* After acknowledging in chapters 11 & 12 of THE ORIGIN OF THE SPECIES that the geological record was imperfect, in chapter 13 Darwin pointed out that, for all its sketchiness, what was known from the geological record was consistent with the evolutionary succession of species. In chapter 14, he presented a particularly strong argument in favor of his concepts, demonstrating that different regions tended to feature distinct groups of organisms that strongly suggested an evolutionary history and were otherwise difficult to account for.
In chapter 15, Darwin closed out his formal argument by discussing how the familial arrangement of organisms in a "tree of life" was consistent with his ideas, with the familial groupings due to common ancestry. That common ancestry was demonstrated by the sometimes remarkable variation in common structures, strong structural hints of ancestry as shown by embryonic development, and the presence of reduced or "vestigial" organs in modern species.
* In chapter 10, Darwin commented that the geological record as it was known did support the notion of evolution:
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Let us now see whether the several facts and rules relating to the geological succession of organic beings, better accord with the common view of the immutability of species, or with that of their slow and gradual modification, through descent and natural selection.
New species have appeared very slowly, one after another, both on the land and in the waters. [...] Species of different genera and classes have not changed at the same rate, or in the same degree. In the oldest [...] beds a few living shells may still be found in the midst of a multitude of extinct forms. [...] Yet if we compare any but the most closely related formations, all the species will be found to have undergone some change.
These several facts accord well with my theory. I believe in no fixed law of development, causing all the inhabitants of a country to change abruptly, or simultaneously, or to an equal degree. The process of modification must be extremely slow. The variability of each species is quite independent of that of all others. [I]t is by no means surprising that one species should retain the same identical form much longer than others; or, if changing, that it should change less.
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Darwin demonstrated his usual cautiousness here, pointing out that some species may hang around with little visible change for a very long time, while others change rapidly and many become extinct. There was no law, as Darwin saw it, that species had to evolve -- if they were well-adapted to their environment, they might well persist in much the same forms indefinitely -- and didn't necessarily change at the same rates. This is established in modern times as the somewhat controversial notion of "punctuated equilibrium", in which specific evolutionary changes arise relatively quickly in response to a change in environment that puts a species under stress, to then settle out once the change has reached a stable point.
Darwin also noted that the competitive nature of evolution by natural selection effectively guaranteed that species often become extinct:
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The theory of natural selection is grounded on the belief that each new variety, and ultimately each new species, is produced and maintained by having some advantage over those with which it comes into competition; and the consequent extinction of less-favoured forms almost inevitably follows.
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Of course, the big punchline for his theory was that, as far as naturalists of the time could see, the extinct forms of the past were clearly related to the living forms of the present:
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[...] all fossils can be classed either in still existing groups, or between them. That the extinct forms of life help to fill up the wide intervals between existing genera, families, and orders, cannot be disputed. For if we confine our attention either to the living or to the extinct alone, the series is far less perfect than if we combine both into one general system.
With respect to the Vertebrata, whole pages could be filled with striking illustrations from our great palaeontologist, [Richard] Owen, showing how extinct animals fall in between existing groups. Cuvier ranked the Ruminants and Pachyderms, as the two most distinct orders of mammals; but Owen has discovered so many fossil links, that he has had to alter the whole classification of these two orders; and has placed certain pachyderms in the same sub-order with ruminants [...]
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Not only that, but there was an evident succession of forms through the fossil record:
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Closely connected with the statement, that the organic remains from an intermediate formation are in some degree intermediate in character, is the fact, insisted on by all palaeontologists, that fossils from two consecutive formations are far more closely related to each other, than are the fossils from two remote formations.
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Darwin also pointed that such successions of forms tended to be more or less unique to different regions. Fossils of giant armadillos and ground sloths are found in the Americas, but not in the Old World or Australia; it is hardly a coincidence armadillos and tree sloths are only found naturally in the Americas as well. The significance of this was emphasized in the following chapters.
He ended the with a tidy summing-up of the current and previous chapters:
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I have attempted to show that the geological record is extremely imperfect; that only a small portion of the globe has been geologically explored with care; that only certain classes of organic beings have been largely preserved in a fossil state; that the number both of specimens and of species, preserved in our museums, is absolutely as nothing compared with the incalculable number of generations which must have passed away even during a single formation [...] All these causes taken conjointly, must have tended to make the geological record extremely imperfect, and will to a large extent explain why we do not find interminable varieties, connecting together all the extinct and existing forms of life by the finest graduated steps.
