Darwin's Origin of species, first edition (1859)
An introduction by Gordon Chancellor
and John van Wyhe
Charles Darwin's Origin of species of 1859 is one of the most important books ever published. In it, he offered two separate but intertwined arguments. His first was that the different species of living things on Earth evolved by natural means from earlier ancestors. In other words, he wanted to convince his readers that evolution is a fact. His second argument was that the process he called 'natural selection' was responsible for how species change and become adapted. The book is called Origin of species because it answered the question, what is the origin or source of new species? Where did they come from? He argued that they were not specially created from nothing but descended from earlier species.
The first part, chapters one to nine, establishes natural selection as a true cause. Of these chapters the first three establish its existence and the fourth makes the case for its ability to explain common ancestry. Chapters five to nine deal with a range of issues and difficulties which impinge on this competence.
The second part of the Origin,chapters ten to thirteen, attempts to cover what exactly natural selection is competent to explain about the history and diversity of life. These chapters lay down what requires the explanation provided in the first four chapters. These last four chapters deal with geology, geography, taxonomy, morphology and embryology. (The boundary between the two parts of the book is not always as clear as implied here, as chapter nine is closely related to ten.) Chapter fourteen is a recapitulation and conclusion.
The following is a summary of the Origin taking each chapter in turn. It is essential, however, to read the whole book to understand its arguments and evidence and therefore its impact.
Darwin's introduction gives a brief account of how he became convinced of evolution following the voyage of the Beagle, and how he had gradually developed his theory. He then moves on to a short summary of his book. A naturalist might become convinced that new species had descended from other species. But such a naturalist would remain unsatisfied without a mechanism to explain all the 'co-adaptations' in nature, such as the intricate ways in which the mistletoe relies on various trees, birds and insects for its existence. This sort of interrelation could not be brought about by species just somehow 'progressing'.
Darwin's last paragraph makes plain that he has barely scratched the surface of understanding how nature works, but states boldly that 'after the most deliberate study and dispassionate judgement' of which he is capable, he is certain that evolution is a fact. Finally he declares his conviction that natural selection 'has been the main but not exclusive means of modification'.
Variation under domestication
This chapter seems designed to show that all the animals and plants that man has bred have been produced by artificial selection. The chapter can be thought of as having three parts. The first deals with issues around fertility and sterility, to show that domesticated varieties can generally produce fertile offspring with one another which reveals their common ancestry. The second is a ten-page essay on the domestic pigeon, seeking to show the incredible variety of forms which breeders have produced from a single wild species by selecting from slight variations in individuals. The third part of the chapter explores how selection by man actually works.
Variation under nature
This chapter is one long statement of the extreme variability of sexually reproducing species. The constant small variations or differences between individuals were, in effect, the raw material available for natural selection. Darwin has to demonstrate that individual differences can gradually accumulate to the point of creating new varieties which diverge away from the parent species. He called these varieties 'incipient species'. These themselves could diverge into further species and in turn up through every level of the taxonomic hierarchy. The one thing they all would share is a common ancestor.
Struggle for existence
This chapter deals with the de facto competition between living things that results from the far greater number that are born than can ultimately survive. Darwin explains that sexual reproduction could double the size of populations each generation. But limited resources and predation usually prevent this. The point of the chapter is to stress the unmitigated struggle for existence in which each individual organism is constantly engaged. Darwin tries to do this by jolting his readers out of any romantic view they may have of the natural world:
We behold the face of nature bright with gladness, we often see superabundance of food; we do not see, or we forget, that the birds which are idly singing round us mostly live on insects or seeds, and are thus constantly destroying life; or we forget how largely these songsters, or their eggs, or their nestlings, are destroyed by birds and beasts of prey; we do not always bear in mind, that though food may be now superabundant, it is not so at all seasons of each recurring year. (p. 62)
Having made the point that competition is incessant, Darwin is ready to show how the tiniest advantage can make the difference between which few individuals survive long enough to reproduce and the vast majority which perish.
Darwin defined natural selection at the start of the chapter. Since all individual organisms vary, those born with any variations or differences that happened to be useful would have an advantage. Whereas “any variation in the least degree injurious would be rigidly destroyed” by predators, starvation etc.” And so “This preservation of favourable variations and the rejection of injurious variations, I call Natural Selection.” The language he uses tended to personify the purely natural process of life or death he called natural selection.
It may be said that natural selection is daily and hourly scrutinising, throughout the world, every variation, even the slightest; rejecting that which is bad, preserving and adding up all that is good; silently and insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life. (p. 84)
Darwin gives numerous examples illustrating the complexities of natural selection, as for example in the way grouse species have different plumage depending on where they live, to protect them from birds of prey. He shows how selection can alter populations of animals and plants at different stages in their life cycles, and he lays down the one law which he says is inviolable: that natural selection can never work to the exclusive benefit of another species. That simply could not work.
