Though Mecca is visited annually by only seventy or eighty thousand pilgrims, it puts into motion a far greater number over the whole Mohammedan world, from westernmost Africa to Chinese Turkestan.[192] Yearly a great pilgrimage, numbering in 1905 eighty thousand souls, moves across Africa eastward through the Sudan on its way to the Red Sea and Mecca. Many traders join the caravans of the devout both for protection and profit, and the devout themselves travel with herds of cattle to trade in on the way. The merchants are prone to drop out and settle in any attractive country, and few get beyond the populous markets of Wadai. The British and French governments in the Sudan aid and protect these pilgrimages; they recognize them as a political force, because they spread the story of the security and order of European rule.[193] The markets of western Tibet, recently opened to Indian merchants by the British expedition to Lhassa, promote intercourse between the two countries especially because of the sacred lakes and mountains in their vicinity, which are goals of pilgrimage alike to Hindu and Tibetan Buddhist. They offer an opportunity to acquire merit and profit at the same time, an irresistible combination to the needy, pious Hindu. Therefore across the rugged passes of the Himalayas he drives his yaks laden with English merchandise, an unconscious instrument for the spread of English influence, English civilization and the extension of the English market, as the Colonial Office well understands.[194] [Sidenote: Historical movement and race distribution.] The forms which have been assumed by the historical movement are varied, but all have contributed to the spread of man over the habitable globe. The yellow, white and red races have become adapted to every zone; the black race, whether in Africa, Australia or Melanesia, is confined chiefly to the Tropics. A like conservatism as to habitat tends to characterize all sub-races, peoples, and tribes of the human family. The fact which strikes one in studying the migrations of these smaller groups is their adherence each to a certain zone or heat belt defined by certain isothermal lines (see map chap. XVII.), their reluctance to protrude beyond its limits, and the restricted range and small numerical strength of such protrusions as occur. This seems to be the conservatism of the mature race type, which has lost some of its plasticity and shuns or succumbs to the ordeal of adaptation to contrasted climatic conditions, except when civilization enables it partially to neutralize their effects. [Illustration: PRIMITIVE INDIAN STOCKS OF SOUTH AMERICA (From Helmolt's _History of the World_. By permission of Dodd, Mead & Co.)] [Sidenote: Migrations in relation to zones and heat belts.]