of teosinte is limited to probably not more than 25 square 
miles of the valley between the elevations of 1720 and 
1920 meters. 
The local inhabitants recognize teosinte as distinct 
from maize and call it mazcillo or maiz silvestre. They 
are also familiar with the maize Xteosinte hybrids and 
universally will claim that if the hybrid is cultivated for 
three years it produces maize. Some feel that hybridi- 
zation even improves the maize. 
Factors limiting gene exchange 
Both teosinte and maize are wind-borne, cross-polli- 
nated plants, and the prevention of hybridization with 
subsequent genetic exchange between the two species is 
dependent on (1) the spatial isolation of the two species, 
(2) the seasonal isolation of the two species, (3) the lack 
of fitness of the hybrid, and (4) the types of selection 
operating on the two parental populations. 
The most effective isolating mechanism in most areas 
where teosinte and maize are sympatric has been the 
mean mid-flowering date, with maize flowering 2-5 
weeks ahead of teosinte. Nobogame teosinte is unique 
in that itis the only teosinte population found in a habi- 
tat other than a cultivated field which is not seasonally 
isolated from maize. Maize of the Nobogame Valley is 
a five month type belonging to the race Cristalino de 
Chihuahua which is planted in May and harvested in 
September before the early killing frost. Both teosinte 
and maize reach mid-flowering in August. 
Hybridization at Nobogame 
In all the areas of both Guatemala and Mexico where 
teosinte and maize occur together there is some evidence 
(only a single F; hybrid in some cases) of hybridization, 
but never has the author found such a large number of 
hybrids and clear and unmistakable effect of teosinte in- 
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