MORGAN, THOMAS HUNT

(b. Lexington, Ken-tucky, 25 September 1866; d. Pasadena, California, 4 December 1945)

embryology, genetics.

Although known best for his studies in heredity with the small vinegar fly Drosophila melanogaster (often called fruit fly), Morgan contributed significantly to descriptive and experimental embryology, cytology, and, to a lesser extent, evolutionary theory. In recog-nition of his work in establishing the chromosome theory of heredity (the idea that genes are located in a linear array on chromosomes), Morgan was awarded the Nobel Prize in medicine or physiology for 1933.

The son of Charlton Hunt Morgan and the former Ellen Key Howard, Morgan came from two prominent family lines. His father had been American consul at Messina, Sicily, in the early 1860’s and had given assistance to Giuseppe Garibaldi and his Red Shirts. John Hunt Morgan, Charlton’s brother, was a colonel and later general in the Confederate Army and leader of his own guerrilla band, “Morgan’s Raiders.” His mother’s maternal grandfather was Francis Scott Key. composer of the national anthem.

As a boy Morgan spent much time roaming the hills and countryside of rural Kentucky. His visits to his mother’s family in western Maryland, provided the opportunity for further explorations during summers, and particularly for collecting fossils. He also worked for two summers in the Kentucky moun-tains with the U.S. Geological Survey. All of these activities gave Morgan an ease and familiarity with natural history which he retained throughout his life.

Morgan entered the preparatory department of the State College of Kentucky in 1880 and, after two years, the college itself (now the University of Kentucky). In 1886 he received a B.S., summa cum laude, in zoology. While an undergraduate Morgan was particularly influenced toward science by one of his teachers, A. R. Crandall, a geologist, and an undergraduate friend, Joseph H. Kastle. Kastle graduated two years ahead of Morgan and went to Johns Hopkins University in 1884 to do graduate work in chemistry. Perhaps through Kastle’s influence, and because his mother’s family lived in and around Baltimore, Morgan was attracted to Hopkins for graduate work. The summer before he entered graduate school (1886), Morgan went to the Boston Society of Natural History’s marine biological station at Annisquam, Massachusetts. This was his first experience in working with marine organisms, an interest he was to continue throughout his life, primarily in association with the Marine Biological Laboratory, Woods Hole, Massachusetts.

At Hopkins, Morgan took courses in general biology, anatomy, and physiology with H. Newell Martin, a former student of Michael Foster and assistant to T. H. Huxley; anatomy with William N. Howard; and morphology and embryology with William Keith Brooks. He concentrated on morphology with Brooks. In 1890 he completed his doctoral work, on sea spiders, and received his Ph.D. He stayed on at Hopkins for a postdoctoral year on a Bruce fellowship; and in the fall of 1891 he went to Bryn Mawr College, where he remained until 1904, when E. B. Wilson offered him the chair of experi-mental zoology at Columbia. He was a member of the Columbia zoology department from 1904 until 1928, when he resigned to found the division of biological sciences at California Institute of Tech-nology. He remained at Cal Tech and was active in scientific and administrative work until his death, after a short illness, in 1945.

During his academic life Morgan was involved not only in research and teaching but also in numerous professional organizations and activities. He was a member of the Genetics Society of America, the American Morphological Society (president, 1900), the American Society of Naturalists (president, 1909), the Society for Experimental Biology and Medicine (president, 1910–1912), and the American Association for the Advancement of Science (president, 1930). He also served as president of the Sixth International Conference on Genetics held in Ithaca, New York, in 1932. He was a member of the American Philo-sophical Society and the National Academy of Sciences (president, 1927–1931); through the National Academy he was intimately involved with the function of the National Research Council, especially in its formative years between 1921 and 1940. In addition to the Nobel Prize, Morgan received numerous scientific honors, including the Darwin Medal (1924) and the Copley Medal (1939) of the Royal Society.

In 1904 Morgan married Lilian Vaughan Sampson, one of his former students at Bryn Mawr. Lilian Morgan was a cytologist of considerable skill who always maintained an active interest in her husband’s work. After the four Morgan children were in school, she returned to the laboratory and made important contributions to the Drosophila work.

Morgan was known to his friends, colleagues, and students as a man of quick mind, incisive judgment, and sparkling humor. While rarely showing his in-ner feelings, he nonetheless enjoyed people immensely and was a personal friend as well as teacher to many of his students. Frequently he paid the salaries of laboratory assistants from his own funds; and he shared his Nobel Prize money with his lifelong assistants and co-workers C. B. Bridges and A. H. Sturtevant, to provide for the education of their children.

Morgan has been described as down-to-earth, practical, and sensitive. He retained an alert inquisi-tiveness and excitement for new ideas throughout his life. An extremely hard worker, Morgan pursued his scientific interests enthusiastically and relentlessly. He seldom took vacations, and used only one sab-batical during his twenty-four years at Columbia (in the year 1920–1921, when he went to Stanford University, where he continued his work in heredity and embryology). Despite his busy schedule and concentration on work, however, Morgan always found a small part of every day to spend with his family.

Early Scientific Work . As a student of W. K. Brooks, Morgan was trained as a morphologist—one who sought to discover evolutionary (phylogenetic) relationships among organisms by studying their comparative anatomy, embryology, cytology, and, to some extent, physiology. Morphologists relied heavily on descriptive and comparative methods, drawing their conclusions by analogy and inference. Such conclusions necessarily were highly speculative, because they could not be tested in any direct way. Brooks had been a student of Louis Agassiz and later Alexander Agassiz, and was thoroughly grounded in comparative anatomy and embryology, two of the hallmarks of late nineteenth-century descriptive biology. Through detailed studies of early and later embryonic stages of various groups of marine organisms, Brooks sought to elucidate phylogenetic relationships which were not apparent simply from examining the adult forms. Marine organisms seemed particularly important to Brooks, because he felt they were the oldest and most basic types of animals, and thus demonstrated most clearly the fundamental principles of animal organization. Like most mor-phologists, he viewed his own special subdiscipline, embryology, less as a field in its own right than as a tool for studying evolutionary relationships.

According to Bateson, Brooks taught his students to see subjects such as heredity not as completed axioms but rather as unsolved problems for further investigation. Brooks elucidated for them the inter-relationships among such disparate areas of biology as heredity, anatomy, embryology, cytology, and evolution. He had, according to Morgan, a wide-ranging and philosophical mind, which, if not always rigorous, was at least provocative.

Morgan’s doctoral dissertation under Brooks involved a study of four species of marine inver-tebrates, the Pycnogonida (sea spiders), focusing largely, but not exclusively, on their comparative embryology. The purpose of this study was to deter-mine whether the Pycnogonida belonged to the Arachnida (a group including spiders and scorpions) or to the Crustacea (including crabs, lobsters, and crayfish). Observing both large anatomical and smaller cellular changes during embryogenesis, Morgan found that the pattern of development more closely resembled that of the Arachnida than that of the Crustacea. He continued this line of work during the first several years after leaving Hopkins, extending his investi-gations of early embryology to other forms such as Balanoglossus and the ascidians (both primitive chor-dates). Morgan had, however, become increasingly dissatisfied with morphology during his graduate days; he objected to the subordination of disciplines such as embryology almost exclusively to phylogenetic and evolutionary problems. Increasingly, he saw em-bryology and other disciplines as having their own sets of problems for study; moreover, he felt that an experimental approach to problem-solving would make it possible to draw more firm and rigorous conclusions than the inferences and speculations that characterized morphology.

Several factors contributed to Morgan’s growing disaffection with the morphological tradition. The first was perhaps his association with the physiologist H. Newell Martin (head of the biology department at Hopkins),¹ an emphatic and vocal exponent of the experimental method. Following Michael Foster’s lead, he had introduced experimental teaching labo-ratories when he came to Hopkins; and he made it clear from the outset that he regarded physiology as the queen of the sciences, with morphology as its servant.² A second factor was Morgan’s early acquaintance with Jacques Loeb. Both joined the faculty of Bryn Mawr in the same year (1891) and maintained a lifelong friendship. Loeb was a strong proponent of the mechanistic conception of life. He believed that (1) organisms function in accordance with the laws of physics and chemistry, so that to understand living phenomena, it is necessary to approach them from a physicochemical standpoint; and (2) only quantitative and experimental methods would allow biologists to get at the fundamental chemical and physical processes involved with life. These methods, in contrast with those of descriptive biologists, would yield rigorous and testable con-clusions. Loeb believed that biologists should emulate the methods used in the physical sciences. Loeb’s views no doubt strongly influenced Morgan at a time when the latter was beginning to turn away, on his own accord, from descriptive methods.

A third, and perhaps crucial, factor which may have caused Morgan to embrace the experimental approach was his association with Hans Driesch, his colleague in 1894–1895 at the zoological station in Naples. Driesch was at the time an enthusiastic proponent of experimental embryology (the school of Entwicklungsmechanik) and had performed some highly controversial experiments on sea urchin eggs. Morgan and Driesch collaborated on experimental studies of development in Ctenophora (published in 1895).³ Not only was Driesch’s influence important, but so was that of the zoological station itself. Morgan had visited the station first in 1890 and had become intrigued with the many possibilities that the institute offered for research on marine forms. During his ten months at Naples, he was excited by the work, the constant stream of visitors, the exchange of ideas, and the emphasis on new modes of thought, such as performing experiments in areas of biology, like embryology, previously approached only descriptively. He wrote in 1896: “No one can fail to be impressed [at the Naples Station] and to learn much in the clash of thought and criticism that must be present where such diverse elements come together.”⁴ By contrast, Morgan found the situation in America more parochial and less exciting: “Isolated as we are in America, from much of the newer current feeling, we are able at Naples as in no other laboratory in the world to get in touch with the best modern work.”⁵

After he returned to the United States in 1895, Morgan’s biological interests expanded in scope; and his research methods became largely experimental for the remainder of his life. Between 1895 and 1902 he focused on experimental embryology; between 1903 and 1910, on evolution, especially heredity and cytology in relation to sex determination; between 1910 and 1925, on problems of heredity in Drosophila; and from 1925 to 1945, on embryology and its relations to heredity and evolution. Yet in none of these periods was Morgan exclusively concerned with a single subject. The breadth of his interests was such that he always worked simultaneously on several problems, often of a divergent nature. At almost any point in his career he moved back and forth between the broad areas of evolution, heredity, and develop- ment with considerable ease and grasp of fundamental concepts.

