respond as does modern pollen to this treatment (Plate 18D). 
This is true also of certain other ancient pollen. Pollen from 
Coxcatlan Cave in the Tehuacan Valley dated at ca. 1900 years 
failed to expand completely with the acetolysis treatment 
(Plate 18C). On the other hand pollen from the same cave at ca. 
1600 years expanded almost like modern pollen (Plate 18B). 
Apparently as the chemical constituents of the pollen grains 
change with age certain of them lose their ability to react with 
the chemicals introduced by the acetolysis treatment (Plate 
I8C). What these constituents might be is a question beyond 
the scope of this discussion. The important point here is to 
recognize the fact that ancient pollen grains differ from modern 
ones in their capacity to respond to certain chemical treat- 
ments. The loss of this capacity seems to begin at about 2000 
years; we do not know at what age it is completely lost. Pollen 
from the Huarmey site in Peru, dated at 3600-4000 years (20) is 
only slightly more collapsed than the pollen shown in Plate 18C 
dated at 1900 years (29). 
The failure of the Bellas Artes pollen to respond to the 
acetolysis treatment combined with the fact that opportunities 
for contamination, discussed above, are minimal, if not ac- 
tually nonexistent, has persuaded us that this pollen is indeed 
ancient and not the product of modern contamination. Having 
previously satisfied ourselves that the pollen has been cor- 
rectly identified as corn pollen, we can now only conclude that 
the fossil pollen is authentic and if so, it is more than highly 
probable that the ancestor of cultivated corn was corn and not 
teosinte. 
IMPLICATIONS FOR CORN IMPROVEMENT 
Eliminating teosinte as the ancestor of corn does not mean, 
however, that it has had no role in the evolution of cultivated 
corn. On the contrary, archaeological remains are consistent in 
showing that although the earliest corn may have been pure 
corn, later corn is the product of hybridization with teosinte. 
And the hybridization still continues (30). 
To the puzzled onlooker the distinction between recognizing 
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