Funny you should ask that, cause I don’t really know the answer these days.
The simple answer would be his entire theory of how life developed on Earth across time, i.e. evolution by natural selection; life began as simple organisms, the lifeforms that were most suited to their environment survived, while the less suited ones died out, and then over long period of time, generation by generation, some species developed into new species, resulting in the plants and animals we currently have today.
There are some things I agree with in that without question. The fact that all living things die, and that those living things which are more suited to their environments are more likely to survive until they reproduce, is undeniable. Also, the fact that there is genetic diversity within species, such that every member within a species is slightly different, (i.e. skin colour, size, shape, number of teeth, etc.) is clearly true. These things are fundamental to Darwin’s theory, but not controversial.
What’s more controversial is whether or not genetic variation plus time is capable of resulting in an entirely new species. It’s very clear that just about all the scientists and laymen in the world believe this now. It has become a true pillar of scientific (and virtually all intellectual) thought these days. This means that either all scientists have been severely led astray or that all creationists are in severe denial.
Basically, my first thought is that, even as a child, the first 3 chapters of Genesis always presented themselves to me as allegorical. I never felt that the writer expected me to believe that the events it describes happened in history, but rather that they were communicating something much deeper and more spiritual than historical. I also had problems with the literal story that took place, such as God’s plan to create all humanity through one man and one woman: such a plan requires incestuous activity, which God doesn’t like.
So given these things I find it very appropriate to take the spiritual information that Genesis gives me, and take my historical information about the descent of man from whatever it is that the scientists discover.
However I’m much less certain about evolution than I used to be. Certain Biblical questions that arise out of taking Genesis 1-3 metaphorically are quite difficult to answer. When in the narrative does the metaphor cease, and the history begin? Genesis says that the world was good when God created it, then after Adam and Eve ate the apple, death came into being: how does this fit in a world where death was an evolutionary requirement for the coming into being of humanity? i.e. how did death exist before sin (where sin is supposed to be a human action).
On top of this, I haven’t directly accessed the evidence for evolution. So I haven’t ever been directly convinced of this scientific theory, I’ve only been convinced by proxy – my trust that scientists know what they’re doing. And the question is, are these scientists reliable?
Before the theory of evolution came about, Atheism was a much more difficult position to hold up. The atheist had to somehow account for the amazing biological diversity found in the world, without a designer to attribute it to. Before evolution they didn’t really have an account, and atheism wouldn’t have gelled very comfortably with this lack. So then for the idea of evolution by natural selection to come about would have been very good news. Suddenly the atheist had a story for the appearance of biological diversity, including humans. Suddenly they could explain life without a designer. What this means is that, no matter how good the evidence was, evolution is a very, very attractive theory to an atheist; it removes the need for God.
So even if the evidence for evolution, which I haven’t been properly exposed to, isn’t very good, I can still understand why largely atheistic scientific communities would have flocked to such an idea – just out of their desire to retain their atheism, in conjunction with their powerful desire to not be utterly perplexed by the world. All this means is that the fact that there is such a consensus about evolution among scientists is not necessarily a testament to the theory’s truth.
So my verdict? Currently pretty confused and unsure. I haven’t seen the evidence for evolution myself, and I have heard it challenged by some scientists (how reliable these scientists are, I don’t know). If I haven’t fully explained myself, hit me back and I’ll try to fill in my gaps.