Joseph Leidy

The American scientist and educator Joseph Leidy (1823–1891) was one of the most renowned researchers of 19th-century America, publishing frequently and teaching in such disparate fields as paleontology and parasitology. Leidy might be characterized, in the words of his biographer, as “the last man who knew everything.”

Although he is little remembered today, Joseph Leidy made contributions of fundamental importance in numerous areas of science. As biographer Leonard Warren noted, he was “the founder of American vertebrate paleontology, the first to describe the dinosaur in America and give it its present-day configuration. America's love affair with these beasts began with Leidy.” Leidy virtually introduced the study of parasites in the United States, and his 1846 discovery of Trichina spiralis as the cause of the serious disease trichinosis was a milestone in public health. In addition to his work as an educator, Leidy was a noted authority on anatomy, a biologist who helped make the microscope commonplace and in one celebrated case used it in a murder investigation, and an expert in mineralogy, botany, and entomology. His curiosity ran from single-celled organisms to human beings.

Library of Congress, Prints & Photographs Division, Reproduction number LC-USZ62-56058 (bw film copy neg.)

Library of Congress, Prints & Photographs Division, Reproduction number LC-USZ62-56058 (bw film copy neg.)

Spoke German in the Home

Joseph Leidy was born in Philadelphia, Pennyslvania, on September 9, 1823, and, except for four trips to Europe, he rarely left the city and its environs. His father, Philip Leidy, was a hatmaker who spoke primarily German, and his family's home was located next door to his shop. Leidy's mother died in childbirth when he was less than two years old, and he was raised by an aunt. As a boy, he was enrolled in a small private school that taught the classics, but his education was spotty: Leidy preferred being outdoors and collecting plants, animals, or rocks to attending class.

Leidy's childhood experiences formed the basis of what became an encyclopedic knowledge of the natural world. Friends who walked along a beach with him in adulthood reported that he would talk nonstop about everything he saw: mosquitoes, semiprecious stones discovered underfoot, shore grasses, clams and their way of hiding themselves in the sand. According to Warren, one of these friends remarked, “You must be the great original who finds tongues in trees and books in the babbling brooks,” to which Leidy answered: “Yes, I do find sermons in stones and good in everything in nature.”

Was Unhappy as a Physician

Leidy had already begun to find research and teaching enjoyable. Assisting several professors at the University of Pennsylvania, he accompanied them to Europe twice, broadening his horizons in the natural sciences in the process. Leidy became adept at using a microscope, which few Americans were at the time, and in one murder investigation he showed that the microscopic molecular structure of blood found on the clothing of an accused killer could not be chicken blood, as the man had claimed. Soon he began to apply himself in diverse other fields. He studied prehistoric North American horse fossils and showed that horses had been present on the American plains before they were brought by Spanish invaders. His research on snails was also widely admired.

One of Leidy's most important accomplishments came during this early stage of his career: Leidy was virtually the only researcher working on parasites in the United States in 1846, when he identified the trichina spiralis parasite as the cause of what became known as trichinosis. The discovery came about as a result of Leidy's characteristic curiosity: he found a small worm in some ham he was eating for breakfast and decided to put it under the microscope. As he later wrote, cooking meat thoroughly would kill this worm, and this advice has been standard with regard to pork products ever since. Leidy's discovery is thought to have saved many hundreds of lives, but he demurred when friends told him that he should demand credit for his discovery because what mattered most was that this knowledge was gained, not who discovered it. Another Leidy discovery of medical significance was his 1851 transplantation of a human cancer into a frog.

Leidy's interest in discovery for its own sake endeared him to generations of students at the University of Pennsylvania, where he served as professor of anatomy beginning in 1853, and later, at Swarthmore College in the Philadelphia suburbs. His textbook An Elementary Treatise on Human Anatomy (1861) was a standard part of medical curricula for several decades. Leidy was known as kind, supportive of other researchers, and self-effacing, more interested in accurate results than in personal renown. He often said he was too busy to accumulate wealth or to devise abstract theories, preferring instead to describe the natural world in as much detail as he could. Leidy's work schedule was strenuous: he often stayed in his lab until 2 a.m. or awoke at 4:30, working from breakfast at 8 a.m. until dinner at 8 p.m. without even a water break.

Reconstructed Dinosaur Skeleton

In the late 1840s and 1850s Leidy turned his attention to paleontology, the study of prehistoric animals and plants. He was the first American to occupy himself with this field of study, and later generations of scholars would refer to him as the Father of American Vertebrate Paleontology. One of his first paleontology papers, “On the Fossil Horse of America” (1847), provided counterexamples that brought into question the prevailing idea that horses were introduced to the Americas by Spain. He also discussed fossils of many other large animals found in the western United States, including those of lions and tigers. Leidy participated in the naming and reconstruction of a skeleton of the large Hadrosaurus foulkii, the first dinosaur to be publicly displayed, at the Philadelphia Academy of Natural Sciences and he correctly guessed that the animal had stood on two feet. He saw the applicability of dinosaur development to Charles Darwin's theory of evolution, which he supported. Like other backers of Darwin's theory, Leidy faced charges of atheism from religious conservatives.

Leidy mentored the younger paleontologist Edward Drinker Cope, who gained wide renown for his rivalry with fellow scholar Othniel Charles Marsh. As the pair scoured the West for dinosaur fossils and started bidding wars over them so as to be able to claim primacy in their discovery, Leidy became increasingly disillusioned by their antics. Although his foundational contribution to the field was rarely acknowledged by either of Cope or Marsh, he backed Marsh against his former student in a celebrated controversy over a misplaced head on a dinosaur skeleton. His knowledge of anatomy enriched the competition and also led him to many insights that were valuable in the growing field. By the 1870s, however, Leidy decided that he was ill-suited to the demands of celebrity science; he withdrew from paleontology and returned to the study of zoology and parasitology.

Although Leidy was most comfortable in the microscopy lab or writing studio, he served during the U.S. Civil War as a military surgeon at the Union Army's Satterlee General Hospital in Philadelphia. After the war's end in May of 1965, he published a large illustrated monograph, Cretaceous Reptiles of the United States. Another major publication, Leidy's Parasites of the Termites (1881), was compiled between 1875 and 1880 and adorned with 211 drawings. In the last years of his life, Leidy returned to the microscope and increasingly concerned himself with protozoology, the study of single-celled organisms.

Although his work left him with little time to devote to his family, Leidy was married, to Anna Harden, whom he wed in 1864. Anna took a strong interest in his work and assisted with some of his experiments. The couple had no children, but in 1876 they adopted a girl, Allwina Franck, who had been orphaned following the death of one of Leidy's colleagues. That same year, Leidy became director of the Zoological Society of Philadelphia, and he helped to establish the city's first zoo.


Warren, Leonard, Joseph Leidy: The Last Man Who Knew Everything, Yale University Press, 1998.


American Heritage, March 1988, p. 68.

Proceedings of the Academy of Natural Sciences in Philadelphia, Volume 75, 1923, p. 73.


Penn Medicine News online (University of Pennsylvania), http:// (February 17, 2015), “Who Was Joseph Leidy?”

Philadelphia Academy of Natural Sciences website, (October 14, 2016), “Joseph Leidy.”

Public Broadcasting System online, (October 14, 2016), “American Experience: Dinosaur Wars” (video).

Strange Science website, (October 14, 2016), “Joseph Leidy.”❑

(MLA 8th Edition)