We often think of scientific ideas, such as Darwin's theory of evolution, as fixed notions that are accepted as finished. In fact, Darwin's On the Origin of Species evolved over the course of several editions he wrote, edited, and updated during his lifetime. The first English edition was approximately 150,000 words and the sixth is a much larger 190,000 words. In the changes are refinements and shifts in ideas — whether increasing the weight of a statement, adding details, or even a change in the idea itself.
The second edition, for instance, adds a notable “by the Creator” to the closing paragraph, giving greater attribution to a higher power. In another example, the phrase “survival of the fittest” — usually considered central to the theory and often attributed to Darwin — instead came from British philosopher Herbert Spencer, and didn't appear until the fifth edition of the text. Using the six editions as a guide, we can see the unfolding and clarification of Darwin's ideas as he sought to further develop his theory during his lifetime.
"On the Origin of Species: The Preservation of Favoured Traces" by Ben Fry is a truly amazing Talmudic site that graphically illustrates in a compact fashion the how the six editions of On the Origin of Species evolved from 1859 to 1872. You just mouse over the changing image (you can set the progression to fast or slow) and you read the text. (Hat tip to Wired.) Fry explains,