Even as the evidence in support of his theory mounted, it was difficult for him to relinquish the idea that species don't transform. It took longer for him to realize the difference between the scope of human existence and the history of all life on Earth.
"When you see the world through his eyes, you begin to understand. … He notices everything," Leroi told ABCNews.com.
And those incredible powers of observation are what led to his success.
"You and I look at a mountain and we see nice scenery. … Darwin looks at it and he sees a theory, he sees a need for a theory to account for the [mountain]," Leroi said. "This is a mind that never stops working."
To find out more about the National Geographic Channel's "Darwin's Secret Notebooks," click here.