Jennings, Herbert Spencer

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(b. Tonica, Illinois, 8 April 1868; d. Santa Monica, California, 14 April 1947),

animal behavior, physiology, genetics, protozoology, zoology, philosophy of science. For the original article on Jennings see DSB, vol. 7.

Since the original DSB article was published, new light has been shed on Jennings’s approach to science, especially his epistemology and experimental method; his views on inheritance; and his political views, especially concerning eugenics.

Herbert Spencer Jennings was a leader in the experimentalization of zoological research in the United States in the first decades of the twentieth century. Beginning with his postdoctoral research on the behavior of lower organisms carried out between 1896 and 1906, and continuing with his studies of heredity and variation from 1907 through the remainder of his career, Jennings championed the use of experiment for biological study. Jennings’s methodological orientation was deeply informed by his adherence to pragmatism, as articulated primarily by the philosophers John Dewey and William James. By 1913 Jennings formulated his own philosophy of biology, termed “radically experimental analysis,” which bore many of the hallmarks of pragmatism, including a relentless commitment to action and to empiricism for the resolution of scientific problems. As a pragmatist, Jennings directed these same priorities to the social arena as well.

Jennings’s pragmatism informed all aspects of his career, from his behavioral and genetics research programs with protozoa, to his substantial philosophical writings, to his public criticisms of eugenics during the 1920s.

Jennings’s contact with pragmatism began early in his education, when he pursued two years of philosophical study with the young Dewey between 1890 and 1892, while an undergraduate at the University of Michigan. Jennings had already had considerable contact with literature and philosophy as a young child who passed much of his time reading widely from the large library of his father, George Nelson Jennings. The senior Jennings was a physician and enthusiastic evolutionist, who named his first-born son for the influential British philosopher, Herbert Spencer, while naming his second son George Darwin. The young Herbert Spencer Jennings came to reject much of the Spencerian thought that he had adopted at an early age when he was exposed to Dewey’s experimentalism. Dewey claimed that ethical behavior was grounded not in natural law, as had been held by Spencer, but in scientific method. While ultimately pursuing a course of study in zoology, culminating in earning his PhD in zoology at Harvard University in 1896, Jennings never lost interest in philosophy and its relevance for political and scientific conduct.

In his dissertation research, Jennings had been deeply interested in the experimental goals and methods but largely critical of the mechanistic orientation of the new experimental program of development mechanics, or Entwicklungsmechanik, championed by Wilhelm Roux and others. Developmental mechanics imported experimental methods from the field of physiology to zoology, which until that time was largely characterized by the observational practices of natural history. Inspired by the new experimentalism, Jennings pursued postdoctoral study in the laboratory of the German physiologist Max Verworn in Jena in 1896–97. There, Verworn introduced Jennings to many of the experimental methods of physiology as well as to the microscopic single-celled organisms, the protozoa, for experimental study. Jennings was also further immersed in the evolutionism that suffused Jena, the epicenter of German evolutionary thought. Throughout his career, Jennings, like Verworn and his Jena colleague, Ernst Haeckel, embraced the protozoa for their biological importance as organisms possessing complex physiological processes and behaviors, yet were at the same time single cells.

Relatively early in his physiological studies of the behavior of protozoa, Jennings emphasized the rudimentary psychological capabilities of the organisms. In the course of this research he characterized the essential features of the avoiding reaction in protozoa, the process by which the microscopic organism stops its forward motion, swims backwards, and then starts off in a new direction when it confronts a new external stimulus. Jennings rejected the idea that external, physicochemical forces causally directed the organism’s motions, as others such as Jacques Loeb claimed. He maintained that the avoiding reaction was driven by a mechanism internal to the organism. During a year of research at the Zoological Station in Naples, Italy, in 1903–04, however, he adopted a more radical position: Jennings now argued that the avoiding reaction in protozoa was an early manifestation of the method of “trial and error” deemed by the evolutionary psychologist C. Lloyd Morgan to be fundamental to the development of intelligence in higher organisms. This research culminated in the publication of his influential text Behavior of the Lower Organisms in 1906. During the same year Jennings accepted an appointment at the Zoological Laboratory at The Johns Hopkins University, which he directed from 1910 until he retired in 1938. (For discussion of the various academic positions held by Jennings prior to moving to Johns Hopkins, please see the original DSB article.)

