Morton claimed 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 was reputed to hold the largest collection of skulls, on which he based his research. He 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 race groups in between. Morton had many skulls from ancient Egypt, and concluded that the ancient Egyptians were not African, but were Caucasian. His results were published in three volumes between 1839 and 1849: the "Crania Americana", "An Inquiry into the Distinctive Characteristics of the Aboriginal Race of America" and "Crania Aegyptiaca". Morton's skull collection was held at the Academy of Natural Sciences of Philadelphia until 1966, when it was transferred to the Penn Museum, where it is presently curated.