Polygenism claimed racial groups had separate origins and ranked them by skull size.

Evidence

Morton was devoted to the classification and ranking of human beings based on race. He was an ardent believer in polygenism, the theory that racial groups do not share a common origin and are thus fundamentally and categorically distinct. … Morton claimed he could place racial groups in a descending order of intelligence by measuring the interior capacity of human skulls. His methodology was a sham, but it conveniently “confirmed” Agassiz’s newfound prejudices by placing black people at the very bottom of the racial hierarchy. — Tamara Lanier, From These Roots, 1037