Behavior of the Lower Organisms attracted considerable criticism both for its critical stance toward mechanistic biology and for its attribution of a developmental-evolutionary continuum “from the lowest organisms up to man” (p. 335) in behavioral features such as perception, choice, attention, and even consciousness. In the years that followed, Jennings dedicated considerable effort to epistemological writings informed by these earlier criticisms. Throughout his philosophical reflections he championed both a natural history-like attention to the individual organism and experiment in biology. Central to his concern was the development of a middle position in biology between vitalism on the one hand and mechanism on the other. Jennings was particularly concerned about the vitalism articulated by the German zoologist Hans Driesch, who maintained that two biological systems that are completely identical could behave differently under identical conditions. Jennings rejected Driesch’s position of experimental indeterminism, because it threatened to halt experimental inquiry in biology altogether. Arising from these concerns was Jennings’ formulation of his methodology of radically experimental analysis, whereby all biological questions are reduced to an experimental situation; in a given experimental situation, a preceding perceptual difference is sought for every existing perceptual difference.

In 1907 Jennings turned his attention from behavior to problems of heredity and variation. Following the rediscovery of Mendel’s laws in 1900, biological interest in heredity quickly blossomed as experimentation grew in prominence among academic biologists. Jennings was in the vanguard of American zoologists to publish experimental research in heredity and genetics in the first decade of the twentieth century. As researchers experimented with a variety of organisms, Jennings initiated experimental studies with the same organisms that had been central to his behavior research: the protozoa, and primarily the ciliate Paramecium. Conceptually, Jennings likewise presented his research program in heredity as a continuation of his behavioral research, focusing on the inheritance of adaptive qualities, a problem that he viewed as intimately related to the development of adaptive behaviors in an individual organism. There was methodological continuity between the two research programs as well; Jennings aimed to maintain analytical focus on the individual organism and on relationships between individual organisms in his hereditary studies. This was quite a distinctive undertaking, as the developing protocols in genetics research also demanded statistical analysis of data derived from large quantities of organisms, a methodological domain in which Jennings was also a leader.

Whereas Jennings engaged in important statistical and theoretical work in the 1910s and 1920s that lent credence to Mendelism and chromosome theory, his own experimental research program was not Mendelian. Like those of other leading zoologists attracted to the study of heredity during the period, the broader intent of Jennings’s research program was to demonstrate evolution experimentally. Furthermore, he aimed to uncover the most fundamental mechanisms of heredity and to advance a generalized conception of heredity. Unlike many, however, Jennings adopted a relatively pluralistic attitude toward the study of heredity. During this period he evenhandedly explored both selection and the inheritance of acquired characteristics as possible mechanisms of heredity and evolution. Jennings placed a great emphasis on the utility of the protozoa for such research, because their single cellularity made their exposure to environmental influences more feasible and hereditary effects in them more readily observable. Likewise, their uniparental form of reproduction made it possible to explore minute sources of heritable variation unobscured by biparental inheritance. Jennings came to view lineages of asexually reproducing protozoa as material embodiments of pure lines. Between 1909 and 1911 he facilitated discussions that helped to solidify the pure line and genotype concepts introduced by Wilhelm Johannsen.

The experience of World War I and the international political upheaval in the years that immediately followed was a transformative experience for Jennings, as it was for many scientists of his generation. Distress over growing American nativism in the postwar years and the associated rise of the eugenics movement prompted Jennings to largely abandon his experimental research program and dedicate his efforts throughout the 1920s to public criticism of eugenics. Jennings employed diverse strategies in this undertaking: He criticized fallacious reasoning and faulty assumptions underlying many eugenic arguments; in concert with social workers and educators, he countered genetic determinism by underscoring the role of environment and education in the development of human potential; and he worked to keep the field of genetics institutionally separate from the broader eugenics movement. At the end of the decade Jennings returned to experimental research with a narrowed research agenda. In a bid to generate new evidence further undercutting the eugenics movement, he reestablished a research program with Paramecium aimed at conclusively demonstrating the inheritance of environmental effects. The research planned by Jennings was largely carried out by his former student and research associate Tracy Sonneborn, and long-time research assistant Ruth Stocking Lynch. This work led to Sonneborn’s discovery of mating types in Paramecium aurelia in 1937.

