In publications issued from 1735 to 1759, Linnaeus classified all the then-known animal forms. He included humans with the primates and established the use of both genus and species terms for identification of all animals. For the human species, he introduced the still-current scientific name Homo sapiens. He listed four major subdivisions of this species, H. americanus, H. africanus, H. europaeus, and H. asiaticus. Such was the nature of knowledge at the time that Linnaeus also included the categories H. monstrosus (which included many fantastical peoples) and H. ferus (“wild man”), an indication that some of his categories were based on tall tales and travelers’ myths.

Blumenbach divided humankind into five “varieties” and noted that clear lines of distinction could not be drawn between them, as they tended to blend “insensibly” into one another. His five categories included American, Malay, Ethiopian, Mongolian, and Caucasian. (He chose the term Caucasian to represent the Europeans because a skull from the Caucasus Mountains of Russia was in his opinion the most beautiful.) These terms were still commonly used by many scientists in the early 20th century, and at least one continued into the 21st century as a major designation of the world’s peoples.

These classifications not only rendered human groups as part of nature but also gave them concreteness, rigidity, and permanence. Moreover, some descriptions, especially those of Linnaeus, included statements about the temperament and customs of various peoples that had nothing to do with biophysical features but were forms of learned behavior that are now known as “culture.” That cultural behavior and physical characteristics were conflated by these 18th-century writers reflects both their ethnocentrism and the limited scientific knowledge of the time.

The institutionalizing of race

Slavery always creates social distance between enslavers and the enslaved, and intellectuals are commonly called upon to affirm and justify such distinctions. As learned men began to write a great deal about the “racial” populations of the New World, Indigenous Americans and Black Africans were increasingly projected as alien. In this way did some Enlightenment thinkers help pro-slavery interests place responsibility for slavery in the “inferior” victims themselves.

Would-be “scientific” writings about the distinctiveness of Black and Indigenous people commenced late in the 18th century in tandem with exaggerated popular beliefs, and writings of this type continued on into the 20th century. The European world sought to justify not only the institution of slavery but also its increasingly brutal marginalization of all non-European peoples, enslaved or free. Science became the vehicle through which the delineation of races was confirmed, and scientists in Europe and America provided the arguments and evidence to document the inferiority of non-Europeans.

About the turn of the 19th century, some scholars advanced the idea that Black people (and perhaps Indigenous people) constituted a separate species from “normal” men (white and Christian), an idea that had been introduced and occasionally expressed in the 18th century but that had drawn little attention. This revived notion held that the “inferior races” had been created at a different time than Adam and Eve, who were the progenitors of the white race. Although multiple creations contradicted both the well-known definition of species in terms of reproductively isolated populations and the biblical description of creation, it is clear that in the public mind the transformation from race to species-level difference had already evolved. In the courts, statehouses, assemblies, and churches and throughout American institutions, race became institutionalized as the premier source, and the causal agent, of all human differences.

Transforming “race” into “species”

One of those whose direct experience of enslaved Africans and assessment of them was given great weight was Edward Long (1734–1813), a former plantation owner and jurist in Jamaica. In a book titled The History of Jamaica (1774), Long asserted that “the Negro” was “void of genius” and “incapable” of civilization; indeed, he was so far inferior as to constitute a separate species of mankind. Long’s work was published as a defense of slavery during a period of rising antislavery sentiment. Its greatest influence came during and after the American Revolutionary War (1775–83), when some southern Americans started freeing their enslaved workers and moving north. Long’s writings, published in popular magazines, were widely read in the United States during the last decade of the 18th century.

In 1799 Charles White, a Manchester physician, published the earliest proper “scientific” study of human races. He described each racial category in physical terms, identifying what he thought were differences in the head, feet, arms, complexion, skin color, hair texture, and susceptibility to disease. White actually measured the body parts of a group of Black and a group of white individuals, lending the semblance of hard science to his conclusions. He not only advocated a gradation of the races, but he provided support for the speculation that Black people, the American Indian, some Asiatic tribes, and Europeans were of different species. His explanation for the presumed savagery of Africans was that they had degenerated from the pure and idyllic circumstances provided in the Garden of Eden while Europeans had made advances toward civilization.

