the same tribe, and where there is only occasional inter- 
change of seeds as the result of war or commerce. That 
more races have not arisen is probably the result of rigid 
and constant selection toward definite standards. These 
standards vary in different districts and even today are 
often regulated by traditions and_ religious beliefs. 
Branched ears are preferred in some parts of Guatemala 
while in other areas different types are chosen. Cere- 
monial customs play a definite part. Large ears are often 
dressed in clothes and placed on an altar or carried about 
during a dance, then kept for planting the next season. 
Giving the best ears to the gods means that a standard 
of quality is maintained, but since many ears are planted, 
there is still room for considerable variation. Present day 
natives are not as careful as their fathers, for in addition 
to forgetting many customs which had a practical foun- 
dation, they have lost some of their hostility toward 
outsiders and frequently interchange products. They 
may even spend part of their time working in mines or 
in rubber or cattle areas, and when they return to their 
own land carry with them seeds of foreign origin. In 
most cases it is possible to determine which are the re- 
cently introduced varieties. 
Prehistoric migrations of crop plants were probably 
slow, but new modes and routes of transportation and 
the destruction of many regional barriers have led to the 
rapid interchange of plants and weeds with the conse- 
quent introduction of new types. Many of the old cul- 
tivated plants have been discarded in favor of the new. 
Even where new plants have not appeared changes in 
customs may handicap the collector of old forms. 
A ease in point is sugar corn in Bolivia. Sugar corn 
was once highly prized in Bolivia for parching and es- 
pecially for making chicha, an alcoholic drink. When the 
Incas ruled and thievery was punished severely, sugar 
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