The teleological debate over human unity or racial diversity that convulsed the science of man after 1750 took its most extreme shape in the hostile opposition of the doctrines known from the mid-nineteenth century as 'monogeny' and 'polygeny'. Did all humanity comprise a single species with common ancestry (monogeny), as neoclassical cosmology assumed and the Church insisted? Or did the present existence of (apparently) morphologically distinct groups signify human descent from more than one independent set of ancestors (polygeny), as popular and scientific opinions increasingly maintained? Arguments for multiplicity flourished, especially in France and the United States, usually in tandem with harsh racial attitudes. These arguments in turn provoked vociferous defence of the orthodox position, especially in Britain where negative racial attitudes were nonetheless widespread.

Such debates were not entirely unprecedented. In the sixteenth and seventeenth centuries, scattered challenges to Biblical dogma on the unitary descent of man were repressed as heretical, notably that of the French deist Isaac de La Peyrère (1596-1676) who was forced to recant his theory of pre-Adamite creations (1655). An empirical case for inherent racial or specific differences between extended human groups was put by Bernier (1684:148 150) whose use of the term race is in some respects decidedly modern. In an anonymous article, he proposed a classification into 'four or five Species or Races of men whose difference is so notable' — and 'essential' or innate in the case of Africans — 'that it can justly serve as the basis for a new division of the Earth'. But this radical argument was largely ignored at the time and if Bernier anticipated the eighteenth-century natural history of man, he seemingly had little direct conceptual influence on its emergence.^[51]

During the eighteenth century, a few sceptical philosophers — notably the Frenchman Voltaire (1694-1778) and the Scots David Hume (1711-1776) and Henry Home, Lord Kames (1696-1782) — contested the prevailing consensus on human unity by projecting the 'perceptible difference in the species of men inhabiting the four known parts of our world' back to nature's 'original' differentiation of plural human 'breeds' or 'species'.^[52] The African slave trade, unsurprisingly, spawned polemical judgments on the matter by opponents and supporters alike. The naval surgeon John Atkins (1685-1757) served on the Guinea coast in the 1720s and was a strong critic of slavery, if no admirer of the 'Way of Living' and mental abilities of 'the Africans'. He nonetheless dismissed the capacity of climate to effect 'this remarkable division of Mankind into Blacks and Whites' and pronounced the opinion that 'White and Black must have descended of different Protoplasts', that they had 'ab origine, sprung from different-coloured first Parents'. Atkins's scientific credentials render his (professedly 'a little Heterodox') verdict especially relevant to this study. In a notably venomous work, the Jamaica plantation owner, historian, and apologist for slavery Edward Long (1734-1813) opined that 'the White and the Negroe had not one common origin'. He concluded that 'the nature of these men, and their dissimilarity to the rest of mankind' proved that Negroes were 'a different species of the same genus'.^[53] It was against the looming threat of such heterodoxy and the atrocities of Negro slavery that Buffon, Blumenbach, and Kant variously sought a scientific resolution to the problem of human diversity without fatal compromise to the established principle of the common origin of the single human species (Zammito 2006). Kant (2001:3, 12) did so pragmatically, invoking the principle of economy in explanation — why posit 'many local creations' and thereby 'unnecessarily duplicate the number of causes'? Blumenbach (1795:73) concurred but the issue for him was primarily ethical.

Phillip Sloan (1995:123, 133, 135) considered a 'slide into polygenism' to be a 'persistent implication' in Linnaean natural history from the tenth edition of Systema Naturae (1758) which posited more than one species of the genus Homo. In the 1780s, the German doctor and anatomist Samuel Thomas Soemmerring (1755-1830) — in a work dedicated to his friend Georg Forster — consigned 'the Moors' (Africans) to 'a lower echelon at the throne of mankind' and produced a catalogue of significant anatomical differences between Europeans and Negroes from which he inferred that 'the brain of a Negro is smaller' (1785:xi, 49-67). Blumenbach (1790:62‑78) criticized the crude biological determinism of Soemmerring's movement from anatomy to intellect but Forster, who took a consistently morphological approach, professed admiration for this 'physiological and anatomical' proof for 'the corporeal difference of Negroes from Europeans'.^[54] In graphic illustration of the liaison of ancient bigotry with a new biology, Forster brought the visual evidence of 'appearance' together with Soemmerring's anatomical argument to speculate that 'the Negro' might be 'a second human species' and 'an originally different stock' from 'white men'.

At the very end of the eighteenth century, the English surgeon and anatomist Charles White (1728-1813) argued for a 'gradation from the European man down to the ape' and located 'the African' much 'nearer to the ape'. He included an appendix of translations of lengthy extracts from Soemmerring's text. White challenged climatic or life style explanations for human variation and concluded that 'material differences in the corporeal organization', hair, and skin colour of 'various classes of mankind' proved that 'various species of men were originally created and separated, by marks sufficiently discriminative': 'the Negro, the American, some of the Asiatic tribes, and the European' were thus 'different species'.^[55] White was arguably the earliest polygenist — though the term itself was well in the future — because he grounded his case systematically in comparative anatomy.^[56] He was also the last British savant for nearly half a century to profess openly a belief in plural human species.