Embryological Studies (1895–1902) . Morgan’s earliest work in experimental embryology largely concerned the factors influencing normal embryonic development. These studies were motivated by the controversy raging in the early 1890’s between Driesch and the founder of the Entwicklungsmechanik school, Wilhelm Roux, on the question of whether the differentiation of embryonic cells is directed by internal (hereditary) or external (environmental) forces. Morgan studied fertilization of egg fragments, both nucleated and nonnucleated, in the sea urchin and in amphioxus. Both types of fragments were able to undergo varying degrees of normal development and even to produce partial larvae. Morgan carried out other studies in which he removed cells from normally fertilized blastulae to produce embryos which, although modified, still developed along the major outlines of their normal course. Other experiments during the same period involved the effects of various salt solutions and of the force of gravity (or lack of it) on the course of development in the eggs of sea urchins, mollusks, and teleost fishes. Beginning in 1902 he published an extensive series of papers on normal and abnormal development in the frog’s egg. Here, Morgan tested the effects of such factors as injury to the egg yolk; varying concentrations of lithium chloride; and injuries to the embryo at various stages, including repetition of Roux’s experiment involving injury to the first blastomere. The results of all these experiments showed Morgan that despite the alterations in development which could be brought about by various physical constraints, the embryo still displayed a tendency to reach its prescribed goal. It became clear to him that environmental influences might shape the embryo’s development within certain limits, but that the overriding factors determining the sequence of events in development must lie within the embryo itself: the interaction of embryonic tissues and of specific embryonic regions with each other.

Coupled with Morgan’s interest in early embryonic development was a corresponding interest in the regeneration of lost or injured tissues (or organs) in adults. While still a student at Hopkins, he had studied regeneration in the earthworm; and in the late 1890’s he pursued these studies in flatworms (Planaria and Bipalium); jellyfish (Gonionemus); bony fishes (teleosts); and ciliate protozoa (Stentor). In 1901 he published his first major book, Regeneration, a compendium of contemporary information on this subject. More than simply a review of the literature, Regeneration provided a foretaste of Morgan’s writing and analytical skill. He saw that the events in regeneration (regrouping of cells in the wound area, despecialization, and renewed differentiation) were the other side of the coin from those of early embryonic development. In regeneration there was a return to the embryonic state. The same essential questions lay behind both processes: How could different components of a cell’s hereditary information be signaled to turn on or off at different periods in its life? Morgan emphasized the relationship between the two processes (he was not alone in making the connection); he saw that any explanation for one must be able to account for the other.

As in most of his later writings, Morgan presented the problem of regeneration as one composed largely of questions—of unknowns—rather than of knowns. He made clear the gaps in contemporary knowledge, in terms of specific experiments or broad interpretations. Morgan sought an understanding of problems such as regeneration (or embryonic differentiation) in terms of underlying (and continuing) processes. He was not content to “explain” one event simply by describing the events or organizational relationships which might lead to it. For example, he felt that those who saw embryonic differentiation as the result of “formative stuffs” already organized in the cytoplasm of the unfertilized egg, or regeneration as simply the work of special cells which congregate at a wound site, really explained nothing. They simply pushed the causal factors back to a further point in the organism’s life history.

It was important to Morgan to view such phenomena less as series of events than as processes. These processes were chemical and physical, and they followed regular laws—if only one could discover what they were. Development and regeneration were to some extent programmed events, but they were not simply the unfolding of preexisting structures. Specific interactions were programmed between structures that gave rise to new and qualitatively different structures. The job of the developmental biologist, he argued, was to seek the general laws governing these interactions. This discovery could not come about simply by describing anatomy—it required experimental analysis as well.

Study of Sex Determination (1903–1910) . In the latter part of the nineteenth and the first years of the twentieth centuries there were two schools of thought on the problem of sex determination. One maintained that the causal factors were environmental: temperature, or amount of food available to the embryo or to the mother during development. This argument derived from the observation that changes in various environmental factors affected the sex ratio in many species, particularly insects. Another school, however, felt that sex was by and large determined at the moment of fertilization, or perhaps even before, by factors internal to the egg or sperm or both. This school emphasized the hereditary, as opposed to environmental, factors in determining sex differentiation.

After 1900 there were several attempts by those favoring the hereditary view to understand sex in terms of the newly discovered Mendelian principles. In 1903 Morgan published a review of the sex determination problem, criticizing all of the existing theories, including those based on Mendel’s laws. His major argument was that there was relatively little evidence substantiating the claims of either the environmentalists or the hereditarians. Most of the current theories of sex determination tried to explain only the customary 1:1 sex ratio found in most species. Any theory of sex determination, however, had to account for a number of other phenomena, such as the process of parthenogenesis, either natural or artificially induced; the appearance of gynandromorphs, often observed in insects (in gynandromorphs, one half of the organism has male characteristics and the other half female characteristics); and sex reversals, as observed in fowl and other species, especially under the influence of hormonal changes.

In his analysis of the sex determination problem, Morgan displayed his deep-rooted embryological bias. He was unwilling to see sex as a primarily hereditary phenomenon, determined at the moment of fertilization, but, rather, he analyzed it as a developmental process, guided by natural laws; he was clearly an epigenesist. He found most of the environmentalists’ experiments inconclusive, but this did not mean that sex could be explained by postulating hereditary units, such as Mendel’s “factors,” or by reference to visible cell structures, such as chromosomes. To Morgan, structures such as chromosomes were only indicators of underlying processes—they were not causal factors themselves. For this reason he was not initially sympathetic to C. E. McClung’s suggestion in 1901 and 1902 that sex was determined by the disposition of the accessory (or X) chromosome. Before 1910 Morgan admitted only that the fertilized egg might inherit a predisposition toward maleness or femaleness. The realization of that sexual potential, however, was largely a result of the same developmental forces involved in differentiation, organogenesis, and regeneration.

Through his interest in sex determination, Morgan carried out important cytological studies on the movement and disposition of chromosomes during the formation of eggs of naturally parthenogenetic forms. Studying in detail two kinds of insects, the aphids and phylloxerans, Morgan was able to demonstrate conclusively that the production of parthenogenetic males was associated with the loss of a chromosome during development from a diploid egg. His papers on this subject, published in 1909 and 1910, show the beginning of Morgan’s realization that chromosomes might actually be related to sex determination.⁶ He did not conclude at the time, however, that the accessory chromosome (X) was a sex determiner. Morgan maintained that the real sex-determining process occurred before the actual loss of the chromosome; the latter was only an indication of this process, not the cause. He wrote in 1909: “The preliminaries of the sex determination for both sexes go on in the presence of all chromosomes … clearly I think the results show that changes of profound importance may take place without change in the number of chromosomes.”⁷

Evidence had been accumulating since the 1807’s that the chromosomes were somehow intimately involved with general hereditary processes. Morgan had remained skeptical of such conclusions, however, not only because the idea had been inferred from circumstantial evidence but also because of his bias against explaining phenomena in terms of preexisting structures. Yet shortly after 1910 increasing experimental evidence led him to change his mind and to accept the chromosomes as important hereditary structures. It was largely work on sex determination that brought Morgan to accept these new ideas. His own studies on chromosomes in aphids and phylloxerans suggested that more attention ought to be paid to the possible role of chromosomes in determining sex. At the same time, between 1901 and 1905, E. B. Wilson, Morgan’s colleague at Columbia, and Nettie M. Stevens, at Bryn Mawr, amassed considerable evidence suggesting that the accessory (X) chromosome was responsible for sex determination. Although Morgan did not accept these findings unequivocally, Wilson’s concern for the hereditary implications of these chromosome studies strongly influenced Morgan’s ideas about sex determination. Morgan and Wilson had been close friends and colleagues for many years and Morgan had great respect for Wilson’s judgment.

Evolution and Heredity (1903–1910) . Morgan had become interested in the Darwinian theory of natural selection, through the influence of W. K. Brooks and through his own studies on regeneration. He reported that he constantly wondered how the regenerative power in higher organisms could have evolved by a mechanism such as natural selection. In 1903 Morgan published Evolution and Adaptation (dedicated to W. K. Brooks), a lengthy attack on the Darwinian theory of natural selection as it was interpreted around the turn of the century by the neo-Darwinians. Morgan believed that Darwin himself was an outstanding naturalist who approached his conclusions with caution, reasoning only within narrow bounds from the data itself. He felt, however, that many of Darwin’s followers had become “ultra selectionists,” investing natural selection with more powers than was legitimate. While maintaining that evolution was a fact, Morgan argued that the theory of the mechanism by which evolution was brought about—natural selection—had many loopholes.

