Darwin devoted this chapter to explaining his ideas about natural selection.
To do this Darwin included a diagram, the only illustration in the original editions of the book. The diagram shows gradually diverging lines representing the offspring of a species becoming more and more distinct through successive generations as natural selection acts on the variation inherent in each individual. Only variations which are in some way profitable to the individual will be preserved or naturally selected, that is, passed on to the next generation.
Through time the gradual accumulation of differences in successive generations results in descendants that are distinct from the ancestral forms. Darwin envisioned this process acting slowly through tens of thousands of generations, and his model for evolution became known as gradualism.
The copy of the diagram shown here is from http://friendsofdarwin.com/docs/origin-1/diagram/
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