I'm currently reading a pre-publication copy of Darwin's Sacred Cause: Race, Slavery and the Quest for Human Origins by Adrian Desmond and James Moore [postscript: review here] . The book describes Charles Darwin's abhorrence of slavery, and how it might have influenced his evolutionary thinking.
In the final chapter of The Voyage of the Beagle, in one of his most outspoken pieces of published writing, the young Darwin wrote:
On the 19th of August  we finally left the shores of Brazil. I thank God, I shall never again visit a slave-country. To this day, if I hear a distant scream, it recalls with painful vividness my feelings, when passing a house near Pernambuco, I heard the most pitiable moans, and could not but suspect that some poor slave was being tortured, yet knew that I was as powerless as a child even to remonstrate […]
[W]hat a cheerless prospect, with not even a hope of change! picture to yourself the chance, ever hanging over you, of your wife and your little children—those objects which nature urges even the slave to call his own—being torn from you and sold like beasts to the first bidder! And these deeds are done and palliated by men, who profess to love their neighbours as themselves, who believe in God, and pray that his Will be done on earth! It makes one's blood boil, yet heart tremble, to think that we Englishmen and our American descendants, with their boastful cry of liberty, have been and are so guilty: but it is a consolation to reflect, that we at least have made a greater sacrifice, than ever made by any nation, to expiate our sin.
From the comfort of the complaisant Twenty-First Century, it is easy for us to shake our heads in astonishment that slavery continued for as long as it did, not being officially abolished in Brazil until 1888:
And then, we turn to today's Guardian and read:
Brazilian authorities rescued more than 4,500 slaves from captivity last year, carrying out a record number of raids on remote ranches and plantations, according to figures released this week by the country's work ministry.
The government said its anti-slavery taskforce, a roaming unit designed to crack down on modern-day slavery, had freed 4,634 workers from slave-like conditions in 2008. The taskforce, which often works with armed members of the federal police, said it had undertaken 133 missions and visited 255 different farms in 2008. The ministry said former slaves had been paid £2.4m in compensation.
One-hundred and seventy-three years after Charles Darwin gratefully left his final slave-country, we've still got an awfully long way to go.