 Early in the voyage he decided that he could write a book about geology, and he showed a gift for theorising.At Punta Alta in Argentina he made a major find of gigantic fossils of extinct mammals, then known from only a very few specimens.He ably collected and made detailed observations of plants and animals, with results that shook his belief that species were fixed and provided the basis for ideas which came to him when back in England, and led to his theory of evolution by natural selection.