Darwin's book...the Theory of Evolution by Natural Selection. Evolution is a fact. Natural Selection was the theory proposed by Darwin to explain evolution. And...fyi...scientists do not consider theories something pulled out of one's backside on a whim. Scientific theories need a lot of evidence before they are taken seriously and considered as the leading explanation. Oh...and in the language of science, the word "law" describes an analytic statement. The law of gravity gives us a formula that tells us what will happen, but not why it happens. For that, scientists refer to Einstein's Theory of General Relativity. A theory. With less evidence than Darwin had for his theory of Natural Selection.
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