148 EDGAR ANDERSON AND WILLIAM L. BROWN 



pictorialized scatter diagram (Fig. 8.8). Using the method of Extrapolated 

 Correlates we were able to reconstruct the probable shank type of the 

 Southern Dents. (We later grew and examined them and verified our predic- 

 tions.) We arranged most of the facts concerning variation in shank type in 

 United States inbreds in a single, easily grasped diagram. All the technical in- 

 formation needed as a background for breeding was available after two days' 

 work by two people. Without the Northern Flint-Southern Dent frame of 

 reference for these miscellaneous facts, we might have worked around the 

 problem for several breeding seasons before comprehending this general, over- 

 all picture. 



SUMMARY 



1. Archaeological and historical evidence shows that the common dent 

 corns of the United States Corn Belt originated mainly from the purposeful 

 mixing of the Northern Flints and the Southern Dents. 



2. Cytological and genetic evidence point in the same direction and were 

 used in the earlier stages of our investigations before the complete historical 

 evidence had been located. 



3. The Northern Flints and Southern Dents belong to races of maize so 

 dififerent that, were they wild grasses, they would certainly be assigned to 

 different species and perhaps to different genera. Such cytological and 

 genetical evidence as is available is in accord with this conclusion. 



4. The significance of these facts to maize breeding problems is outlined. 

 In the light of this information, the heterosis of Corn Belt maize would seem 

 to be largely the heterosis acquired by mingling the germ plasms of the 

 Northern Flints and Southern Dents. It is pointed out that most breeding 

 programs have been so oriented as to be inefficient in assembling the dis- 

 persed heterosis of the open-pollinated varieties of the Corn Belt. The possi- 

 bility of gathering more heterosis from the same sources is discussed and it is 

 suggested that more might be obtained, particularly among the Northern 

 Flints. 



5. Morphological characters of dents and flints, if carefully chosen, should 

 be useful criteria for specific combining ability. The problem of selecting 

 such characters is described. Two seasons' results in correlating combining 

 ability and flint-dent differences are reported. They are shown to be statisti- 

 cally significant and of probable practical importance. 



6. The practical advantages of understanding the flint-dent ancestry of 

 Corn Belt maize are discussed and illustrated by example. In brief these facts 

 provide a "frame of reference" for detecting, organizing, and understanding 

 much of the manifold variability in Corn Belt maize. 



