420 E. J. WELLHAUSEN 



months. Different temperatures due to changes in elevation and different 

 amounts of rainfall may occur in areas separated only by a few miles. Such 

 conditions are conducive to the development of many different varieties of 

 corn. 



As a result of the above evolutionary factors operating over a period of at 

 least four thousand years, there is a greater variation in the corns of Mexico 

 today than in any country in the world. Without doubt the greatest single 

 factor in the development of the modern high yielding agricultural types in 

 Mexico has been the introduction of exotic types from the south. These exotic 

 types were largely big-grained flour corns, which no doubt brought in a series 

 of genes for higher yield that had not existed in Mexico before. 



The various processes and types of gene action involved in the develop- 

 ment of higher yielding varieties from the reciprocal introgression betwieen 

 the indigenous and exotic types, plus introgression of teosinte, are not easily 

 explained. 



Gene Combinations 



These processes probably involved a gradual sifting of the gene combina- 

 tions brought together by hybridization, and continuous backcrossing or re- 

 hybridization of resulting hybrids or segregants. The complex pedigree of 

 some of the modern high yielding varieties in Mexico, taken from Wellhausen 

 elal.m collaboration with Mangelsdorf (1951), are shown in Figures 27.1-27.4. 

 In these pedigrees each product of the indicated hybridization between two 

 different races, or species in the case of teosinte, was higher yielding or better 

 adapted to its native habitat than either one of the putative parents. For ex- 

 ample, in Figure 27.1 Conico is a better corn than either Palomero Toluqueno 

 or Cacahuacintle, and Tuxpeno is a more productive corn than either Olotillo 

 or Tepecintle. Chalqueno, which is somewhat more recent in the evolutionary 

 scale, is more productive than either Conico or Tuxpeno. 



This does not necessarily mean that the same races crossed today would 

 all show considerable heterosis in Fi. As a matter of fact many of the crosses 

 indicated in the diagrams have been made and studied. In certain cases the 

 Fi hybrid, when tested in the environment best suited to one or both parents, 

 showed considerable heterosis. In other cases it was no better than the better 

 parent or was intermediate between the two parents. In some crosses the Fi 

 was definitely unadapted to the environment of either parent. 



In the natural development of higher yielding corns from the intercrossing 

 of different races, there were no doubt many instances in which the Fi hy- 

 brids that first occurred between a native and an introduced variety were 

 very poorly adapted to native conditions and showed no heterosis. A 50 per 

 cent random dosage of an introduced variety is often more than sufficient to 

 completely upset the physiology of a native variety that has adapted itself 

 to a fixed environment over a long period of natural selection. Under natural 

 conditions, however, any crossing that might take place between two varie- 



