(Previous in the series: Introduction, Overview, On Species, Peculiarities, More Peculiarities. Full text of Cartwright’s paper can be found here in three parts on pages 64, 209, 331 and with a response to critics on page 504.)
We left Cartwright after having weighed his claims about the skeletons of black people. Those differences would matter, had they withstood scrutiny. They instead told a story of the physical cost of slavery. But Cartwright’s thesis rests less on physical differences between black and white and more on mental differences. He insisted, after all, that black people resisted and ran from slavery due to mental illnesses, Dysaethesia Aethiopica and Drapetomania, respectively. Facts about skeletal arrangement might support his position that black and white made two species, but the key peculiarities for Cartwright’s position reside above the neck:
According to Soemmerring and other anatomists, who have dissected the negro, his brain is a ninth or tenth less than in other races, his facial angle smaller, and all the nerves going from the brain, as also the ganglionic system of nerves, are large in proportion than in the white man. […] Soemmering remarks, that the negro’s brain has in a great measure run into nerves.
In Cartwright’s time as in our own, if not to the same degree or in the same way, biology understood that brains at the very least had a great deal to do with minds. If black people truly had smaller brains and a different ratio of neurons in the central nervous system vs. the peripheral nervous system, one would expect mental differences. Different species do in fact have different arrangements of neurons. Cephalopods have something like the distributed nervous system that Soemmering, per Cartwright, assigned to black people so we can at least say he hasn’t asserted a natural impossibility.
The arrangement of the nervous system, like the arrangement of the skeleton, does not leave much room for speculation. Dissection, as Cartwright notes, would reveal the differences to anybody and settle the matter at once. Soemmerring did the dissection.
Cartwright goes on to say
His [a black person’s] imitative powers are very great, and he can agitate every part of the body at the same time, or what he calls dancing all over. [Italics in original.] From the diffusion of the brain, as it were, into the various organs of the body, in the shape of nerves to minister to the senses, everything, from the necessity of such a conformation, partakes of sensuality, at the expense of intellectuality. Thus music is a mere sensual pleasure with the negro.
Cue the jokes about black people having rhythm that whites lack. I leave finding impressive dance performances by people of all races as an exercise to the reader. Cartwright made no such jokes, but continues
a deficiency of cerebral matter in the cranium, and an excess of nervous matter distributed to the organs of sensation and assimilation, that is the true cause of that debasement of the mind which has rendered the people of Africa unable to take care of themselves.
The conclusion follows from the premise: One would expect people with smaller brains and radically different nervous systems to, at the very least, have very different behavior. Cartwright rolls together his diagnosis and the position that slavery uplifts black people at the same time. They, the poor, unfortunate species, need us to help them take care of themselves and in exchange they help us by filling our pockets and generously provide us with a population we can beat, whip, mutilate, rape, and sell at will. Everyone “wins”.