Jennings served in many leadership capacities and received many honors in the course of his career. He served as president of the American Society of Zoologists (1909) and the American Society of Naturalists (1910), First Chairman of the Genetics Society of America (1922), Vice-President of Section F (Zoology) of the American Association for the Advancement of Science (1925), and member of the Council of the National Academy of Sciences (1934–1940). He served as a founding member of the editorial boards of The Journal of Experimental Zoology and Genetics and on the editorial boards of several other journals. He was a Terry Lecturer at Yale University (1933), Vanuxem Lecturer at Princeton University (1934), Eastman Visiting Professor at Oxford University (1935–1936), Leidy Lecturer at the University of Pennsylvania (1940), and Patten Lecturer at Indiana University (1943).



Herbert Spencer Jennings Papers. Philadelphia, PA: American Philosophical Society Library. Contains extensive correspondence, notebooks, journals, manuscripts and lectures.

“Diverse Ideals and Divergent Conclusions in the Study of Behavior in Lower Organisms.” The American Journal of Psychology 21 (1910): 349–370.

“Doctrines Held as Vitalism.” The American Naturalist 47 (1913): 385–417.

“Causes and Determiners in Radically Experimental Analysis.” American Naturalist 47 (1913): 349–60.

“Heredity and Environment.” Scientific Monthly 19 (1924): 225–38.

“Diverse Doctrines of Evolution, Their Relation to the Practice of Science and of Life.” Science 65 (1927): 19–25.


Kingsland, Sharon. “A Man Out of Place: Herbert Spencer Jennings at Johns Hopkins, 1906–1938.” American Zoologist 27 (1987): 807–817.

Pauly, Philip J. “The Loeb-Jennings Debate and the Science of Animal Behavior.” Journal of the History of the Behavioral Sciences 17 (1981): 504–515.

Schloegel, Judith Johns. “Intimate Biology: Herbert Spencer Jennings, Tracy Sonneborn, and the Career of American Protozoan Genetics.” PhD dissertation. Bloomington: Indiana University, 2006.

———, and Henning Schmidgen. “General Physiology, Experimental Psychology and Evolutionism: Unicellular Organisms as Objects of Psychophysiological Research, 1877–1918.” Isis 93 (2002): 614–645.

Sonneborn, Tracy M. “Herbert Spencer Jennings, April 8, 1868–April 14, 1947.” Biographical Memoirs of the National Academy of Sciences 47 (1975): 143–223. A detailed biographical resource including a complete bibliography of Jennings’s publications and listings of positions, honors, and distinctions.

Judith Johns Schloegel

Jennings, Herbert Spencer

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Jennings, Herbert Spencer

(b. Tonica, Illinois, 8 April 1868; d. Santa Monica, California 14 April 1947)


Jennings was the son of George Nelson Jennings, a physician, and the former Olive Taft Jenks. After attending local schools he studied at Illinois Normal (now Illinois State University) and taught school near Tonica. In 1889-1890 (at the age of twenty-one and without a degree), through the influence of a former teacher he was appointed assistant professor of botany and horticulture at the Agricultural and Mechanical College of Texas. From 1890to1893 he attended the University of Michigan, where he met Jacob Reighard, a young zoologist and ichthyologist. After obtaining the B.S. from Michigan and spending a year in graduate study there, Jennings went on to work with Reighard’s teacher, E. L. Mark, at Harvard. There he took the M.A. in 1895 and the Ph.D. in 1896, submitting a thesis on the morphogenesis of a rotifer. After finishing his doctorate Jennings held the Parker traveling fellowship, which took him to Jena in the winter of 1896-1897, where he worked with Max Verworn, a pioneer student of the behavior of protozoans. In the spring he went to the zoological station at Naples (to which he returned in 1903-1904 as one of the first scientists subsidized by the Carnegie Institution of Washington).

