How fleeting are the wishes and efforts of man! how short his time! and consequently how poor will his products be, compared with those accumulated by nature during whole geological periods. Can we wonder, then, that nature’s productions should be far “truer” in character than man’s productions; that they should be infinitely better adapted to the most complex conditions of life, and should plainly bear the stamp of far higher workmanship?
Darwin now focuses on natural selection, returning to his discussion of a breeder selecting desired characteristics for animals. If breeders can select characteristics they wish to breed in their animals and perpetuate these characteristics in their domestic populations, is it possible for nature to act in the same way? Darwin believes it is. Variations that give one species an advantage, no matter how slight, over another species, allow organisms a better chance of surviving and therefore of leaving more offspring. While breeders can select variations that are beneficial for them, omniscient nature has the power to develop and select variations that are imperceptible to man and beneficial to species in ways that man might never have considered. Nature can make any slight advantage increase an organism’s likelihood of surviving over another, and since these advantageous variations are heritable, they can be perpetuated in future generations. Conversely, nature can also limit species by not bestowing advantageous characteristics on them, therefore making the species less likely to survive and putting them at risk of extinction.
Darwin gives a number of examples of variations that could be advantageous to an individual organism, passed along to subsequent generations, and preserved in the future of the species. For instance, a wolf with slimmer features might be able to run faster and therefore to escape from its predators. A bear might be born with the instinct to pursue a more abundant type of prey, which will be easier to obtain than less abundant types. In a more complex case, a plant might contain a sweet nectar or colorful petals that attract more insects than other plants attract, which will spread the plant’s pollen more frequently. Similarly, an insect might inherit a smaller body type, allowing it to obtain food more easily from a plant.
Adding to the concept of natural selection, Darwin briefly outlines the concept of sexual selection, which is another factor in species evolution. Because having offspring is key to species’ survival, male individuals with variations that allow them to successfully attract females are more likely to reproduce and therefore to produce more offspring than others. These advantageous sexual characteristics are also variations that would be passed on to offspring. Darwin gives examples of tactics that species of male birds use to attract females: singing to their potential mates, displaying various brightly colored feathers, and performing antics to attract attention. If any variation in these characteristics were to give male birds a better chance of mating, that variation would likely spread to the birds’ offspring and, as a result, would become predominant in the population in subsequent generations. Variations of this sort, such as color, aptitude for singing, or ornamentation, may have no direct impact on the ability of an individual organism to survive in nature; rather, the variations’ utility in helping an organism reproduce is what allows these variations to be selected and perpetuated in a species.
Darwin goes on to explain the geographical requirements for the propagation of advantageous traits, noting that isolation is key to natural selection, as it creates a situation in which few individuals live in a specific area in which no new organisms will be introduced. In isolated areas, variations that allow organisms to survive in that particular environment win out over disadvantageous ones. In time, these variations spread throughout the species, making the organisms appear perfectly adapted to their environment. The workings of natural selection in geographical isolation explain why floral and animal life is unique in island environments. On the other hand, large geographical areas in which plant and animal populations are not isolated allow species to spread and propagate widely, increasing the species’ numbers and its overall chances of survival in a range of environments.
Finally, Darwin relates the concept of natural selection to divergences of character and the gradual creation of new species, thus explaining the origin of species. As particular variations are selected over others, these variations lead to a divergence in characteristics from the parent species. Additional variations then lead to more divergences, and as some variations propagate and expand, others die out as a result of the struggle for existence and the limits of population growth. Darwin provides a chart to illustrate this process, demonstrating how these divergences create “branches” of related species. Some branches end as species become extinct, while other branches multiply as more subspecies develop and a new species becomes a parent species. Eventually the divergences become so great that new species come about. Darwin says this process can take tens of thousands of years.
Chapter IV is perhaps the most important chapter in The Origin of Species, because it lays out the principle of natural selection, on which Darwin’s theory of evolution is based. Darwin does not answer every question about the origin of species in his theory. For example, he does not explain how variations actually occur in a species population. He simply assumes that new variations in organisms must be proven to be advantageous and that, over time, they change the characteristics of the species. How these variations occur remained a mystery until Hugo de Vries discovered genetic mutation in 1903. Additionally, Darwin’s explanation of how variation leads to the creation of species is not based on clear evidence, because the distinction between varieties and species had not yet been defined. The holes in his theory, however, do not take away from the brilliance of his identification of natural selection as the mechanism for evolution. Significantly, Darwin answers the question of how species become so well adapted to their environment without relying on environmental adaptation. With natural selection, a species’ adaptation to its environment occurs through a hereditary process—a concept that would eventually connect Darwin’s evolutionary theory to discoveries in genetics and genetic mutation in the twentieth century.