He who rejects these views on the nature of the geological record, will rightly reject my whole theory. For he may ask in vain where are the numberless transitional links which must formerly have connected the closely allied or representative species, found in the several stages of the same great formation. [...]
Passing from these difficulties, all the other great leading facts in palaeontology seem to me simply to follow on the theory of descent with modification through natural selection. We can thus understand how it is that new species come in slowly and successively; how species of different classes do not necessarily change together, or at the same rate, or in the same degree; yet in the long run that all undergo modification to some extent. [...]
We can understand how the spreading of the dominant forms of life, which are those that oftenest vary, will in the long run tend to people the world with allied, but modified, descendants; and these will generally succeed in taking the places of those groups of species which are their inferiors in the struggle for existence. [...]
We can understand how it is that all the forms of life, ancient and recent, make together one grand system; for all are connected by generation. We can understand, from the continued tendency to divergence of character, why the more ancient a form is, the more it generally differs from those now living. [...] The more ancient a form is, the more often, apparently, it displays characters in some degree intermediate between groups now distinct; for the more ancient a form is, the more nearly it will be related to, and consequently resemble, the common progenitor of groups, since become widely divergent. [...]
The succession of the same types of structure within the same areas during the later geological periods ceases to be mysterious, and is simply explained by inheritance.
If then the geological record be as imperfect as I believe it to be, and it may at least be asserted that the record cannot be proved to be much more perfect, the main objections to the theory of natural selection are greatly diminished or disappear. On the other hand, all the chief laws of palaeontology plainly proclaim, as it seems to me, that species have been produced by ordinary generation: old forms having been supplanted by new and improved forms of life, produced by the laws of variation still acting round us, and preserved by Natural Selection.
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Even the sketchy fossil record known to Darwin reinforced his evolutionary ideas, demonstrating a clear succession of forms diversifying, branching, over time, with some branches surviving to the present day and others becoming extinct. Those that became extinct were not seen again.
BACK_TO_TOP* While Darwin admitted the geological record was imperfect -- it might be more accurate to say he belabored the fact -- he was not so reserved about the evidence for his theory provided by the geographical distribution of organisms, or what is now called "biogeography", a concept he explored in detail in chapters 11 & 12. Why, he wondered -- and he was not the first to do so -- did separated regions have distinct flora and fauna? As he pointed out, the differences between the New World and the Old World seemed highly systemic:
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In considering the distribution of organic beings over the face of the globe, the first great fact which strikes us is, that neither the similarity nor the dissimilarity of the inhabitants of various regions can be accounted for by their climatal and other physical conditions. [...]
The case of America alone would almost suffice to prove its truth: for if we exclude the northern parts where the circumpolar land is almost continuous, all authors agree that one of the most fundamental divisions in geographical distribution is that between the New and Old Worlds; yet if we travel over the vast American continent, from the central parts of the United States to its extreme southern point, we meet with the most diversified conditions; the most humid districts, arid deserts, lofty mountains, grassy plains, forests, marshes, lakes, and great rivers, under almost every temperature.
There is hardly a climate or condition in the Old World which cannot be paralleled in the New -- at least as closely as the same species generally require; for it is a most rare case to find a group of organisms confined to any small spot, having conditions peculiar in only a slight degree; for instance, small areas in the Old World could be pointed out hotter than any in the New World, yet these are not inhabited by a peculiar fauna or flora. Notwithstanding this parallelism in the conditions of the Old and New Worlds, how widely different are their living productions!
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Similarly, the marine organisms on each side of the Isthmus of Panama were distinctly different, even though they were only separated by a short distance of land -- while, at the same time, the marine organisms on each side remained generally similar far out to sea.
Add to this the fact, as Darwin pointed out in chapter 10, that there were clear relationships among the organisms within a region, but with generally much more distant relationships with organisms elsewhere. Monkeys in the Old World and the New World are clearly distinct groups, with naturalists able to easily classify a specimen of monkey as a New World or Old World monkey even if they had never seen it before and had no idea where it came from. There are no baboons in the New World; there are no marmosets in the Old World. What Darwin saw was that the monkeys were two branches of a common ancestral family that split apart a long time ago.
The large island of Madagascar is noted for its unusual species. Madagascar has been geologically isolated for 90 million years; although it is not all that far from Africa in the current era, its wildlife is almost entirely different from that of Africa, in particular lacking any large native species of ungulates. It includes a fascinating set of species of lemurs, which are only represented in Africa by the bush babies and pottos, while there are no monkeys or apes in Madagascar.
