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Usually: argument (390) · detachment (203) · curiosity (164) · skepticism (116) · wonder (30) · reflection (29) · narrative (15) · crisis of belief (14) · censure (12) · commentary or gloss (10)
“That large animals require a luxuriant vegetation, has been a general assumption which has passed from one work to another; but I do not hesitate to say that it is completely false, and that it has vitiated the reasoning of geologists on some points of great interest in the ancient history of the world.” Charles Darwin, The Voyage of the Beagle · insight
“To admit that species generally become rare before they become extinct--to feel no surprise at the comparative rarity of one species with another, and yet to call in some extraordinary agent and to marvel greatly when a species ceases to exist, appears to me much the same as to admit that sickness in the individual is the prelude to death--to feel no surprise at sickness--but when the sick man dies to wonder, and to believe that he died through violence.” Charles Darwin, The Voyage of the Beagle · insight
“There is one marine production which, from its importance, is worthy of a particular history.” Charles Darwin, The Voyage of the Beagle · observation
“What can be more remarkable that to see a plant-like body producing an egg, capable of swimming about and of choosing a proper place to adhere to, which then sprouts into branches, each crowded with innumerable distinct animals, often of complicated organizations.” Charles Darwin, The Voyage of the Beagle · observation
“From these several facts we may, I think, conclude, first, that the wildness of birds with regard to man, is a particular instinct directed against him, and not dependent upon any general degree of caution arising from other sources of danger; secondly, that it is not acquired by individual birds in a short time, even when much persecuted; but that in the course of successive generations it becomes hereditary.” Charles Darwin, The Voyage of the Beagle · insight
“Although no one has a right to speculate without distinct facts, yet even with respect to the Chatham Island mouse, it should be borne in mind, that it may possibly be an American species imported here.” Charles Darwin, The Voyage of the Beagle · insight
“May this difference not be caused, by the greater facility with which the eggs of lizards, protected by calcareous shells might be transported through salt-water, than could the slimy spawn of frogs?” Charles Darwin, The Voyage of the Beagle · insight
“After the most deliberate study and dispassionate judgment of which I am capable, that the view which most naturalists entertain, and which I formerly entertained—namely, that each species has been independently created—is erroneous.” Charles Darwin, On the Origin of Species By Means of Natural Selecti · insight
“Perhaps the correct way of viewing the whole subject, would be, to look at the inheritance of every character whatever as the rule, and non-inheritance as the anomaly.” Charles Darwin, On the Origin of Species By Means of Natural Selecti · insight
“The key is man’s power of accumulative selection: nature gives successive variations; man adds them up in certain directions useful to him.” Charles Darwin, On the Origin of Species By Means of Natural Selecti · precept
“From these remarks it will be seen that I look at the term species, as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other, and that it does not essentially differ from the term variety.” Charles Darwin, On the Origin of Species By Means of Natural Selecti · insight
“Owing to this struggle for life, any variation, however slight and from whatever cause proceeding, if it be in any degree profitable to an individual of any species, in its infinitely complex relations to other organic beings and to external nature, will tend to the preservation of that individual, and will generally be inherited by its offspring.” Charles Darwin, On the Origin of Species By Means of Natural Selecti · insight
Darwin explains how his Beagle observations of South American distribution and geology led him to propose descent with modification, published because Wallace independently reached similar conclusions, and he argues that adaptation cannot be explained by external conditions alone.
expressing my deep obligations to Dr. Hooker, who for the last fifteen years has aided me in every possible way by his large stores of knowledge and his excellent judgment. In considering the Origin of Species, it is quite conceivable that a naturalist, reflecting on the mutual affinities of organic beings, on their embryological relations, their geographical distribution, geological succession, and other such facts…
Darwin outlines his argument that variation enables natural selection via a Malthusian struggle for existence, describes extinction and divergence as outcomes, and concludes that species are not independently created.
will have a better chance of surviving, and thus be _naturally selected_. From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form. This fundamental subject of Natural Selection will be treated at some length in the fourth chapter; and we shall then see how Natural Selection almost inevitably causes much Extinction of the less improved forms of life and induces …
Darwin argues that cultivated plants and animals show greater individual variability than wild ones because domestication involves less uniform living conditions and can require several generations to produce noticeable change.
