HETEROSIS IN A NEW POPULATION 427 



ficial selection. Often this was precisely the case. Out of 240 samples of corn 

 collected in Zones 2 and 3 from areas with elevations ranging from 1400 to 

 2000 meters, and tested under conditions representative of these two zones, 

 15 varieties were outstanding. These consisted of three early, five medium, 

 and seven late maturing varieties in relation to the normal growing or rainy 

 season of the area in which tests were made. The best early varieties (Well- 

 hausen, 1947) yielded from 15 to 49 per cent more than the average of 26 

 varieties of the same maturity. The best medium maturing varieties yielded 

 from 30 to 54 per cent higher than the average of 57 varieties of medium ma- 

 turity. The best late varieties yielded from 25 to 60 per cent more than the 

 average of the 62 late varieties of similar maturity included in the tests. 



A similar situation was found to exist in the valleys at higher elevations 

 in the high plateau (Zone 1) among both the late and early varieties. A late 

 variety commonly grown under irrigation in the fertile valleys of the State 

 of Hidalgo was found to yield about 20 per cent more than a variety called 

 Chalco widely used for irrigation plantings in the States of Mexico and 

 Puebla (Wellhausen and Roberts, 1948). The best early varieties for Zone 1 

 were found in the State of Mexico, north of Mexico City, and these yielded 

 from 15 to 20 per cent more than the average early varieties grown in Zone 1 . 

 On the whole, from 15 to 20 per cent increase in yield often could be obtained 

 in some parts of all zones through the wider distribution of the best open-pol- 

 linated variety found within each zone. 



Fundamental Methods in the Formation of Good Inbred Lines 



Steps 2 to 4, as outlined above, imply the formation and use of inbred lines, 

 and the degree of improvement one might expect depends upon the isolation 

 of good, vigorous, disease-resistant lines that combine well with each other 

 when used in synthetics or hybrids. In the formation of lines we have, in gen- 

 eral, adhered to the following principles: 



1. The use of diverse varieties adapted within a particular zone. 



2. Rigid selection based on vigor and desirable agronomic characters. 



3. Tests for combining ability after one generation of selfing. 

 Approximately 35 different varieties of corn belonging to eight different 



races have been inbred in the improvement program for central Mexico. 

 These eight races are listed as follows in order of adaptation to elevation: 



Zone to 

 Which 

 Race Adapted 



Conico 1 



Chalqueno 1 



Conico Norteno 2 



Celaya 3 and 4 



Bolita 3 and 4 



Tabloncillo 3 and 4 



Vandeno 4 and 5 



Tuxpeno 5 



