One of the pieces of evidence for evolution that Charles Darwin collected on his travels aboard the HMS Beagle was the observation of fossils of extinct animals and similar but noticeably different current species. Giant ground sloth fossils were very similar in shape to the bones of sloths that inhabited the regions of South America he was exploring. The main difference was the size, the ground sloth fossils were significantly larger than the bones of the sloths he captured alive. What was a conclusion that Darwin was able to develop based on this piece of evidence?