HETEROSIS IN A NEW POPULATION 425 



It would seem, therefore, that heterosis is the result of the combined effect 

 of individual gene action (for growth) plus the interaction of all genes within 

 the genotype (its genetic environment) in relation to the sum-total of all ex- 

 ternal influences acting upon the organism and governing expression of its 

 gene complex. If heterosis in a cross between A X Bis measured as the excess 

 vigor or yield over the average of A and B, there may be no heterosis in the 

 environment to which ,4 or B is best adapted as shown with Chalqueno. Yet 

 in some new^ environment different from that in which A and B developed, 

 the excess vigor may be great, even exceeding that of A and B in their native 

 environment, if such comparison could be made. Certainly the genotype is 

 no different in any of the areas. The difference must be due to different inter- 

 actions between over-all gene action and environment. It is no wonder that so 

 many different ideas exist when it comes to the explanation of heterosis. It 

 has no simple explanation. 



IMPROVEMENT THROUGH BREEDING 



In the evolution of corn in Mexico, the different varieties and races were 

 brought together in a haphazard manner. Relatively few of the total combi- 

 nations of races and varieties possible have been made, and when two varie- 

 ties or races come together in a specific region by chance, there is no reason 

 to believe that the particular combination was the best that could have been 

 made for the area. Although some fairly productive varieties did develop, 

 especially in the more fertile areas with higher rainfall, the possibilities of 

 further over-all improvement are astonishing and offer a challenge to modern 

 corn breeders. 



The cooperative corn improvement program of the Mexican Government 

 and the Rockefeller Foundation was begun about six years ago. Its objective 

 was to provide higher yielding varieties or hybrids for the many different en- 

 vironments in the main corn producing areas. In most of Mexico, selection 

 has been going on for many centuries for adaptation to low soil fertility and 

 extreme climatic conditions. A variety or strain capable of producing some- 

 thing in the years of extreme drought or early frost was highly prized by the 

 native Indians, even though it produced only a little more in good years. Low 

 yields meant hunger, but a crop failure meant starvation. With modern 

 means of transportation, crop failures in a region no longer mean starvation 

 for the people of that region, and low fertility can and must be remedied be- 

 fore corn production can be greatly increased. The breeding program, there- 

 fore, was geared to the development of productive varieties or hybrids adapt- 

 ed to the average climatic conditions of a particular region, and a level of 

 fertility that could permit maximum production under these average condi- 

 tions. More productive varieties would pay the cost of soil improvement and 

 pave the way for a generally higher level of corn production. 



The program also is based on a gradual improvement year by year. As 