VARIATION UNDER DOMESTICATION. Causes of Variability. Effects of Habit. Correlation of Growth. Inheritance. Character of Domestic Varieties. Difficulty of distinguishing between Varieties and Species. Origin of Domestic Varieties from one or more Species. Domestic Pigeons, their Differences and Origin. Principle of Selection anciently followed, its Effects. Methodical and Unconscious Selection. Unknown Origin of our…
He discusses when variability arises, suggesting it often comes from changes to reproductive elements before conception, explaining sterility and differing breeding success under confinement.
ceasing to be variable under cultivation. Our oldest cultivated plants, such as wheat, still often yield new varieties: our oldest domesticated animals are still capable of rapid improvement or modification. It has been disputed at what period of life the causes of variability, whatever they may be, generally act; whether during the early or late period of development of the embryo, or at the instant of conception. …
Darwin claims rare sudden “sports” in plants support variability coming from ovules or pollen rather than direct conditions at generation, and he argues reproduction and inheritance outweigh direct effects.
their reproductive system has not been thus affected; so will some animals and plants withstand domestication or cultivation, and vary very slightly—perhaps hardly more than in a state of nature. A long list could easily be given of “sporting plants;” by this term gardeners mean a single bud or offset, which suddenly assumes a new and sometimes very different character from that of the rest of the plant. Such buds c…
He explains correlation of growth: changes in embryos or larvae tend to affect mature parts, so selection on one trait can modify other traits through linked development and heredity.
view suggested by some authors, that the drooping is due to the disuse of the muscles of the ear, from the animals not being much alarmed by danger, seems probable. There are many laws regulating variation, some few of which can be dimly seen, and will be hereafter briefly mentioned. I will here only allude to what may be called correlation of growth. Any change in the embryo or larva will almost certainly entail ch…
Darwin argues inheritance is the rule and non-inheritance is an anomaly, while noting inheritance laws are unknown and describing reversion and age-specific expression of traits.
be inheritable. Perhaps the correct way of viewing the whole subject, would be, to look at the inheritance of every character whatever as the rule, and non-inheritance as the anomaly. The laws governing inheritance are quite unknown; no one can say why the same peculiarity in different individuals of the same species, and in individuals of different species, is sometimes inherited and sometimes not so; why the child…
He challenges claims that domestic varieties inevitably revert to wild types when run free, arguing there is little evidence and that long domestication does not contradict species-level inference.
as in the crossed offspring from a short-horned cow by a long-horned bull, the greater length of horn, though appearing late in life, is clearly due to the male element. Having alluded to the subject of reversion, I may here refer to a statement often made by naturalists—namely, that our domestic varieties, when run wild, gradually but certainly revert in character to their aboriginal stocks. Hence it has been argue…
Comparing domestic races to species, Darwin says domestic groups are usually less uniform, sometimes show extreme differences in particular parts, and he discusses uncertainty about whether races have one or multiple parent species.
do change, variations and reversions of character probably do occur; but natural selection, as will hereafter be explained, will determine how far the new characters thus arising shall be preserved. When we look to the hereditary varieties or races of our domestic animals and plants, and compare them with species closely allied together, we generally perceive in each domestic race, as already remarked, less uniformi…
He argues that domestication does not require innate “extraordinary” variability, discusses ancient records from Egypt and early pottery, and concludes domestic dogs likely come from several wild species.
made to breed for an equal number of generations under domestication, they would vary on an average as largely as the parent species of our existing domesticated productions have varied. In the case of most of our anciently domesticated animals and plants, I do not think it is possible to come to any definite conclusion, whether they have descended from one or several species. The argument mainly relied on by those …
Darwin criticizes extreme claims that every true-breeding domestic race must have its own wild prototype, arguing that crossing cannot easily recreate extreme wild forms, citing difficulties in forming stable intermediate breeds.
ducks and rabbits, the breeds of which differ considerably from each other in structure, I do not doubt that they all have descended from the common wild duck and rabbit. The doctrine of the origin of our several domestic races from several aboriginal stocks, has been carried to an absurd extreme by some authors. They believe that every race which breeds true, let the distinctive characters be ever so slight, has ha…
To study variation systematically, Darwin explains his pigeon project, describing how breeds differ drastically in beaks, skulls, behaviors, and other inherited traits across many sub-breeds.
very distinct* breeds could not be got without extreme care and long-continued selection; nor can I find a single case on record of a permanent race having been thus formed. *On the Breeds of the Domestic Pigeon*.—Believing that it is always best to study some special group, I have, after deliberation, taken up domestic pigeons. I have kept every breed which I could purchase or obtain, and have been most kindly favo…
Darwin lists many variable anatomical traits in domestic pigeons, noting that these differences can be so great an ornithologist might rank several breeds as well-defined species.
kept expanded, and are carried so erect that in good birds the head and tail touch; the oil-gland is quite aborted. Several other less distinct breeds might have been specified. In the skeletons of the several breeds, the development of the bones of the face in length and breadth and curvature differs enormously. The shape, as well as the breadth and length of the ramus of the lower jaw, varies in a highly remarkabl…
He argues that most pigeon breeds likely descend from the rock-pigeon (Columba livia), because producing many breeds from multiple wild stocks would be improbable and would predict feral forms that do not appear.
