The three principal factors in the evolution of maize 
have probably been: (1) a reduction in the pressure of 
natural selection; (2) a change in the alleles at the T'w- 
tu locus; (8) an introgression of teosinte germplasm into 
maize. 
The principal effect of the first factor has been a tre- 
mendous increase in total variation. Variations, which in 
nature would have been rigidly selected against, survive 
and accumulate in a man-made environment where the 
struggle for existence is reduced to intra-specific compe- 
tition. There is no evidence that human selection was 
an important factor in the early evolution of maize under 
domestication. Thus, in the absence of natural selection, 
and before human selection became a factor, the trend 
was one of creating a wider and wider range of variation. 
The changes in the alleles at the 7'u-tw locus have al- 
ready been mentioned, but the full significance of these 
changes cannot be appreciated without considering the 
present-day products of this evolutionary trend. It has 
already been pointed out that changes from the higher 
to the lower alleles at the 7'u-tu locus result in a reduc- 
tion in the size of the glumes and an increase in the size 
of the rachis. These are the primary effects. The second- 
ary effects, which are perhaps even more important, in- 
clude a substantial increase in the vascular system of the 
ear, and a reduction in the length of the rachillae upon 
which the spikelets are borne. Cutler and Cutler (1948) 
state that the rachilla in teosinte, and in some species 
of Tripsacum, is compacted. They state further that in 
races of maize with long, slender rachillae the grains 
yield to pressure and can be pushed into the cob, while 
in races with compact rachillae the grains are firm. 
All of these characteristics can easily be studied, with 
no more equipment than a pair of calipers and a rule, by 
an examination of the cross section of an ear exposed by 
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