Answer: I'm literally learning this now so hopefully I get this right or close enough, because I don't have my book with me right now so I'm depending on my brain.
Explanation: I believe that during Darwin's time he believed that organisms came from pre-existing organisms, they changed over the years, and share a common ancestor. With the age of the Earth he has changed his estimates from being 300 million years to being at least several hundred million years old.