Top: A paleontologist removes fossils from rock. Center: Minerals have replaced the plant matter to form this petrified wood. Bottom: This insect became trapped in tree sap, which then hardened into amber. 
Darwin found many types of remains of ancient organisms. In addition to fossil layers, he saw other fossils, bones, insects in amber (hardened tree sap), and petrified wood. Another type of preserved organism, which Darwin did not find, is animals such as mammoths frozen and preserved in ice.
During his journey, Darwin found the bones of an extinct giant sloth, Megatherium. He realized that animals can become extinct and that life is not unchanging, and he also saw similarities between extinct and living animals. Darwin wondered how they could be related.
Darwin and scientists today have discovered that the ancient organisms whose remains they find look like organisms alive today because they are the living organisms' ancestors or evolved from a common ancestor. For example, megatherium was probably an ancient ancestor of tree sloths that exist today.
Today, fossils are still being studied to find out more about life in the past and its relation to life in the present. They provide valuable information about evolution and how life formed. Unlike in Darwin's time, now scientists can date these fossils and remains to get a more exact picture of when different organisms evolved. We are still learning new things from a valuable source that is literally millions of years old.