Why are Lincoln's and Darwin's influences still alive and kicking?

Many reasons.

But here is a particularly charmingly put one:

The things that intrigued and worried them and made them stay up nights were the same things that most other intelligent people in their day worried about, the same kinds of things that keep us up nights, too. An entire mountain range of minds rises between them and around them, most of the rest submerged by history. But they are high peaks within it, and they look out toward each other. And from on top of one you can see the other. They are still above water because what they made of those worries was something big, a permanent mountain of meaningful anxiety.

-- Adam Gopnik, Angels and Ages

[Bold mine. Less elegant as it goes, but sometimes the awkward bits say it best--also the bolded part made me laugh.]