Two centuries ago, arguably the greatest naturalist was born in England. What Democritus did for philosophy, Copernicus for Astronomy and Isaac Newton for Physics, Charles Darwin achieved in the filed of Biology. His unifying theory of natural selection based on spontaneous mutation outlined in the 1859 classic On the Origin of Species has profoundly affected the way we think about the relationships between organisms and environment.
National Geographic has an excellent story and TV show on Darwin's adventures and work.
NPR discusses the cultural controversies surrounding his theory.