Morgan’s many objections to natural selection have been discussed in detail in the secondary literature; but one major criticism deserves mention here. Morgan shared with many prominent biologists (especially embryologists) the view that the Darwinian theory (as stated by Darwin or modified in the 1890’s by his followers) was incomplete because it lacked a concept of heredity. Although Darwin had emphasized that selection acts on slight individual variations (what some people at the time came to call “continuous variations”), more recent evidence had suggested that such variations were not usually heritable. It was a cardinal principle to Darwin and the neo-Darwinians that the only variations upon which selection could act were hereditary ones. Thus, Morgan and many of the less orthodox Darwinians came to believe that variations of evolutionary significance must be largescale, or discontinuous, because these were the only ones which appeared to be inherited. Morgan maintained that in the face of this dilemma, the neoDarwinians, rather than abandoning the idea of small, individual variations as the raw material for evolution, interpreted selection itself as the creative agent. Morgan believed that selection was only a negative factor, however, which sorted out the favorable from the unfavorable variations already present. It could not, as some neo-Darwinians believed, create new variations in the germ plasm.

Morgan’s view of evolution was like his view of heredity and development, in that it was fashioned by a skepticism about single answers or mechanisms for which experimental proof was inconclusive. From his graduate days on he felt that heredity was in some ways central to an understanding of all biological phenomena, especially development and evolution. Recognizing the lack of what seemed to be any coherent theory of heredity in the period before 1910, he was skeptical of any attempts to explain processes such as cell differentiation or the origin of species by analogies, inferences, or speculative hypotheses. A change in Morgan’s ideas led him to the dramatic discoveries with Drosophila. A brief examination of his ideas on heredity, especially in relation to cytology and evolution between 1900 and 1910, will be useful in understanding this change.

Three concepts of heredity, representing several lines of reasoning and experimentation, had become well-known to most biologists by the first decade of the twentieth century. The first of these was the newly discovered Mendelian laws, based on data from plantbreeding experiments. The second was the chromosome theory of heredity, based on cytological studies of chromosome movement during gametogenesis in both animals and plants. Third was the publication of The Mutation Theory, a monumental treatise on heredity, variation, and evolution by the Dutch botanist Hugo de Vries.

Although by 1902 several workers had suggested the possible relationship between chromosome movements and the segregation of Mendel’s alternate factors, there was no agreement that this relationship was anything more than coincidental. Morgan’s objections to the Mendelian scheme can be summarized as follows:

1. If the Mendelian theory were correct, and if Mendelian “factors” (what Mendel more commonly called Anlagen, and what later became known as genes) were actually associated with chromosomes, then breeding results ought to show large groups of characteristics inherited together (as many groups as there were chromosome pairs). Because few “linkage groups” had been observed in the period before 1910 (Bateson and Punett in England had shown some in 1905 and 1906), the identification of Mendel’s factors with chromosomes seemed less than likely.

2. The results of animaland plant-breeding experiments showed that many characteristics in an offspring were a mixture of parental types, and not simple dominance or recessiveness. Thus, Mendel’s “laws” might apply only to special, exceptional cases.

3. The Mendelian theory of dominance and recessiveness could not explain the normal 1:1 sex ratio. According to Mendel’s scheme, the sex ratio would be 3:1 (if one sex factor were dominant over the other) or 1:2:1 (if incomplete dominance were involved). Since neither sex ratio occurred in nature, Mendel’s laws provided no clear way to account for the important phenomenon of sex inheritance.

4. On methodological grounds, Mendel’s laws called for too neat a set of categories among the offspring of any cross. Since such categories seldom occurred in nature, Morgan claimed that Mendelians often placed borderline organisms into whichever category was necessary to give the expected ratios.

5. On a more philosophical level, both the Mendelian and the chromosome theories seemed to be preformationist in character; they referred basic hereditary characteristics to preexisting particles or units in the cell. Morgan felt that, like all preformationist theories of the past, the Mendelian and chromosome doctrines simply pushed a basic problem back further in the life history of the organism.

6. In addition, the Mendelian and chromosome theories seemed to Morgan to be based too much on speculation, and too little on sound experimental evidence. They reminded him of the speculative theories—especially those of Ernst Haeckel and August Weismann—that attempted to explain all of biology that abounded during his student years. Morgan was inalterably opposed to speculation that could not be subjected to experimental tests.

Skeptical of both the Mendelian and the chromosome theories, Morgan was, however, an outspoken advocate of de Vries’s mutation theory (published in a two-volume work between 1901 and 1903). De Vries proposed that large-scale heritable variations occurring in one generation could produce offspring that were of species different from their parents. De Vries’s evidence was based largely on experiments with the evening primrose (Oenothera lamarckiana). What he called “mutations” are now known to be the result not of actual changes in genetic material, but complex chromosome arrangements which are peculiar to Oenothera. Thus they did not produce species-level changes in a single generation, as de Vries claimed. Nevertheless, the mutation theory is historically important, for Morgan and others saw in it, as did de Vries himself, an answer to the perplexities of Mendelian heredity and Darwinian selection. It accounted for the origin of new variations which were definite enough to be of evolutionary significance (that is, would not be lost by swamping), and yet were also heritable. Furthermore, Morgan’s acceptance of the mutation theory was influenced by the sound experimental evidence behind de Vries’s work. De Vries had a large experimental garden where he grew his plants and made crosses under carefully controlled conditions. New mutants could be isolated and shown to breed true. Thus de Vries not only provided a new concept that made evolution conceivable; he also provided an experimental approach by which his conclusions could be tested.

Morgan’s Work With Drosophila . Morgan appears to have begun breeding the fruit fly Drosophila melanogaster somewhere around 1908 or 1909. It is not clear how he came to use this organism, or where he obtained his original cultures. Drosophila seems to have been an organism favorable for laboratory studies, however, between 1900–1910. It was used in Castle’s laboratory at the Bussey Institution (Harvard) as early as 1900–1901; by W. J. Moenkhaus at Indiana in 1903; by F. E. Lutz at the Carnegie Institution Laboratory (Cold Spring Harbor, New York, and after 1909, when Lutz was at the American Museum of Natural History); by Nettie Stevens at Bryn Mawr in 1906; and by Fernandus Payne and L. S. Quackenbush in the Columbia laboratory itself prior to 1909. Morgan’s original purpose had been to test de Vries’s mutation theory in animals. He exposed Drosophila cultures to radium in an attempt to induce the formation of new mutants, but he never obtained mutations of the magnitude which de Vries claimed for Oenothera.

In 1910 Morgan discovered a small, distinct variation in one male fly in one of his culture bottles. This fly had white, as opposed to the normal (wildtype) red, eye color. This variation did not make a new species, but Morgan thought he would try to breed the fly with its red-eyed sisters to see what would happen. All of the offspring (F₁) were red-eyed. Brother-sister matings among the F₁ generation produced a second generation (F₂) with some whiteeyed flies—all of which, Morgan noticed with astonishment, were males. Further matings showed that while the white-eye condition almost always occurred in males, occasionally a white-eyed female would appear. Morgan noted that the white-eye and red-eye conditions behaved as typical Mendelian factors, with red being dominant over white.

The limitation of the white-eye condition largely, but not exclusively, to males presented a very curious problem. Morgan found that the only way he could explain this phenomenon was to assume that the red-and white-eye conditions were determined by Mendelian factors, and that these somehow associated with the element which determined sex in the cell. In his first paper on heredity in Drosophila, Morgan refrained from identifying the eye color with chromosomes in general, or the accessory chromosomes in particular.⁸ Within a year, however, he concluded that such caution was unwarranted. The cytological studies on chromosomes and sex determination by Wilson and others, and his own work with Drosophila, convinced Morgan that chromosomes could in fact be the real bearers of Mendelian factors. Much to his credit, he rejected his skepticism about both the Mendelian and the chromosome theories when he saw from two independent lines of evidence (breeding experiments and cytology) that one could be treated in terms of the other.

Morgan called the white-eye condition sex-limited (later sex-linked), meaning that the genes for this character were carried on (linked to) the X chromosome. Sex-linked genes, if recessive to their wild-type alleles, will show up almost exclusively in males, who do not have a second X chromosome to mask genes on the first. Sex linkage was found to hold for all sexually reproducing organisms and accounted for many other perplexing hereditary patterns, including red-green color blindness and hemophilia in man. Morgan’s Drosophila work showed for the first time the clear association of one or more hereditary characters with a specific chromosome.

Early in 1910 Morgan had taken into his laboratory several enthusiastic Columbia undergraduates: A. H. Stutevant and Calvin B. Bridges, both juniors in the college and Hermann J. Muller, a graduate student of E. B. Wilson’s. With Morgan these men quickly developed the Drosophila work into an intensively active project. As more breeding experiments were initiated, new mutants began to appear. Careful records were kept of the mutants, and their hereditary patterns were studied through various crosses and backcrosses. It would be impossible to describe or list all of the new findings which emerged from the Drosophila studies. A few major developments will illustrate the enormous breakthroughs which Morgan and his colleagues were able to make with this new experimental organism.

At first the relationship between Mendelian genes and chromosomes was purely inferential. While it was not possible to make that relationship more concrete (no one could “see” a gene on a chromosome), a means appeared by which the inference could be tested. In 1909 the Belgian cytologist F. A. Janssens had published a careful series of cytological observations of what he called chiasmatype formation (intertwining of chromosomes during meiosis).⁹ Janssens believed he could show that occasionally homologous chromosome strands exchanged parts during chiasma. Morgan was familiar with Janssens’ concept and applied it to the conception of genes as parts of chromosomes. He reasoned that the strength of linkage between any two factors must be related in some way to their distances apart on the chromosome. The farther apart any two genes, the more likely that a break could occur somewhere between them, and hence the more likely that the linkage relationship would be disturbed. During a conversation with Morgan in 1911, Sturtevant, then still an undergraduate, suddenly realized that the variations in strength of linkage could be used as a means of determining the relative spatial distances of genes on a chromosome. According to Sturtevant’s own report, he went home that night and produced the first genetic map in Drosophila for the sex-linked genes y, w, z, m, and r. The order and relative spacing which Sturtevant determined at that time are essentially the same as those appearing on the recent standard map of Drosophila’s X chromosome.

