of more than 6500 feet presumably because it confers no 
advantage at these elevations. Wellhausen and Prywer’s 
conclusion is as follows: 
.. these data suggest that there is a relationship between knob 
number and yield factors. At low altitude, the high-knobbed inbred 
lines tend to be better combiners than the low-knobbed ones. At high 
altitudes, the reverse seems to be true; the low-knobbed lines tend 
to be the best combiners. It is now almost certain that many of the 
good, high yielding, open-pollinated varieties in Mexico during their 
evolution picked up some favorable characters from teosinte or Trip- 
sacum. The number of knobs a variety has may well be indicative of 
the amount of germplasm it has received from these two species. 
It is possible that the Indians of western Mexico have 
long recognized the beneficial effect of teosinte introgres- 
sion, for Lumholtz (27) reported their practice of inter- 
planting maizillo (probably teosinte) and maize for the 
purposes of improving the latter. This, however, is quite 
in contrast with the attitude of the natives of the Valley 
of Mexico who contend that the presence of teosinte 
plants in the field causes the maize to ‘‘run down.’” Here 
again it may be a matter of altitude. 
The question may be raised whether or not the intro- 
gression of teosinte into maize which has occurred in 
Mexico and Guatemala has any influence on corn beyond 
the borders of these countries. There is little doubt that 
it has. Practically all of the corn varieties of the United 
States owe their origin to Mexican and Guatemalan 
races. ‘he Corn-Belt Dent, the principal type in the 
United States, is a hybrid of the Southern Dents and 
Northern Flints (1). The Southern Dents in turn are de- 
rived from Mexican lowland corns (6) which are highly 
tripsacoid races believed to be the product of teosinte 
introgression (53). 
The only direct evidence of the improvement effected 
by controlled introgression of teosinte is furnished by the 
data of Reeves (43), who introduced teosinte germplasm 
[ 895 ] 
