In the first comprehensive biography of Joseph Leidy, Leonard Warren surveys Leidy’s wide-ranging scientific work. Although Leidy may not have known quite [End Page 715] everything, his work did range through the whole realm of zoology, with occasional excursions into botany, and he made detailed studies of a great variety of organisms, both living and fossil.
As a boy, Leidy was fascinated by the natural world. He spent much time exploring the countryside around his Philadelphia home, collecting and studying animals and plants. In 1840, at the age of seventeen, he entered the University of Pennsylvania Medical School. There he studied anatomy under the brilliant Paul B. Goddard, who introduced him to the new achromatic compound microscope that was then revealing the microscopic world with striking clarity. In the winter of 1842 Leidy attended Charles Lyell’s lectures on geology at the Philadelphia Museum. The lectures aroused Leidy’s interest in paleontology, and through Goddard he met Lyell. They became lifelong friends.
On receiving his medical degree in 1844, Leidy became prosector to the professor of anatomy at the University of Pennsylvania, William Horner. In that year he also contributed a chapter, illustrated by sixteen beautifully detailed drawings of the anatomy of snails, to Amos Binney’s The Terrestrial Air-Breathing Molluscs of the United States. The work, together with Leidy’s papers on new species of Tertiary fossil shells, resulted in his election at the age of twenty-two to both the Boston Society of Natural History and the Academy of Natural Sciences of Philadelphia. The latter body, of which he became librarian, curator, and ultimately president, was Leidy’s scientific home. At the Academy in 1847 he identified certain fossil teeth as belonging to two extinct species of horse—one known previously from South America, the other new. The fossils showed that horses had lived in North America during the Pleistocene period, becoming extinct long before the arrival of Europeans.
Leidy’s identification of fossil horses was the earliest of his long series of important discoveries in paleontology. Collectors throughout the country began to send him fossil bones, and by 1853, when he was appointed professor of anatomy at the University of Pennsylvania, he had published fifteen papers on them. The same year he published his classic work, The Ancient Fauna of Nebraska, in which he showed that in addition to the horse, species of lion, tiger, camel, and rhinoceros had formerly lived in the American West. Leidy laid the foundations of American vertebrate paleontology on which Edward Cope of Philadelphia and Othniel Marsh of Yale were to build so successfully.
Much of Leidy’s zoological work was done with the microscope. In 1846 he identified Trichina spiralis in pork, showing thereby how this dangerous parasite might be transmitted to human beings. In 1853, his annus magnus, he published, in addition to his work on Nebraska fossils, his book A Flora and Fauna within Living Animals—a comprehensive treatise on parasitism. In 1879 he published a classic work on freshwater rhizopods, the group of protozoa that includes Amoeba proteus, so familiar to beginning students of biology.
Leonard Warren is troubled by two anomalies in Leidy’s otherwise massive achievement: Leidy did not advance biological theory, and he did not become an experimental biologist. The latter anomaly is the easier to understand. It is difficult to do experiments on organisms of whose structure, relationships, and [End Page 716] life history you are ignorant. In Leidy’s lifetime, the horizons of the living world were expanding in both space and time, through geographical exploration and paleontology, while the microscope revealed a new living world of boundless richness and variety. Joseph Leidy’s great contribution was to reveal portions of such previously unknown natural worlds in exact and beautiful detail. If biologists today tend to depreciate descriptive natural history, they continue to rely on the work of such naturalists of the past as Leidy.
In regard to the first anomaly, Warren shows that Leidy’s avoidance of theory reflected in part his...