Following the initial success of this technique, positions were determined for many other genes. The Drosophila group depended upon the appearance of mutants to determine the existence and chromosomal location of specific genes. Thus the initial work of the group took two directions: the location of mutants and the maintenance of a stock for each mutant (or group of mutants), and the mapping of these mutant gene positions on the appropriate chromosomes. The success of the mapping technique added further weight to the inferred relationship between genes and chromosomes and at the same time provided an increasingly clear picture of the architecture of the germ plasm. The major outcome of the mapping work was the idea that genes are arranged in a linear fashion and occupy specific positions, or loci, on the chromosomes. While the direct and final proof of this relationship had to wait until proper cytological materials (the giant salivary glands of Drosophila) and techniques were developed by T. S. Painter and others in the 1930’s, the mapping work firmly established the inference in the years between 1912 and 1915.

As the work progressed, other problems arose. Genes were discovered which, when combined in the homozygous condition, caused the embryo to die before birth (so-called lethal genes). Various traits proved to be determined by a number of alternative genes (alleles) at the same locus, which could be combined in various forms to give a series of phenotypes (multiple alleles). Because crossover frequencies did not always turn out as predicted, they arrived at the idea of crossover interference, in which segments of a homologous chromosome pair showed little or no crossing over, often as the result of alterations in chromosome structure which prevented normal intertwining during chiasma. A furor among orthodox Mendelians was aroused by Sturtevant’s suggestion that the expression of a given gene was affected by its position on the chromosome (the “position effect”). Position effect became the target of one of the most persistent attacks on the Mendelian and chromosome theories to be launched in the twentieth century, by Richard Goldschmidt, for many years director of the Kaiser Wilhelm Institute for Biology in Berlin-Dahlem. Goldschmidt argued that the suggestion that a gene’s effect could be modified by a change in its position along the chromosome (that is, by what genes were on either side of it) violated the basic Mendelian conception of the purity of the gametes. The necessity of invoking a hypothesis such as position effect was, to Goldschmidt, tantamount to an admission that the Mendelian and chromosome theories were not compatible, and that a new conception had to be substituted. Yet position effect and its cytological basis, as worked out in the l930’s by Muller, Prokofieva, Bridges, and others, proved to be a valid conception—a modification, if not a contradiction, of orthodox Mendelian theory.

Among the most important ideas to emerge from the Drosophila work was the balance concept of sex, developed largely by Bridges between 1913–1925 through an analysis of the cytological phenomenon of nondisjunction. Nondisjunction is a condition occurring during oogenesis, in which the X chromosomes fail to segregate, so that a haploid egg may end up with two X chromosomes. Bridges’ work of 1916, in particular, showed clearly that sex was determined not simply by the inheritance of one or two X chromosomes but, rather, by the ratio of X chromosomes to autosomes (the other, nonsex chromosomes in the nucleus). According to this idea, organisms could inherit various degrees of sexuality based upon variations in this ratio. The genes governing male and female characteristics (such as production of testes or ovaries) are found in both sexes and apparently are not located exclusively on the sex chromosomes but throughout the genome. Which of these sets of genes express themselves is a result not simply of their presence or absence but, rather, of some complex and little-understood relationship between the sex chromosomes and autosomes.

The major early findings of the Drosophila group were summarized in an epoch-making book, The Mechanism of Mendelian Heredity, published by Morgan, Bridges, Sturtevant, and Muller in 1915. They presented evidence to suggest that genes were linearly arranged on chromosomes and that it was possible to regard the Mendelian laws as based on observable events taking place in cells. Most important, however, they demonstrated that heredity could be treated quantitatively and rigorously. For almost the first time since the advent of experimental embryology in the 1880’s, a previously descriptive area of biology had proved itself accessible to quantitative and experimental methods. Through The Mechanism of Mendelian Heredity, the new science of genetics reached many teachers, students, and specialists in other areas.

All of the early work on Drosophila between 1910 and 1925 was carried out in the winter in Morgan’s small laboratory space, called the “fly room,” at Columbia, and during the summers at the Marine Biological Laboratory in Woods Hole, Massachusetts. Although Morgan was considerably older than his co-workers, there was a give-and-take atmosphere in the “fly room” that precluded formal barriers and rigid teacher-student distinctions. There was little consideration of priority in new ideas or discoveries at the time (although some did emerge in later years); and each was free to criticize anyone else openly, and sometimes vehemently. Sturtevant has described the relationship among the workers in the “fly room” as follows:

As each new result or new idea came along, it was discussed freely by the group. The published accounts do not always indicate the source of ideas. It was often not only impossible to say but was felt to be unimportant, who first had an idea. A few examples come to mind. The original chromosome map made use of a value represented by the number of recombinations divided by the number of parental types as a measure of distances; it was Muller who suggested the simpler and more convenient percentage, the recombinance formed of the whole population. The idea that “crossover reducers” might be due to inversions of sections was first suggested by Morgan, and this does not appear in my published account of the hypothesis. I first suggested to Muller that lethals might be used to give an objective measure of the frequency of mutation. These are isolated examples, but they represent what was going on all the time. I think we came out somewhere near even in this give and take, and it certainly accelerated the work.¹⁰

However, all was not idyllic within the Drosophila group. H. J. Muller, perhaps Morgan’s most independent and brilliant student, felt that Morgan had a tendency to use his students’ ideas without fully acknowledging them. While recognizing Morgan’s unsurpassed abilities as a leader, his fiery and quick imagination, and his frequently penetrating insights, Muller claimed that Morgan was frequently confused about rather fundamental issues involved in the work—such as the theory of modifier genes, or the supposed swamping effect of dominant genes in a population. According to Muller, Morgan frequently had to be “Straightened out” on such issues by hardheaded arguments with his students—mostly Muller and Sturtevant, with occasional help from E. B. Wilson. Sturtevant concurs with this evaluation at least with regard to the idea of natural selection, which he claims Morgan persisted in misunderstanding until as late as 1914 or 1915.

What is clear from an analysis of the reports of many people who worked in the “fly room” during the years 1911–1915, was that Morgan’s primary role was that of leader and stimulator. He was constantly coming up with ideas—some wrong, others right—and throwing these out to the eager and brilliant group of young people whom he had working with him. That many of the most far-reaching ideas (such as a quantitative method of making chromosome maps, crossover interference, modifier genes) were first proposed by his students, not directly by Morgan, is also clear. His genius in the development of the Drosophila work may have rested more in bringing together the right group of people, in working together with them in a democratic and informal way, and in letting them alone, than in producing all the major ideas himself. In fact, it is clear from an analysis of Morgan’s published work that he frequently proposed ideas “off the top of his head” and was not always careful to work out their details or implications.

Morgan’s laboratory became the training ground for a school of Mendelian genetics—one generation of which emphasized particularly the relationship between genes and chromosomes. Besides Bridges, Sturtevant, and Muller, Morgan’s students or postdoctoral associates at Columbia included Alexander Weinstein, E. G. Anderson, H. H. Plough, Theodosius Dobzhansky, L. C. Dunn, Donald Lancefield, Curt Stern, and Otto Mohr. These workers, and many others, developed what has come to be called “classical genetics”—that is, genetics at the chromosome level.

Morgan’s mind ranged freely over the broad areas of genetics, embryology, cytology, and evolution. Soon after the Drosophila work had gotten under way, he saw that the Mendelian concept could throw considerable light on the problem of natural selection. In 1916 Morgan published his second major work on evolution, A Critique of the Theory of Evolution (revised in 1925 as Evolution and Genetics), showing clearly his altered views about Darwinian selection. Although he had previously regarded de Vries’s mutation theory as an alternative to natural selection, Mendelism now provided a mechanism for understanding the Darwinian theory itself. Mendelian variations (called also “mutations” by Morgan) were not as large or as drastic as those postulated by de Vries. Yet they were more distinct and discontinuous than the slight individual variations which Darwin had emphasized. Most important, they could be shown to be inherited in a definite pattern and were therefore subject to the effects of selection. The Mendelian theory filled the gap which Darwin had left open so long before.

Morgan found it more difficult to make explicit the relationships which he instinctively knew existed between the new science of heredity and the old problems of development (such as cell differentiation or regeneration). In 1934 Morgan attempted to make these connections in a book titled Embryology and Genetics. The work proved to be less an analysis of interrelated mechanisms and more a summary of efforts in the two separate fields. Morgan knew well that the time was not ripe for understanding such problems as how gene action could be controlled during development. Yet Embryology and Genetics served an important function of keeping before biologists the idea that ultimately any theory of heredity had to account for the problem of embryonic differentiation. Morgan wisely refrained from drawing conclusions or proposing hypotheses which could not be experimentally verified. One of the most important characteristics of his genius was the ability to restrict the number and kinds of questions which he asked at any one time. For example, by focusing primarily upon the relationships between the Mendelian theory and chromosome structure, he was able to work out the chromosome theory of heredity in great detail. In contrast, other workers, such as Richard Goldschmidt, tried to make those relationships more explicit than the evidence at the time would allow. Consequently, they were often drawn into realms of speculation where no concrete advances could be made.

