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Adding on to Darwin's Voyage
Darwin's ideas were inspired by the observations that he had made on the second voyage of HMS Beagle (1831–1836). One of his most notable observations were on the beaks of the many finches the he encountered on the Galapagos Islands. Due to his theory on natural selection, he made the discovery that different finches could only survive in the different environments based on the type of food that was available to them. For instance, the finches that congregate on the ground have bills used for crushing, because of this, their diet mainly consists of seeds. Finches that like to stay high in the trees like the insectivorous finch have bills that are used for grasping things like insects, so seeds wouldn't be an option for them to eat. If an insectivorous finch were placed in a habitat that mainly consisted of seeds, then those finches would die off over time because the shapes of their beaks are not "best-fit" for the environment. Aside from finches, Darwin observed the eating habits of giant tortoises and how the shape of their shells determined their food sources. There were two major groups of tortoises: those with dome-shaped shells and those with saddle-back shells. Tortoises with saddle-back shells had the advantage of eating foods that were at greater heights like tree-cactus because the front of the shell is raised, allowing for more mobility of the neck. The tortoises with a dome-shaped shell, however, have to rely on food sources that are loser to the ground because the front of the shell restricts the neck's range to move. Darwin's ideas were also influenced by the work of a political economist, Thomas Robert Malthus, who, in An Essay on the Principle of Population (1798), noted that population (if unchecked) increases exponentially, whereas the food supply grows only arithmetically; thus, inevitable limitations of resources would have demographic implications, leading to a "struggle for existence". When Darwin read Malthus in 1838 he was already primed by his work as a naturalist to appreciate the "struggle for existence" in nature. It struck him that as population outgrew resources, "favorable variations would tend to be preserved, and unfavorable ones to be destroyed. The result of this would be the formation of new species." Darwin wrote: If during the long course of ages and under varying conditions of life, organic beings vary at all in the several parts of their organization, and I think this cannot be disputed; if there be, owing to the high geometrical powers of increase of each species, at some age, season, or year, a severe struggle for life, and this certainly cannot be disputed; then, considering the infinite complexity of the relations of all organic beings to each other and to their conditions of existence, causing an infinite diversity in structure, constitution, and habits, to be advantageous to them, I think it would be a most extraordinary fact if no variation ever had occurred useful to each being's own welfare, in the same way as so many variations have occurred useful to man. But if variations useful to any organic being do occur, assuredly individuals thus characterized will have the best chance of being preserved in the struggle for life; and from the strong principle of inheritance they will tend to produce offspring similarly characterized. This principle of preservation, I have called, for the sake of brevity, Natural Selection.

— Darwin summarizing natural selection in the fourth chapter of On the Origin of Species