The previous lesson left a fact without a mechanism: faunas succeed one another in a fixed order over an enormous span of time, and nothing explains how one gives rise to the next.
Charles Darwin supplied the mechanism, and the striking thing about it is how little it needs. There is no new force, no vital principle, no drive towards complexity. There are four observations, all of them agreed on by his opponents, and the conclusion follows from them by ordinary reasoning. This lesson takes the argument apart, states what it establishes, and is precise about the three things it could not supply in 1859.
The domesticated animals come first
On the Origin of Species was published on 24 November 1859 in a printing of 1,250 copies, and it does not begin with fossils or with the Galapagos. It begins with pigeons.
That is a deliberate choice of ground. Darwin had kept fancy pigeons since 1855 and had joined two London pigeon clubs. The breeds are grotesquely different: the pouter with an inflatable crop and elongated body, the fantail with thirty or forty tail feathers instead of twelve and its head bent back to touch them, the short-faced tumbler with a beak like a finch, the carrier with wattled skin around the eyes and bill. An ornithologist shown their skeletons alone would file them as several genera. Yet every fancier knows they are all Columba livia, the rock dove, because they are interfertile and because crossing two breeds throws back to a slate-blue bird with black wing bars.
Darwin's use of this is precise. Breeders did not design those birds. They selected, generation after generation, from whatever variation turned up, keeping the birds nearest the direction they wanted and killing or selling the rest. Nobody planned the fantail's vertebral count. What the case establishes is that undirected variation plus a sustained bias in who reproduces is sufficient to produce structural change of a magnitude that would be called generic in the wild. The word Darwin uses for the bias is selection, and he took it from the breeders.
The analogy has a well-known weakness, which he states himself: in the domestic case there is a selector with an intention, and in nature there is not. The argument therefore has to supply something that plays the selector's role without wanting anything. That is what Malthus gave him.
Geometric increase against finite resources
Thomas Malthus's Essay on the Principle of Population of 1798 argues that populations increase geometrically while their subsistence increases at best arithmetically, so that population is always pressing against its limit and is always cut back. Darwin read it for amusement in September 1838 and recognised the missing piece at once.
The biological version is stronger than the human one, because reproduction in most species is not close to replacement, it is enormously above it. Darwin makes the point with the slowest breeder he can find.
Example. Darwin's later editions state that from a single pair of elephants, breeding from age 30 to age 90 and producing six young, there would be nearly nineteen million alive after 745 years. What annual rate of increase does that imply, and what is the doubling time?
Starting from 2 and reaching is a factor of , so and per year, about 2.2 per cent. The doubling time is years. Two per cent a year is a rate no one would describe as explosive, and it is what the slowest-breeding land animal in the world achieves if nothing kills it. The conclusion Darwin wants follows immediately: since elephants do not cover the earth, almost every elephant born must fail to leave a descendant.
Now you. A bacterium of mass g divides every 20 minutes. How long until its descendants weigh as much as the earth, g?
Answer
The required increase is fold, and the number of doublings is . At 20 minutes each that is 2,640 minutes, or 44 hours. Under two days. The number is absurd, which is the point: unrestrained geometric increase is not merely fast, it is impossible, so the restraint is universal and continuous. Every population you can see is a population being held down, and the interesting question is which individuals it is held down on.
The argument in four premises
With those two pieces the argument can be stated compactly, and it is worth writing it out because most disputes about it are really disputes about one premise.
First, organisms produce far more offspring than can survive, as the elephant arithmetic shows. Second, individuals within a species vary, in every character anyone bothers to measure, and the variation is not confined to trivia. Third, some of that variation is heritable: offspring resemble their parents more than they resemble the population, which is why breeding works at all. Fourth, some of the heritable variation affects the chance of surviving and reproducing in the circumstances the organism actually faces.
From these four, the conclusion is forced. If more are born than can live, and they differ, and the differences are passed on, and some differences make survival likelier, then the composition of the next generation is not a random sample of the last. The characters that helped are over-represented. Repeat for enough generations and the population is no longer what it was. Darwin called the process natural selection, and later, adopting Herbert Spencer's phrase in the fifth edition of 1869, "survival of the fittest".
Nobody in 1859 denied any of the four premises. What was denied, and reasonably, was that the process could accumulate far enough to build an eye or to split a lineage. Darwin's answer occupies the other thirteen chapters: it consists of showing that the process, if real, would leave a specific set of traces, and then showing that those traces are present. Geographical distribution, the peculiarities of island faunas, rudimentary organs, the classification hierarchy of the previous lesson, and the imperfection of the fossil record are all handled this way. He called the book "one long argument", and it is a fair description of a structure that never has a single decisive experiment in it.
Example. A critic says that selection cannot create anything, since all it does is remove the unfit, and removal is not creation. Which premise is being attacked, and is the objection sound?
None of the four is being attacked, which is the first thing to notice: the objection is aimed at the conclusion by way of a claim about what the word "creation" can mean. It is unsound, and the reason is the third premise. Selection removes individuals, but what persists across generations is not individuals, it is heritable variants, and a variant that was rare can be made common. Combine that with the fact that variation keeps arising in each generation, and a favourable combination that no single ancestor possessed can be assembled step by step, because each step is retained by heredity while the next is being waited for. The process is cumulative, and it is the cumulation, not the removal, that does the building. Darwin's own answer was the breeder's: nobody says a fancier created nothing merely because all they did was choose which birds to keep.