pouter, and fantail in the same genus; more especially as in each of these breeds several truly-inherited sub-breeds, or species as he might have called them, could be shown him. Great as the differences are between the breeds of pigeons, I am fully convinced that the common opinion of naturalists is correct, namely, that all have descended from the rock-pigeon (Columba livia), including under this term several geog…
Darwin explains pigeon color and marking patterns as reversion to ancestral characters when breeds are crossed, and he stresses that domestic pigeon hybrids are perfectly fertile.
picked out extraordinarily abnormal species; and further, that these very species have since all become extinct or unknown. So many strange contingencies seem to me improbable in the highest degree. Some facts in regard to the colouring of pigeons well deserve consideration. The rock-pigeon is of a slaty-blue, and has a white rump (the Indian sub-species, C. intermedia of Strickland, having it bluish); the tail has …
He concludes that multiple considerations—difficulty of multiple domesticated origins, abnormal traits tied to the rock-pigeon, occasional blue markings, and fertile mongrels—support a single origin from Columba livia.
as to suppose that species, aboriginally as distinct as carriers, tumblers, pouters, and fantails now are, should yield offspring perfectly fertile, *inter se*, seems to me rash in the extreme. From these several reasons, namely, the improbability of man having formerly got seven or eight supposed species of pigeons to breed freely under domestication; these supposed species being quite unknown in a wild state, and …
Darwin adds that C. livia can be domesticated in Europe and India and that breed extremes form continuous series, then surveys long historical human care and valuation of pigeons worldwide.
circumstance for the production of distinct breeds, that male and female pigeons can be easily mated for life; and thus different breeds can be kept together in the same aviary. I have discussed the probable origin of domestic pigeons at some, yet quite insufficient, length; because when I first kept pigeons and watched the several kinds, knowing well how true they bred, I felt fully as much difficulty in believing …
He describes how breeders and plant cultivators are convinced each breed comes from distinct species, and he warns naturalists against neglecting long chains of accumulated differences.
descended from the same parents—may they not learn a lesson of caution, when they deride the idea of species in a state of nature being lineal descendants of other species? *Selection*.—Let us now briefly consider the steps by which domestic races have been produced, either from one or from several allied species. Some little effect may, perhaps, be attributed to the direct action of the external conditions of life,…
Darwin introduces selection as the key mechanism: nature produces variations, while humans accumulate useful ones, likening breeding to a magician’s wand and describing skilled breeders’ methods.
of accumulative selection: nature gives successive variations; man adds them up in certain directions useful to him. In this sense he may be said to make for himself useful breeds. The great power of this principle of selection is not hypothetical. It is certain that several of our eminent breeders have, even within a single lifetime, modified to a large extent some breeds of cattle and sheep. In order fully to real…
He claims horticulture shows selection’s cumulative effects, citing gooseberry growth and comparisons of flowers, leaves, and fruit among varieties to argue for small changes building into distinct races.
if he wants any of these qualities, he will assuredly fail. Few would readily believe in the natural capacity and years of practice requisite to become even a skilful pigeon-fancier. The same principles are followed by horticulturists; but the variations are here often more abrupt. No one supposes that our choicest productions have been produced by a single variation from the aboriginal stock. We have proofs that th…
Darwin argues selection is not modern, pointing to ancient Chinese and Roman references, Genesis animal color attention, and examples of breeders’ unconscious selection over centuries.
I cannot doubt that the continued selection of slight variations, either in the leaves, the flowers, or the fruit, will produce races differing from each other chiefly in these characters. It may be objected that the principle of selection has been reduced to methodical practice for scarcely more than three-quarters of a century; it has certainly been more attended to of late years, and many treatises have been publ…
He gives further cases of gradual improvement via unconscious selection—pointers, racehorses, English cattle—and illustrates how long-pure breeding can still yield strikingly different strains.
effectually, that, though the old Spanish pointer certainly came from Spain, Mr. Borrow has not seen, as I am informed by him, any native dog in Spain like our pointer. By a similar process of selection, and by careful training, the whole body of English racehorses have come to surpass in fleetness and size the parent Arab stock, so that the latter, by the regulations for the Goodwood Races, are favoured in the weig…
Darwin transitions to variation in nature, discussing variability among species, the challenge of defining “species” versus “variety,” and whether even environmentally induced forms like Baltic shell dwarfs could be inherited.