Upon returning to the United States, Jennings was without a position. In August 1897, he was called to the Agricultural College of Montana in Bozeman (now Montana State University) as professor of botany and horticulture. The next summer he married Mary Louise Burridge, who did many of the illustrations in his publication. He spent the academic year 1898-1899 at Dartmouth as instructor in zoology; and in 1899 he rejoined Reighard at Michigan as instructor in zoology. By 1901 he had advanced to assistant professor, but in 1903 he departed for an identical appointment at the University of Pennsylvania. Finally, in 1906, Jennings accepted an associate professorship of zoology at Johns Hopkins University. In 1910 the title was changed to professor of experimental zoology. He was named Henry Walters professor and director of the zoological laboratory in 1910. After his retirement in 1938, he become research associate at the University of California, Los Angeles. His wife died in 1937;and in 1939 he married Lalu Plant Jennings, the window of his brother. At Johns Hopkins, Jennings was noted for his dedication to research and graduate instruction. Although he did not have a great number of students, among them were T. M. Sonneborn, William Taliaferro, and Karl Lashley.

Jennings concentrated his attention upon only two types of microorganisms, the Rotifera and the Protozoa; but his research nevertheless mirrored the change that were taking place in the shifting mainstream of biology. His first works were descriptive and systematic; he then turned his attention to physiology and adaptation; and finally took up the question of variation and reproduction and made major contributions to genetics.

The earliest phase of Jenning’s work is reflected in his publications for the Michigan State Board of Fish Commissioners and the U. S. Fish Commission, Working under the direction, in each case, of Reighard, who saw to his employment during several summers. Jennings’ descriptions and classifications of Rotifera, including new species, hold a respectable place in the systematic and morphological literature. At Harvard, C. B. Davenport helped kindle his enthusiasm for experimental methods;and Jennigs’work with the paramecium, undertaken with Verworn at Jena in 1897, began a decade of study of the behavior of the very simple organisms. This research resulted in a number of papers and, ulrimately, in a book, Behavior of the Lower Organisms (1906), a classic of zoology and comparative psychology.

Behavior of Lower Organisms had an impact in three areas particularly. First, Jennings, unlike previous workers on the protozoa, studied the reactions of individual organisms rather than generalized or group behavior. He thus was able to raise questions about the specific processes and patterns involved when stimuli were followed by responses. Second, the book (and an earlier report of 1904) presented the first clear challenge, with experimental evidence, to the theoryof physicochemical tropisms, of which Jacques Loeb was the chief exponent. Since most of the workers supporting the existence of tropistic behavior had utilized metazoic organisms, Jenning’s demonstration in even more primitive one-celled animals of phenomena that the concept of tropistic responses could not encompass was devastating to the theroy, which eventually languished.

Finally, through his book Jennings did much to bring very simple and one-celled organisms into the relam of psychology. Jennings now adapted to paramecia the use of experiment in comparative psychology, largely pioneered by E. L. Thorndike in 1898;and he asserted the identity of the basic nature of activity and reactivity in all animals, from protozoans to man. The idea was not new but Jennings’ experimental evidence was, and it appeared at the time to be conclusive.

With the completion of the book, Jennings turned away from the subject of animal adaptation and functioning. Even the course on animal behavior at Johns Hopkins was turned over to his new colleague and ally in the field, S. O. Mast. Jennings now embarked on four decades of research in the the recently opened field of genetics, still utilizing the very smallest animals. Although again basing his work upon characteristics of individual organisms (aggregated statistically rather than studied in groups or swarms), he devoted himself “to what happens in the passage of generations in these creatures; to a study of the biology of races rather than of individuals, to life, death, mating, generation, heredity, variation and evolution in the Protista”(Life and Death, p. 19).

In the course of his inquiry Jennings made numerous important contributions. He contended, chiefly against Gary Calkins, that conjugation was not necessarily essential in maintaining the vitality of strains of one-celled animals, an opinion that he reversed at the end of his life. As he systematically applied Mendelian theory, he helped to found mathematical genetics through his calculations of expectable radios of traits in various types of inheritance. Of all of Jennings’ work in genetics however, the most momentous investigated the questions of variation and evolution.

Between 1908 and 1916 Jennings and his students published a number of papers on the constancy and variability of protozoan lines of inheritance. He was able to show that within a given species there exist a number of distinctive strains whose traits persist over many generations—essentially the kinds of variavariation that Darwin had discussed originally as providing the basis for the effects of natural selection. But Jennings also observed the spontaneous development of this type of variation, usually only a very slight—but a persisting—alteration. A university publication characterized Jennings—not entirely hyperbolically—as the first scientist “actually to see and control the process of evolution among living things.”This work did much to modify the theory of mutations, because the “saltations”that Jennings reported were very slight indeed and suggested that evolution must proceed gradually by very small changes.

