at this time, though it is far from clear whether he actually made this journey or composed his published account of it based on other extant reports of travels through inner Asia and China The second collapse of the Silk Road after the end of Mongol domination in inner Asia stimulated the search for new trade routes with China in western Europe East Asia The heartland of China is the fertile plain dominated by river valleys stretching along the coast from Korea to Southeast Asia The main rivers (from north to south) are the Yellow, the Yangzi, and the Pearl The abundant waters of these rivers and excellent soils in their floodplains led to the development of agriculture by about 7000–5000 b.c.e and early states by about 2000 b.c.e However, China’s rivers are more prone to Fujiyama from Okitsu, by Yoshida Hiroshi; woodblock print, Japan, 1928 (Arthur M Sackler Gallery, Smithsonian Institution, Purchase and partial gift of H Ed Robison, S1988.254)