corn and ‘Tripsacum chromosomes, when associated in 
the same nucleus, has been confirmed. 
With few exceptions, annual teosinte, the species most 
comparable to modern corn, proves to be intermediate 
between corn and Tripsacum in number and position of 
chromosome knobs. 
It has become increasingly clear that a correlation 
exists in corn varieties between frequency of chromo- 
some knobs and proximity of their native locality to 
Guatemala and southern Mexico. As corn and teosinte 
hybridize naturally, this correlation constitutes further 
circumstantial evidence on the phylogenetic relationship 
between corn and teosinte. It is explicable on the as- 
sumption that pure corn without knobs was already wide- 
spread over the American continent when teosinte orig- 
inated. Later corn and teosinte began hybridizing, and, 
in fact, are still doing so; in this way, a slow ‘‘diffusion’’ 
of corn with knob-bearing chromosomes from Guatemala 
and southern Mexico has been occurring for several cen- 
turies, but an equilibrium in knob frequency throughout 
the continent has not been reached. 
The hypothesis of the hybrid origin of teosinte has 
been vigorously challenged on the grounds that many of 
the chromosome knobs of corn are intercalary, whereas 
those of Tripsacum are terminal, and that changes in 
knob positions would require structural chromosomal re- 
arrangements which probably have not occurred. Struc- 
tural rearrangements not previously recognized have 
been demonstrated since 1939, however, and some forms 
of Tripsacum actually have been shown to possess a few 
internal knobs. In any event, our statement of 1989 still 
stands: that the theory of common ancestry is confronted 
with this same problem as to the origin of the different 
knob positions. 
Considerable evidence from archaeology and_ paleo- 
[ 382 | 
