Charles Darwin was the man who went to an island called New Guinea. He observed a lot of the animals on different parts of the island and found differences in the same species this was found in all of the animals he observed. A giraffe with a short neck will not be able to eat leaves off of a tall tree, so it dies. It is not supposed to be there so it will evolve. This theory is known as B.
I really hope that this will help you out in the future. :)