Later Work (1925–1945) . After the mid-1920’s Morgan’s interest shifted away from the specific Drosophila work. His new concerns took two forms. One was the attempt to summarize the conclusions deriving from his genetic studies. To this category belong those broader works relating heredity to development and evolution. His other interest turned him to some of the original problems of development and regeneration which had launched his career thirty-five years previously. During the summers at Woods Hole, and especially after his move to California in 1928, Morgan returned to studies of early embryonic development. The cleavage of eggs; the effects of centrifuging eggs before and after fertilization; the behavior of spindles in cell division; preorganization in the egg; self-sterility in ascidians; and the factors affecting normal and abnormal development were some of the problems in experimental embryology. They represented the type of biological work that Morgan was most interested in. Although he approached the Drosophila studies enthusiastically, the mathematics of mapping and many other highly technical problems were less interesting to him than working directly with living organisms. Morgan had the naturalist’s love of whole organisms and of studying organisms in their natural environment. He was a good naturalist with a knowledge of many species.¹¹ His strong interest in laboratory and experimental work in no way detracted from his interest in whole systems. He was not in spirit a mechanist, although he recognized the value of studying systems in isolation to obtain rigorous and useful data.

Methodology in Science . Being a thorough experimentalist, Morgan saw that unbounded speculation was detrimental to the development of sound scientific ideas. He did not object to the formulation of hypotheses for he saw them as essential to developing new concepts and experimental ideas. For Morgan, however, the only acceptable hypotheses were those which suggested experimental tests.

Yet Morgan was not a mere empiricist—that is, one who simply tries to amass large amounts of basically similar kinds of evidence before drawing a conclusion. As an experimentalist he drew conclusions most readily when several different types of data sources were available (for example, determining the existence of a chromosomal deletion from breeding data and from cytological examination of chromosome preparations). By 1909 considerable breeding data suggested that Mendel’s laws had wide application. Yet Morgan remained skeptical because there was no evidence (in his mind) that Mendel’s “factors” had any reality. What began to change his mind was not the finding that he could apply the Mendelian theory to yet another organism (Drosophila), but that he could test the Mendelian theory (studied by breeding experiments) with evidence from a wholly different area—cytology—in the observed behavior of chromosomes during gametogenesis. As soon as he saw that the white-eye mutation acted as if it were part of the X chromosome, he began to view the Mendelian theory in a completely different light.

That Morgan saw Mendel’s “factors” as having a possible material basis in the chromosome does not imply that he automatically accepted the idea that genes were physical entities; nor was he primarily concerned with determining how much of the chromosome a mutant gene occupied whenever a new mutation was discovered. The physical existence of genes was unnecessary for the validity of the original Mendelian theory and for much of the Drosophila work. The Mendelian-chromosome theory was largely a formalism: it stood on its own as a consistent scheme without necessarily being tied to observable physical structures. Until cytological techniques materials were developed by Painter and others in the late 1920’s, it was impossible to determine a point-by-point correspondence between genetic maps (determined by crossover frequencies) and chromosome structure (determined cytologically). Nevertheless, from the outset Morgan was never content to deal with a purely formalistic theory. In the preface to The Mechanism of Mendelian Heredity, he and his coauthors admitted that Mendel’s theories could be viewed independently of chromosomes. But they hastened to point out that this was not the course they were going to follow:

Why then, we are often asked, do you drag in the chromosomes? Our answer is that since the chromosomes furnish exactly the kind of mechanism that the Mendelian laws call for; and since there is an everincreasing body of information that points clearly to the chromosomes as the bearers of the Mendelian factors, it would be folly to close one’s eyes to so patent a relation.¹²Preliminary evidence suggested that genes were real entities on chromosomes, even though it could not be proved conclusively.

As an experimentalist Morgan urged other biologists to employ the quantitative and rigorous methodology which had been so successful in experimental embryology and in his own work on heredity. For biology to attain the same level of development as the physical sciences, it was necessary to adopt the same standards. Yet Morgan did not believe that biology should be reduced simply to expressions of physical and chemical interactions. He believed too much in the naturalist’s view of living systems. Reductionism was too simplistic for Morgan; he could never follow Loeb to the logical conclusions of the mechanistic conception of life. What Morgan did believe, however, was that biology should be placed on the same footing as the physical sciences: that is, that the criteria for evaluating ideas in biology should be the same as those in physics and chemistry (quantitative measurement, experimentation, and rigorous analysis).

The California Institute of Technology . In 1927 George Ellery Hale invited Morgan to come to the California Institute of Technology to establish its first division of biology. After weighing the matter for a short time, Morgan accepted with enthusiasm. Although he had doubts about his abilities as an administrator (he wrote to Hale that he was a “laboratory animal, who has tried most of his life to keep away from such entanglements”),¹³ the opportunity of heading a new department seemed to far outweigh the possible administrative problems. This move offered several advantages to Morgan, who was then sixty-two. Because the Kerckhoff Laboratory had a generous endowment (from the Kerckhoff family) as well as assistance from the Rockefeller Foundation, Morgan was able from the start to attract a first-rate staff. At Caltech, Morgan developed a modern department based on the concept of biology as he thought it should be studied and taught, where the new experimentalism could play a predominant role. Moving to Caltech also provided Morgan with the opportunity of achieving on a permanent basis the kind of scientific interaction and cooperation which he found so productive first at Naples and later during summers at the Marine Biological Laboratory, Woods Hole. As he wrote to Hale: “The participation of a group of scientific men united in a common venture for the advancement of research fires my imagination to the kindling point.”¹⁴

In the Caltech period Morgan’s influence in genetics extended beyond the Drosophila work and the classical chromosome theory. Although he did not pioneer in the newer biochemical and molecular genetics that began to emerge in the 1940’s, he nourished that trend. Both George Beadle, as a National Research Council fellow in 1935, and Max Delbrück, as an international research fellow in biology of the Rockefeller Foundation in 1939, worked with Morgan’s group at Caltech; both saw that the next logical questions arising out of the Drosophila work were those of gene function. It was their work on the relationships between genes and proteins in simple organisms, such as yeasts and bacteriophages, that prepared the way for the revolution in molecular genetics during the 1950’s and 1960’s.

Morgan’s influence was central to the transformation of biology in general, and heredity and embryology in particular, from descriptive and highly speculative sciences arising from a morphological tradition, into ones based on quantitative and analytical methods. Beginning with embryology, and later moving into heredity, he brought first the experimental, and then the quantitative and analytical, approach to biological problems. Morgan’s work on the chromosome theory of heredity alone would have earned him an important place in the history of modern biology. Yet in combination with his fundamental contributions to embryology, and his enthusiasm for a new methodology, he can be ranked as one of the most important biologists in the twentieth century.

NOTES

1. A. H. Sturtevant, “Thomas Hunt Morgan,” p. 285.

2. D. M. McCullough, “W. K. Brooks’ Role in the History of American Biology,” in Journal of the History of Biology, 2 (1969), 411–438, esp. p. 420.

3. T. H. Morgan and Hans Driesch, “Zur Analysis der ersten Entwickelungsstadien des Ctenophoreneies. I. Von der Entwickelung einzelner Ctenophorenblastomeren. II. Von der Entwickelung ungefurchter Eier mit Protoplasmadefekten,” in Archiv für Entwicklungsmechanik der Organismen, 2 (1895), 204–215, 216–224.

4. T. H. Morgan, “Impressions of the Naples Zoological Station,” in Science, 3 (1896), 16–18.

5.Ibid.

6. T. H. Morgan, “A Biological and Cytological Study of Sex Determination in Phylloxerans and Aphids,” in Journal of Experimental Zoology, 7 (1909), 239–352; “Chromosomes and Heredity,” in American Naturalist, 44 (1910), 449–496.

7. T. H. Morgan, “A Biological and Cytological Study …,” p. 263.

8. T. H. Morgan, “Sex-Limited Inheritance in Drosophila,” in Science, 32 (1910), 120–122.

9. F. A. Janssens, “La théorie de la chiasmatypie,” in La Cellule, 25 (1909), 389–411.

10. A. H. Sturtevant, A History of Genetics (New York, 1965), pp. 49–50.

11. A. H. Sturtevant, “Thomas Hunt Morgan,” p. 297.

12. T. H. Morgan, A. H. Sturtevant, H. J. Muller, and C. B. Bridges, The Mechanism of Mendelian Heredity, p. viii.

13. Morgan to George Ellery Hale, 9 May 1927, G. E. Hale papers, California Institute of Technology Archives, microfilm roll 26, frame 29.

14.Ibid.

BIBLIOGRAPHY

I. Original Works. A complete bibliography of Morgan’s published writings can be found in Sturtevant’s “Thomas Hunt Morgan” (see below). Among the more important books and articles are “The Relationships of the Sea-Spiders,” in Biological Lectures Delivered at the Marine Biological Laboratory of Woods Hole in the Summer Session of 1890 (Boston, 1891), pp. 142–167; “Regeneration: Old and New Interpretations,” in Biological Lectures Delivered … Summer Session of 1899 (Boston, 1900), pp. 185–208; Regeneration (New York, 1901); Evolution and Adaptation (New York, 1903); “Recent Theories in Regard to the Determination of Sex,” in Popular Science Monthly, 64 (1903), 97–116; “The Assumed Purity of the Germ Cells in Mendelian Results,” in Science, 22 (1905), 877–879; Experimental Zoology (New York, 1907); “A Biological and Cytological Study of Sex Determination in Phylloxerans and Aphids,” in Journal of Experimental Zoology, 7 (1909), 293–352; “What Are ‘Factors’ in Mendelian Explanations?,” in American Breeders, Association Report, 5 (1909), 365–368; “Chromosomes and Heredity,” in American Naturalist, 44 (1910), 449–496; and “Sex-Limited Inheritance in Drosophila,” in Science, 32 (1910), 120–122.

