Darwin’s theory of the importance of variations emerged in part from his immersion in natural theology and the work of Reverend William Paley. Paley argued that the beauty and intricacy of the natural world implied the presence of a magnificent Creator, just as “a watch [...] implies a watchmaker” (49). Paley’s influence motivated Darwin to learn what forces shaped the seemingly miraculous forms found in nature. Darwin theorized that variations provided the raw material upon which these forces worked, selecting only the most beneficial attributes in a species. However, Darwin offered no experimental evidence towards this point, and advocates of the theory clashed with its detractors well into the 20th century.

In 1938, the ornithologist David Lack traveled to the Galápagos to study the finches directly. He found the finches bred exclusively with their own kind, confirming their status as distinct species, not “hybrid swarms.” Further, he saw that species with nearly identical beaks did not live together on any island, and when species with similar beaks shared an island, they diverged from the average more significantly than when they were not neighbors. He believed that the differences in the finches’ beaks had secured each species’ place on its island; where competition between similar-beaked birds may have existed, it seemed that

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