The survey continued to the south in Patagonia . They stopped at Bahía Blanca , and in cliffs near Punta Alta Darwin made a major find of fossil bones of huge extinct mammals beside modern seashells, indicating recent extinction with no signs of change in climate or catastrophe. He identified the little-known Megatherium by a tooth and its association with bony armour, which had at first seemed to him to be like a giant version of the armour on local armadillos . The finds brought great interest when they reached England.  
Darwin's exposure to specimens all over the globe raised important questions. Other naturalists believed that all species either came into being at the start of the world, or were created over the course of natural history. In either case, the species were believed to remain much the same throughout time. Darwin, however, noticed similarities among species all over the globe, along with variations based on specific locations, leading him to believe that they had gradually evolved from common ancestors. He came to believe that species survived through a process called "natural selection," where species that successfully adapted to meet the changing requirements of their natural habitat thrived, while those that failed to evolve and reproduce died off.