Darwin attempts to explain how variations in species occur, which was one of the main questions left unanswered in his analysis of natural selection. Variations do not occur by chance; rather, they are somehow related to the conditions of life that exist in species’ geographical environments. Though actual environmental factors such as the availability of food and the climate of a region do not directly create variations in species, variations will develop in animals and plants that will allow them to better survive in an environment. These variations will be naturally selected over less beneficial variations, prompting the advantageous variations to reappear in subsequent generations.

Darwin provides a few ideas about how these variations might occur. First, he imagines that use and disuse of particular organs play a role. He gives numerous examples of animals with unused organs, such as ground-living birds, insects with wings that are useless for flight, underground-living rodents with eyes that have been covered by skin or fur, and cave-living animals that are blind. Darwin says lack of use may cause organs to be modified. Similarly, large wings can be found in flying birds and insects, which may be a result of the animals’ increased use of the organs. Darwin accounts for these variations as “effects of habit.” In other words, he suggests that significant use of an organ can modify its structure in an advantageous way. These modifications from use and disuse must be inherited for natural selection to work. Ultimately, the heredity of variations, no matter how it occurs, drives natural selection, and Darwin argues that natural selection of hereditary variations can take precedence over the effects of habit.

Darwin also points out that variations in one organ of a species may cause variations in a different organ of the same species. These variations often occur in homologous organs—organs that form together during embryonic development, such as arms and legs. However, Darwin cannot explain other coexisting variations, such as the relationship between blue eyes and deafness in cats. This mode of variation is important, as it explains how some modifications that seem useless to the species may have occurred. The useless modification simply remains, while another more advantageous modification develops, and both modifications are incorporated into the species through natural selection. Variations, therefore, often correspond, even if naturalists do not understand why.

Darwin also notes that variations seem to occur mostly in a particular species’ highly developed organs. The organs that a species uses the most, such as wings in flying birds, have been susceptible to variation in the past and still exhibit high levels of variability. The comparison between the variability of a particular organ in one species and the lack of variability of an organ in a similar species marks the relationship between the two species: Both must have descended from a common ancestor, with the variation in the organ of one causing its divergent development from the other. Darwin notes that occasionally a species will revert to the characteristics of its early ancestor. For example, stripes can appear on horses descended from zebras, or bluish colors can appear on pigeons descended from the blue rock pigeon. While the reasons for these reversions remain unknown, Darwin hypothesizes that the tendency toward these past characteristics must remain in a species, even if the characteristics are dormant for hundreds of years.

Darwin argues that these laws of variability call into question the concept of the independent creation of species. If each species were independently created, wouldn’t all organs be equally susceptible to variation, rather than the ones most highly developed and most useful to the species? And how would variations from one species randomly occur in another species in the same pattern and color? Darwin argues that these patterns of variation illustrate the validity of his theory of natural selection and disprove the notion that species were created separately.


Although he attempts to explain how variations occur, Darwin presents hypotheses that are ultimately confusing and contradictory, because he himself does not fully understand the mechanisms of variation and heredity. Darwin concludes that variations must be hereditary for natural selection to work. However, without understanding the laws of heredity, Darwin cannot provide us with much of an explanation for how variations even occur, let alone how they are passed from generation to generation. He argues that the environment cannot itself create the variations—but then, what does? Darwin attempts to explain the creation of variations by falling back on some of Lamarck’s notions of use and disuse of organs. Still, he clearly does not believe Lamarck’s explanation to be sufficient—it is the mechanisms of heredity that drive natural selection, not adaptation to the environment. Darwin knows that if his theory of natural selection is to hold any water, he must present at least some potential explanation for the causes of variation. But ultimately, because he cannot muster a convincing case for the causes of variation, his analysis remains weak and open to attack.

Darwin does a much better job of analyzing the relationships between species through their differences in variation. He illustrates his case for descent with modification by explaining how developed organs in particular species are highly variable compared to the undeveloped organs in similar species, and by describing how modification can separate species over time. Darwin’s discussion of the reversion to characteristics from previous species shows evidence of how one species can be related to—or, in his terms, descended from—a parent species through the reappearance of characteristics from the parent species in subsequent generations. Although Darwin still does not explain how variations occur, his analysis of hereditary variation helps to bolster the theory of natural selection by explaining how variations link separate species through heredity. Here, Darwin the scientist is at his best, using particular facts and examples to draw conclusions that create broad scientific principles (an effective usage of inductive reasoning) and showing the ways variations help to prove the relationship between species.

Darwin also uses this analysis to attack the separate creation of species theory. Somewhat paradoxically, he invokes religion to bolster his own theory of the origin of species. He argues that some naturalists who might ignore his theory and cling to the notion of the separate creation of species are making a “mockery” of the “work of God.” Darwin challenges natural theologians—those naturalists who believe that the natural world shows the beauty of God’s independent creation of each species—by suggesting that their theory of independent creation is not the only one that can invoke the greatness of God. Implicitly, Darwin argues that the system of natural selection is just as brilliant and wondrous as a system of independently created species.

Darwin implies that religion must not turn its back on scientific reasoning. Facts and analysis have led him to his theory, and the evidence cannot simply be ignored because of devotion to a set dogma, be it religious or scientific in nature. Anticipating the resistance to his theory, Darwin emphasizes that a shift in scientific and theological reasoning must occur before his theory can be accepted as scientifically valid.