solution to the problem, for, according to the theory of 
common ancestry, all corn, as well as teosinte and Trip- 
sacum, is traceable eventually to a single ancestor, even 
though the line of descent might pass through one or 
more intermediate ancestral forms. Somewhere in such 
a line of descent there must have arisen many changes 
in number and position of knobs. Yet Randolph insists 
that, according to the tripartite theory, structural re- 
arrangements would be required to account for the dif- 
ferent knob positions, without recognizing that they 
would be required equally by the theory of common 
ancestry. 
A few literature reports will now ke mentioned indi- 
cating that some varieties of corn contain structural 
chromosomal differences not usually recognized. Rhoades 
and Dempsey (42), working on 90 exotic races of corn, 
did not find indications of large structural differences 
among races when pollen abortion was used as the index, 
but they did find significant differences in the amount of 
crossing over in heterozygotes involving diverse races. 
Ono and Suzuki (30) found six different karyotypes in 11 
corn accessions out of 69 from Nepa, the karyotypes 
differing in total length or in arm ratio, or both, in cer- 
tain chromosomes. Blanco (5) reported various structural 
abnormalities in corn, following self fertilization and sub- 
sequent crossing. Clark (9) recognized six translocations 
and two inversions occurring naturally among the entries 
in corn-yield tests at the Connecticut Agricultural Ex- 
periment Station in a single season. 
No final conclusion has been reached, except by Ran- 
dolph (82), that abnormal chromosome 10 is anything 
other than the result of a simple attachment of a natural 
end of chromosome 10 to a fragment of another chromo- 
some. Such an attachment to a terminal knob would 
change the position of the knob from terminal to inter- 
[87s] 
