When I finally got around to reading The Origin of Species, the first thing that struck me about Darwin was how readable he is. The second thing, over the course of my reading, was how carefully he structures the book-length argument he presents.
Darwin says in his introduction that he early suspected that studying variation in domesticated animals would give him the clue to “that mystery of mysteries,” the origin of species. It seems likely, though, that he reached his conclusions by looking at variations in nature, and especially in the controlled conditions of species isolation provided by islands, in his time on the Beagle in 1831-36—and that he returned to man-made selection as a way of substantiating his theories and, more important, a safe place to start his argument. Everyone can observe what the plant or animal breeder can do in the way of passing on and enhancing traits that show up in his stock. Of course he can’t create variations, but he can select for them. The cause of the occurrence of variations Darwin could not be expected to guess, since no one at the time knew about DNA or how small accidents in its replication show up as mutations. Darwin speculates, here and in later chapters, that changes in external conditions act beforehand on the reproductive systems of parents to cause variations in offspring. He also, if I read him correctly, leaves open the possibility of the inheritance of acquired characteristics (“who can say that . . . the thicker fur of an animal from far northwards, would not in some cases be inherited for at least some few generations?”).
When he moves from “Variation Under Domestication” to “Variation Under Nature,” he has still to tackle the question whether variations, selected by man or by nature, amount to the coming about of new species. He muddies the waters (naturalists can’t always agree on whether two forms are varieties or distinct species) before announcing his own opinion that “a well-marked variety may be justly called an incipient species” and that there exists a continuum of small variations, varieties, sub-species, and species. He offers as confirmation the fact that larger genera have not only more species but also more very common species and more varieties than smaller genera even though “if we look at each species as a special act of creation there is no apparent reason why more varieties should occur in a group having many species, than in one having few.”
Then Darwin applies the Malthusian argument to “the whole animal and vegetable kingdom”: every species, allowed to reproduce without check, would cover the earth in a few thousand generations. They do not because of “The Struggle for Existence,” which also ensures that any favorable variation in any individual tends to preserve that individual and its offspring, a principle Darwin calls, “to mark its relation to man’s power of selection,” natural selection.
In his chapter on “Natural Selection,” Darwin is careful to make clear what it can and cannot do: it can “modify the structure of the young in relation to the parent” and vice versa, it can “adapt the structure of each individual for the benefit of the community,” but it cannot “modify the structure of one species, without giving it any advantage, for the good of another species.” Sexual selection works not by “death to the unsuccessful competitor, but few or no offspring.” Darwin points out that since intercrosses retard natural selection, isolation can allow for new varieties to be slowly improved, but he also believes that modification and new varieties will occur faster where there is a large area rather than, for example, an island. Natural selection operates by modifications that work well and by the extinction of those that do not. Gradually the small differences between varieties “become augmented into the greater difference between species.” Darwin introduces a diagram at this point to show variations within a species can ultimately grow into distinctly different species and form a new genus. He not only uses the schematic tree diagram, but he employs the tree metaphor at the chapter’s end to express the idea of natural selection operating through geologic time.
Although “our ignorance of the laws of variation is profound,” Darwin devotes a chapter to them, pointing to the way natural selection works 1) to encourage the strengthening and enlargement of parts that are used and the diminishment and disappearance of those that are not, 2) to select those variations best adapted to their climates, and 3) to work toward correlation of the parts of organisms that are adapting. “Species very rarely endure for more than one geological period” is a pronouncement he throws out during this chapter. Sexual selection is less drastic than natural selection—you don’t necessarily die; you just don’t find a mate. This chapter ends with an interesting case of reversion: lots of horses and mules show, at least when young, zebra-like stripes. Darwin says if you believe in independent creation of species, you have to see such species senselessly imitating each other. “To admit this view is . . . to reject a real for an unreal, or at least for an unknown, cause. It makes the works of God a mere mockery and deception.”
