What was significant about the fossils darwin found?

Finding evidence such as giant sloths, breed of horses, and hippos in the environment which no longer existed. 

There were special/specific traits in birds that were very similar but lived in vastly different environmental conditions. This helped lead to the idea that animals adapt to their surroundings, and those who don't die off. As was previously mentioned, it also helped show the earth in it's earlier years, with fossils of deep-sea creatures showing up in deserts where they don't belong.