were responsible for the increased attempts at European colonization of Africa during the 19th century. In his 1969 book The Atlantic Slave Trade: A Census, Curtin researched the sources of frequently used estimates of the number of individuals transported across the Atlantic Ocean by the slave trade. His analysis of shipping contracts and data from the ports of entry enabled him to estimate between 9 and 10 million individuals being transported on slave ships, with a margin of error of 20%, out of the 20 to 30 million that had been loaded aboard at ports in Africa. Prior to Curtin's research, estimates of the number of individuals brought from Africa as slaves ranged from 3.5 million to numbers as high as 100 million individuals. A widely cited number of 15 million slaves used by W. E. B. Du Bois, who had gotten the number from abolitionist Edward Dunbar. Another widely quoted estimate of 20 million slaves was based on calculations using data concerning slaves in Jamaica that was adjusted for the entire Atlantic slave trade, though the original data used to make the calculations has since been lost. His 1989 book Death by Migration combined medical and population history, tracing the effects of tropical diseases on Europeans in tropical Africa during the time before medicines were available to treat these conditions effectively. A controversial opinion piece published in a 1995 issue of The Chronicle of Higher Education titled "Ghettoizing African History" criticized the frequent equation of African and African American scholars in college and university departments of history with jobs concerning the history of Africa. Although Curtin mentioned that this practice might discourage some caucasian academicians from specializing in African studies, his comments were also an argument for more opportunities for African-American scholars. While many visitors to Africa have been to Gorée Island in Senegal, described as a site where as many as 20 million Africans were fattened for shipment across the Atlantic Ocean from the Slave House after being shackled there in dank cells, Curtin debunked the traditional account, stating that "[t]he whole story is phony". Curtin stated that the Slave House, one of the most beautiful houses on the island, would not have been used for storing slaves, that the rocks near the shore would make docking boats perilous and estimated that a total of no more than 50,000 slaves had passed through the island. Senegalese academics criticized