gradual replacement of the higher alleles in the T'w-tu 
series by the lower. Mangelsdorf (1948) has shown that 
both Zw and tuk have relatively high mutation rates. It 
is evident from the Bat Cave material that mutations at 
the 7'u-tu locus have occurred and that there has been 
a gradual decline in the frequency of the higher alleles 
and an increase in the frequency of the lower. This, in 
turn, has had far-reaching effects upon the size of the ra- 
chis, on the size of the entire cob, on the space available 
for the vascular system, upon the length of the rachillae 
which bear the kernels and undoubtedly upon the ulti- 
mate yield of grain. ‘The importance of the changes in 
relative frequency of the different 7'w alleles can scarcely 
be over-emphasized. 
Teosinte Introgression. The hypothesis of Mangels- 
dorf and Reeves (1989) with respect to the origin of maize 
postulates that teosinte is the product of a natural hy- 
bridization of maize and ‘Tripsacum which was followed 
by an introgression of teosinte (or Tripsacum) germplasm 
into maize. The origin of teosinte as the progeny of maize 
and ‘Tripsacum has not yet been finally proved, although 
much additional evidence in support of the hypothesis 
has been accumulated since 1989 (cf. Mangelsdorf, 1946). 
For the purpose of this paper, however, it is not neces- 
sary to debate the question of the origin of teosinte. We 
are here concerned primarily with the question whether 
there is evidence of such an introgression of teosinte into 
maize. The answer is that there is strong evidence of 
such introgression, but only in the later strata of the Bat 
Cave material. 
The scores for teosinte introgression represent esti- 
mates involving personal judgment which in turn is based 
upon an extensive experience in studying derivatives of 
maize-teosinte hybrids produced experimentally. When 
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