After 1910 there appeared “An Attempt to Analyze the Constitution of the Chromosomes on the Basis of SexLimited Inheritance in Drosophila,” in Journal of Experimental Zoology, 11 (1911), 365–412; “Random Segregation Versus Coupling in Mendelian Inheritance,” in Science, 34 (1911), 384; “The Explanation of a New Sex Ratio in Drosophila,” ibid., 36 (1912), 718–719; Heredity and Sex (New York, 1913); “Multiple Allelomorphs in Mice,” in American Naturalist, 48 (1914), 449–458; The Mechanism of Mendelian Heredity (New York, 1915; reiss,. New York, 1972), written with A. H. Sturtevant, H. J. Muller, and C. B. Bridges; A Critique of the Theory of Evolution (Princeton, 1916), rev. as Evolution and Genetics (Princeton, 1925); Sex Linked Inheritance in Drosophila, Carnegie Institution Publication no. 237 (Washington, D.C., 1916), written with C. B. Bridges; “The Theory of the Gene,” in American Naturalist, 51 (1917), 513–544; “The Origin of Gynandromorphs,” in Contributions to the Genetics of Drosophila Melanogaster, Carnegie Institution Publication no. 278 (Washington, D.C., 1919), 3–124, written with C. B. Bridges; The Physical Basis of Heredity (Philadelphia, 1919); “Chiasmatype and Crossing Over,” in American Naturalist, 54 (1920), 193–219, written with E. B. Wilson; “The Evidence for the Linear Order of the Genes,” in Proceedings of the National Academy of Sciences of the United States of America, 6 (1920), 162–164, written with A. H. Sturtevant and C. B. Bridges; ‘The Bearing of Mendelism on the Origin of Species,” in Scientific Monthly, 16 (1923), 237–247; “The Modern Theory of Genetics and the Problem of Embryonic Development,” in Physiological Reviews, 3 (1923), 603–627; The Theory of the Gene (New Haven, 1926); “The Relation of Physics to Biology,” in Science, 65 (1927); The Scientific Basis of Evolution (New York, 1932); Embryology and Genetics (New York, 1934); and “The Conditions That Lead to Normal or Abnormal Development of Ciona, in Biological Bulletin, 88 (1945), 50-52.

There is no single collection of Morgan’s letters, notebooks, or other unpub. materials. Numerous Morgan letters can be found, however, in the papers of Ross G. Harrison (Yale University), Edwin Grant Conklin (Princeton University), William Bateson (American Philosophical Society), and George Ellery Hale (Mount Wilson and Palomar Observatories Library, Pasadena). The American Philosophical Society Library, Philadelphia, is collecting the papers of important American geneticists; Morgan letters appear prominently in many of these collections.

II. Secondary Literature. The fullest account of Morgan’s life to date remains A. H. Sturtevant, “Thomas Hunt Morgan,” in Biographical Memoirs National Academy of Sciences33 (1959), 283-325. Selected writings about Morgan and his work include G. E. Allen, “Thomas Hunt Morgan and the Problem of Natural Selection,” in Journal of the History of Biology, 1 (1968), 113–139; “T. H. Morgan and the Emergence of a New American Biology,” in Quarterly Review of Biology, 44 (1969), 168-188; “T. H. Morgan and the Problem of Sex Determination,” in Proceedings of the American Philosophical Society, 110 (1966), 48 T. H. Morgan, Richard Goldschmidt and the Opposition to Mendelian Theory 1900–1940,” in Biological Bulletin, 139 (1970), 412–413; and a slightly fuller treatment of this same material, “Richard Goldschmidt’s Opposition to the MendelianChromosome Theory,” in Folia Mendeliana, 6 (1971), 299–303. See also Edward Manier, “The Experimental Method in Biology. T. H. Morgan and the Theory of the Gene,” in Synthese, 20 (1969), 185–205; and A. H. Sturtevant, “The Fly Room,” ch. 6 of A History of Genetics (New York, 1965). An analysis of the work of the Drosophila group from Muller’s point of view is given in E. A. Carlson, “The Drosophila Group; the Transition From the Mendelian Unit to the Individual Gene,” in Journal of the History of Biology (in press).

Background material on much of the development of Mendelian genetics after 1900 can be found in three general historical studies: E. A. Carlson, The Gene, a Critical History (Philadelphia, 1966); L. C. Dunn, A Short History of Genetics (New York, 1965); and Sturtevant’s History of Genetics.

Garland E. Allen

MORGAN, THOMAS HUNT

(b. Lexington, Kentucky, 25 September 1866; d. Pasadena, California, 4 December 1945),

embryology, genetics. For the original article on Morgan see DSB, vol. 9.

The original DSB article presented Thomas Hunt Morgan as a specialist in embryology and genetics. Garland E. Allen painted Morgan as a man “known to his friends, colleagues, and students as a man of quick mind, incisive judgment, and sparkling humor” (p. 515). The reader sees a Morgan who won a Nobel Prize for his work in genetics, but also the study of heredity as a logical and consistent part of a more complex Morgan excited about natural history and organisms generally. Allen introduced Morgan’s most important students, but the emphasis remained on Morgan and his ideas and methods. The picture was consistent with that presented in formal obituaries and earlier biographical sketches, though Allen was much more aware of the larger context in which Morgan worked than most previous authors.

In his much longer biography of Morgan published two years later, Allen developed this picture further and added some additional themes that he pursued in other work in more detail. Chapter 3 of Thomas Hunt Morgan: The Man and His Science (1978) laid out the idea of a “revolt from morphology” that had formed a foundation for his textbook, Life Science in the Twentieth Century. Thus, Morgan became the exemplar for a broad interpretation of trends in the history of biology.

Work at Bryn Mawr . Importantly, the years that Allen characterized in terms of the “revolt” and that he tied to Morgan’s endorsement of the experimental embryological program of “Entwicklungsmechanik” were also the years that Morgan was on the faculty at Bryn Mawr College. Allen acknowledged the significance of this appointment at a leading women’s college in a couple of pages, but later scholarship by Margaret Rossiter and Helen Lefkowitz Horowitz has carried exploration of the role of women in science, and the role of Bryn Mawr and Martha Carey Thomas in particular, much further.

The fact that Morgan and fellow leading biologists Edmund Beecher Wilson and Jacques Loeb each began his career at Bryn Mawr in the last two decades of the nineteenth century deserves more attention. Despite the limited archival materials in the scientists’ collections concerning this early period, there may well be instructive materials available at Bryn Mawr or in collections of outstanding students there. Researchers have little understanding of what Morgan, Wilson, and Loeb gained from Bryn Mawr, though it is clear that Morgan gained a wife. Lilian Vaughan Sampson Morgan became Morgan’s collaborator on many projects, and a researcher in her own right.

It is known what scholarly work Morgan carried out at Bryn Mawr, but there is little about his teaching or interactions with students in his lab. One can hope for future insight along these lines to illuminate the way science was carried out in that time and place and to learn more about what the growing study of women in science reveals about scientific careers more generally. Because the building of a collaborative team became a central feature of Morgan’s research approach, and because Morgan did work with such outstanding young women as Nettie Maria Stevens during his Bryn Mawr days, one can only

speculate about whether he began to learn such lessons there and carried them with him.

Morgan as Naturalist . In 1904, Wilson recruited Morgan to join him at Columbia University where Morgan remained until he was again lured away, this time at age sixty-two, to the California Institute of Technology (Caltech). In his full biography, Allen portrayed Morgan as a passionate and energetic researcher driven by questions and pursuing productive methodologies. Allen’s Morgan was never a single-minded geneticist, but rather a naturalist always fascinated by questions about development, evolution, heredity, and the way that all those forces work together. From the 447-page biography, the reader learns about the details of Morgan’s 22 books and more than 370 published papers. Allen worked through the diverse lines of research that Morgan pursued, sketching the driving questions, the ideas, the organisms studied, and Morgan’s results and interpretations. His was a first-rate biography consistent with the highest scholarly standards of the time.

Allen concluded his study by making clear that this is not an example of the “great man” view of history, where science and other successes are driven by one great man after another, each doing great things. It was important to Allen not to focus on the Nobel Prize–winning work, nor on the Caltech lab where Morgan worked from 1928 until his death in 1945, but rather all aspects of the whole career of the whole man. As Allen explained

It is in the qualities of the interaction of human and intellectual spheres that Morgan has contributed much to our understanding not only of genetics, but also of the process of science. Morgan was a product of his times, perhaps more than he would have been willing to admit. But he left his imprint on modern biology; he influenced the subsequent course of work in the field in a way that made it quite different from what had gone before. A lucid experimentalist himself, he also possessed that special personal quality that enabled him to work cooperatively. It is in this way, more perhaps than in his technical finds, that he represented the most profound wave of the future. (1978, pp. 399–400)

As a Marxist, Allen saw it as important to emphasize the social and institutional contexts, and to demonstrate that what matters for the processes of science is the social interactive accumulation of contributions. For Allen, this meant not only that Morgan’s own contributions were replaced and interpreted by others, but that Allen’s own interpretations would be replaced as well. He explained that he saw his biography as only a beginning, as raising more questions than it answered. “My hope,” Allen wrote,

is that it will help to focus some of the important questions for other historians of biology to pursue in depth. Topics such as the relationship between embryology and genetics, the growth of eugenics and theories of the genetic basis of human and animal behavior, the development of population genetics, and genetics and biochemistry are but a few areas which have only been touched upon, but not developed, in the present volume. If this book serves any long-lasting function, it will be more in the questions it asks, than the answers it provides. (p. xiii)

Allen’s portrait of Morgan has had lasting impact, and is likely to remain the most important starting point for study of Morgan for some time. Yet his study did stimulate further questions and explorations, just as he wished, and those have provided a richer understanding of the history of science and of Thomas Hunt Morgan as a part of that scientific enterprise. Some of the studies that expand the understanding of Morgan and of the history of science include Robert E. Kohler’s emphasis on the material culture of the Morgan lab at Caltech, Jane Maienschein’s examination of Morgan’s epistemology, and Frederic L.

