152 THE ANDES OF SOUTHERN PERU 



ble. Thus for the sturdiest and most useful forest growth the 

 one-wet-one-dry season zone of the plains has alternately too 

 much and too little water. The rubber tree is most tolerant toward 

 these conditions. Some of the best stands of rubber trees in Ama- 

 zonia are in the southwestern part of the basin of eastern Peru 

 and Bolivia, where there is the most typical development of the 

 habitat marked by the seasonal alternation of floods and high 

 temperatures. 



When tropical agriculture is extended to the plains the long 

 dry season will be found greatly to favor it. The southwest- 

 ern quadrant of the Amazon basin, above referred to, is the 

 best agricultural area within it . The northern limits of the 

 tract are only a little beyond the Pongo. Thence northward the 

 climate becomes wetter. Indeed the best tracts of all extend from 

 Bolivia only a little way into southeastern Peru, and are coinci- 

 dent with the patchy grasslands that are there interspersed with 

 belts of woodland and forest. Sugar-cane is favored by a climate 

 that permits rapid growth with a heavy rainfall and a dry season 

 is required for quality and for the harvest. Rice and a multitude 

 of vegetable crops are also well suited to this type of climate. 

 Even corn can be grown in large quantities. 



At the present time tropical agriculture is almost wholly con- 

 fined to the mountain valleys. The reasons are not wholly cli- 

 matic, as the above enumeration of the advantages of the plains 

 suggests. The consuming centers are on the plateau toward the 

 west and limitation to mule pack transport always makes distance 

 in a rough country a 'very serious problem. The valleys combine 

 with the advantage of a short haul a climate astonishingly like the 

 one just described. In fact it is even more extreme in its seasonal 

 contrasts. The explanation is dependent upon precisely the same 

 principles we have hitherto employed. The front range of the 

 Andes and the course of the Urubamba run parallel for some dis- 

 tance. Further, the front range is in many places somewhat 

 higher than the mountain spurs and knobs directly behind it. 

 Even when these relations are reversed the front range still acts 

 as a barrier to the rains for all the deep valleys behind it whose 



