Shull, Aaron Franklin

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(b Miami County, Ohio, 1 August 1881: d. Ann Arbor. Michigan, 7 November 1961)

genetics, evolution

Shall was one of eight children born to Harrison and Catherine Ryman Shull. Although they had relatively little formal education, both parents encouraged intellectual activity among their children. The father was a farmer and lay minister; the mother had strong interest in serious reading, especially topics relating to natural history, and in her later years became an accomplished horticulturist with a firsthand knowledge of plants. Because the family moved from farm to farm in South-central Ohio, Shull’s early schooling was largely informal. In 1904 he enrolled as an undergraduate at the University of Michigan, completing his A. B. degree in 1908. In the summer following his graduation he worked with the Michigan Biological Survey, and in the fall of that year he entered Columbia University for graduate work in zoology. Stimulated by the work of T. H. Morgan and E. B. Wilson, both at Columbia, Shall became interested in problems of heredity, particularly sex determination, on which he wrote his thesis. After obtaining his Ph.D. in 1911, he returned to the University of Michigan as an instructor in zoology (1911–1912), assistant professor (1912–1914), associate professor (1921–1951). His only break with this university occurred in the summer of 1938. when he was a visiting professor at the University of California, Berkeley. Although a researches of considerable merit. Shall perhaps is best remembered in American biology for his teaching and writing. A stimulating lecturer, he was also a prolific writer of monographs and textbooks that introduced countless students to modern, experimental biology and to rigorous concepts in general biology, heredity, and evolution.

Shull studied in considerable depth the life cycle of and sex determination problem in rotifers, a subject that had been variously interpreted by workers in the last part of the nineteenth and first part of the twentieth centuries. A study of developmental physiology and sex determination in aphids, expanded the earlier work to the problem of sex determination in general. In the 1920’s and 1930’s he experimented with induction of crossing over in Drosophila by physical factors, most notably heat. In all these studie, Shull’s focus was on the relationship between heredity and environment in determining the phenotype of an organism. His interests centered on some of the highly controversial questions, which received so much prominence at the time, concerning the influence of environment on heredity. A Mendelian from his earliest days, Shull was interested not only in working out specific problems of heredity—was food a factor in determining sex, or does heat induce crossing-over in Drosophila?—but also in the relationships between these phenomena and the process of organic evolution. Recognizing earlier than many biologists that one side of the evolutionary coin was the problem of the origin of hereditary variation (the other side was the effect of selection on these variations), he concentrated much of his research on determining the relatinship between genetic and environmental factors in producing the phenotypes on which selection acted.

In his studies and writings on evolution, Shull was incisive and penetrating in his analyses of such subsidiary problems as the nature and origin of mimicry. A fervent advocate of drawing conclusions only from the data at hand, he severely criticized the concept of mimicry advanced by G. D. Hale Carpenter. To Shull, phenotypic properties such as mimicry (in which one species comes to resemble another as a means of protection against predators) could not always be judged as being of survival value simply because, to man, the two species appeared to resemble each other. He pointed out that two forms resembling each other in human vision might appear quite distinct to some predator (such as a bird). Shull cited the work of W. L. McAtee, who, in 1912, had shown from analysis of the contents of birds’ stomachs that mimics were eaten, like nonmimics, roughly in proportion to their availability. Thus, whatever resemblance the mimics may or may not have borne to the model appeared to be of little value of helping the prey escape its predator. Shull’s opposition to the theory of warning coloration and mimicry was based largely on what he considered the tendency to misuse evidence, to interject anthropomorphism and subjective speculation into an area where the collection of solid facts would be more beneficial. A strong empiricist and experimentalist, Shull was one of a group of younger biologists who rejected the nonrigorous and speculative tradition in biology represented by the older morphologists and neo-Darwinians. Although a strong proponent of Darwin’s theory of natural selection, Shull felt that among evolutionists there was a preponderance of purely descriptive, nonrigorous, and nonanalytical thinking that did not take into consideration the recent findings in such experimental areas as physiology, development, and genetics.

Shull carried his concern for new methods in biology into his teaching an educational writings. A strong advocate of the “principles” approach to biology, he organized his courses and teaching monographs in terms of generally applicable biological concepts: cell structure and function, transport, unity an control,embryonic development, genetics, systematics, and geographic distribution. To the modern reader this may appear to be a customary and routine type of organization; it was in fact quite novel in Shull’s day. During the first three decades of the twentieth century, most biology courses were organized along phylogenetic and “type specimen” lines. Students were given a detailed, descriptive, and anatomically oriented survey of animal or plant types (poriferans, annelids, crustaceans, echinoderms, vertebrates, tracheophytes, angiosperms, and so on). To Shull, this method of organization not only was uninteresting for most students but also offered a limited view of biology. Why not, he asked, study the characteristics of cells by using examples of many cell types from many different specie of animals and plants? Biological principles are what the student is going to remember most; the anatomical detail will be important if—and only if—the student decides to pursue some area of biology in depth. Besides, he argued, the old “type specimen” course did not even do what it claimed to do: give the student a picture of evolutionary development. Shull pointed out, for example, that most such courses involved detailed consideration of a modern echinoderm (starfish, sand dollar, sea urchin); yet echinoderms as a group represent an evolution ary offshoot that was by no means to be found in the mainstream of animal phylogeny. In Principlesof Animal Biology (1934), Evolution (1936), and Heredity (1938), Shull gave this approach a concrete form that he hoped would aid others in developing a “principles” approach to biology at the undergraduate and graduate levels.

During his academic career Shull received numerous honors and was a member of many professional societies. He was a fellow of the American Association for the Advancement of Science and was appointed Russel lecturer for 1951. He was a member of the American Society of Naturalists, serving as secretary (1920–1926), vice-president (1929), and president (1934). He also belonged to the American Society of Zoologists, the Genetics Society of America, the Society for the Study of Evolution, the Eugenics Society (London, and the National Association of Biology Teachers; was a fellow of the Entomological Society of America; and served as president of the Michigan Academy of Sciences (1921–1922). In 1911 Shull married Margaret Jeffrey Buckley; they had four children.


I. Original Works. Shull wrote a large number and variety of scholarly papers on sex determination, heredity, evolution, and education. A few of the most notable are “Color Sport Among Locustidae,” in Science, 26 (1907), 218–219; “Nutrition and Sex Determination in Rotifers,” ibid., 38 (1913), 786–788; “Biological Principles in the Zoology Course,” ibid., 48 (1918), 648–649; “Crossovers in Male Drosophila melanogaster Induced by Heat.” ibid., 80 (1934), 103–104, written with Maurice Whittingill; “Wisemann and Haeckel: One Hundred Years,” ibid., 891 (1935), 443–452); and “Needs of the Mimiery Theory,” ibid., 85 (1937), 496–498.

II. Secondary Literature. There is no detailed biographical sketch of Shull’s life or work. He is listed in American Men of Science, 9th ed. (Lancaster, Pa., 1955). 1030. Letters to and from Shull can be found at the American Philosophical Society, in the Jennings, Blakeslee, Demerec, Dunn, and Davenport papers.

Garland Allen