BIG BRAIN ANSWER:
Before Darwin, a naturalist named Jean-Babtiste Lamarck believed that the environment altered the shape of individuals and that these acquired changes were then inherited. Meanwhile, Charles Darwin was a naturalist on a ship called the HMS Beagle, and he discovered that the beak shape varies among finch species in the Galapagos Islands. He found that the beak of an ancestral species of finch had somehow been transported to these islands, and adapted over time to acquire different food sources that were available, causing variation, and that nature just selected for the most suitable beak shape and against less useful ones.