CULTURE-PERIOD AND NATIVE HOME 123 
Fic. 121.—Map showing, by shaded areas, the three primitive centers of 
agriculture. 
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 native 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- 