On page 87 Darwin introduces 'sexual selection', which he regards as auxiliary to natural selection. As males compete for access to females they may have to fight each other and this leads to the development of weapons only found in the male, such as the antler of the stag or the spur of the cock fowl. Even when there is no actual combat the male may have to compete with other males by being more attractive to the females, as for example by possessing more colourful plumage.
Darwin then focuses on the extraordinary interdependence of plants on insects to ensure their cross fertilisation. This leads into the reasons why cross fertilisation is so important in sexually-reproducing organisms. Darwin is convinced that although self-fertilisation is possible for many hermaphrodite organisms, it is absolutely essential that at least occasionally they mate with another individual. Darwin believes this ensures sufficient variation for natural selection to work on. Were a species not to vary, it would be unable to change to adapt to an ever changing world, and would eventually go extinct.
Darwin devotes several pages to the elaborate contrivances found in plants to ensure that they cannot normally self-fertilise, even though they produce both pollen and seeds. Darwin explains how insects such as bees are drawn by the scents and colours of flowers which produce sweet nectar. To suck the nectar the insects are forced by the structure of the flower to push inside and in so doing inadvertently get pollen stuck to them. They move on to the next flower of the same plant species and when reaching the nectar, deposit the pollen on the stigma, thus cross-fertilising the seed.
Darwin moves on to explain how animals tackle the need to cross-fertilise in different ways from plants. He says he knows of no case of hermaphrodites on land habitually self-copulating, and in aquatic animals the currents ensure that sperm will always be able to reach the eggs of another individual. Where this is not sufficient, other elaborate mechanisms have evolved, such as the extraordinary 'complemental' males Darwin discovered in the bodies of some female barnacles.
In the next section Darwin discusses the circumstances which are best for the action of natural selection. He starts by saying that copious variation is generally favourable and this obviously depends on large numbers of individuals covering large areas, or at least exploiting many different niches. Darwin believes that continents are for this reason better 'manufactories' of species than islands.
Darwin concludes this section with a word about 'living fossils', for example the duck-billed platypus, which have 'primitive' features, in this case a mammal which we now know lays eggs. He explains that such species often inhabit small areas, such as freshwater ponds, where he says they have been subject to less competition than in the sea, say, and have therefore evolved slowly or very little.
The final section is a full fifteen pages on 'divergence of character'. Darwin observes that the maximum amount of life in any given environment results when there is the maximum variety of forms, all exploiting different niches. He cites examples of small patches of soil which can provide homes to an astonishing number of species of plants from widely differing genera, families and even orders. Darwin called this tendency for natural selection to favour the emergence of distinct varieties adapted to different ecological niches the 'principle of divergence'. He regarded it as one of the two 'keystones' of his theory of evolution, capable of explaining the entire history of life and the hierarchy of classification, precisely like the divergence of twigs eventually creating an entire tree. Darwin explains the principle of divergence with the aid of a fold-out diagram, the only illustration in the Origin.
Darwin's diagram shows a series of fourteen horizontal dotted lines which represent time horizons getting younger from the bottom. Darwin says we can think of the vertical spaces between the lines as representing a thousand generations for some unnamed large and variable genus. The genus consists at the start of eleven species, each designated a capital letter A to L. The eleven species are more or less loosely grouped by common characters; A-D, E-F and G-L, with the most variable species (A and I) at or near the far ends of the diagram and the least variable in the middle. As time moves 'up', natural selection affects the eleven species in different ways.
The two variable species (A and I) 'throw off' varieties which fan out to exploit what we would today call different ecological niches. A few of these varieties (a, f, m, w and z) prosper and eventually produce two or three new species each, which have shared group characteristics and can therefore be regarded as five new genera, or perhaps even new families. Note that it tends, according to Darwin, to be the more extreme variants in the fan of descendants which go on to produce more fans. This tendency causes divergence in particular directions. Darwin argues that new variants exploit new 'ecospaces'.
None of the less variable species evolve much by natural selection of favourable varieties and all but one of them become extinct after different periods of time, B-D, H, K and L in the first three thousand generations. G carries on till the sixth thousand and E till the tenth, showing how extinction is more likely for species which do not change. Ultimately, at the top of the diagram, we are left with fifteen species of which only F was there at the beginning, although it is still intermediate between the descendants of A and I and because the latter have evolved into new genera F appears less closely related than it was to the ancestral A and I.
Darwin says we could consider the time lines to represent a hundred million generations so that the diagram would then represent geological history in which the time lines would be rock strata and some of the extinct species would only be known as fossils. Furthermore, he says, the nested hierarchy of varieties, species, genera, families, orders, classes and so on makes perfect sense when explained as the result of natural selection as shown in the diagram, whereas it would make no sense at all if species were special creations unrelated to other closely similar species.
