John Wilkins has posted a link to his essay in Resonance. Go to his blog (Evolving Thoughts), click on the link, read, and enjoy.
I can't think of an essay/article that better captures the essential reason why Darwin made such an important contribution to science. It takes a philosopher to make the case.1
Here's a teaser ... there's much more were this comes from ....
What we remember Darwin for is a synthesis and the empirical support he brought in its defense. He brought together many ideas that were `in the air', so to speak, reading more widely than almost anyone else as well as doing his experimental and anatomical work, and more importantly, managed to filter out most of the bad ideas.
Darwin's achievement was to identify crucial questions and offer a coherent theoretical account that answered them . For instance, in the first half of the nineteenth century, the reasons for the systematic arrangements of plants and animals, why they were arranged 'group with in group' as he put it, was being explored by idealists like William Swainson  and William Macleay , who offered Pythagorean accounts based on similarities and magic numbers. Darwin offered a general account—which we call common descent—that explained why this was a fact, but also why it was not regular (for example, extinction is not evenly distributed across all groups).
The broader point I want to make here is about the nature of science. Often as not, it is the synthesizers who reorganize how we view things, and as David Hull  and others (e.g., Ellegard ) have shown, within ten years of the publication of the Origin, nearly all specialists in the sciences concerned had adopted common descent and transmutation (descent with modification). It was the closest any science has ever come to an actual Kuhnian paradigm shift.
1. Perhaps I should say a "good" philosopher since there are others (Dennett, Ruse) who seem to have missed the point.