CULTURE-PERIOD AND NATIVE HOME 123 



Fig. 121. — Map sho\\'inj^, hy 



ihaded areas, 

 a^rriculture. 



the three primitive centers of 



bus, an important agricultural center was established on the 

 highlands of tropical America, and formed the basis of those 

 remarkable civilizations of the Nahuas and Incas which the 

 Spanish invaders overthrew. 



These three primitive centers of agriculture (Fig. 121) 

 are important for us to remember, since, when taken in con- 

 nection with what is known of the native homes of cultivated 

 plants, they help us to understand why certain species have 

 been cultivated so much longer than others, and why they 

 have come to be so important. 



47. Relation between culture-period and native home. 

 It may be laid down as a rule that, other things being equal, 

 the nearer the nati\'e home of a cultivated species is to the 

 region forming one of the primitive centers of agriculture, 

 the longer has that species been under cultivation; and, 

 conversely, the more remote its native home from an agri- 

 cultural center, the more recently has it come to be cultivated. 

 This, indeed, is what we should expect in view of the probable 

 beginnings of agriculture already considered in our study of 

 the grains (section 17). Reference to the foregoing tabular 

 view will afford .some interesting confirmations of this general 

 principle, which in turn will help us to an orderly (and there- 



