low altitudes generally benefit from teosinte introgression 
(as indicated by chromosome knob number), while those 
grown at high altitudes suffer from introgression (Well- 
hausen and Prywer, 1954). 
In the archaeological material, the strongest correla- 
tions between the characters studied have been between 
teosinte introgression and cob length; but the two curves 
expressing this relationship in the Tonto and Richards’ 
Caves material are different, at least in regard to the ex- 
tension of the curve. For example, the relationship be- 
tween cob length and introgression at Richards’ Caves 
is described as parabolic curvilinear, while at Tonto it is 
reciprocal curvilinear. A parabolic curvilinear correlation 
suggests that more than one casual factor is involved; 
and, in light of the evidence from modern material, it is 
explicable in this case in terms of teosinte homozygosity 
and heterozygosity. That is, the short Tripsacoid cobs 
represent a product of homozygosity for teosinte germ- 
plasm, while the long Tripsacoid cobs are a result of 
maize-teosinte heterosis. The reciprocal curvilinear cor- 
relation between these factors in ‘Tonto maize appears to 
correspond to the left half of the parabolic curve for 
Richards’ Caves maize. After considering the experi- 
mental data on modern maize, the lack of this apparent 
heterosis in Tonto cobs might be accounted for in one or 
more of several ways, such as: 1) introgression from a 
different type of teosinte germplasm; 2) different residual 
inheritance from maize; and 3) a breeding restriction on 
the spread of introgression. The last possibility is sug- 
gested because of the significantly lower average level of 
introgression at Tonto. 
This ‘“‘heterotic effect’? on length of the cobs from 
Richards’ Caves appears to be reflected in the kernel-row 
numbers of non-fasciated cobs. In this case, the heterotic 
stimulation (right side of the parabolic curve) is not as 
[ 120 ] 