It appears that the ancestors of Madagascar's lemurs arrived tens of millions of years ago by a "rafting" event, in which they floated over the sea on uprooted trees or the like. That sounds like an implausible notion, but it has been observed in modern times, and in fact hurricanes can tear up so much vegetation that the rafts look like small islands, swarming with various animals. Such events are admittedly rare, but over millions of years events of low probability become an effective certainty.
In any case, since that time, in Africa the lemurs have been pushed to one side by the monkeys and apes, while they thrived in Madagascar. There was once a huge ground-living lemur there, the size of a bear, and in the current day the family includes, in sadly small and shrinking numbers, the bizarre nocturnal "aye-aye", named for its cry. It is the ecological equivalent of a woodpecker, with forepaws featuring a long and thin middle finger, which it uses to search for and dig out insects burrowed into trees.
There were peculiar puzzles in the patterns of geographic distributions. Camels were found in Africa and Asia, while clearly related llamas were found in South America -- but no camel-like creatures were found in North America. Similarly, the big ground-living birds known as "ratites" -- including the ostrich of Africa, the rhea of South America, and the emu of Australia -- had a distribution that was hard to explain.
The puzzle of camels was soon resolved through the discovery of fossils of tall giraffe-like camels in North America, with the conclusion being that camels had arisen there and become extinct after they migrated northwest and south. The puzzle of the distribution of ratites was much harder to figure out until the drifting of continents was understood a century later, helping to vindicate Darwin's notions about biogeography. The ratites are a very ancient lineage, and in the days of their origins all the continents on which they are now found were linked together.
* Where Darwin's evidence became particularly impressive was with isolated and remote island groups that had never been associated with a large landmass, and were inaccessible to organisms that cannot cross large stretches of salt water. Darwin keenly recognized the importance of such cases, with the distribution of finches among the separate islands of the Galapagos ending up being a particularly interesting example. The pattern of the species of Darwin's finches suggested that ancestral finches arrived from South America and became established, radiating out into species differing in appearance and habit.
The only organisms that could reach these islands were those that were mobile enough to get there, that could fly to them, swim to them, or somehow hitch a ride to them on things that could fly, swim, or float. That is why such isolated islands rarely featured mammals aside from bats or seals, at least before the arrival of humans and their camp followers. Reptiles were also unusual -- the Galapagos is unusual in featuring significant numbers of iguanas, with all the iguana species there clearly closely related.
Such isolated island had birds, of course, and insects that floated in on the breeze; plants were restricted to those whose seeds could be blown in on the winds, float over the sea water, or be carried on the toes and feathers of birds -- Darwin conducted a number of experiments along such lines, for example carefully testing how long seeds of various sorts could float in salt water and still germinate.
When the islands were clustered in chains, as in the Galapagos, the results were particularly striking, with different variants of organisms derived from common stock on different islands, each island in the chain featuring its own unique organisms, but with those organisms related between islands. The Hawaiian Islands demonstrate a similar pattern of species, particularly the birds known as "honeycreepers" -- which diversified in their isolation into a much wider range of variation than the Galapagos finches.
The notion that the local species were specifically designed for their environments doesn't fly very well, since invasive species -- like cats, rats, and pigs brought by humans -- do very well in such environments, and have tended to shove the locals out. Most species of Hawaiian honeycreepers are extinct, and there are real worries that those that remain won't be around much longer.
Darwin summarized his thoughts on biogeography at the end of chapter 12:
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[I believe that] all the grand leading facts of geographical distribution are explicable on the theory of migration (generally of the more dominant forms of life), together with subsequent modification and the multiplication of new forms. We can thus understand the high importance of barriers, whether of land or water, which separate our several zoological and botanical provinces. We can thus understand the localisation of sub-genera, genera, and families; and how it is that under different latitudes, for instance in South America, the inhabitants of the plains and mountains, of the forests, marshes, and deserts, are in so mysterious a manner linked together by affinity, and are likewise linked to the extinct beings which formerly inhabited the same continent. [...]