Such works as those of Long and White initiated a debate among scholars and scientists that had long-range implications for European attitudes toward human differences. The issue, as expressed by mid-19th-century scientists, was “the Negro’s place in nature”—that is, whether “the Negro” was human like Europeans or a separate species nearer to the ape.

Samuel Morton, a Philadelphia physician and founder of the field of craniometry, collected skulls from around the world and developed techniques for measuring them. He thought he could identify racial differences between these skulls. After developing techniques for measuring the internal capacity of the skull, he concluded that Black Africans had smaller brains than white Europeans, with Indigenous American brains intermediate between the two. Because brain size had long been correlated with intelligence in both the popular mind and science, Morton’s findings seemed to confirm that Black individuals were also less intelligent than white ones. In publications of 1839 and 1844, he produced his results, identifying the Native Americans as a separate race from Asians and arguing from his Egyptian materials that these ancient peoples were not Black. His findings magnified and exaggerated the differences between racial populations, imposing meaning on the differences that led to the conclusion that they were separate species.

Morton soon became the center of a network of scholars and scientists who advocated multiple creations (polygeny) and thus contradicted the long-established biblical view of one single creation from which all humans descended (monogeny). The most influential of the scientists involved in this debate was Louis Agassiz, who accepted a position at Harvard University and revolutionized the field of natural science. Agassiz converted from monogenism to polygenism after moving to the United States from Switzerland in 1846. It was then that he saw Black people for the first time. He was also impressed with Morton’s work with skulls, and eventually he became the most important advocate of polygenism, conveying it in public lectures and to generations of students, many of whom took leading intellectual roles in American society.

One result of the mid-19th-century concern with documenting racial distinctions by means of body measurements was the establishment of the “scientific” enterprise of anthropometry. During the Civil War the U.S. Sanitary Commission and the provost marshall general’s office collected data on the physical condition of military conscripts and volunteers in the army, navy, and marines. Using anthropometric techniques, they produced massive tables of quantitative measurements of the body dimensions of tens of thousands of whites, Blacks, mulattoes, and Indians. Scientists interpreted the data in a way that strengthened the argument that races were fundamentally distinct and that confirmed that Blacks, Indians, and mulattoes were inferior to whites. Anthropometry flourished as a major scientific method for demonstrating race differences well into the 20th century.

The false assumptions of anthropometry

For the second half of the 19th century, scholars continued to debate, as they termed it at the time, “the Negro’s place in nature.” But the debate over multiple or single origins receded after 1859, when the publication of Charles Darwin’s theory of evolution led to a more dynamic understanding of human diversity. Evolution produced a new perspective on the causes of Black people’s (supposedly) innate condition; the central problem became whether they evolved before or after white Europeans. By the 1860s Black primitiveness was assumed without question. “The Negro,” in fact, had become the new savage, displacing Indigenous peoples and the Irish, and the ideology proclaimed that his savagery was intrinsic and immutable.

The use of metrical descriptions, while they seemed objective and scientific, fostered typological conceptions of human group differences. From massive quantitative measurements, experts computed averages, means, and standard deviations from which they developed statistical profiles of each racial population. These profiles were thought to represent the type characteristics of each race expressed in what seemed to be impeccable scientific language. When statistical profiles of one group were compared with those of others, one could theoretically determine the degree of their racial differences.

The activities of typologists carried a number of false assumptions about the physical characteristics of races. One was that racial characteristics did not change from one generation to another, meaning that averages of measurements such as body height would remain the same in the next generations. Another false assumption was that statistical averages could accurately represent huge populations, when the averaging itself obliterated all the variability within those populations.

Expressed alongside existing myths and popular racial stereotypes, these measurements inevitably strengthened the assumption that some races were “pure” and some not so “pure.” Scholars argued that all the major races were originally pure and that some races represented the historical mixing of two or more races in the past. “Racial types” were conceived as representing populations with certain inherited morphological features that were originally characteristic of the race; every member of a race thus retained such traits. These beliefs attempted to validate the image of races as internally homogeneous and biologically discrete, having no overlapping features with other races.