Darwin isn't really bad. The Descent of Man, being less abstract, is even more readable. It even has lots of cool pictures.
The real delight among old-timey writers for me is Gibbon, he of The Decline and Fall of the Roman Empire. My biggest complaint of him is he deals with truly sexy material by hinting at the content in English, then putting the expansion in a quote from his original source in the original language (typically Latin or Greek) in a footnote for his scholarly readers. Anyone educated enough to be able to read the footnotes was presumed not to be a bluenose moralist and thus able to deal with the material without clamoring to get him banned like jec41.
Gibbon and Darwin are both great writers. However, the cheering Gibbon gives to the persecution of Christians by Marcus Aurelius (in other respects a great man) is pretty shameful. His theories about the fall of the Empire are original and true, but where they are true (the loss of civic virtue) they are unoriginal and where original (Christianity killed the Empire) they are not true. Still, Decline and Fall makes for great reading, as you say. I wish "Gladiator" has stuck closer to the facts therein.