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.[9] 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.[9] 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.[10]