2. Such cobs, because of their greater lignification, 
also resemble modern teosinte-contaminated maize in 
having a higher specific gravity and greater resistance to 
the action of sulphuric acid. 
3. On the basis of induration or lignification of the 
cob, estimates were made of the amount of teosinte in- 
trogression in the archaeological specimens and correla- 
tions were calculated between these estimates and various 
morphological features of the cob. 
4. The most significant correlations found were be- 
tween teosinte introgression and cob length, but the 
curves expressing this relationship in the Tonto and 
Richards’ Caves material are different. 
The Richards’ Caves material shows that teosinte in- 
trogression is associated with both longer and shorter 
cobs, indicating that maize-teosinte heterosis produces 
beneficial effects, while the homozygous expression of 
teosinte germplasm has detrimental effects. There is 
also an indication of the same sort of effects on kernel- 
row number, although no relation was found between 
diameter and introgression. 
In the Tonto material there is no evidence of heterosis 
in regard to length and row number, although diameters 
do increase with introgression. 
The different effects of introgression within and be- 
tween these archaeological collections emphasize the 
complexity of the effects of teosinte introgression—some- 
times beneficial, at other times detrimental or ineffective. 
5. This material also has a bearing on the problem of 
the role of fasciation and teosinte introgression in the 
evolution of extreme polystichy of the ear. The results 
indicate that introgression modifies the expression of 
[122 ] 
