Darwin’s emphasis on the gradual process of natural selection proved to be a double-edged sword, although it did provide a persuasive argument for his greater theory of evolution. On the one hand, the idea of gradual change enabled Darwin to fight critics who argued that scientists’ inability to observe evolution in current populations means that it is not occurring. If change is gradual, a scientist might not observe any change in his lifetime, even though change is constantly occurring. In defense of this concept, Darwin borrows from Lyell’s Principles of Geology (1833), which suggests that geological change occurs gradually over time. He relates Lyell’s notion of geological evolution to his own theory of species evolution: Just as geological change cannot be seen by observers in one lifetime, species change is also unobservable in one’s lifetime, even as it is gradually happening.

However, at the time of the publication of The Origin of Species, Darwin’s ideas about the gradual nature of evolution also opened his theory to criticism from the scientific community. The notion that evolution happens gradually would later be proven incorrect. The fossil record showed that species seemed to have appeared and become extinct rapidly, with long periods of continuity between years of rapid change. The rediscovery of Mendelian genetics and mutation theory at the turn of the twentieth century would help resolve this, as Gregor Mendel (often called the father of modern genetics) discovered the patterns of inheritance that explain how traits are passed from parents to offspring. Hugo de Vries later discovered how genes can mutate, introducing new genes into a population that would cause the variations necessary for natural selection to take hold. The discovery of genetic mutations proved that species change occurrs rapidly once a mutation appears.

Darwin explicates natural selection by using metaphors that relate it to human society, a tactic that unknowingly paved the way for the Social Darwinist movement that would spring up years later. In discussing geographical isolation, Darwin uses the term immigrants to describe the introduction of new species into a geographic area. In the late nineteenth and early twentieth century, the rapid increase in immigration (and its effects on society) became a major concern of policymakers in the United States. Some of those calling for limitations on immigration applied Darwin’s theory of population change to human society, arguing that the increase of immigrants in the United States was leading to the degeneration of society. Darwin’s explanation of sexual selection was influenced by, and had an influence on, human society. By arguing that male organisms contain the variations to attract females, Darwin perpetuates the notion that males are the pursuers of females, with the female playing no active role in the reproductive relationship. Darwin also suggests that males, not females, pass on characteristics to their offspring. Darwin did not consider that females might also contain variations, such as color and shape, that could better attract males and be sexually selected. He therefore implies that males, not females, are the founders of society.

In his discussion of natural selection, Darwin revisits the concept of the glory of nature, which contrasts with the “good versus evil” portrayal of the struggle for existence. Darwin’s description of natural selection characterizes nature as a wise force: Because advantageous characteristics are selected and perpetuated, species become perfectly adapted to their environments, minimizing nature’s potential to destroy. Natural selection suggests an advantageous and therefore positive end—the perfect adaptation of species to their environment—rather than a struggle to survive, with extinction as an end. Although destruction will happen in the process of natural selection, the overall result is one of survival. Darwin concludes that nature is wise in its selection—even wiser, Darwin notes, than man himself, who cannot always detect advantageous characteristics in his own attempts to breed animal populations. This portrayal of nature complicates Darwin’s previous discussion of nature’s capacity for both good and harm. By placing man beneath nature in a hierarchy of wisdom and perfection, it also complicates man’s relationship to nature itself.