Darwin's work during the Beagle expedition let him study at first hand 
geology, fossils and a multitude of living organisms as well as meeting 
native peoples. He methodically collected an enormous number of 
specimens, many new to science, which established his reputation as a 
naturalist and made him one of the precursors of ecology. His detailed 
notes formed the basis for his later work as well as providing social, 
political, and anthropological insights into the areas he visited.

    * From reading Charles Lyell's Principles of Geology postulating 
    gradual processes over huge periods of time, Darwin wrote home that 
    he was 'seeing' land-forms as if he had the eyes of Lyell.
    * He discovered fossils of gigantic extinct South American Megatheriums 
    and Armadillos in strata which showed no signs of catastrophy or change 
    in climate, and found later that these were relatives of creatures still 
    living in the area.
    * Argentinian Rheas, and mockingbirds on different Galápagos Islands, 
    formed distinct species in nearby territories. On return he would find 
    that this also applied to Galápagos tortoises and finches.
    * The natives of Tierra del Fuego appeared to him savages little above 
    animals, yet three natives returning with them as missionaries had 
    become civilised in two years. When revisited after a year, the one 
    they met preferred savagery to a return to civilisation.
    * Stepped plains of shingle and seashells in Patagonia appeared to be 
    raised beaches. In Chile he experienced an earthquake raising the land, 
    then saw seashells high in the Andes.
    * While in South America he contracted Chagas' disease from insect 
    bites. For the rest of his life this caused recurring episodes of 
    illness and disability.
    * An Australian marsupial rat-kangaroo and a platypus made him think 
    that an unbeliever might exclaim "Surely two distinct Creators must 
    have been [at] work".
    * He theorised that coral atolls formed on sinking volcanic mountains, 
    and this was confirmed by survey in the Cocos (Keeling) Islands.