On the Origin of Species

Darwin reiterates a point that he considers crucial to the thinking that his theory requires: “Let it be borne in mind how infinitely complex and closefitting are the mutual relations of all organic beings to each other and to their physical conditions of life” (600). Thus, the most minute of useful variations propagate, and all problematic variations quickly disappear. This is the process of natural selection.

The author notes that no environment on Earth prevents species from improving itself to take further advantage of natural resources in its struggle. He then poetically muses on the superiority of natural selection to the works of human endeavor (breeding), stating that the products of nature “bear the stamp of far higher workmanship” (603). He notes that much about this process is completely unknown to human observers. Slight modifications have unknowable ramifications for further modifications later. As examples, he cites the beaks of birds and the jaws of insects, which the organisms use to break out of eggs and cocoons, respectively, and which subsequent modifications enable secondary uses, giving the animal additional (and perhaps unexpected) advantages.

Darwin then introduces another critical idea: sexual selection, a form of natural selection based on the struggle to procreate rather than the struggle for life. Darwin notes that competition is most strenuous between males of polygamous animals. Although the males that fail to reproduce aren’t killed, they don’t propagate offspring and therefore aren’t selected for. The sexual selection process also selects for traits that give individual males advantages in mating. Darwin writes that these can include “weapons, means of defense, or charms” (607).

The author then notes how natural selection can produce different varieties, each better suited to a respective environment, as for wolves that live in the mountains versus those that live in the valleys. Additionally, the division of labor can be selected for among the complex parts of flowers and other plants. Flowers and bees, he observes, can co-adapt given their close interdependence. He concludes that natural selection works via “infinitesimally small inherited modifications, each profitable to the preserved being” (612).

Next, Darwin discusses how plants are often hermaphrodites, elaborating on the importance and frequency of intercrossing with other individuals. Nevertheless, he notes, the hermaphrodites that exist in the animal kingdom all interbreed. He concludes that “both in the vegetable and animal kingdoms, an occasional intercross with a distinct individual is a law of nature” (616). Darwin emphasizes the importance of variation, both among individuals and in sexual selection.

He then discusses the circumstances that influence to natural selection, such as the number of individuals in a population, how isolated that population is, the amount of terrain it occupies, and its number of competitors. Unfavorable circumstances lead to diminishment and extinction. Rare species, unsurprisingly, often go extinct, especially when another, competitive species has recently undergone beneficial modifications.

Darwin refers to the divergence of character, which he previously discussed in domestic breeding programs, to explain the lineage of appreciable subspecies. This principle states that divergences in species tend to accrue because the diversity of traits in the species leads to an advantage over competitors in the struggle for existence. These divergences eventually reach a degree that indicates an appreciable basis for a new species. This subsection of Chapter 4 contains (on pages 630-31) the book’s only diagram. This diagram presents a speculative, abbreviated, and simplified history of the process of natural selection among several species. Darwin explains how this process works, and he references the diagram throughout the remainder of the book. He discusses the relationship of intermediate forms to their parent forms, their ancestors, and one another. This process is known as “descent with modification” (635). He notes the predictive capacity of the divergence of character principle to show what will likely occur in future generations of a species. Darwin concludes by summarizing the chapter’s theoretical points and waxes poetic, referring to the great “tree” of life that connects all species.