Leidy, Joseph

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Leidy, Joseph

(b. Philadelphia, Pennsylvania, 9 September 1823; d. Philadelphia, 30 April 1891)


A descriptive scientist of unusual range, Leidy made contributions of lasting value in human anatomy, parasitology, protozoology, and paleontology of mammals and reptiles.

Leidy was the second son and third child of Philip Leidy, a Philadelphia hatter and member of a family of mechanics and artisans who immigrated to Bucks Country, Pennsylvania, from Wittenberg in the early eighteenth century, and Catherine Mellick, of Columbia Country, Pennsylvania, following whose death in 1823 Philip Leidy married her cousin Christiana T. Mellick of Philadelphia. Joseph Leidy attended a private day school and was influenced to study natural history by a visiting lecturer on minerals. This early taste deepened under the guidance of a nearby horticulturist and during rambles in the countryside. A little book of drawings of shells accompanied by both scientific and common names, done by Leidy when he was ten, may be seen at the Academy of Natural Sciences of Philadelphia. Encouraged by his stepmother to become a student of anatomy, he matriculated at the University of Pennsylvania, where Paul B. Goddard introduced him to microscopy and supervised his dissertation on the comparative anatomy of the vertebrate eye, for which he was awarded the M.D. in 1844.

Leidy failed in an effort to build a private medical practice and seemed much better suited to close study. In the words of his biographer, W. S. W. Ruschenberger:

It may truly be said that Dr. Leidy was born to be a naturalist. To his innate ability to perceive the minutest variations in the forms and color of things was united artistic aptitude of a high order. These natural faculties, in continuous exercise almost from his infantile days, and his love of accuracy, enabled him to detect minute differences and resemblances of all objects, and to correctly describe and portray them [p. 147].

His acuteness was exemplified by his noticing minute specks in a slice of pork he was eating, and thus discovering that Trichinella spiralis, known to be a parasite of man, infested the raw flesh of pigs and that it could be killed by boiling. In 1853 Leidy was appointed professor of anatomy at the University of Pennsylvania. He published a widely admired manual on human anatomy in 1861 and served during the Civil War as an army surgeon, performing many autopsies. In 1864 he married Anna Harden of Louisville, Kentucky; they adopted one daughter.

Leidy seems to have had an unusual degree of patience that allowed him to observe organisms for longer than was strictly necessary in order to publish a description. When writing up the discovery of an unusual planarian in a spring in Pennsylvania, he added notes on behavior and feeding habits and conducted experiments on its capacity for regeneration. In 1848 he commenced studies on the intestinal flora and fauna of healthy animals, an intricate and difficult subject which prompted reflections on spontaneous generation and the origin of life. Leidy found no difficulty in step-by-step reasoning from close study of intestinal microorganisms to considerations of life in the universe, and he offered a calmly worded suggestion that all life had evolved from comparably simple circumstances. His discovery in 1848 that the larval eye persisted into the adult stage of the Cirripedia attracted the attention of Charles Darwin and exemplified Leidy’s capacity for detail, for these organs are very minute and rudimentary.

In 1848 Leidy was elected to the Academy of Natural Sciences of Philadelphia, then the foremost American institution for the deposit of paleontological specimens. In 1847 he described the fossil remains of the horse in America with the accuracy and minuteness that became his hallmarks. Charles Lyell urged Leidy to devote himself to paleontology. Spencer Baird turned over to him the vertebrate paleontology collections of the Smithsonian Institution and by 1855 Leidy had inspected most of the important finds from the American West. He proved to be a talented and perceptive comparative anatomist, writing meticulous descriptions of fossil mammals that have endeared him to subsequent workers. One typical appraisal is that he was “a paleontologist’s paleontologist,” whose monographs eschewed speculation and laid the foundation for understanding the diverse North American fossil vertebrates. “The Extinct Mammalian Fauna of Dakota and Nebraska” (1869), probably his greatest single work, was thought by Henry Fairfield Osborn to be “with the possible exception of Cope’s Tertiary Vertebrata, the most important paleontological work which America has produced.” The infant science of paleontology became embroiled in controversy among its promoters; and Leidy, who had no appetite for acrimony, gradually shifted his interests to protozoology.

Leidy devoted the years 1874 to 1878 to studies of freshwater Protozoa in a diverse array of habitats, ranging from the Uinta Mountains of Utah to the bogs of the Southeast. His monograph Fresh-Water Rhizopods of North America (1879), illustrated with superb lithographs of his field drawings, was the equal of any previous work in the field, constituting a remarkable portrayal of the diversity of unicellular life in microhabitats.

Leidy was a devoted worker who missed only five days in thirty-eight years as professor of anatomy, and his steadfastness contributed greatly to the strength of the institutions he served. He was president of the Academy of Natural Sciences for the last ten years of his life and in 1885 became president of the Wagner Free Institute of Science and director of its museum. His lectures there were popular discourses revealing an unaffected humanity—the quality that prompted him, while visiting the fish market in search of scientific specimens, to tell fish sellers the scientific names of uncommon varieties for them to copy onto their signs for the public.

His dislike for the controversies that beset vertebrate paleontology did not prevent Leidy from seeking the wider scientific implications of facts that he regarded as established. He drew connections from observations of parasites to concepts of contagion or primordial origins of life, for example, and exercised great powers of observation. His perceptions were marked by sober appreciation of the vastness and diversity of life, as in these comments on the Bridger Basin in Utah:

The utter desolation of the scene, the dried-up watercourses, the absence of any moving object, and the profound silence which prevailed, produced a feeling that was positively oppressive. When I then thought of the buttes beneath my feet, with their entombed remains of multitudes of animals forever extinct, and reflected upon the time when the country teemed with life, I truly felt that I was standing on the wreck of a former world [Contributions to the Extinct Vertebrate Fauna of the Western Territories, pp. 18-19].


I. Original Works. Leidy’s writings include “A Flora and Fauna Within Living Animals,” in Smithsonian Contributions to Knowledge, 5 (1853); “The Ancient Fauna of Nebraska, a Description of Extinct Mammalia and Chelonia From the Mauvaises Terres of Nebraska,” ibid.,6 (1854); “Cretaceous Reptiles of the United States,” ibid.,14 (1865); “The Extinct Mammalian Fauna of Dakota and Nebraska …,” which is Journal of the Academy of Natural Sciences of Philadelphia, 2nd ser, 7 (1869); Contributions to the Extinct Vertebrate Fauna of the Western Territories, Report of the U.S. Geological Survey of the Territories (Washington, D.C., 1873); and Fresh-Water Rhizopods of North America, ibid (Washington, D.C., 1879).

MS collections in the Academy of Natural Sciences, Philadelphia, include diaries and correspondence and a 373-page typescript of excerpts from these prepared by Joseph Leidy, Jr., covering the period 1823-1869. Medical correspondence is at the College of physicians, philadelphia.

II. Secondary Lliterature See W. S. W. Ruschenberger, “A Sketch of the Life of Joseph Leidy, M.D., L.L.D …,” in Proceedings of the American Philosophical Society,30 (1892), 135-184, with bibliography; and Joseph Leidy, Jr., ed., “Researches in Helminthology and Parasitology by Joseph Leidy,” in Smithsonian Miscellaneous Collections46 no. 1477 (1904), with bibliography.

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