This document provides an overview of Charles Darwin's theory of evolution by natural selection. It discusses how Darwin made careful observations as a naturalist which led him to question how the diversity of life arose and why so many species had become extinct. His theory proposed that life changes over generations through a process of natural selection, where traits that aid survival and reproduction are passed on. Offspring often outnumber resources, creating competition, and only the best adapted individuals survive to reproduce and pass on their traits. Over long periods of time, this process of gradual change leads to the descent of species with modification.