Darwin collected and described thousands of animals and plants. In South America he observed the adaptations of organisms to a variety of habitat from jungle to grassland to mountain habitats. In the temperate regions the species resembled more closely the species of the tropical regions of South America rather than the corresponding species of the temperate regions of Europe. For example, in the grasslands of Argentina there are no rabbits, however, there are rodents that resemble rabbits; these rodents are unrelated to European rabbits but are similar to other rodents in South America. Moreover, the fossils in South America are dissimilar to European fossils but have similarities with extant (i.e. currently living) plants and animals in South America.
Darwin was particularly intrigued by the finches on the islands of Galapagos which are located approximately 500 miles from the mainland of South America. These finches, although unique to these islands, were clearly related to mainland species. There were 14 different species or genera of Galapagos Finch and their bills were adapted for particular diets. Darwin amassed these and other data including observations on variability in domestic animals (for example, dogs) which had been brought about by generations of selective breeding.
As well as drawing on his own observations, Darwin drew from the work of Linnaeus, Cuvier, Hutton, Lyell, Malthus and Lamarck. In the hierarchial classificatory system of Linnaeus there is a tacit acknowledgement of relatedness, for example, species belonging to one genus have more in common with each other than they do with species belonging to another genus. Linnaeus was a creationist -- as evidenced by his egotistical proclamation "God creates, Linnaeus arranges". Cuvier, also a creationist, was a comparative morphologist (he described the similarity/dissimilarity in anatomy of diverse animals). Cuvier founded the science of paleontology and described the differences between the fossil flora and fauna in different strata of rock: he observed that the more recent strata had fossils that more closely resembled extant organisms. Cuvier believed that the discontinuities between fossils in different strata were brought about by catastrophes such as floods which caused the extinction of many species living at a particular time. This interpretation of earth's history is termed catastrophism and was also held by many contemporary geologists. By contrast, Hutton and subsequently Lyell held that geological processes are slow and subtle but that over prolonged periods of time (millions of years) these can lead to major changes; implicit in this viewpoint is an age for the earth radically different from the 6,000 years of the biblical creationists.
Other key influences on Darwin were Malthus who had concluded that war and famine were inevitable as the human population grew more rapidly than available resources, and Lamarck who had proposed a theory of evolution based...