"This fundamental subject of Natural Selection will be treated at some length in the fourth chapter; and we shall then see how Natural Selection almost inevitably causes much Extinction of the less improved forms of life and induces what I have called Divergence of Character."

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There are innumerable writing problems in an extended work. One book took a little more than six years. You, the writer, change in six years. The life around you changes. Your family changes. They grow up. They move away. The world is changing. You're also learning more about the subject. By the time you're writing the last chapters of the book, you know much more than you did when you started at the beginning.

— David McCullough