reasons for studying them. Since these plants are basic 
to our present cultures as well, a more complete under- 
standing of them allows us knowingly to reshape and im- 
prove our sources of food and raw materials in a way 
which was not possible for our ancestors. 
In the present paper particular attention will be given 
to the geographic origins of the plants of this area. A 
number of different criteria have been used in attempts 
to determine the centers of origin of the plants which are 
more widely distributed in cultivation. These criteria 
have largely been developed and elaborated by de Can- 
dolle (85) and Vavilov (205, 206, 207); among the more 
important ones may be listed the following: 
1. Distribution of the same species or its apparent an- 
cestor ina wild state. This is a point of great importance, 
if one can be certain that the ‘‘wild’’ plants are not nat- 
uralized from cultivation. his, however, is not easy, 
particularly considering the grave doubts which exist as 
to the occurrence of ‘primeval forest’’ in much of trop- 
ical America. If, of course, a useful wild plant occurs 
over a wide area, then it is not only possible but quite 
probable that it will have been cultivated independently 
or simultaneously in two or more areas. The avocado 
(Persea americana) may be an example of this. 
2. Distribution of primitive forms within the culti- 
vated species. ‘This, too, is of great value, if properly in- 
terpreted. The primitive forms of a group may, however, 
occur either in the center of origin or on the periphery 
of the plant’s range. Each case must be decided on its 
own merits. 
3. Center of diversity. This valuable concept, devel- 
oped principally by Vavilov (205, 206, 207), is based on 
the idea that the greatest diversity within a species will 
occur in the area where it has been in cultivation for the 
longest time. Also, the area where varieties of the wild 
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