On these same principles, we can understand [...] why oceanic islands should have few inhabitants, but of these a great number should be endemic or peculiar [...] We can see why whole groups of organisms, [such as] terrestrial mammals, should be absent from oceanic islands, whilst the most isolated islands possess their own peculiar species of aerial mammals or bats. We can see why there should be some relation between the presence of mammals, in a more or less modified condition, and the depth of the sea between an island and the mainland. We can clearly see why all the inhabitants of an archipelago, though specifically distinct on the several islets, should be closely related to each other, and likewise be related, but less closely, to those of the nearest continent or other source whence immigrants were probably derived.
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BACK_TO_TOP* Chapter 13 is something of a "grab bag" of topics, being fully titled "Mutual Affinities Of Organic Beings; Morphology; Embryology; Rudimentary Organs". All were important elements in Darwin's case -- he just didn't have as much to say about them as he did for other elements, such as geographical distribution.
* The concept of "mutual affinities" was Darwin's read on the familial relationships of organisms. In the previous century, the Swedish naturalist Carolus Linnaeus had developed the first modern classification system for organisms, arranging them in a hierarchy of groupings based on a careful assessment of their similarities. A species of cat belonged to the "genus" of felines (along with civets and hyenas); the genus of felines belonged to the "order" of carnivores (along with canines, bears, weasels, and so on); the order of carnivores belonged to the "class" of mammals (along with rodents, bats, and so on); while the class of mammals belonged to the "kingdom" of animals (along with reptiles, birds, fish, and so on). As Darwin put it:
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From the first dawn of life, all organic beings are found to resemble each other in descending degrees, so that they can be classed in groups under groups. This classification is evidently not arbitrary like the grouping of the stars in constellations. The existence of groups would have been of simple signification, if one group had been exclusively fitted to inhabit the land, and another the water; one to feed on flesh, another on vegetable matter, and so on; but the case is widely different in nature; for it is notorious how commonly members of even the same subgroup have different habits.
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This hierarchy of groupings could be represented by a branching "tree" of relationships, which in hindsight suggests a familial pattern of ancestry -- that is, the different branches of the tree traced back to common ancestors, with the modern species of the Earth having evolved from earlier forms. As Darwin pointed out, it might be argued that organisms were created in such groups, but that would be an arbitrary assumption, all the more so because functions were not distinct to each group. Mammals included flying insect-eaters (bats), ground-living predators (cats), ground-living herbivores (deer), and sea-living predators (seals). All such forms were also found among birds, the corresponding analogues being swallows, secretary birds, ostriches, and penguins. To Darwin, the tree of life was clear evidence for evolution:
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Naturalists try to arrange the species, genera, and families in each class, on what is called the Natural System. But what is meant by this system? Some authors look at it merely as a scheme for arranging together those living objects which are most alike, and for separating those which are most unlike; or as an artificial means for enunciating [...] the characters common, for instance, to all mammals, by another those common to all carnivora, by another those common to the dog-genus, and then by adding a single sentence, a full description is given of each kind of dog.
The ingenuity and utility of this system are indisputable. But many naturalists think that something more is meant by the Natural System; they believe that it reveals the plan of the Creator; but unless it be specified whether order in time or space, or what else is meant by the plan of the Creator, it seems to me that nothing is thus added to our knowledge.
[The belief that] the characters [characteristics of organisms] do not make the genus, but that the genus gives the characters, seem to imply that something more is included in our classification, than mere resemblance. I believe that something more is included; and that propinquity of descent -- the only known cause of the similarity of organic beings -- is the bond, hidden as it is by various degrees of modification, which is partially revealed to us by our classifications.
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* Darwin's discussion of morphologies focused on the tendency of organisms to share features, but with alterations, sometimes significant ones. Simply sharing features was not particularly significant to his case; if organisms had been designed, it would have been efficient to have simply reused the same components in different designs. Darwin's point was subtler: that there were common structures among sets of organisms that had been modified, sometimes in seemingly awkward and improvised ways, to a range of purposes. For example, the basic structures of flowers generally had much in common, but they featured wild elaboration in practice, with the same elements sometimes used for different functions.
If these features had been specifically designed, there was no sense in relying on the modification of old features to come up with new ones, particularly when there was no advantage or even some disadvantage in doing so. As Darwin put it, the only argument that advocates of design could suggest would be that "so it is; that it has so pleased the Creator to construct each animal and plant" in such a fashion. To Darwin, it was far better evidence for evolution:
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The explanation is manifest on the theory of the natural selection of successive slight modifications -- each modification being profitable in some way to the modified form, but often affecting by correlation of growth other parts of the organisation. In changes of this nature, there will be little or no tendency to modify the original pattern, or to transpose parts. The bones of a limb might be shortened and widened to any extent, and become gradually enveloped in thick membrane, so as to serve as a fin; or a webbed foot might have all its bones, or certain bones, lengthened to any extent, and the membrane connecting them increased to any extent, so as to serve as a wing: yet in all this great amount of modification there will be no tendency to alter the framework of bones or the relative connexion of the several parts.
