I was very glad of your P.S. on the state of your country; one values a private note far more than a dozen public letters. After carefully reading Olmstead's last Book I never doubted the North would conquer the South. But then what is to follow? From Olmstead & Russell's letters in Times, I cannot believe that the South would ever have fellow-feeling enough with the North to allow of government in common. Could the North endure a Southern President? The whole affair is a great misfortune in the progress of the World; but I shd not regret it so much, if I could persuade myself that Slavery would be annihilated. But your president does not even mention the word in his Address.— I sometimes wish the contest to grow so desperate that the north would be led to declare freedom as a diversion against the Enemy. In 50 or 100 years your posterity would bless the act.— But Heaven knows why I trouble you with my speculations; I ought to stick to Orchids.
The president in question was Darwin's twin, Abraham Lincoln; his address was before a special session of the United States Congress on 4 July 1861.
The North, it turned out, could indeed endure a Southern president. How much more surprised (and, I presume, pleased) would Darwin have been to learn that the South would one day accept a black president?
See also: Books review: Darwin's Sacred Cause