138 EDGAR ANDERSON AND WILLIAM L BROWN 



tury. They resulted in a large measure from deliberate crossing and re- 

 crossing of two races of maize (the Northern Flints and the Southern Dents) 

 so different that, were they wild grasses, they would be considered as totally 

 different species and might well be placed in different genera. The origin of 

 two so-different races within cultivated maize is an even larger problem and 

 one outside the scope of this discussion. It may be pointed out parentheti- 

 cally that the Tripsacum hypothesis (Mangelsdorf and Reeves, 1945) would 

 not only account for variation of this magnitude, it would even explain the 

 actual direction of the difference between these two races of maize. However, 

 the relation between maize and Tripsacum on any hypothesis is certainly a 

 most complicated one (Anderson, 1949). It would be more effective to post- 

 pone detailed discussions of this relationship until the comparative morphol- 

 ogy of the inflorescences of maize and of Tripsacum is far better understood 

 than it is at present. 



SIGNIFICANCE TO MAIZE BREEDING 



Derivation of the commercial field corns of the United States by the de- 

 liberate mingling of Northern Flints and Southern Dents is a fact. Unfortu- 

 nately, it is a fact which had passed out of common knowledge before the 

 present generation of maize breeders was educated. From the point of view 

 of practical maize breeding, either hybrid or open-pollinated, it is of central 

 importance. Briefly, it means that the maize germ plasms now being worked 

 with by plant breeders are not varying at random. They are strongly 

 centered about two main centers or complexes. Such practical problems as 

 the development and maintenance of inbreds, the detection of combining abil- 

 ity, and the most effective utilization of hybrid vigor need to be rethought 

 from this point of view. Detailed experiments to provide information for such 

 practical questions already are well under way. While these experiments are 

 not yet far enough along to give definite answers, they have progressed far 

 enough to allow us to speak with some authority on these matters. 



HETEROSIS 



The heterosis of American Corn Belt dents acquires a new significance in 

 the light of these results, and practical suggestions as to its most efficient 

 utilization take on a new direction. We are immediately led to the hypothesis 

 that the heterosis we are working with is, in part at least, the heterosis ac- 

 quired by mingling the germ plasms of the Northern Flints and the Southern 

 Dents. 



Insofar as hybrid vigor is concerned, the hybrid corn program largely has 

 served to gather some of the dispersed vigor of the open-pollinated dents. 

 Preliminary results indicate that this has not been done efliciently in terms 

 of what might be accomplished with somewhat more orientation. 



The early days of the hybrid corn program were dominated by the hy- 



