now sympatric only in Central America and Mexico, and 
for this reason all of them probably originated from a 
common ancestor which also occurred there. When the 
fallacy of placing such strong reliance on this present-day 
distribution is recognized, little or no factual evidence 
for the theory of common ancestry remains. Stebbins 
(36) reviews the literature on the theory that centers of 
diversity correspond to centers of origin. He points out 
that the theory has many pitfalls, except when the group 
in question is young and the selective forces of the en- 
vironment have been operating in about the same man- 
ner throughout its evolutionary history. Stebbins shows 
that related genera, congeneric species and even conspe- 
cific populations might be expected to have widely dis- 
junct ranges. He cites numerous examples of a single 
species with two ranges separated by half the distance 
around the earth. He shows also that examples of closely 
related disjunct taxa are not rare exceptions but are nu- 
merous and that some of them have been known since 
the time of Darwin. 
Much of the validity of the Montgomery-W eatherwax 
theory depends upon the assumed correlation between 
the common ancestry and the common place of origin 
of the three groups. If one or two of the groups were 
shown to have originated elsewhere than in the present 
center of diversity (and this may yet prove to be true 
especially of Tripsacum), this would seriously weaken the 
theory. In addition, the proponents of the theory assume 
the previous existence of a pre-maize and of the common 
ancestor of pre-maize, teosinte and Tripsacum without 
one iota of evidence. The tripartite theory, in contrast, 
is more flexible and much less dependent upon com- 
pletely unknown ancestors of corn. Indeed, it requires 
no ancestral types other than forms still in existence. 
Types of pod corn very similar to our hypothetical an- 
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