Darwin closes this long chapter with a summary of all the key points. He has shown how all these establish the competence of natural selection to explain the history and diversity of life on Earth which he finally illustrates with the ancient metaphor of the Tree of Life:
As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever branching and beautiful ramifications. (p. 130)
Laws of variation
This chapter serves two functions. Firstly, it bolsters Darwin's case that species are not fundamentally different from varieties. Secondly, it continues Darwin's theme of establishing natural selection's credentials as the most likely cause of evolution. Darwin has to pull together a vast array of disparate observations about the way organisms vary, how they live, grow and reproduce, and with how offspring inherit their traits. In this chapter he shows that variation suggests underlying similarities between allied species but gives no support to the view of species as independent creations.
Chapter five opens with a survey of how animals and plants vary. His first sub-section is devoted to 'use and disuse' and explores how a change in the habits of a species can cause a character to grow, or wither. He is particularly interested in the loss of wings in insects and birds on islands which no longer benefit from flying, and the loss of sight in cave dwelling animals.
Darwin closes with an elegant summary of the chapter:
Whatever the cause may be of each slight difference in the offspring from their parents—and a cause for each must exist—it is the steady accumulation, through natural selection, of such differences, when beneficial to the individual, that gives rise to all the more important modifications of structure, by which the innumerable beings on the face of this earth are enabled to struggle with each other, and the best adapted to survive. (p. 170)
Difficulties on theory
Darwin opens chapter six with a disarmingly frank admission:
Long before having arrived at this part of my work, a crowd of difficulties will have occurred to the reader. Some of them are so grave that to this day I can never reflect on them without being staggered; but, to the best of my judgment, the greater number are only apparent, and those that are real are not, I think, fatal to my theory. (p. 171)
Darwin starts with the lack of transitional forms preserved in the fossil record. Natural selection automatically forces the less successful varieties to go extinct so that they may not last long. And, as a geologist, he shows that the fossil record is extremely incomplete, so that one must not expect to find as fossils more than a tiny fraction of the forms which have ever lived.
Geography, Darwin says, is constantly changing. Climate fluctuates and barriers to migration keep coming and going. Thus, over time, descendants from a common ancestor become adapted to different ways of life. Hence the marsupial animals of Australia are clearly related to the extinct fossil marsupials of that country.
Darwin then moves on to how an organism can change from one way of life to another, for example to be able to fly through the air. How could such varied creatures as insects, fish, reptiles, birds and mammals have managed to acquire this power? His strategy is to show that once again the problem is illusory and that there are gradations all around us between non-flying and flying forms.
Darwin's next section is on the apparent 'difficulty' of organs of extreme perfection and he chooses to illustrate his solution with the human eye. This example had been Paley's paradigm for the 'proof' of God's design in his Natural theology (1802). Paley said that the eye was so extremely perfect that it can only have been designed by an intelligent designer. Darwin asks on page 188, however, if we have 'any right to assume that the Creator works by intellectual powers like those of man?' He turns Paley's logic upside down by showing that the eye is not perfect and is the result of the unplanned process of natural selection. He shows that there is in nature an abundance of intermediary stages between complex eyes and simple light sensitive tissues, as for example in many molluscs.
Darwin's next example is the fish's swim bladder which he cites as an example of an organ which has multiple functions, chiefly buoyancy and breathing, and formed a step on the transition from gills to lungs.
In all these cases, however, Darwin is confident that the intermediate steps can indeed exist by the modification of existing organs. He uses this fact to turn special creation on its head by arguing that if every organism were designed for perfection there would be no reason for such intermediates. In fact, he says, the very existence of intermediates is strong evidence for evolution.
The final group of difficulties are 'organs of little apparent importance'. Darwin cautions his readers not to assume their lack of importance; a fly-swatting tail can mean the difference between life and death to a mammal plagued by blow flies on the African plains, and if not now, an organ may have been important in the past. What right have we to think we understand everything that is going on in nature? We might assume for example that woodpeckers had to be green if we had never known that there are black and white woodpeckers.
In answering these difficulties Darwin provides a catalogue of amazing examples to show that natural selection will always enhance and never erode the net reproductive success of a species. If it makes a species more beautiful to us humans, that is just a by-product of its usefulness to the species. He introduces the concept of what we might call relative, or 'good enough' adaptation, whereby natural selection creates adaptations which are less than perfect but still better than what went before, as proof against special creation which should produce perfect adaptation. The endemic species of New Zealand, for example, show 'good enough' adaptations in isolation but are not good enough to defend them against introduced species evolved in larger countries where the competition has been tougher.
