Explaining adaptation

The next important step was to invent a theory to explain why species change. The notebooks Darwin kept at the time still survive. They reveal how he struggled with several ideas, including Lamarckism, but rejected them all because they failed to explain a crucial fact: adaptation. His theory would have to explain not only why species change, but also why they are well designed for life. In Darwin's own words (in his autobiography):

"It was equally evident that neither the action of the surrounding conditions, nor the will of the organisms could account for the innumerable cases in which organisms of every kind are beautifully adapted to their habits of life,- for instance a woodpecker or tree-frog to climb trees, or a seed for dispersal by hooks or plumes. I had always been much struck by such adaptations, and until these could be explained it seemed to me almost useless to endeavour to prove by indirect evidence that species have been modified."