2 CONWAY ZIRKLE 



exception. Practically all of its factual background was reported before 

 Mendel's great contribution was discovered. Even workable methods for 

 utilizing hybrid vigor in crop production were known, but it was not until 

 the classic post-Mendelian investigations of Shull, East, and Jones were 

 completed, that heterosis took its proper place in genetics. The following 

 discussion of the importance of heterosis will be confined to its pre-Men- 

 delian background. 



Heterosis can be described as a special instance of the general principles 

 involved in inbreeding and outbreeding. To fit it into its proper niche, we will 

 trace first the evolution of our ideas on the effects of these two contrasting 

 types of mating. Since our earliest breeding records seem limited to those 

 of human beings and primitive deities, we will start with the breeding 

 records of these two forms. 



Hybrid vigor has been recognized in a great many plants during the 

 last two hundred years. We will therefore describe briefly what was known 

 of its influence on these plants. Because heterosis has reached its greatest 

 development in Zea mays, we will trace briefly the pre-Mendelian genetics 

 of this plant, and show how the facts were discovered which have been of 

 such great scientific and economic importance. 



The ill effects of too-close inbreeding have been known for a long time. 

 Indeed, Charles Darwin (1868) believed that natural selection had pro- 

 duced in us an instinct against incest, and was effective in developing this 

 instinct because of the greater survival value of the more vigorous offspring 

 of exogamous matings. One of his contemporaries, Tylor (1865), noted that 

 many savage tribes had tabooed the marriage of near relatives, and he 

 assumed that they had done so because they had noticed the ill effects of 

 inbreeding. The Greeks looked upon certain marriages between near rela- 

 tives as crimes. This has been known almost universally ever since Freud 

 popularized the tragedy of King Oedipus. At present, we outlaw close in- 

 breeding in man, and our custom is scientifically sound. 



We are apt to be mistaken, however, if we read into the standards of our 

 distant preceptors the factual knowledge which we have today. The in- 

 tellectual ancestors of European civilization approved of inbreeding and 

 actually practiced it on supposedly eugenic grounds. The fact that their 

 genetics was unsound and their eugenic notions impractical is irrelevant. 

 They had their ideals, they were conscientious and they did their duties. 

 The Pharaohs married their own sisters when possible so that their god- 

 like blood would not be diluted. Marriage between half brother and sister 

 was common in other royal families of the period. Actually, as we shall see, 

 the two great pillars of European thought, Hebrew morality and Greek 

 philosophy, endorsed inbreeding as a matter-of-course. 



The Hebrews, who derived mankind from a single pair, were compelled 

 to assume that the first men born had to marry their sisters — as there were 



