Morton believed that people could be divided into five races and that these represented separate acts of creation. The races had distinct characters, which corresponded to their place in a divinely determined hierarchy. Morton's "craniometry" showed, he claimed, that whites, or "Caucasians," were the most intelligent of the races. East Asians--Morton used the term "Mongolian"--though "ingenious" and "susceptible of cultivation," were one step down. Next came Southeast Asians, followed by Native Americans. Blacks, or "Ethiopians," were at the bottom. In the decades before the Civil War, Morton's ideas were quickly taken up by the defenders of slavery.

"He had a lot of influence, particularly in the South," says Paul Wolff Mitchell, an anthropologist at the University of Pennsylvania who is showing me the skull collection, now housed at the Penn Museum. We're standing over the braincase of a particularly large-headed Dutchman who helped inflate Morton's estimate of Caucasian capacities. When Morton died, in 1851, the Charleston Medical Journal in South Carolina praised him for "giving to the negro his true position as an inferior race."

Today Morton is known as the father of scientific racism. So many of the horrors of the past few centuries can be traced to the idea that one race is inferior to another that a tour of his collection is a haunting experience. To an uncomfortable degree we still live with Morton's legacy: Racial distinctions continue to shape our politics, our neighborhoods, and our sense of self.

This is the case even though what science actually has to tell us about race is just the opposite of what Morton contended.

Morton thought he'd identified immutable and inherited differences among people, but at the time he was working--shortly before Charles Darwin put forth his theory of evolution and long before the discovery of DNA--scientists had no idea how traits were passed on. Researchers who have since looked at people at the genetic level now say that the whole category of race is misconceived. Indeed, when scientists set out to assemble the first complete human genome, which was a composite of several individuals, they deliberately gathered samples from people who self-identified as members of different races. In June 2000, when the results were announced at a White House ceremony, Craig Venter, a pioneer of DNA sequencing, observed, "The concept of race has no genetic or scientific basis."