On the Origin of Species

Darwin examines the development of domestically bred species over generations. He notes that species must receive consistent exposure to the same “conditions of life”—that is, fundamentally equivalent life circumstances—for several generations to accrue “any appreciable amount of variation” (543). Darwin attributes variability in offspring to changes in the reproductive organs of parents before the act of sexual reproduction. While he considers the conditions of life indirectly crucial, he sees the “laws of reproduction, and of growth, and of inheritance” as more directly relevant. For Darwin’s study of natural selection, any variation that isn’t inherited (but is instead acquired) is irrelevant.

As natural conditions of life change, the process of natural selection—which Darwin details in a later chapter—accounts for the variations (and reversions to ancestral forms) that propagate. He notes that given the improbability of specific variations in any individual, the fact that variations are so likely to recur in that individual’s offspring indicates that natural law rather than chance dictates inheritance.

Darwin then discusses the history of breeding. He speculates on the ancient breeding of semidomesticated dogs and makes similar observations about ducks and rabbits. He then details various breeds of pigeons, which he finds notable for their extreme diversity. Darwin posits that all pigeons descended from the same parent species, the rock-pigeon. He refers to his personal experiences breeding pigeons and describes an instance in which he bred two pigeons that produced offspring substantially different in coloration than either parent. Additionally, he notes that the various breeds of pigeons can breed fertile hybrids. Based on these and other observations, he concludes that all pigeons share a common ancestor, the Columbia livia rock pigeon.

Next, Darwin discusses domestication: “One of the most remarkable features in our domesticated races is that we see in them adaptation, not indeed to the animal’s or plant’s own good, but to man’s use or fancy” (560). Thus, Darwin identifies the purpose of selection for improved traits as a fundamental difference between natural and domestic selection. He discusses how breeders talk about the plasticity of animal structure, which they can adapt over time to their own ends. He acknowledges the virtues of a skilled breeder and the difficulty of that line of work.

Darwin touches on the laws of growth correlation, which he discusses several times throughout the book. These laws dictate how one part of an organism’s structure changes when another part changes. He notes that these principles of selection aren’t new discoveries and have been practiced by people for a long time; for instance, Egyptians bred domesticated pigeons 3,000 years ago.

He notes that his primary interest is instead in the “unconscious” breeding of the best individuals of a species. He notes how a “gradual process of improvement” via the “occasional preservation of the best individuals” (566) in various species of domesticated plants, over time, caused them to differ so substantially from their wild counterparts that they were unrecognizable as the same species. Darwin alludes to the concept of natural selection in describing how domestically selected species in different countries (with concurrently different conditions of life, such as environment) may form two sub-breeds.

He notes how little is known about the domestic origin of various breeds and then discusses what he considers the pros and cons of domestic selection. High degrees of variability are very useful but require a large population. Animals and plants that undergo breeding can benefit, as a species, from intensive attention to minute “deviation in the qualities or structure of each individual” (569). He notes that this is crucial to his study and states how important physical enclosures are for breeders, especially because they prevent unwanted interbreeding. He realizes that some domesticated animals, like cats and donkeys, have few distinct breeds but claims that this is simply breeding programs for these species receive little attention. Darwin ends the chapter by summarizing his findings regarding domestication and reiterating his views on the power of selection.

Before applying the principles of variability and selection to the state of nature, Darwin clarifies the terms “species” and “variety.” Although no universally accepted definitions of the two key terms exists, he notes, species generally denotes a “distinct act of creation,” whereas variety refers to some “community of descent” (572) among a set of organisms. In some cases, widespread disagreement arises about what constitutes a new species and what constitutes a mere variety: “[In determining] whether a form should be ranked as a species or a variety, the opinion of naturalists having sound judgment and wide experience seems the only guide to follow” (575). Referring to his experiences on the Galapagos Islands and the South American coast, Darwin recalls the arbitrary distinction among varieties and species of birds, concluding, “Certainly no clear line of demarcation has as yet been drawn between species and sub-species” (578). Eventually, he settles on the view that, among other things (like expert opinion), the degree of variation is an important criterion in distinguishing a species from a mere variety.

What most generally agree on is the significant individual variation among animals within a species. This variation provides the grounds for accumulation of naturally selected traits for various purposes. The gradations of intermediate varieties between species, Darwin notes, are crucial to his study. He uses natural selection to articulate the flow of species into varieties that, with accumulated variation, become “incipient species,” thereby noting a process of species mutability. Varieties form through exposure to strenuous physical conditions and, more essentially, competition with other organisms. Both these variables are classifiable as conditions of life. Darwin observes that competitive conditions of life spur variation and that natural selection theory—but not by the opposing independent creation theory—can easily explain this dynamic: “All that we want to show is that where many species of a genus have been formed, on an average many are still forming; and this holds good” (581).

At the heart of the problematic distinction between species and variety, Darwin notes, is “Divergence of Character” (582). This principle, he thinks, explains how small differences among lesser varieties can accrue and, over time, result in large differences between species. One thing that can increase divergence of character is the range (or amount of territory) that a species occupies. Larger ranges lead to greater variation. He ends the chapter by briefly reiterating his main conclusions.