The bee's sting, Darwin says, evolved from ovipositors for boring into wood and has been 'co-opted' for defence, but this is not a perfect adaptation as it often kills the bees. The stinging function will, however, be retained by natural selection because it has a net benefit to the bee's extended family.
Darwin's chapter summary returns to the 'two great laws' which were widely believed to underlie all living things. 'Unity of type' was the homologies across different species in a group, such as the five finger bones in the vertebrates. The 'conditions of existence' were believed to determine an animals structure. For example an aquatic creature like a whale has fins for swimming.
In his closing and arguably most profound paragraph in the entire Origin, Darwin - for the first time in the history of biology - sweeps these two great laws together. Unity of type, he says, is explained by unity of descent, while the conditions of existence act mainly through natural selection, constantly adapting organisms to their ways of life.
In this chapter Darwin shows that instincts vary at least as much as other features of animals. Having always been a countryman Darwin was extremely knowledgeable about the behaviour of domestic animals, such as horses, dogs and pigeons, and he can easily point to examples of the occasional individual which behaves differently. A new behaviour might lead an organism to get more mates and leave more offspring, some of which may inherit their parent's behaviour. Citing 'Natura non facit saltum' (nature does not make jumps), he says that this variation of behaviour is usually a matter of degree rather than of large differences.
That instincts are inherited Darwin easily demonstrates with numerous examples and he goes to some trouble to show that habits acquired during life are not necessarily inherited. As an example of an almost certainly inherited instinct he cites the genius of Mozart.
We are all familiar with the degrees of tameness of birds. Small birds may feed from our bird tables but birds of prey will rarely visit our gardens because they have had thousands of generations of persecution from man and his pets. Darwin had seen the almost comical tameness of birds on islands where they have little experience of man, but the native Galápagos hawk was the most difficult bird there to catch. Domesticated animals gradually lose their wild instincts because man will not breed from individuals which display them. Darwin cites dogs from Tierra del Fuego which when brought to England kill chickens because they have had no previous experience of domestic poultry. Like other traits, Darwin says that natural selection could never produce an instinct for the exclusive benefit of another species.
From the many natural instincts he could give as examples of evolution, Darwin chooses three to discuss in detail: the parasitic egg laying habits of cuckoos; the slave-making instinct of ants, and the bee's ability to make honeycomb.
Darwin next deals with the problem of the existence of 'castes' in the sterile 'worker' females in insect colonies. Since the workers are sterile, how could they pass on their characteristics and instincts? He argued that in the insect colony, the fertile members will flourish if their sterile worker offspring are useful. Then the fertile insects will have more sterile offspring with the desirable worker traits, and so on. Darwin then tackled the problem of how to explain the origins of castes in the first place. To do this he pointed to several species of ants which exhibited varying degrees of caste formation from none at all to partial castes to those with highly developed and distinct castes.
The chapter ends with a triumphant claim that it is not special creation which accounts for all the wonderful instincts possessed by animals. Instead they are all “small consequences of one general law, leading to the advancement of all organic beings, namely, multiply, vary, let the strongest live and the weakest die.” (p. 244)
In this chapter Darwin addresses the ability of separate species either to produce sterile or fertile offspring or none at all. Almost all other naturalists in 1859 believed that species were specially and separately created and were therefore fundamentally different from varieties. Varieties, or races, were a local population descended from a parent species, and differing from it somewhat in colour or other traits.
Darwin argued that there is no essential difference between varieties and species. The difference is one of degree, not of kind, clearing the path for his view that species are formed by natural selection and that the tree of life follows from the simple branching of two twigs or lineages.
Darwin's contemporaries believed that the usual sterility of hybrid offspring from two species is of advantage to the two species. Many believed that by this means species were kept distinct and separate and a confusing blend of everything mixed together was prevented.
But Darwin argued that natural selection could never create sterility as it is of no benefit to the hybrids to be sterile. Instead, sterility must be an incidental consequence of the accumulated differences between the two species since they diverged from a common ancestor. The longer they had been separated, the less likely that they could produce fertile or even infertile offspring.
He ends the chapter having demonstrated conclusively that there is no way to make an absolute distinction between a variety and a species. It is simply a matter of the degree of divergence from ancestors. A small amount of change we call a variety. A more substantial amount of change we call a species. That is all he needs to defeat the view, traceable back to Plato's Timaeus of c. 360 BCE, that species are unchangeable entities.
On the imperfection of the geological record
Darwin's aim in this chapter is to negate the obvious objection that the fossil record did not clearly reveal many of the intermediate forms that his theory predicts must have existed. He explained that the fossil record, whilst compatible with his theory, is so fragmentary that it cannot be used to disprove it. But he goes further and implies that the fossil record is so fragmentary that it cannot be used to prove it either, so that establishing the truth of his theory must be based on its ability to explain the phenomena of life as we see it today.
