Numbers 901-929 in Morton's catalogue are "Thirty Skulls of genuine unmixed NEGROES born in Africa. This interesting series series was collected by Don José Rodriguez Cisnerso, M. D. of Havana, in the island of Cuba, and by him presented to me for the purpose of ascertaining the internal capacity of the cranium in the pure AFRICAN race."

Morton believed that he could define the intellectual ability of a race by the skull capacity. A large volume meant a large brain and high intellectual capacity, and a small skull indicated a small brain and decreased intellectual capacity. He also claimed that each race had a separate origin, and that a descending order of intelligence could be discerned that placed Caucasians at the pinnacle and Negroes at the lowest point, with various other races in between. Considered the origin of scientific racism, this theory provided a "scientific" justification for slavery.

 "Samuel George Morton is often thought of as the originator of "American School" of ethnography, a school of thought in antebellum American science that claimed the difference between humans was one of species rather than variety and is seen by some as the origin of scientific racism.^[6]

"Morton argued against the single creation story of the Bible (monogenism) and instead supported a theory of multiple racial creations (polygenism). Morton claimed the Bible supported polygenism, and within working in a biblical framework his theory held that each race had been created separately and each was given specific, irrevocable characteristics.^[7]

"After inspecting three mummies from ancient Egyptian catacombs, Morton concluded that Caucasians and Negroes were already distinct three thousand years ago. Since the Bible indicated that Noah's Ark had washed up on Mount Ararat, only a thousand years ago before this, Morton claimed that Noah's sons could not possibly account for every race on earth. According to Morton's theory of polygenesis, races have been separate since the start^{[7] " (Wikipedia article on Samuel George Morton, accessed 01-2017).}

Digital facsimile of the 1840 edition from the Internet Archive at this link. Morton continued to develop and expand his collection, which reached 1512 human and animal skulls in the third edition of his catalogue published in 1849. That catalogue contained an introduction, some illustrations, and a complete index. Digital facsimile of the 1849 edition from the Internet Archive at this link.

Morton's last paper on the measurement of cranial capacity and its relationship to intellectual ability may have been "Observations on the size of brain in various races and families of man," Proceedings of the Academy of Natural Sciences, October, 1849.  Digital facsimile from the Internet Archive at this link.