Holmes’s look at Morgan in the context of the history of genetics and biochemistry.

Other biographical studies, such as Ian B. Shine and Sylvia Wrobel’s Thomas Hunt Morgan: Pioneer of Genetics (1976) or an essay by Eric Kandel, have emphasized Morgan’s contributions to genetics, and have largely set aside the connections of those studies with other areas of biology. These serve their purpose, but in emphasizing the emergence of genetics, they miss the connectedness across areas of evolution, development, and heredity that Allen’s approach revealed and that helps illuminate the complexity of both life and the science that seeks to understand the phenomena of life.

Kandel’s 1999 essay, “Thomas Hunt Morgan at Columbia University: Genes, Chromosomes, and the Origins of Modern Biology,” was intended to highlight one of Columbia’s great men for the Columbia Magazine. Yet rather than seeing Morgan as a biologist, concerned with a range of questions about life, Kandel gave the impression that Morgan cared only about genes and suggested that Morgan was in search of the gene, which he identified in the “year of discovery,” 1910. As Kandel put it, the great breakthrough of twentieth-century biology was

the discovery that the gene, localized to specific positions on the chromosome, was at once the unit of Mendelian heredity, the driving force for Darwinian evolution, and the control switch for development. This remarkable discovery can be traced directly to one person and to one institution: Thomas Hunt Morgan and Columbia University. (p. 2)

Such exaggerations are typical because posterity often looks back at what are considered great ideas and great successes by great men. While understandable in the context, they are highly problematic because they give mistaken impressions of how science works. Morgan was not single-mindedly looking for “the gene,” and indeed he rejected the Mendelian theory of heredity and the theoretical and unverifiable “gene” until late 1910. Even then, as Maienschein has demonstrated, his epistemological commitments required a high standard of evidence in favor of theoretical claims. And as Holmes explained in the first chapter of his Reconceiving the Gene, Morgan’s understanding of what a gene might be changed over time and was complex at any rate.

Furthermore, study of genetics did not bring together study of heredity, evolution, and development during Morgan’s lifetime. Morgan continued to struggle to see how genetic inheritance might connect with embryology. When the young Boris Ephrussi wrote to Morgan that his 1934 Embryology and Genetics did not, in fact, bring the two fields together, Morgan responded that the book did indeed include both and that he had not promised to show how they are connected. It was too early to understand how genetics and development intersect. Therefore, the picture is much more complex than the linear progress toward discovery of fact that Kandel presented.

Morgan as Lab Director . The most important revisionist look at Morgan’s place in the history of science has been Robert E. Kohler’s Lords of the Fly: Drosophila Genetics and the Experimental Life (1994). As the title suggests, this is a study of the fly and the layers of what is became popular to call the “material culture” of scientific study of this fruit fly that became so important to the understanding of genetics. Kohler explained clearly that he sought to examine the fly room as a particularly instructive example of “the nature of experimental life”—particularly instructive because of the focus on a famous community that had had important results certified as successful. As Kohler noted, “Few laboratory creatures have had such a spectacularly successful and productive history as Drosophila.” He saw Morgan’s fly room, first at Columbia and then at Caltech, as “an archetypical experimental community” (p. 1). Kohler did not intend to write a biographical study of one man, nor an intellectual tracing of one man’s or even one scientific school’s ideas. Instead, he sought to illuminate science by looking at an exemplary laboratory and all the people and interactions and practices within it. What also emerged from Kohler’s book was a revised picture of Morgan and his contributions.

Whereas Allen’s Morgan was a fine man, inspiring to his students and successful in putting together collaborative teams, Kohler’s Morgan was more the representative of an older generation whose team of younger students carried out many of the intellectual and most of the technical innovations in the lab. Allen’s Morgan had disagreements with others and taught students such as Hermann J. Muller who had disputes with their advisor. But the lab remained largely a happy and productive place in Allen’s study.

Perhaps the differences arose partly because Allen concentrated more on the earlier Columbia years while Kohler looked more closely at the Caltech years. But the differences are not just minor matters of emphasis or focus. For Allen, Morgan’s contributions were intellectual and social. Morgan built up a remarkably successful lab, and Morgan’s group carried out outstanding scientific work there. Once Allen had detailed that work, however, Kohler could shed light on the cracks in the system. He shows that a collaborative team in science, where so much hinges on who gets the credit, is likely to experience disputes or at least uneasiness over credit. His readers see that while Morgan provided intellectual drive, he contributed relatively little technical innovation concerning how to study the fly, or how to get at the genes and their structure and function and effects.

Kohler beautifully laid out the work in the fly room, and the interactions of the “boss and boys” (p. 62), in ways that raise questions about what it meant to work as a team, to mentor younger students, to work together for so long in such different ways. Remember that Morgan was already near official retirement age by the time he moved to Caltech, yet he continued his research almost until his death. Kohler discussed how the relationships changed, and how the dynamics of the fly room evolved.

Through Kohler’s study, the researcher comes to understand “the experimental life” in new ways. Kohler provided an outstanding complement to Allen’s more traditional focus on the leading man full of ideas and questions. Together, these studies depict a picture of Morgan as a complex scientist in an increasingly complex scientific community and set of practices. Together, they raise new questions that suggest that even more additions and revisions of an understanding of Morgan can be expected in the future. They also show that in producing scientific biography of the twentieth century and later, the individual scientist must be placed in the larger context not only of social and institutional constraints but of choices and material culture and laboratory practices. Ideas, technology, organisms and their sometimes errant real life behaviors, scientists, and assumptions all work together to shape and define science—and to define the contributions and biographical interpretations of any one contributor.

SUPPLEMENTARY BIBLIOGRAPHY

WORK BY MORGAN

Embryology and Genetics. New York: Columbia University Press,1934.

OTHER SOURCES

Allen, Garland E. Life Science in the Twentieth Century. New York: Wiley, 1975; Cambridge, U.K.: Cambridge University Press, 1978.

_____. Thomas Hunt Morgan: The Man and His Science Princeton, NJ: Princeton University Press, 1978..

Holmes, Frederic Lawrence. Reconceiving the Gene: Seymour Benzer’s Adventures in Phage Genetics, edited by William C. Summers. New Haven, CT: Yale University Press, 2006.

Horowitz, Helen Lefkowitz. The Power and Passion of M. Carey Thomas. Urbana: University of Illinois Press, 1999.

Kandel, Eric R. “Thomas Hunt Morgan at Columbia University: Genes, Chromosomes, and the Origin of Modern Biology.” Columbia Magazine (Fall 1999). Available from http://www.columbia.edu/cu/alumni/Magazine/Legacies/Morgan/.

Kohler, Robert E. Lords of the Fly: Drosophila Genetics and the Experimental Life. Chicago: University of Chicago Press, 1994.

Maienschein, Jane. “T. H. Morgan’s Regeneration, Epigenesis, and (W)holism.” In A History of Regeneration Research: Milestones in the Evolution of a Science, edited by Charles Dismore. Cambridge, U.K.: Cambridge University Press, 1991.

_____. Transforming Traditions in American Biology,1880–1915. Baltimore, MD: Johns Hopkins University Press, 1991.

Rossiter, Margaret. Women Scientists in America: Struggles and Strategies to 1940. Baltimore, MD: Johns Hopkins University Press, 1982.

Shine, Ian B., and Sylvia Wrobel. Thomas Hunt Morgan: Pioneer of Genetics. Lexington: University of Kentucky Press, 1976.

Jane Maienschein

Morgan, Thomas Hunt

Geneticist 1866-1945

Thomas Hunt Morgan proved the validity of the chromosomal theory of heredity and led a research group whose insights into the physical nature of inheritance propelled genetics into the center of biology in the twentieth century.

Training and Early Interests

Morgan was born and raised in Kentucky, and received his bachelor's degree from the State College of Kentucky in 1886. He pursued graduate study atJohns Hopkins University in Baltimore, and eventually became a professor of biology at Bryn Mawr College in 1891. His early interests were in developmental biology and evolution. After moving to Columbia University in 1901 and coming under the influence of the great cell biologist Edwin Wilson, Morgan turned his attention to understanding the physical basis of inheritance, which he saw as a means to test theories about the role of mutation in evolution.

At the time Morgan began his work, chromosomes had been seen in cells, but their significance was unknown and not widely considered. A student of Wilson's, Walter Sutton, had recently proposed that chromosomes carried the genetic material, but had little evidence to support this important hypothesis. At the time, the gene itself was an abstract concept with no known physical correlate, and many scientists thought it was not a physical entity at all, but only a convenient fiction for describing some experimental results. In fact, it was Morgan's use of the term "gene" that helped bring it into general use in science.

To attack the issue of heredity, Morgan chose to work with the fruit fly, Drosophila melanogaster. This fly requires little space, breeds quickly, has many observable characteristics, and has only four chromosomes, making it an ideal model organism for genetics studies. Morgan also gathered a trio of very bright students, Hermann Muller, Alfred Sturtevant, and Calvin Bridges, and cultivated an egalitarian system of collaboration that was unknown in most other labs. The combination of the right question, the right model, the right collaborators, and some luck allowed Morgan and his group in their lab, dubbed "The Fly Room," to make their fundamental discoveries. Beginning in 1908, they proved that chromosomes do indeed carry the genes, that genes are discrete physical things arranged on chromosomes like beads on a string, that genes change places on chromosomes, that genes can be mutated and those mutations are faithfully inherited, and that mutations can be caused by exposure to high-energy radiation or other environmental phenomena.

