CONWAY ZIRKLEexception. 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-Mendelian 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 produced 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 relatives as crimes. This has been known almost universally ever since Freud popularized the tragedy of King Oedipus. At present, we outlaw close inbreeding 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 intellectual 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 godlike 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 EARLY IDEAS ON INBREEDING AND CROSSBREEDING 3 then no other women on the e...

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Drportmcnt of Animal and Plant Pathology, Thc RocLcfcllc'r Inslaiulc for T W O FIGURESThe rneiot ic divisions arc customarily regarded as fixed means of separating the paired parent a1 chromosomes, together with their contained genes, into haploid packets. The beautifully precise and almost uniformly consistent results of cytological observations of these processes in many different orders have contributed to this view. The success of genetic research in utilizing this postulate to foretell breecling results further strengthened it. The observed regularity of these processes has itself obscured tlie mechanism by which the nicety of this control is accomplished. In 1918, an exception t o the universal expectation of the maturation processes in the female Drosophila was found(1, 2) which showed that tlie processes through which the chromosomes pass during the meiotic divisions are in part, a t least, subject to the same specific gene regulation that guides other bodily development and inheritance. The description of the forms produced by the different types of meiosis(3) and the physical effects of these changes on duration of life of the different classes of individuals thus produced (4, 5) have been given elsewhere.The genetic data for this aberrant type of meiosis are herein presented.Three aspects of meiosis, ordinarily regarded as distinct, are significant : 1) crossing over ; 2) chromosome reduction ;

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striking utilization of this fact has come in corn breeding, where the hybrid is often able to fix food materials in excess of 2 or 3 times that of the inbred parents. But the observation is not limited in any way to corn. It is true in most instances for small grains, vegetables, fruits, flowers, economic animals, domestic birds and man.Universal as this phenomenon is, exceptions do occur.These exceptions are as interesting as the occurrences, for they serve to demonstrate that there is much behind hybridity we do not understand.The purpose of this series of papers is to study the mechanisms which lead to this striking capacity of race crosses to metabolize. Drosophila melanogaster egg production was chosen for study.The work was begun in the early part of 1937 when an analysis of heterotic effects as related to cytoplasm and chromosomes was approached through the -use of inversion test stocks for the formation of homozygous races.