After 1916 Jennings did much less laboratory work, instead producing a notable series of works popularizing genetics and discussing philosophical questions raised by the newer methods and discoveries in experimental biology. He discussed particularly the fundamental finding of his own lifework, that life processes are identical throughout all of the animal kingdom, for he had extended his contention from areas of reactivity to include inheritance. In the 1940’s he even wrote about social phenomena among unicellular beings, a conception that grew out of the writings of W. C. Allee as well as his own renewed burst of laboratory research, an inquiry into the nature of sexuality in the paramecium.

As a youngster Jennings had at first been interested in the humanities, and this interest appeared later in his published philosophical discussions. As early as the 1910’ he had written in opposition to Driesch ’ vitalism because, he asserted, he was afraid that biological experimentation that was purely scientific would be inhibited by vitalistic beliefs. He later wrote in a more positive vein about broader questions. For example, in the Terry lectures (published in 1933) on the relation between biology and religion, he affirmed the finality of death but also stressed the purposiveness of life and the compatibility of ethics with a strictly biological viewpoint.

Jenning’s popularizations of experimental biology were very widely read and cited, but the precise influence of The Biological Basis of Human Nature (1930) and other writings on Western culture is very hard to determine. The impact of Jennings upon science, however, can be measured, at least in part, by his place in the literature. Experimental methods that he introduced were still in use a generation or two later. Lines of investigation in genetics that he started or fostered were exciting and productive for at least half a century. For years he was the most conspicuous figure in a new genetics developedin protozoology, complementary to but different from the classic Drosophila work. His results in the Behavior of the Lower Organisms were still fundamental in that field in the 1960’s. His thorough experimental procedures and clear thinking gained him respect among his colleagues that was reflected in prizes, honorary degrees, lectureships, and membership in the American Philosophical Society, the National Academy of Sciences, and similar bodies. Jennings assisted in editing a number of major journals and, through his influence within the powerful inner circles of American science, helped to develop the best traditions of experimental work in the United States.


I.Original Works . Two books sum up most Jennings’ contributions: Behavior of the Lower Organisms (New York, 1906) and Life and Death, Heredity and Evolution in Unicellular Organisms (Boston, 1920). He reviewed his own work in Genetics of the Protozoa (The Hague, 1929). The most important popularization are The Biological Basis of Human Nature (New York, 1930) and The Universe and Life (New Haven, 1933). Other major books include Genetic Variations in Relation to Evolution, A Critical Inquiry Into the Observed Types of Inherited Variation, in Relation to Evolutionary Change (Princeton, 1935) and Genetics (New York, (1935). No complete bibliography is known to exist, but a good list can be compiled from standard bibliographical guides and citations in Jennings’ works. The Jennings papers in the American Philosophical Society Library contain a number of unpublished materials, the most importent of which are a diary covering 1906-1943 and a lecture. “History of Zoology in America, Partly as Reminiscences“(1929). Few letters exist in that collection, but a number may be found in the Robert M. Yerkes papers in the Yale University Medical Library. An autobiographical fragment is “Stirring Days at A.and M.,” in Southwest Review, 31 (1946), 341-344.

II. Secondary Literature. The best biographical sketch is T. M. Sonneborn, “Herbert Spencer Jennings, April 8, 1868-April 14, 1947,” in Genetics, 33 (1948), 1-4. see also Samuel Wood Geiser, “Herbert Spencer Jennings, Apostle of the Scientific Spirit,” in Bios (Iowa), 16 (1946), 3-18; National Cyclopaedia of American Biography, XLVII, 92-93; and Donald D. Jensen, “Foreword to the 1962 Edition,” in H. S. Jennings, Behavior of the Lower Organisms (Bloomington, Ind., 1962), pp. iX-Xvii. Newspaper obituaries are not helpful. For Jennings’ work at Johns Hopkins, in addition to materials there and official university reports, see Carl Pontius Swanson, “A History of Biology at the Johns Hopkins University,” in Bios (Iowa), 22 (1951), esp. 245-248.

J. C. Burnham

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