When Charles Darwin was a younger man, he did not yet have the full, iconic beard he is generally known for. Instead, he had some mighty fine sideburns. And even as the years rolled by, his theory of evolution developed and his hairline receded, he still maintained only the sideburns.
But, while ill in 1862, he underwent a change that might be considered a form of punctuated equilibrium, and began growing a full beard. By the time he reappeared in public in 1866, it was full and powerful and even though he still had merely the sideburns when he published "On the Origin of Species" in 1859, one hundred and fifty years later the image that is used on the back cover of all subsequent reprints and the vision that leaps to our mind's eye whenever someone mentions the name of Charles Darwin, is as he looked with a fully evolved beard.
So great was his beard, one of the most famous beards in history, that the Natural History Museum in London displayed a lock of it in celebration of his 200th birthday.