To admit this view is, as it seems to me, to reject a real for an unreal, or at least for an unknown, cause. It makes the work of God a mere mockery and deception; I would almost as soon believe with the old and ignorant cosmogonists, that fossil shells had never lived, but had been created in stone so as to mock the shells now living on the sea-shore.
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.