coid, Harinoso de Ocho, and Cristalino de Chihuahua. 
All four types are present in Swallow, Slab, and Olla 
Caves. Tau Cave and Dark Cave each lack specimens of 
one of these types, Harinoso de Ocho and Tripsacoid 
respectively, but in both cases the total number of speci- 
mens is small and the absence of one of the four types 
has no significance. 
THE EvoLuTIONARY SEQUENCE 
The sequence of steps in the evolution of maize in 
northwestern Mexico is reasonably clear. All of the early 
maize, whether from Swallow Cave in Chihuahua or 
Dark Cave in Sonora, is related to the living, but still 
primitive, race, Chapalote. The earliest maize of this type 
(from Levels 18 and 14 of Swallow Cave) is smaller and 
more primitive than modern Chapalote and is a precursor 
of it. During the period (perhaps a very long one) rep- 
resented by the difference between Levels 18 and 14, and 
Levels 1 and 2 in Swallow Cave, there was, except for 
a slight increase in size, little change in this race. In this 
respect, the situation is similar to that described by 
Mangelsdorf et al (1956) in the maize of northeastern 
Mexico, where another ancient indigenous Mexican race, 
Nal-Tel, remained remarkably constant during a long 
period of time. 
This gradual evolution within a single race was sud- 
denly interrupted when two new entities, an eight-rowed 
maize originally from South America and ‘Tripsacoid 
maize, became involved in the evolutionary sequence. 
With almost explosive rapidity, these three elements 
now fused to produce an entirely new and highly ad- 
vanced race of maize similar to the modern race of Cris- 
talino de Chihuahua. This spectacular evolutionary spurt 
can be accounted for by three genetic phenomena: ge- 
netic recombination, heterosis, and the mutagenic effects 
[ 172 ] 