Darwin opens by pointing out that we should not assume that the intermediates we are looking for as fossils will look 'midway' between previously discovered fossil species. They are more likely to resemble the common ancestor of two similar species and for reasons explained in chapter four such intermediates are often pushed to extinction too fast to leave abundant fossils.
Darwin then reminds the reader of the almost inconceivable time that the Earth has existed. He had seen at first hand the mind-boggling piles of strata exposed in the Andes. He had studied in detail the crawlingly slow rates at which coastlines and valleys are eroded which resulted in the depositing of sediments under the sea- which is how new sedimentary strata are formed. He tries to give some idea of the immensity of geological time by calculating the thickness of strata across Britain as 72,584 feet, or 'very nearly thirteen and three quarter British miles' (p. 284). He then uses an estimate for the rate of sedimentation by the Mississippi to translate that thickness into years.
Darwin then demonstrates the scale of the age of the Earth by citing the thickness of the sediments in the Weald of Kent and Sussex as 1,100 feet. Using an average rate for coastal erosion he arrives at the figure of 300 million years since the Weald emerged from the sea, an event which occurred during the Tertiary. This is long after life had started looking like life today, so the Earth must be unimaginably older than that.
Unfortunately his calculation was based on so many assumptions that it backfired on him and within days of publication he halved the estimate while making the corrections for the second edition. This was too late to stop it being challenged in a review of the first edition, so Darwin omitted it entirely from the next available edition (the third). Darwin's methodological mistake had been to assume that the erosion of the Weald was overwhelmingly by marine action, when in fact erosion while on land would have been much more important.
The next section looks more closely at 'the poorness of our palaeontological collections', for if we 'now turn to our richest geological museums… what a paltry display we behold!' (p. 287). Darwin recites a litany of reasons for this: Europe is the only continent which has been closely examined for fossils; only hard-bodied species are preserved; sediment only forms in a few areas; sea shore species such as barnacles are usually smashed by the waves; the rain dissolves shells from the sediment if it survives the uplift and land-dwelling species are unlikely to be buried in such sediments in the first place. In fact, as Darwin quotes himself as concluding in his Voyage of the Beagle of 1845 (pp. 344-345), the only places where sediment is likely to preserve fossils are where the seabed is gradually subsiding.
Darwin goes on to show how the chances of ever seeing the formation of a new species preserved in the sequence of fossils is negligible. He explains the difficulties for geologists of his day: if a species enters or leaves the rock sequence it may not record speciation or extinction but may just record migration; how are we to assess the precise relationships of the species and how can we identify which may be an ancestor? He illustrates his point with a thought experiment for the Malay Archipelago, a region the size of Europe with perhaps more species than anywhere else on Earth, but because of the geography almost none of them will be fossilised. He declares that the lack of intermediate fossils is 'probably the gravest and most obvious of all the many objections which may by urged' against his views.
Darwin opens the next section by saying that his old Cambridge geology mentor Adam Sedgwick had declared the sudden appearance of groups of allied fossil species to be a fatal objection to any theory of evolution. Darwin explained this as mainly an unfortunate result of the incompleteness of the fossil record. The abrupt appearance of a novel group must be an artefact of poor preservation of the group's ancestors, and he can cite barnacles as proof of this. When he had published his monograph the sudden appearance of acorn barnacles in the Tertiary was a 'sore trouble' to him until one was reported (mistakenly, as it later turned out) from the Chalk. He leaves the reader with the striking caveat that taking the fossil to be complete is as rash as landing 'for five minutes on some one barren point in Australia' and then discussing the number and range of that vast country's species (p. 306).
In his final section Darwin declares 'the sudden appearance of groups of allied species in the lowest known fossiliferous strata' is a much graver difficulty, because the sheer range of fossils in these rocks proves that the world must have teemed with ancestral species before this, none of which has ever been found. He predicts that pre-Silurian fossils will eventually be found in abundance, as indeed they have since been.
Darwin closes the chapter with a one-page summary and a wonderful simile for the incompleteness of the fossil record:
I look at the natural geological record, as a history of the world imperfectly kept, and written in a changing dialect; of this history we possess the last volume alone, relating only to two or three countries. Of this volume, only here and there a short chapter has been preserved; and of each page, only here and there a few lines….On this view, the difficulties above discussed are greatly diminished, or even disappear. (pp. 310-311)
On the geological succession of organic beings
In this chapter Darwin shows how the fossil record is not just consistent with evolution but actually reveals much about the evolutionary history of life. The species one finds preserved in the most recent formations resemble most closely those still living. The resemblance between present and past life decreases as one descends into the rock record, as expected if evolution has occurred.
