Darwin's three ideas are these:
- Most fundamentally, that it is possible to come up with a scientific explanation of how species change. (This is actually the second argument in the book, but it is the most important.) Although most of the book is a massing of evidence in favour,he takes one or two moments to tilt against any idea that species originate through acts of supernatural creation. "To admit this view is, as it seems to me, to reject a real for an unreal, or at least for an unknown ,cause. It makes the works of God a mere mockery and deception; I would almost as soon believe with the old and ignorant cosmogonists, that fossil shells had never lived, but had been created in stone so as to mock the shells now living on the sea-shore."
- Darwin works up to this gradually, reviewing in considerable detail how humans change domesticated species of both animals and plants through artificial selection. He is not in general as lucid a writer as Gould, or even Dawkins. But the first four chapters on their own set out a compelling case that the answer to the question posed by the book's title is that species generally originate by descent from other species with modification by natural selection. (The word "evolution" is not used.)
- The third new element introduced by Darwin is the enormity of geological time. Having proposed a very slow process of change, he needs long ages to have passed for it to operate. This also helps him with another difficulty - he needs the fossil record to be incomplete in order to account for the fact that we don't find many intermediate stages of species change (though there are other factors here). He is coy about the details but does propose that the Weald has taken 300 million years to erode to its present situation. I think today's estimate is more like 100 million years, but he's within an order of magnitude.
It is worthing also noting what Darwin doesn't do. He doesn't use the word "evolution"; he says little about God and nothing about the Bible; he has precisely one sentence about the relevance of this to humanity, saying that as a result of his book, "Light will be thrown on the origin of man and his history." But the implications are pretty clear.
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