Darwin relied on his notebooks. In them, he jotted private ideas, questions and fragments of conversations related to his thinking on "transmutation"--what we now call "evolution." The notebooks reveal a great mind homing in on a great idea: Plants and animals are not fixed and unchanging. Instead, all species are related through common ancestry, and they change over time.
Once Darwin started thinking seriously about evolution, he grasped its essentials with astonishing speed. Only a month or so elapsed between the time he opened the first full transmutation notebook, in about July 1837, and the time he drew a crude--but unmistakable--evolutionary tree. This drawing, with the most ancient forms at the bottom and their descendants branching off irregularly along the trunk, reveals that Darwin understood all plants and animals are related. Above his tree Darwin wrote firmly, "I think."
Darwin's Evidence: Structure
Limbs that look very different and serve different functions--"the hand to clasp, the bat's wing to fly . . . the porpoise to swim" --are often much alike in skeletal design. For Darwin, this resemblance was further evidence that large classes of organisms, such as mammals, shared a common ancestry. Here, each of the bones in this bat wing has a counterpart in the bones of a human hand.