expression. In either case, as the indurative effects of 
this introgression’ declined, the average cob size retro- 
gressed somewhat toward that of the original pure Chapa- 
lote. The reduction in “‘introgression’’ was slow at first, 
being only 8% at this stratum (Level 3) and not in pro- 
portion to the far greater recovery in kernel row number, 
cob diameter, and cob length of 18%, 22%, and 21% 
respectively (Plate XXIV, figs. 4, 5, 6). 
But even as the direct effects of teosinte introgression 
were apparently diminishing, the variability in cob size 
which was introduced by this introgression in the pre- 
vious Level (4), continued to increase in higher levels. 
For cob diameter, the standard deviations which measure 
degree of variability, for Levels 5 through 1, were 1.48 
mm., 2.12 mm., 2.56 mm., 2.54 mm., and 2.10 mm., 
respectively. Increases in diameter variance are signifi- 
cant up to Level 3. But for cob length, the expanding 
variation proceeds one level higher, as shown by the 
standard deviations for Levels 5 to 1, respectively, as 
follows: 1.01 cm., 1.89 cm., 2.27 cm., 2.39 cm., and 
1.86 cm. 
There are several possibilities which may, as a whole 
or in part, account for the continued increase in varia- 
bility after a reduction in teosinte introgression. If there 
was some variability injected by a new non-Tripsacoid 
race from elsewhere, its effect must have been insignifi- 
cant because the continuity of the population was not 
disrupted by a complete break from the features of Chap- 
alote. [In addition to an actual reduction in teosinte germ- 
plasm, the accumulating variability may have brought 
about some modification of its indurative effects. Man- 
gelsdorf (1958) suggested, on the basis of experimental 
"In order to facilitate further discussion, we shall assume that our 
estimate of teosinte introgression, according to the degree of indura- 
tion, represents a relative measure of its intensity. 
[175 ] 