Darwin brings up difficulties and objections to his theory, and in two chapters he responds to these difficulties. The first problem is that of transitions, and there are several parts to it: how “is it possible that an animal having, for instance, the structure and habits of a bat, could have been formed by the modification of some animal with wholly different habits?” “Why, if species have descended from other species by insensibly fine gradations, do we not everywhere see innumerable transitional forms?” And finally, “can we believe that natural selection could produce . . . organs of trifling importance, such as the tail of the giraffe . . . and . . . organs of such wonderful structure, as the eye?” Darwin points to flying squirrels and the flying lemur as possible way stations on the road from quadruped to bat. But he points out that we do not have lots of transitional forms because they, along with the parent forms, “will generally have been exterminated by the very process of formation and perfection of the new form.” We do not find evidence of them in the geological record because it is very incomplete, a topic that he takes up in chapter 9. As to trifling versus wonderful organs, Darwin points out that we can’t know the giraffe’s fly-swatter tail doesn’t give it a competitive advantage in an environment of many disease-carrying and harassing insects, and that we are not justified in supposing “that any organ could not possibly have been produced by successive transitional gradations.”
Darwin looks at instinct as a difficulty, but illustrates that complicated instincts might be selected, using as examples the fairly sloppy hives of humble-bees, the mathematically perfect hives of hive bees (in terms of greatest storage for least possible use of wax and wall space) and a group of Mexican bees with intermediate hive-making efficiency.
In “Hybridism,” Darwin concludes that the received view that crossed species (as opposed to crossed varieties) have infertile offspring so that species will not become confused oversimplifies the actual facts about crossings and infertility, so that “the facts . . . do not seem to me to be opposed to, but even rather to support the view, that there is no fundamental distinction between species and varieties.”
In “On the Imperfections of the Geologic Record,” Darwin struggles with the absence of transitional forms in the geologic record, the seemingly sudden appearance of groups of species within single strata, and the paleontologists and geologists who are unanimously lined up against the mutability of species (though he thinks Lyell might be changing his mind). Darwin has nothing but “elevation” and “subsidence” to explain geologic movement, and he knows how inadequate his picture of the earth over geologic time must be. Then, in the next three chapters, “On the Geological Succession of Organic Beings” and “Geographical Distribution” I and II, Darwin comes back from his diffidence on the imperfection of the geologic record to underline that everything else points toward his theory. The slow appearance of new species, differing rates of change in species of different classes, and the fact that extinct species never recur, all support natural selection. Extinction is one of its key elements, because all forms are in competition. Size is not an advantage, rarity precedes extinction, and competition is most severe among forms most like each other. The forms of marine life—not identical species but families—change simultaneously throughout the world, and this is how we know we’re in the Cretaceous, for instance. More ancient forms tend to connect with characteristics widely separated in present forms, and all of these observations agree with a theory of descent with modification.
In geographical distribution, natural barriers produce species differences on either side of mountain ranges and different shores of continents. At the same time, various methods of dispersal ensure that species are similar on islands and the continents they are close to. Glacial dispersion explains why similar plants and animals to those we see in the north are left on mountaintops farther south. Oceanic islands sometimes lack a whole class of animals while other classes take its place—reptiles in the Galapagos or wingless birds in New Zealand replacing mammals, for instance.
In the penultimate chapter, “Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Organs,” Darwin argues that the classification of animals and plants isn’t arbitrary, but that the “Natural System” taxonomists have tried to find is itself a plan of relation by descent rather than a divine plan of creation or a scheme of like characteristics such as appearance or life habits. Homologous parts and organs—that is, the way “members of the same class resemble each other in the general plan of their organization” to show “unity of type” is, according to Professor Flower, “powerfully suggestive of true relationship, of inheritance from a common ancestor.” Darwin could not agree more. The extraordinary likeness among embryos of mammals, birds, and lizards may show the “condition of the progenitor of the whole group.” Rudimentary organs and limbs seem to show evidence of modification through disuse.
In his last chapter, Darwin recapitulates his arguments, speculates about why they might have been resisted by naturalists and geologists, and writes “I see no good reason why the views given in this volume should shock the religious feelings of any one.” Well, he could hardly be right about everything. In fact, of course he was wrong about a lot of things, but none of his errors really weakens the amazing structure of his argument. I wonder how many people who profess not to “believe” in evolution might be convinced by a candid reading of the book.
