172 HAROLD H. SMITH 



Houser (1911) originally suggested the use of first generation intervarietal 

 tobacco hybrids on a commercial scale to increase yields. He presented breed- 

 ing results on cigar filler types, dating back to 1903, in which the hybrids 

 outyielded the parent types by as much as 57 per cent. Plant breeders in 

 various tobacco-growing areas of the world have observed hybrid vigor 

 among first generation hybrids of commercial varieties (Ashton, 1946), and 

 have suggested its use in practice to increase production. Currently, con- 

 sideration is being given to improving the yield of flue-cured varieties by 

 this method (Patel et al., 1949). 



The results of Hayes (1912), Hayes, East, and Beinhart (1913), and East 

 and Hayes (1912) showed that by intervarietal hybridization, selection, and 

 inbreeding the number of leaves, an important factor in yield of tobacco, 

 could be fixed at a level exceeding the parents or Fi. Regarding the use of 

 Fi hybrids on a commercial scale, they stated (Hayes, East, and Beinhart, 

 1913), 



While it is doubtless true that by this method the yield could be somewhat increased, 

 the yield factor, for cigar wrapper types at least, is only of secondary importance com- 

 pared with quality. Because of the great importance of quality it seems much more reason- 

 able to suppose that further advance can be made by the production of fixed types which in 

 themselves contain desirable growth factors, such as size, shape, position, uniformity, vena- 

 tion, and number of leaves, together with that complex of conditions which goes to make 

 up quaHty, than by any other method. 



The problem of producing higher yielding varieties of N. tabacum with 

 acceptable quality characteristics of the cured leaf remains today. Kosmo- 

 demjjanskii (1941) bred four families from the cross Dubec 44XTrebizond 

 1272, two Russian varieties of .Y. tabacum, which, he reported, were uniform 

 for morphological characters and flavor and maintained transgression in 

 plant height and number of leaves to the F; generation. 



While first generation hybrids between selected parents may be of use as 

 a temporary measure to improve self- fertilizing crop plants, it would appear, 

 in so far as can be generalized from the results on Nicotiana, that production 

 of fixed types with favorable transgressive characteristics offers a better long- 

 time solution. Within any one type of tobacco, such as flue-cured, there are 

 currently available a number of high quality inbred varieties which, though 

 of similar phenotype, may be expected to differ by genes of a multifactorial 

 system affecting size characteristics (Emerson and Smith, 1950). Selections 

 from intervarietal crosses may be expected, therefore, to yield fixed types of 

 increased size without presenting undue difficulties to the breeder attempt- 

 ing to maintain quality. 



In order to discuss the hereditary basis for experimental results on hetero- 

 sis and inbreeding, current concepts of the genetic and evolutionary mecha- 

 nisms involved are briefly presented. In the evolution of naturally crossbred 

 organisms, mutation and selection result in the accumulation of dominant 

 favorable genes, hidden deleterious recessives, and alleles or complexes of 



