Forming a Theory

Darwin began to understand. To survive on the islands, the plants and animals had to change. This is called adaptation. These changes happened slowly. Then, over time, new species formed. He called his idea the theory of evolution.

Española cactus finch

large ground finch

He wrote a book about his idea. He wrote that some plants and animals have traits that work best in some places. These traits helped the living things survive. His thinking helped to change how people saw the natural world. 

Charles Darwin