By the 19th century, the scientific debate focused on whether human biological difference was just a racial variation, or represented an entirely different species. The "species" theory, polygenism, held that human "races" were of different lineages and suggested a hierarchy outlined in the "Chain of Being" that positioned Africans between man and lower primates.
Polygenism was the antithesis of monogenism, which espoused a single origin theory of humanity consistent with the Bible. Ironically, proponents of slavery were, for the most part, monogenists, because polygeny was incompatible with the Bible.
Edward Long published History of Jamaica in 1774 in England; excerpts were reprinted in the U.S. in 1788. In History of Jamaica, Long compared blacks to animals and outlined a racial hierarchy where blacks were situated between Europeans and orangutans. Long, along with Dr. Charles Whites' Account of the Regular Gradation in Man in 1799, provided the "empirical science" for the species theory. White defended the theory of polygeny by refuting French naturalist George Louis de Buffon's interfertility argument—the theory that only the same species can interbreed—pointing to species hybrids such as foxes, wolves and jackals, which were separate groups that were still able to interbreed.
Naturalist Charles Pickering was the librarian and a curator of the Academy of Natural Sciences. In 1843, he traveled to Africa and India to research human races. In 1848, Pickering published Races of Man and Their Geographical Distribution, which enumerated eleven races.
In the 1820s and 30s, a Philadelphia physician named Samuel G. Morton collected and measured hundreds of human skulls in order to confirm that there were differences among the races — in particular, a difference in brain size. His systematic large-scale experiments made him a pioneer of American race science and physical anthropology. Morton was a proponent of polygenism, which theorized that the different races were different species, with separate origins. Morton amassed a large collection of human skulls from all around the world. He believed he could identify any skull's racial origin simply by measuring it, and developed tables based on his experiments, which involved pouring lead pellets into skull cavities. Morton assigned the highest brain capacity to Europeans—with the English highest of all. Second was the Chinese, third was Southeast Asians and Polynesians, fourth was American Indians, and the smallest brain capacity was assigned to Africans and Australian aborigines. The collection of skulls is now in the museum of the Philadelphia Academy of Natural Sciences. He wrote Crania Americana (1839), An Inquiry into the Distinctive Characteristics of the Aboriginal Race of America and Catalogue of Skulls of Man (1840), and Crania Egyptica (1844). Although Morton was a scientist, he used his influence to make the case for black inferiority to bolster U.S. Secretary of State John Calhoun's efforts to negotiate the annexation of Texas as a slave state. Calhoun was a pro-slavery advocate from South Carolina.
In 1854, Josiah Clark Nott published his racial theories in a book of essays written with George Robins Gliddon, an Egyptologist and follower of Samuel George Morton, entitled Types of Mankind or Ethnological Research. It popularized the polygenist theory. In 1856, Nott and Confederate propagandist Henry Hotze translated Arthur Gobineau's 1853 essay on racial inequality, but with significant omissions and distortions.
In the wake of Types of Mankind, abolitionists and for the first time, African American scholars, engaged in the race science debate. In the political discourse leading up to the Civil War, prominent statesman and abolitionist Frederick Douglass challenged the leading theorists of the American School of Anthropology, which included Nott, Gliddon, Agassiz and Morton, among others. Work by early "race scientists" tried to prove that blacks were not the same species as whites, and alleged that the rulers of ancient Egypt were not Africans. In Douglass' 1854 address, "The Claims of the Negro Ethnologically Considered," he argued that "by making the enslaved a character fit only for slavery, [slaveowners] excuse themselves for refusing to make the slave a freeman ...."