In the Galapagos Archipelago he found only one species of terrestrial mammal, a new species of mouse, and that only on the most easterly island of the group. On the South American continent there were at least forty species of mice, those east of the Andes being distinct from those on the west coast. Of land-birds he obtained twenty-six kinds, twenty-five of which were to be found nowhere else. Among these, a hawk seemed in structure intermediate between the buzzard and polyborus, as though it had been modified and induced to take over the functions of the South American carrion-hawk. There were three species of mocking-thrush, two of them confined to one island each. There were thirteen species of finches, all peculiar to the archipelago. In the different species of geospiza there is a perfect gradation in the size of the beaks, only to be appreciated by seeing the specimens or their illustrations.
Few of the birds were of brilliant coloration. The same was true of the plants and insects. Darwin looked in vain for one brilliant flower. This was in marked contrast to the fauna and flora of the South American tropics. The coloration of the species suggested comparison with that of the plants and animals of Patagonia. Amid brilliant tropical plants brilliant plumage may afford means of concealment, as well as being a factor in the securing of mates.
Darwin found the reptiles the most striking feature of the zoölogy of the islands. They seem to take the place of the herbivorous mammalia. The huge tortoise (Testudo nigra) native in the archipelago is so heavy as to be lifted only by six or eight men. (The young naturalist frequently got on the back of a tortoise, but as it moved forward under his encouragement, he found it very difficult to keep his balance.) Different varieties, if not species, characterize the different islands. Of the other reptilia should be noted two species of lizard of a genus (Amblyrhynchus) confined to the Galapagos Islands. One, aquatic, a yard long, fifteen pounds in weight, with "limbs and strong claws admirably adapted for crawling over the rugged and fissured masses of lava," feeds on seaweed. When frightened it instinctively shuns the water, as though it feared especially its aquatic enemies. The terrestrial species is confined to the central part of the group; it is smaller than the aquatic species, and feeds on cactus, leaves of trees, and berries.
Fifteen new species of sea-fish were obtained, distributed in twelve genera. The archipelago, though not rich in insects, afforded several new genera, each island with its distinct kinds. The flora of the Galapagos Islands proved equally distinctive. More than half of the flowering plants are native, and the species of the different islands show wonderful differences. For example, of seventy-one species found on James Island thirty-eight are confined to the archipelago and thirty to this one island.
In October the Beagle sailed west to Tahiti, New Zealand, Australia, Keeling or Cocos Islands, Mauritius, St. Helena, Ascension; arrived at Bahia, Brazil, August 1, 1836; and finally proceeded from Brazil to England. Among his many observations, Darwin noted the peculiar animals of Australia, the kangaroo-rat, and "several of the famous Ornithorhynchus paradoxus," or duckbill. On the Keeling or Cocos Islands the chief vegetable production is the cocoanut. Here Darwin observed crabs of monstrous size, with a structure which enabled them to open the cocoanuts. They thus secured their food, and accumulated "surprising quantities of the picked fibres of the cocoanut husk, on which they rest as a bed."
In preparing his Journal for publication in the autumn of 1836 the young naturalist saw how many facts pointed to the common descent of species. He thought that by collecting all facts that bore on the variation of plants and animals, wild or domesticated, light might be thrown on the whole subject. "I worked on true Baconian principles, and, without any theory, collected facts on a wholesale scale." He saw that pigeon-fanciers and stock-breeders develop certain types by preserving those variations that have the desired characteristics. This is a process of artificial selection. How is selection made by Nature?
In 1838 he read Malthus' Essay on the Principle of Population, which showed how great and rapid, without checks like war and disease, the increase in number of the human race would be. He had seen something in his travels of rivalry for the means of subsistence. He now perceived "that under these circumstances favorable variations would tend to be preserved, and unfavorable ones to be destroyed. The results of this would be the formation of a new species." As special breeds are developed by artificial selection, so new species evolve by a process of natural selection. Those genera survive which give rise to species adapted to new conditions of existence.
In 1858, before Darwin had published his theory, he received from another great traveler, Alfred Russel Wallace, then at Ternate in the Moluccas, a manuscript essay, setting forth an almost identical view of the development of new species through the survival of the fittest in the struggle for existence.