We just finished the kickoff lecture of the Darwin150 campaign, "The World Before Darwin", by the wonderful Professor Emeritus Everett Mendelsohn. Credited with being a cofounder of the social history of science, Professor Mendelsohn has taught and wowed students at Harvard for 50 years.
You can download the mp3 file and listen to his great lecture here.
And you can sign-up for the rest of the Darwin150 lecture series here: http://Darwin150.com/events
UPDATE: The video from the webcast is available for viewing here.