If evolution is true, a species can never reappear once it has gone extinct. Darwin explains that the rare cases where such an event appears to have happened from the fossils must be due to species having migrated away then later returning. He extends such generalisations to groups of species and in several passages he speaks of the waxing and waning of groups over time in the same way that in earlier chapters he had talked of the comings and goings of varieties. Thus the fossil record, patchy as it is, exactly reflects what one would expect if evolution has occurred since the origin of life: "Each formation, on this view, does not mark a new and complete act of creation, but only an occasional scene, taken almost at hazard, in a slowly changing drama." (p. 315)
Darwin argues that extinction is a gradual process as decline leads to rarity then, when there are too few individuals to maintain a viable population, extinction inevitably follows. Darwin relates hisexperience of unearthing extinct mammals in South America. Perhaps his most remarkable find was a single horse's tooth embedded with the extinct beasts thereby proving that horses must have existed in the Americas then gone extinct, long before the Spanish reintroduced them in the sixteenth century. Why did those native horses die out if South America is obviously a suitable environment for horses today? What Darwin took from this case in his Voyage of the Beagle in 1845 was that we can rarely know why a species died out in the distant past.
The next section deals with one of the most remarkable features of the fossil record, namely the simultaneous changes of whole fossil marine life across the world. Indeed, this 'correlation' of fossil species is the basis on which geologists compare the ages of rocks across the globe, even when the sediments which contain the fossils may be strikingly different. Darwin argues that such global events are exactly what is predicted by his theory and are also strong arguments against special creation of species to suit particular environments.
Opening a new section 'on the affinities of extinct species to each other and to living forms' Darwin declares that all life is part of one continuous system and there are no major groups of fossil species which do not have living relatives. This declaration is based on the findings of the best comparative anatomists, none of whom were guided by a prior belief in evolution. Darwin then states that the further back one traces any group, the less it resembles living members but the more it resembles its nearest neighbouring groups; thus reptiles in Palaeozoic times were more like amphibians and further back still, both are more like some kinds of fish.
Darwin has recourse to his chapter four diagram to show why, when read 'downwards', that is backwards in time, groups tend to converge in this way. He likens the history of a group to tree branches joining a trunk, although of course many twigs encountered on the way will have no living descendants. This also explains why, he says, the fossils will generally appear intermediate to those in the preceding and succeeding formations.
The next section is a brief discussion of whether the fossil record supports the view, commonly held by Victorian naturalists, that life tends to 'progress'. Darwin's approach to this question is more or less to dismiss it as untestable and teleological and therefore of no value to science. He does, however, explain that progress is a fair term for some trends in evolution inasmuch as fitter forms replace their parents.
This is followed by another short section 'on the succession of the same types within the same areas during the later Tertiary periods'. This was the 'succession of types', mainly of mammals found in Australia, South America and Europe, in deposits laid down since the Ice Ages. The point is that in any one area the successive species are all closely related but are quite distinct from the types found in other areas. Darwin points out that succession of types is remarkable because there is no obvious correlation between the characters of the species and their environments. It must therefore be due to inheritance within groups of species isolated by geography from other groups. It is proof of evolution.
Certain types of animals and plants thrive best in certain climates, but if that was all there was to explain we would expect the same types of tropical creatures, say, in the jungles of South America as we see in the jungles of South East Asia. In fact there are very few species common to these two parts of the world.
Darwin notes that the land areas of the world are divided into great biological regions, the most distinct being the continents of Europe, Africa and Asia on the one hand and of the Americas on the other. But how to explain the biological differences between these Old and New Worlds? Darwin says the answer is the 'second great fact', that animals and plants have evolved separately in the two regions because the oceans are barriers to migration between them.
Darwin now arrives at his 'third great fact', that on any one continent there are striking similarities between all the species within any particular group, birds for example, which is almost the opposite to their differences with the same group on other continents. It was the 'Americanness' of the Galápagos fauna and flora which had struck Darwin so forcefully when first examining the archipelago. Why, he wondered, if species were created to suit their environments were the Galápagos species not like those he had encountered in the Cape Verde archipelago off the African coast? The answer had to be that each archipelago had been populated by migration from its neighbouring continent and that its endemic species had evolved from the migrants; in other words they had not been created in situ as Lyell had claimed. But the bird story was even more subtle, because Darwin had discovered that there were two very obviously similar species of Rhea occupying distinct parts of South America. So what is the bond which exists between these species? 'The naturalist' says Darwin on p. 350 'must feel little curiosity, who is not led to enquire what this bond is'. That bond is 'simply inheritance'.
Darwin next discusses in some detail the actual means by which migration occurs in nature. He recounts some of his experiments on the surprising abilities of seeds to germinate after salt-water immersion, indicating how they can travel great distances across the sea. He points to dispersal by birds as a potent mechanism.