Now you. A second critic accepts all four premises but says the conclusion holds only for small changes within a species, not for the origin of new ones. Is that a coherent position, and what would settle it?
Answer
It is entirely coherent, it was the majority view among naturalists for decades after 1859, and it cannot be refuted from the four premises alone. They license the claim that populations change; they do not by themselves license the claim that the change can accumulate without limit, or that it can produce two lineages incapable of interbreeding. What settles it is evidence of three separate kinds: a demonstration that no internal barrier stops the accumulation, which needs a theory of heredity Darwin did not have; direct observation of reproductive isolation arising, which came later; and the historical record of the intermediates, which is what the fossils and the genomes supply. Three of the remaining lessons in this course exist to answer this objection, and taking it seriously rather than dismissing it is the reason the answer is worth having.
Darwin against himself
Chapter six is titled "Difficulties on Theory", and it contains the strongest statements of the case against that anyone wrote in the century. This is not modesty, it is tactics: an objection you have stated better than your opponent cannot be used against you.
The famous one is the eye. Darwin writes that to suppose the eye, with all its inimitable contrivances, could have been formed by natural selection seems, he freely confesses, absurd in the highest possible degree. Then he answers his own point in two moves. If a graded series of eyes exists, each useful to its possessor, and if the variation is heritable, then the difficulty is only apparent. He goes through the series the invertebrates actually provide: a pigmented spot, a pit, a deeper pit that gives direction, a narrowed aperture that makes a pinhole image, a filled chamber, a lens. Every stage in that list is the working eye of some living animal, which is what makes it an argument rather than a story.
Example. Why is Darwin's move here logically stronger than simply asserting that the eye evolved gradually?
Because it converts a claim about the possible into a claim about the actual. The objection is that intermediate stages of an eye would be useless, so selection could not preserve them. If you can exhibit living animals at each intermediate stage, thriving, the objection's premise is false as a matter of observation and no argument about probability is needed. The structure recurs throughout the book: identify what your opponent claims to be impossible, then find it living somewhere. The limitation is equally clear, and Darwin does not hide it: showing that a series of viable intermediates exists does not show that any actual eye passed through that series, which is a historical claim needing evidence of a different kind.
Now you. Chapter six also raises the problem of sterile worker castes in ants, which leave no offspring at all. Why is this a sharper problem for the argument than the eye, and what does Darwin's answer commit him to?
Answer
The eye is a question of degree, but the workers look like a direct contradiction: a character that reduces its bearer's reproduction to zero cannot be favoured by a process defined in terms of its bearer's reproduction. Darwin calls it the one special difficulty which at first appeared to me insuperable and actually fatal to my whole theory. His answer is that selection can act on the family rather than the individual, since a well-provisioned colony leaves more fertile queens and the queen carries the tendency to produce such workers. That commits him to selection operating on a unit larger than the organism, which is exactly the ground on which the theory would be fought over for another century. The last lesson of this course returns to it with the arithmetic Darwin lacked.
Wallace, and what independent discovery shows
On 18 June 1858 Darwin, who had been accumulating evidence for twenty years without publishing, received a manuscript from Alfred Russel Wallace, then collecting specimens in the Moluccas. It set out selection acting on variation under the pressure of population, from Malthus, and asked Darwin to forward it to Lyell if he thought it worth anything. Darwin wrote that he never saw a more striking coincidence, and that if Wallace had his 1842 sketch he could not have made a better short abstract of it.
The compromise arranged by Lyell and Joseph Hooker was a joint reading at the Linnean Society on 1 July 1858, of Wallace's essay together with extracts from Darwin's unpublished 1844 essay and an 1857 letter. It attracted no attention whatever; the society's president reported at the end of the year that nothing striking had occurred. Darwin then wrote the Origin in thirteen months as an "abstract" of the much larger book he had planned.
The joint discovery is worth a moment. Wallace and Darwin differed in temperament, class, income and continent, and shared two things: field experience of geographical variation, and Malthus. That two people reasoning independently from the same premises reached the same conclusion is weak evidence that the conclusion follows from the premises rather than from the person, which is the most that such coincidences ever show. They later disagreed sharply, Wallace holding that natural selection could not account for the human intellect, so their agreement was not a matter of shared prejudice.
The three things the argument did not have
It is easy to read the Origin as the end of the story. It is better read as an argument with three specified holes, all of which Darwin knew about.
It had no theory of heredity. Darwin needed offspring to resemble parents and could not say why they do. His own attempt, pangenesis, published in 1868, supposed that every part of the body sheds particles called gemmules that collect in the reproductive organs; it is a blending theory, it is Lamarckian in effect since a modified organ would shed modified gemmules, and it is wrong. Worse, blending of any kind is fatal to the argument, for a reason a Scottish engineer would state in 1867 and the next lesson takes up.
It had no account of where variation comes from. Darwin treats variation as a given, uses the phrase "the laws governing inheritance are quite unknown", and is explicit that his ignorance is profound. Since the whole process is a filter, and a filter cannot produce what is not fed into it, this is a gap in the middle of the mechanism and not at its edge.
It had no time, in the sense of the previous lesson: Kelvin's twenty to forty million years was not enough, and Darwin's own estimate of 300 million years for the erosion of the Weald, printed in the first edition, was attacked so effectively that he removed it from later ones.
Two of those three would be filled by the same discovery, made three years after the Origin appeared, in a monastery garden in Brno, and ignored for thirty-four years.