If we suppose that the ancient progenitor, the archetype as it may be called, of all mammals, had its limbs constructed on the existing general pattern, for whatever purpose they served, we can at once perceive the plain signification of the homologous construction of the limbs throughout the whole class. So with the mouths of insects, we have only to suppose that their common progenitor had an upper lip, mandibles, and two pair of maxillae, these parts being perhaps very simple in form; and then natural selection will account for the infinite diversity in structure and function of the mouths of insects.
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Darwin, always careful, was quick to add that there could be variations in the underlying inherited structures:
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[It] is conceivable that the general pattern of an organ might become so much obscured as to be finally lost, by the atrophy and [ultimate loss] of certain parts, by the soldering together of other parts, and by the doubling or multiplication of others -- variations which we know to be within the limits of possibility.
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Darwin believed that any useful natural system of classification of organisms would necessarily consider the matter from an evolutionary viewpoint. This would actually be later judged to be somewhat getting the cart before the horse. It is true that the classification of organisms does mirror their evolutionary paths of descent; however, the traits used for classification, most significantly the patterns of genomes, establish the classification, which then highlights the evolutionary paths of descent. Evolution explains taxonomy, but taxonomy in practice necessarily avoids assumptions of evolution, instead simply looking for the arrangement of resemblances.
* Darwin's discussion of embryos pointed out that the embryonic forms of organisms provided clues to their relationships and ancestry:
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It [can be observed] that certain organs in the individual, which when mature become widely different and serve for different purposes, are in the embryo exactly alike. The embryos, also, of distinct animals within the same class are often strikingly similar: a better proof of this cannot be given, than a circumstance mentioned by [the Swiss-American naturalist Louis Agassiz], namely, that having forgotten to ticket the embryo of some vertebrate animal, he cannot now tell whether it be that of a mammal, bird, or reptile. The [...] larvae of moths, flies, beetles, etc., resemble each other much more closely than do the mature insects; but in the case of larvae, the embryos are active, and have been adapted for special lines of life.
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He discussed pertinent observations about embryos, summarizing the intriguing facts neatly:
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Of course, to Darwin the solution was straightforward: "I believe that all these facts can be explained" by "descent with modification." The similarities between embryos suggested familial relationships and so common descent; and the occasional wide divergence of unusual adult organisms from their embryonic forms suggested ancestors that were not so different from relatives more ordinary in form.
It should be noted that studies of embryos and embryonic development can be taken too far -- and were, under the archaic doctrine of "ontogeny recapitulates phylogeny", or that the past evolutionary history of an organism is demonstrated in the course of development of an embryo. For example, the human embryo has gills at one point, then a tail at another point. That turned out to be overstating the case: ancestral features can show up in embryonic development, but they don't always do so. Overall it might be said that examination of embryonic development provides interesting clues to past evolutionary history, but it doesn't reveal more than clues.
* Darwin's final argument focused on "vestigial" or rudimentary organs, which were clearly "leftovers" that had been useful to ancestral forms, but had ceased to be useful, or had become useful for some secondary purpose. Darwin pointed out that in some snakes "there are rudiments of the pelvis and hind limbs", and pointed out "the presence of teeth in foetal whales, which when grown up have not a tooth in their heads" -- this second observation buttressing his comments on embryonic development.
Vestigial organs tend to be striking evidence of evolutionary descent, since they would be difficult to explain if organisms were specifically designed. It must be emphasized that vestigial organs are not always or even generally useless; they may serve a function, but clearly not the function for which they originally emerged. Flightless ostriches have wings, but they are only used for sexual display and cooling.
* Taken in sum, Darwin concluded that the arguments presented in chapter 13:
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... 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.
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In fact, though we have learned much since Darwin's time, nothing we have discovered undermines his argument, instead expanding on it and altering it in some cases. We would be justified in accepting Darwin's argument as he laid it out in 1859 even if we had learned nothing new. Darwin's theory of evolution by natural selection has survived all challenges and, in the 21st century, is now more firmly established than it ever was.
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