teosinte as an ancestor of modern corn while vigorously deny- 
ing it to be sie ancestor may seem to be no more than an 
exercise in semantics. From the standpoint of theoretical gen- 
etics and practical plant breeding, however, it is much more 
than that. If teosinte is fe ancestor of corn and the only one, 
then modern corn contains only one stream of germplasm. But 
if the ultimate ancestor of corn was corn, as the fossil pollen 
and the archaeological remains show, and modern corn Is the 
product of repeated introgression from teosinte, then two dis- 
tinct streams are involved and modern corn, although a diploid, 
has some of the attributes of an allopolyploid (8). In this respect 
it is comparable to the allopolyploid cereals like bread wheat, 
one of the world’s most productive food plants, the product of 
hybridizing three species, two of which have never been rec- 
ognized as worthy of cultivation and a third, einkorn, which 
being generally quite unproductive, is but little grown. 
The interactions of these two streams of germplasm, corn’s 
and teosinte’s, has produced profound effects, including gen- 
etic recombination, heterosis — hybrid vigor — and mutagene- 
sis (3/). These, combined with corn’s preadaptation to changes 
in ear length, by virtue of its large pollen, have resulted in the 
explosive evolution (23) illustrated in Plate 19. It is doubtful if 
such rapid evolution could have occurred were teosinte the 
only ancestor of corn. Then the hybridization of cultivated 
corm, originating from teosinte, with its ancestor, teosinte 
would have produced a minimum of genetic recombination and 
heterosis and probably no mutagenesis, and any increase in ear 
length would have been inhibited by teosinte’s lack of preadap- 
tation because of its small pollen. 
To the practical plant breeder the difference in the two 
theories may determine the most promising methods of 
employing genes from teosinte. Shall he hybridize corn directly 
with teosinte or shall he hybridize strains of one race of corn 
containing teosinte germplasm with strains of another race 
containing a somewhat different combination of teosinte 
genes? In both strains the teosinte genes or blocks of genes will 
have been absorbed into the corn genotype by the natural 
selection of modifying genes that suppress the undesirable 
characters of teosinte and allow the desirable ones to be ex- 
250 
