Link Relations of Southwestern Asia 257 



15 to 20 inches up to 45 or 50. With a 

 rainfall of 50 inches few civilizations 

 exist. With a rainfall of over 50 inches 

 civilization is drowned out. A rainfall 

 of less than 10 or 15 inches renders cul- 

 tivation impossible unless irrigation is 

 conducted on a large scale, and this in- 

 volves either complete isolation from 

 disturbance on the part of small com- 

 munities or a share on the part of a 

 small community in the security and 

 capital of a larger nation, with suffi- 

 cient resources to carry out an exten- 

 sive project of irrigation. If, as in 

 this map,'" which presents rainfall, the 

 average annual precipitation be distrib- 

 uted into a rainfall of under 25 centi- 

 meters, from 25 to 50 centimeters, from 

 50 to 100 centimeters, and over 100 

 centimeters, it will be seen that the civ- 

 ilized and developed regions of Asia and 

 Europe have a rainfall of from 50 to 100 

 centimeters; but there lies between them 

 a broad area of a rainfall of under 25 

 centimeters; and that the region which 

 we have been considering has over it a 

 strip of rainfall of 25 to 50 centimeters — 

 a mean between the rainfall in which 

 civilization is impossible and that under 

 which it best flourishes. Where a rain- 

 fall as small as this falls over a broad 

 tract of uniform modeling, it will be so 

 distributed and diffused as to do little 

 more than create brief green patches in 

 the winter and spring. Where, how- 

 ever, it meets any medium mountain 

 range creating valley areas, a rainfall of 

 this character will be so collected as to 

 give fertile river valleys essentially in- 

 sular in their character, which will be 

 sheltered from disturbing invasion by 

 stretches around less easily traversed 

 and in some respects, as in the desert 

 west of Egypt, a greater protection than 

 any ocean deep. Such a stretch of re- 

 duced rainfall over a mountain tract 

 would constitute, therefore, another of 



^"Jahrliche Niederschlage Mengenin "Grund- 

 zuge Physischen Erdkunde," Supan, L-, 1897, 

 Taf. XI. 



the link conditions which unite the 

 heavier precipitation under which civ- 

 ilization develops. 



RAINFALL AND MOUNTAIN TRACTS 



So far as precipitation is concerned, 

 therefore, the three centers of the Eura- 

 sian system are separated by regions 

 of insufficient rainfall north and south. 

 Across these, just north of that high 

 barometric area along the thirtieth par- 

 allel, which constitutes so important a 

 climatic influence in the North Temper- 

 ate Zone, stretches a region of medium 

 rainfall for which the mountain system 

 of southwestern Asia gives exactly the 

 conditions which permit the early devel- 

 opment of isolated civilizations in a re- 

 gion where the development of man is 

 not impeded, as it is over the forest 

 region which once stretched from the 

 Pacific to the Atlantic across the Eura- 

 sian system or by the desert region to the 

 south . At this point, therefore, the hyp- 

 sometric conditions cooperate with the 

 rainfall' produced by the distribution of 

 isobars and other causes to create in this 

 linked region the opportunities, not for 

 extensive and heavy population, but for 

 nests and centers of population. Climate, 

 which is rainfall plus place and temper- 

 ature, enforces this condition still more 

 clearly. The Mediterranean basin con- 

 stitutes a distinct climatic region, sepa- 

 rated on the one side from the steppe 

 climate of eastern Europe and from the 

 moderate climate, due to warm currents 

 of air, in western Europe. India and the 

 island world to the southeast constitute 

 another climate, not unlike in its uniform 

 conditions to the Mediterranean basin, 

 though wholly unlike in its tempera- 

 ture and precipitation. This has to the 

 north the steppe climate of central Asia 

 and the climate of China, as with that of 

 Europe, modified by air drifts. Be- 

 tween these two regions, as the distri- 

 bution of climate by Supan shows, the 

 mountain region, extending from the 



