The elements of strength necessary to support this greatly enlarged 
inflorescence have come from teosinte, which contributes genes for 
hardness and toughness when it is hybridized with corn. ‘Teosinte is 
to the modern ear of corn what steel is to the modern skyscraper. 
THe MuraGcenic Errects or 
TEOSINTE INTROGRESSION 
Hybridization of maize and teosinte not only produces 
new genetic combinations, some of which are favorable, 
but it also has mutagenic effects. These have been re- 
cently described by Mangelsdorf (81). Most of the muta- 
tions, like spontaneous mutations or mutations produced 
by irradiation, are deleterious, but some appear to be 
beneficial. It is entirely possible that these mutagenic 
effects of teosinte introgression have been an important 
factor in the evolution of cultivated corn, as important 
perhaps as the creation of new genetic combinations fol- 
lowing hybridization. 
INTROGRESSION DIRECTLY FROM TRIPSACUM 
After carefully examining the objections to the idea 
of teosinte introgression, as well as the evidence support- 
ing it, we see no reason to doubt that corn is undergoing 
introgression from teosinte now and that this process has 
been going on for centuries. If teosinte is, as we have 
postulated (85), a hybrid of corn and Tripsacum, then 
the introgression is ultimately from Tripsacum. We have 
not assumed that there has been any direct introgression 
of Tripsacum into maize; our assumption has been that 
the hybridization of maize and ‘Tripsacum which gave 
rise to teosinte needed to have occurred only once (35). 
Evidence is now accumulating, however, to indicate that 
maize may have hybridized directly with Tripsacum re- 
peatedly, although only once did such hybridization pro- 
duce teosinte. The evidence for this is of two kinds: (a) 
tripsacoid characteristics in races of maize which have not 
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