Usually on Darwin day I spend some time leafing through "On the Origin of Species" or "Voyage of the Beagle", or any other of his writings, to find an aspect about Darwin that is new to me. Here are my entries from 2012 - Darwin on cruelty - and 2011 - Mockingbirds and Darwin's original thought. But this year, since I don't have much time over and I've just finished writing my PhD thesis, I thought I'd share a quick quote from "Origin" that I used in the introduction.
We have formerly seen that parts, many times repeated, are eminently liable to vary in number and structure; consequently, it is quite probable that natural selection, during a long-continued course of modification, should have seized on a certain number of the primordially similar elements, many times repeated, and have adapted them to the most diverse purposes. And as the whole amount of modification will have been effected by slight successive steps, we need not wonder at discovering in such parts or organs a certain degree of fundamental resemblance, retained by the strong principle of inheritance.
This is from chapter XIII of "Origin", where Darwin compares the morphologies of related structures between organisms - like the human hand, the wings of bats and the flippers of dolphins - and recognizes the result of common origin and descent with modification. The brilliance of this quote is that Darwin is hinting at a significant mechanism of evolutionary innovation that he couldn't possibly have known anything about - the duplication of genes. Duplication, be it of individual genes or of the entire genome, is a force to be reckoned with in evolution since it generates new genetic "raw material" that mutation and selection can act on to generate new gene functions. Or as Darwin put it - "it is quite probable that natural selection, during a long-continued course of modification, should have seized on a certain number of the primordially similar elements, many times repeated, and have adapted them to the most diverse purposes."
Darwin knew nothing about genes or genetics, yet we can find numerous hints to a modern understanding of biology in Darwin's evolutionary theory. Here he is talking about repeated morphological structures such as vertebrae in the spinal column or the whorls of a flower, but the same applies for duplicated genes.