On the Origin of Species

Darwin continues to unpack the problems of geography that he engaged in the previous chapter. He discusses, among many other things, the distribution of freshwater fish. He thinks they modify slowly, as do all creatures of “low” development. He again discusses the role of birds in dispersing the seeds and eggs of other species.

Next, Darwin turns his attention to islands and their endemic species. He mentions the Galapagos Islands, with which he’s famously familiar, and notes the specificity of the land-based birds on the island. Many small mammals exist on islands, he notes, when the islands are close to a mainland. He notes that the mammals in Great Britain are the same as those on the European mainland, presumably because the British isle is separated from the continent only by a narrow sea. In addition, Darwin thinks the depth of the sea, like the distance of an island from the shore, inversely relates to the number of mammalian lifeforms that the island and mainland share.

All these remarks tie back to the idea that species can migrate to diverse places over “the long course of time” (842). Darwin notes, “The most striking and important fact for us in regard to the inhabitants of islands, is their affinity to those of the nearest mainland, without being actually the same species” (843). Again using the Galapagos Islands, Darwin notes the similarities (and differences) the species there share with those on the South American mainland. Darwin points out that whereas natural selection theory easily explains such close relationships between varieties of species, independent creation theory can in no way account for them and would have to pass over them in silence.

Darwin then summarizes this chapter (and the previous one), focusing on the nature of migration: “I think all the grand leading facts of geographical distribution are explicable on the theory of migration, together with subsequent modification and the multiplication of new forms” (850). The closeness that organisms share in geographical distance and in natural history, he concludes, is a strong sign of common genealogical roots.

In the penultimate chapter, Darwin investigates the problem of taxonomic classification. He discusses how organisms ought to be classified and investigates areas of study that can help determine true affinities between species: morphology, embryology, and the study of rudimentary organs.

First, he notes that these classifications can’t be arbitrary. Instead, species and genera should be organized into one “Natural System” (855). While some naturalists claim that the organizing principle should be the “plan of the Creator,” rather than the “mere resemblance” of species, Darwin posits that the truest organizing principle is the “propinquity of descent” (856). This alone, he writes, is what binds the similarity among organisms. He then develops rules for proper classification. One rule is that the “physiological importance of an organ doesn’t determine the classificatory value” (857). Another rule states that no individual trait of an organism is sufficient for proper classification. Embryonic characteristics, he claims (following others) are of special relevance in classification. Additionally, he notes that while the natural system of classification isn’t arbitrary, the levels of distinction (order, family, genera, etc.) are. The true natural system is a genealogical account of the history of life.

Unwittingly or not, Darwin holds that all naturalists make descent a part of their classificatory scheme when they assume that the sexes of a species should be classified together despite wide morphic difference in some cases. In addition, he notes that while attending to resemblances between species is important, these may be the results—not the cause—of true affinity. He notes that “one great natural system” (867), at the end of the genealogical tree, unites all species. The theory of descent with modification yields an explicit adherence to genealogical account, which provides the ability to comprehend the rules for proper classification. He again refers to the diagram in Chapter 4 to illustrate his point.

Darwin uses the remainder of the chapter to discuss morphology, embryology, and rudimentary organs. As the study of morphology shows, natural selection theory has greater explanatory power than independent creation theory because natural selection can organize and systematize the morphological differences among varieties and species.

In addition, Darwin considers embryology a very useful classification and notes a “law of common embryonic resemblance” (876). Since embryos are less developed than the adult version of an organism, their structure more readily reveals its ancestry. Animals that appear very different in adulthood may share a line of descent if they undergo the same embryological development. He notes that hereditary information is often embedded in a creature even before the embryo forms and that the display of a trait in adulthood doesn’t mean that the trait didn’t originate at a much earlier point. Darwin concludes this study of embryos by writing, “Embryology rises greatly in interest, when we thus look at the embryo as a picture, more or less obscured, of the common parent-form of each great class of animals” (884).

In concluding the chapter, Darwin briefly reviews how organisms’ rudimentary (underdeveloped or atrophied) organs can inform species classification. He notes that observation of rudimentary organs commonly helps determine an organism’s history. Darwin notes that natural selection theory easily explains the existence of rudimentary organs: Disuse is the main reason they lose relevance for the organism’s ability to function. He compares such organs to the letters of a word that we no longer use in its pronunciation; these letters can still speak to the history of the language in important ways. Darwin briefly reviews the chapter and reiterates that his theory makes a wide array of facts both intelligible and useful for classification.

In the final chapter, as the title suggests, Darwin recapitulates the book’s arguments and is most notable for Darwin’s philosophical reflections on natural selection theory. He reiterates some of the main benefits of natural selection and recounts his work in explaining the geological record and the reasons for peculiar geographical distribution. The author notes the variability that occurs under domestication and the laws that cause variability. He reiterates that the struggle for existence is essential to the development of naturally selected traits. In addition, he restates his fundamental uneasiness about strong distinctions between species and varieties, which, he holds, exist on a continuum: “Species be only well-marked and permanent varieties” (902). This, like so much of his theory, opposes independent creation theory. He outlines many of the things that his theory can easily explain—and that completely stump the opposing theory, independent creation. Relationships between organisms, he emphasizes, are more important than physical conditions of life.

Darwin notes that the proponents of independent creation theory haven’t been as rigorous in evaluating their own view as they’ve been in bashing theories of the mutability of species. Darwin notes that they use phrases like “plan of creation” (907) to mask their lack of rigor. He calls on others who support his views to put this in writing to overturn prejudices. Darwin then explicitly speculates on the ultimate origin of all species: “I should infer from analogy that probably all the organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed” (909). Thus, Darwin notes the possibility of a single parent organism situated at the base of the great tree of life. Darwin notes that when scientists start to think in terms of natural selection instead of independent creation, a “grand and almost untrodden field of inquiry will be opened” (910).

Darwin ends by reflecting on the vastness of time and the amazing feat of long, slow, gradual transmutation of form. He notes that the future will likewise be mind-blowingly extensive. He recounts the essential laws of variability and notes the “grandeur” of the existential perspective that natural selection theory enables. He then subtly reiterates that natural selection is an ongoing process with an open future.