Charles Darwin proved his theory by showing the different birds he'd seen from different islands. They were the same type of bird but something was different because of the way they lived. For instance, he went to an island where there was small flowers. The birds had longer necks which was much easier to get the flower juice which the birds liked.
He went on an expedition and on the islands he visited there were the same plants and animals. But they had very important differences to adapt to were they lived. Charles Darwin also thought that we were apes that turned into humans over the years.