A Lucky Discovery

This long string of seminal discoveries began with the discovery of a single male white-eyed fly among the many thousands of normal red-eyed ones. Morgan bred this mutant male with a red-eyed female. All the offspring were red-eyed, indicating the white form of the gene (called the white allele ) was "recessive" to the dominant red allele: Flies carried the mutant allele, but its effects did not show up. When these offspring were crossed, the ratio of red to white was 3:1, just what would be expected for a classical recessive trait.

However, Morgan noted an unusual fact about the white-eyed flies—all of them were male. Morgan knew that the female Drosophila had two so-called X chromosomes, while the male had only one. Combining this fact with his discovery that only males showed the white-eye trait, he reasoned that the white-eye mutant allele must be on the X chromosome. Males show the white-eye trait because the mutant white allele is the only one they have—they don't have a second X chromosome with a normal red allele. Females rarely show the white-eye trait, because they have a normal redeye allele on the other X chromosome.

Morgan's results showed that the white-eye allele is inherited on the X chromosome, and confirmed the discovery that the X chromosome helps determine sex, first shown in 1905 by Sutton and Nettie Stevens. In one step, his discovery proved that genes, the factors governing inheritance, are carried on chromosomes, and that specific genes are carried on specific chromosomes. This provided the crucial evidence that genes are indeed discrete physical objects.

Linkage and Chromosome Mapping

The discovery of more mutated genes allowed Morgan's group to explore how genes are arranged on the chromosome, and to discover an exception to one of Mendel's laws of inheritance. Mendel had proposed the Law of Independent Assortment, stating that the alternative forms of different traits (such as round versus wrinkled pea seeds and short versus tall plant height) separate and recombine independently of each other, so that, for instance, obtaining a wrinkled tall plant is just as likely as obtaining a wrinkled short plant.

Morgan found this was not always true. Rather, certain combinations of alleles are very unlikely to be separated from each other, a fact he attributed to co-inheritance of the two alleles on the same chromosome. While alleles on separate chromosomes assort independently, as Mendel predicted, those on the same chromosome travel together unless separated.

To explore this, Morgan crossed a red-eyed fly with normal-length wings with a purple-eyed fly with stubby wings. After two generations, Mendel's laws predicted that all possible combinations of eye color and wing length should be equally likely. Instead, Morgan found that most flies had the original trait combinations, while red-eyed, stubby-winged flies were rare, as were purple-eyed, normal-winged flies. He concluded that the genes for wing length and purple eye color were on the same chromosome. Like passengers traveling on the same ship, once the particular alleles were together, they tended to stay linked. (Note that the purple eye-color gene is not the same one as the red-white eye-color gene he discovered previously, and is not on the X chromosome.)

However, Morgan noted specific allele combinations didn't always stay together: There were a few flies whose stubby-wing allele and purple-eye allele had become separated from each other. This led Morgan to propose that chromosomes sometimes exchange segments, allowing their passengers to change vessels, so to speak. This phenomenon is known as crossing over, and was later conclusively demonstrated in maize by Barbara McClintock.

Crossing over is now known to occur only during meiosis, the chromosome division that leads to formation of eggs and sperm. During meioisis, homologous chromosomes originally donated from the mother and father pair up for an extended period. In this period, called synapsis, the maternal and paternal chromosomes randomly exchange several segments, resulting in a pair of chromosomes with a mix of maternally derived and paternally derived alleles. These then separate to form the eggs and sperm.

Morgan's student Sturtevant reasoned that the likelihood of two alleles becoming separated during crossing over was proportional to the distance between them. In other words, the closer they are, the more likely they will stay together, and the further apart they are, the more likely they will separate. If A, B, and C are on the same chromosome, and A stays with B more often than it stays with C, then the distance from A to B is shorter than the distance from A to C. In this way, the relative distances of genes can be determined, providing a "linkage map" of the chromosomes. The unit of relative distance is called the morgan, in honor of Morgan himself. Calvin Bridges later devised a method to determine the absolute distance between genes, relying on the distinct banding patterns seen in Drosophila chromosomes in the larval stage.

Morgan's Legacy

In 1915 Morgan, Bridges, Sturtevant, and Muller published The Mechanism of Mendelian Heredity, a highly influential textbook laying out the evidence for the chromosomal theory of heredity and illustrating their methods so others could apply them in further research. In 1928 Morgan moved to the California Institute of Technology to found the Division of Biology. Sturtevant and Bridges went with him. Five years later Morgan was awarded the Nobel Prize in physiology or medicine for his work in genetics. He shared the prize money with Sturtevant and Bridges. Besides his own discoveries, Morgan's intellectual legacy includes the historically important researchers who trained with him, including Theodosius Dobzhansky, who applied the new genetics to an understanding of evolution. Another of his students was George Beadle, who discovered that mutations affect the working of proteins, and proposed the "one gene-one enzyme" definition of the gene.

see also Fruit Fly: Drosophila ; Linkage and Recombination; Mcclintock, Barbara; Meiosis; Mendel, Gregor; Muller, Hermann.

Richard Robinson

Bibliography

Allen, Garland E. Thomas Hunt Morgan: The Man and His Science. Princeton, NJ:Princeton University Press, 1978.

Judson, Horace F. The Eighth Day of Creation: The Makers of the Revolution in Biology. New York: Simon & Schuster, 1979.

Morgan, Thomas Hunt, et al. The Mechanism of Mendelian Heredity. New York: HoltRinehart & Winston, 1915. Reprint, with an introduction by Garland E. Allen, New York: Johnson Reprint Corporation, 1978.

Sturtevant, Alfred H. A History of Genetics. New York: Harper & Row, 1965.

Thomas Hunt Morgan

The American zoologist and geneticist Thomas Hunt Morgan (1866-1945) established the theory of the gene which helped clarify the process of evolution and formed the modern basis of heredity.

Thomas Hunt Morgan, born on Sept. 25, 1866, in Lexington, Ky., was the son of Charlton and Ellen Morgan. He was descended on both sides from English Cavalier stock. In 1886 he entered the State College of Kentucky and later studied at Johns Hopkins University, where he divided his time between morphology and physiology. In 1890 he received his doctorate for a paper on the embryology and phylogeny of sea spiders. In 1891 he served as professor of biology at Bryn Mawr College, after which he went to Europe for further study, first in Germany and then at the famous zoological station at Naples, Italy. There he met Hans Driesch, the philosopher-scientist who believed in "vitalism." Morgan, however, favored a mechanistic approach to the solution of biological problems.

Upon his return to the United States in 1904, Morgan accepted a professorship at Columbia University which lasted until 1928. While there he undertook a series of breeding experiments to assess the reality of genes as the particles of heredity. Morgan chose the fruit fly (Drosophila melanogaster) for his experiments because it was a short-lived organism that could easily be bred in the laboratory under changing condition and could complete its life cycle in about 10 days, supplying as many as 30 generations a year.

Morgan's experiments were so successful that by 1914 he had proved the chromosome theory of heredity as a result of breeding and cytological examination. In 1910 he found his first mutant and proceeded to cross this fly with a normal one. The percentages of normal and mutant off-spring were in accordance with Mendel's law of inheritance. Morgan found many mutant characters and soon discovered that certain characteristics not only were sex-linked but also tended to appear together in certain flies. From this he postulated that all sex-linked characters tended to be inherited together because they were associated as a unit on a single chromosome in the nucleus of the original cell. Morgan called these characters linkage groups. By the summer of 1914 three linkage groups had been discovered. He used the word "gene" to represent each character unit, and the exact positions of these genes in the chromosomes was worked out by Alfred Henry Sturtevant, one of Morgan's former students and a member of his research staff. In 1915 Morgan and his assistants published The Mechanism of Mendelian Heredity to describe the system of genes. Later he published The Theory of the Gene (1926), his culminating work on the subject which discussed at length the chromosome theory of heredity.

In 1928 Morgan established the Kerckhoff Laboratories of Biological Sciences at the California Institute of Technology in Pasadena, which became the leading center for research genetics. In 1933 he received the Nobel Prize in physiology or medicine in recognition of the significance of his theory of heredity for physiology and for the part that the new genetics was destined to play in the future of medicine.

In 1941 Morgan retired as active head of his department at Cal Tech. However, he continued to work on problems in embryology which he had first approached in 1903—trying to find out why the spermatozoon of the common hermaphroditic sea squirt almost never fertilizes the egg of the same individual (self-sterilization) but does fertilize eggs of all other sea squirts. On Dec. 4, 1945, the grand old man of genetics passed away.

Further Reading

No full-length biography or autobiography of Morgan has been published. A detailed account of his life and work is in Bernard Jaffe, Men of Science in America (1944; rev. ed. 1958). A biography also appears in National Academy of Sciences, Biographical Memoirs, vol. 33 (1959). Short studies of Morgan are in Theodore L. Sourkes, Nobel Prize Winners in Medicine and Physiology, 1901-1965 (1953; rev. ed. 1967); Katherine Binney Shippen, Men, Microscopes and Living Things (1955); Jay E. Greene, ed., 100 Great Scientists (1964); and Nobel Foundation, Physiology or Medicine: Nobel Lectures, including Presentation Speeches and Laureates' Biographies (3 vols., 1964-1967). □

Morgan, Thomas Hunt (1866–1945) US geneticist, who held professorships at Bryn Mawr College (1891–1904), Columbia University (1904–28), and the California Institute of Technology (1928–45). He established that chromosomes were the carriers of Mendel's `factors' of inheritance (genes). Working with fruit flies (Drosophila), he demonstrated the phenomenon of linkage and modified Mendel's law of independent assortment by stating that this could only apply to genes located on different chromosomes. He showed that linkage could be broken by crossing over and went on to produce the first chromosome maps. For his work Morgan was awarded the 1933 Nobel Prize for physiology or medicine.