426 E. J. WELLHAUSEN 



soon as a variety or hybrid was found to be superior to native varieties in a 

 particular area, it was increased and distributed, although it still may have 

 had many minor defects. When better varieties or hybrids become available, 

 they are substituted for those previously released in the increase and distribu- 

 tion program. 



The different steps involved in this gradual improvement program for any 

 particular area have been as follows: 



1. Variety testing. In this way good open-pollinated varieties were some- 

 times isolated for immediate distribution and as basic material for the breed- 

 ing program to follow. 



2. The improvement of the best native open -pollinated varieties through 

 the formation of synthetics which could be propagated through open-pollina- 

 tion. 



3. The formation and distribution of double cross hybrids which were good 

 not only as double crosses but also as synthetics in advanced generations. For 

 this purpose one generation selfed lines were used. 



4. Finally, after many of the farmers have learned how to use hybrid corn, 

 greater emphasis may be devoted to the formation of more specific, higher 

 yielding uniform hybrids with highly selected and proven inbred lines. 



Areas in Which Improvement Work Has Been Concentrated 



The methods used and results obtained may be understood more clearly 

 if the areas in which improvement programs were initiated are identified. 

 Although corn is grown everywhere in Mexico, the most important commer- 

 cial corn growing areas are found on the central plateau between 18 and 22 

 degrees latitude. It is within this area that the breeding work has been con- 

 centrated. To facilitate the work still further, the area was divided into five 

 zones on the basis of elevation as follows: 



Zone 1 — 2200-2600 meters elevation 



a) Late varieties with six months' growing season planted under irrigation 



b) Early varieties with four months' growing season under natural rainfall conditions 

 Zone 2—1800-2200 meters 



Zone 3—1400-1800 meters 

 Zone 4—1000-1400 meters 

 Zone 5 — 0-1000 meters 



The main breeding stations for these five zones are located in Zone 1 at 

 2200 meters, Zone 3 at 1600 meters, and Zone 4 at 1200 meters. With these 

 three main stations and with the cooperation of farmers in making yield tests 

 in outlying regions, it has been possible to cover the central plateau and cer- 

 tain tropical areas fairly completely. 



Utilization of Good Native Open-pollinated Varieties 



Since it is entirely possible that the original gene populations in the many 

 different isolated valleys of a particular zone were not the same, one might 

 e.xpect to find a different variety in each valley as a result of natural and arti- 