Darwin now addresses the three greatest biogeographical difficulties raised by his theory that species only have one origin in one place. The first difficulty is the discontinuous distribution of alpine species which he explains as resulting from climate change. He starts with the curious distribution of 'relict' alpine species on the tops of mountains. These are now separated by temperate lowlands over which they could not easily migrate and he attributes their distribution not to independent creations but to warming of the climate since the Ice Ages leaving the alpine species stranded on the mountains.
Geographical distribution continued
Darwin begins this chapter with what he calls the second of his greatest difficulties, namely the wide distribution of freshwater species. Obviously freshwater species can't migrate across the sea. Darwin's solution is that because freshwater is relatively ephemeral, only those species adapted to migration across land will have survived to evolve. He describes their great migratory powers in some detail, largely based on his own research. Birds are a crucial agency as they eat, or have accidentally attached to them, freshwater snails, seeds and eggs which they often transport rapidly over remarkable distances between water bodies. Flying water-beetles too, as Darwin attests, may be borne by the wind over land and sea, and fish containing species they have swallowed may manage to swim down one river and up another. By these agencies the best adapted freshwater species can spread far and wide.
In the next section he plays one of his trump cards: his solution to the peculiar fauna and flora of oceanic islands. Darwin starts by ruling out 'land bridges' as an explanation for the similarity of island species to their neighbouring continents. Next he states that island life is lacking in numbers or variety of species compared to continents and this cannot be due only to the relative simplicity of island environments. On the other hand the proportion of unique or endemic species on islands is far higher than elsewhere. Mammals other than bats, and freshwater species are generally rare or absent, in the first case because their metabolic needs make long journeys difficult and in the second case because they are killed by sea water. Also, island floras are often dominated by species with seeds well adapted for dispersal, such as umbillifers (the celery, carrot or parsley family). All these facts make sense on the theory of evolution but not on the belief that species are created to suit their environments.
Darwin notes that the degree of difference between the species across an archipelago is often in proportion to the depth of the sea between the islands, as for example in the Malay Archipelago where there are Asian land species in the west and Australian species in the east and coincides with a deep water channel. Of course the obvious explanation is that the depth betrays the geological age of the different blocks of which the islands are an above-sea expression, and prevents migration even when sea level is low.
Darwin finally focuses on the differences he had discovered between species across the Galápagos, and their similarities to those on the South American mainland. These facts were crucial for his own path to the theory of evolution. Darwin focuses on the differences between the mockingbird species, which were different on different islands, yet obviously closely related to species on the mainland whence he argues they originated.
In his summing up of the four 'geo' chapters Darwin cites the view of Edward Forbes (who had died too young in 1854) that 'there is a striking parallelism in the laws of life throughout time and space' and he declares that this parallelism follows from the theory of evolution by natural selection.
Mutual affinities of organic beings; morphology; embryology; rudimentary organs
In this substantial chapter Darwin deals with the relationships between and the structure of organisms and what these can tell us about evolution. Unlike all the other chapters this one is more concerned with what organisms inherit than with how they interact with each other and the world around them.
In the first half of the chapter Darwin looks at how his theory makes sense of the classification that naturalists used in order to show the similarities between group of animals and plants. There are nested hierarchies of species, genera, families, orders and classes. Once more Darwin refers to the Origin's only diagram to remind readers how some groups have few members while others are more diverse and how over time the tree of life evolves. The accepted groupings, he says, constitute the only true natural system because they are the result of 'propinquity of descent', that is evolution from common ancestors, and not as many had claimed, because they fit the plan of the Creator.
Darwin explains that to group organisms together naturally one must identify the organs which are most stable because they are most likely to reflect common ancestry. The great anatomist Richard Owen, Darwin says, lays stress on the sex organs for this reason. Rudimentary organs are also good clues to ancestry even though they may have become useless. One must also learn to understand the way organs may have been modified by adaptation to different ways of life; as for example the mammalian limb which has the same underlying structure in a bat as it does in a seal. Embryonic characters, as Henri Milne Edwards and Louis Agassiz insist, are also of high importance for classification, as for example in the basic division of plants depending on whether they have one or two seed leaves.
The crucial issue in classification is not one of amount of difference, Darwin says, but of arrangement. Referring to his diagram, if we take the bottom to be the Silurian and compare a species alive then with its descendant species alive today they may look very different but are still 'cousins to the millionth degree'. Most species will have left no descendants but some may be 'living fossils' which have barely changed. Darwin makes a fascinating comparison with human languages, some which may have changed little for thousands of years while others have evolved beyond recognition. The only meaningful classification of languages, he argues, would be the one which best reflected the historical relationships of the peoples who originated those languages.
Darwin reminds us how the extraordinary diversity of artificial pigeon varieties, when grouped carefully, reveals how they have all descended from the rock pigeon. He explains that even human races follow the same rules.
Darwin says that naturalists have always, perhaps without realising it, used the natural system in their classifications. Even when the males and females of a species are extremely different, as often happens, they are not given different scientific names. Likewise for the different life stages, for example the butterfly which has passed through egg, larva and pupa before the adult stage. But what, he asks, should the naturalist do if a species of kangaroo was found to have evolved from a bear? Should the kangaroo then be re-named a bear in this 'preposterous' case? Darwin answers in the affirmative, because a true classification must reflect descent.
Darwin distinguishes homologies from analogies and says that he is here following in Lamarck's footsteps. Homologies are characters shared because of common descent, such as the vertebrate limb, which should be the basis of classification. Analogies are characters shared because of similar mode of life, such as fins of whales and fishes, which are therefore less important for classification. Finally, he reminds us, again with reference to his diagram, that extinction has been the main reason why organisms fall into distinct groups rather than appearing as one continuum.
In the next section Darwin deals with morphology, or the 'unity of type' within groups of organisms, a subject Darwin calls the 'very soul' of natural history. He says that the mouth parts of many different types of insects are all constructed in the same way, just as Owen had shown was the case in his Nature of limbs (1849)with vertebrates, and he asks:
What can be more curious than that the hand of a man, formed for grasping, that of a mole for digging, the leg of the horse, the paddle of the porpoise, and the wing of the bat, should all be constructed on the same pattern, and should include the same bones, in the same relative positions? (p. 434)
Darwin explains that the reason for the underlying pattern is gradual modification from an ancient common ancestor.
The next section deals with embryology. Darwin shows how Karl Ernst von Baer's great generalisation - that organisms start simple and grow more complex, is evidence for evolution. He recites many cases of organisms in different groups resembling each other more in their earlier life stages when they show features of little use to them as adults: kittens having stripes or rows of spots and barnacle larvae having legs. He cites Agassiz's story of finding an unlabelled vertebrate embryo specimen which he couldn't decide whether it was a mammal, bird or reptile. Darwin then recaps the evidence from chicks that pigeon breeds are all the same species. This is to illustrate how features are more variable in the adult because the younger stages are not subject to such ruthless natural selection.
The reason for all this, Darwin says, is that 'community in embryonic structure reveals community of descent'. Which is to say that similarity in embryonic structure reveals similar ancestry. Darwin ends the section on wonderful flourish: "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." (p. 450)
The final section focuses on the purpose of 'rudimentary, atrophied or aborted organs'. They are evidence of natural selection in the past, before the organs in question fell into disuse. He cites male nipples as a classic example. Other examples quoted are the limb bones sometimes found in snakes and the proverbially rare 'hens' teeth' which are sometimes found in the embryo state, giving unmistakeable evidence of their reptilian ancestry.
Darwin finally reminds us that an organ can become a liability if life habits change, as they take precious energy and resources, as in the case of eyes in cave animals or wings in island animals. In such cases natural selection may atrophy or abort the organ which may still reveal its original use like 'the letters in a word, still retained in the spelling, but become useless in the pronunciation, but which serve as a clue in seeking for its derivation.' (p. 455) The word 'knee' is an example of this.
The closing paragraph of Darwin's chapter summary is worth quoting in full:
Finally, the several classes of facts which have been considered in this chapter, seem to me to proclaim so plainly, that the innumerable species, genera, and families of organic beings, with which this world is peopled, have all descended, each within its own class or group, from common parents, and have all been modified in the course of descent, that I should without hesitation adopt this view, even if it were unsupported by other facts or arguments. (pp. 457-458)
Recapitulation and conclusion
Darwin used the analogy of artificial animal and plant breeding to explain how selection works in nature. He has shown that selection in nature has the power to modify varieties to the point where they are effectively new species. This is the natural origin of species. Thirdly, Darwin has presented a vast range of evidence to show that living things have all descended with modification from one or a few common ancestors. He has anticipated and answered every conceivable objection. He reminds the reader of the enormous weight of evidence in favour of natural selection as the mechanism by which species have originated and become adapted.
To conclude, he asks why species have hitherto been thought of as special creations, and answers that it is our inability to appreciate the vast timescale over which life evolves. He says there is a rising generation of naturalists who will examine the matter objectively and he calls those he has convinced not to hold their tongues. He wonders how far the theory can be extended and cautiously suggests that all life will be found to have a common origin.
Darwin leaves the reader with some of the most poetic passages ever to appear in a work of science, including this justly famous understatement:
In the distant future I see open fields for far more important researches. Psychology will be based on a new foundation, that of the necessary acquirement of each mental power and capacity by gradation. Light will be thrown on the origin of man and his history. (p. 488)
Finally, Darwin closes with this very famous closing sentence:
There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.
Darwin, C. R. 1859. On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. London: John Murray. [1st edition] Text Image PDF F373
See Francis Darwin's annotated presentation copy of the first edition, here.