VARIATION UNDER NATURE. Variability. Individual differences. Doubtful species. Wide ranging, much diffused, and common species vary most. Species of the larger genera in any country vary more than the species of the smaller genera. Many of the species of the larger genera resemble varieties in being very closely, but unequally, related to each other, and in having restricted ranges. Before applying the principles a…
He argues that individual differences within the same species supply raw material for natural selection, noting that even important organs can vary, and that polymorphic groups like Rubus and Rosa complicate species boundaries.
from far northwards, would not in some cases be inherited for at least some few generations? and in this case I presume that the form would be called a variety. Again, we have many slight differences which may be called individual differences, such as are known frequently to appear in the offspring from the same parents, or which may be presumed to have thus arisen, from being frequently observed in the individuals …
Darwin says doubtful forms closely allied to others and linked by intermediates often drive the hardest decisions about species versus variety, with classification sometimes relying on analogy and majority expert judgement.
structure which are of no service or disservice to the species, and which consequently have not been seized on and rendered definite by natural selection, as hereafter will be explained. Those forms which possess in some considerable degree the character of species, but which are so closely similar to some other forms, or are so closely linked to them by intermediate gradations, that naturalists do not like to rank …
He proposes a continuum view where species are arbitrarily named convenience groups and varieties are less distinct steps toward species, potentially accumulating from individual differences via natural selection.
or between lesser varieties and individual differences. These differences blend into each other in an insensible series; and a series impresses the mind with the idea of an actual passage. Hence I look at individual differences, though of small interest to the systematist, as of high importance for us, as being the first step towards such slight varieties as are barely thought worth recording in works on natural his…
Using tabulations, Darwin claims dominant and widely diffused species most often produce well-marked varieties (incipient species), and he tests this across countries and larger versus smaller genera.
it necessarily here is with much brevity, is rather perplexing, and allusions cannot be avoided to the “struggle for existence,” “divergence of character,” and other questions, hereafter to be discussed. Alph. De Candolle and others have shown that plants which have very wide ranges generally present varieties; and this might have been expected, as they become exposed to diverse physical conditions, and as they come…
He reports that in his tables larger genera show a higher proportion of varieties and incipient species than smaller genera, arguing this supports species as strongly marked varieties continuing to form.
at each species as a special act of creation, there is no apparent reason why more varieties should occur in a group having many species, than in one having few. To test the truth of this anticipation I have arranged the plants of twelve countries, and the coleopterous insects of two districts, into two nearly equal masses, the species of the larger genera on one side, and those of the smaller genera on the other si…
Darwin argues that species in large genera cluster like satellite varieties, with varieties and species sharing characters but differing mainly by degree of difference and often having restricted ranges.
are now manufacturing, many of the species already manufactured still to a certain extent resemble varieties, for they differ from each other by a less than usual amount of difference. Moreover, the species of the large genera are related to each other, in the same manner as the varieties of any one species are related to each other. No naturalist pretends that all the species of a genus are equally distinct from ea…
Darwin introduces struggle for existence as the foundation for natural selection, explaining that useful variations are preserved because only a few individuals survive and reproduce amid universal competition.
STRUGGLE FOR EXISTENCE. Bears on natural selection. The term used in a wide sense. Geometrical powers of increase. Rapid increase of naturalised animals and plants. Nature of the checks to increase. Competition universal. Effects of climate. Protection from the number of individuals. Complex relations of all animals and plants throughout nature. Struggle for life most severe between individuals and varieties of the …
He expands the struggle for existence as a broad metaphor—including dependence and reproductive success—then argues Malthusian pressure makes destruction inevitable, citing rapid population growth examples.
we shall hereafter see, is a power incessantly ready for action, and is as immeasurably superior to man’s feeble efforts, as the works of Nature are to those of Art. We will now discuss in a little more detail the struggle for existence. In my future work this subject shall be treated, as it well deserves, at much greater length. The elder De Candolle and Lyell have largely and philosophically shown that all organic…
Darwin continues that geometrical tendency to increase is universal for plants and animals, but must be checked by destruction, with average population tied more to survival losses than egg counts.
disseminate its seeds rather than those of other plants. In these several senses, which pass into each other, I use for convenience sake the general term of struggle for existence. A struggle for existence inevitably follows from the high rate at which all organic beings tend to increase. Every being, which during its natural lifetime produces several eggs or seeds, must suffer destruction during some period of its …
He explains that checks can include indirect effects where favored competitors or enemies expand, gives an example from his travels with latitude and mountains, and emphasizes that extreme regions rely on elemental struggle.
as with the elephant and rhinoceros, none are destroyed by beasts of prey: even the tiger in India most rarely dares to attack a young elephant protected by its dam. Climate plays an important part in determining the average numbers of a species, and periodical seasons of extreme cold or drought, I believe to be the most effective of all checks. I estimated that the winter of 1854-55 destroyed four-fifths of the bir…
Darwin argues that complex relations among organisms create shifting outcomes, illustrating with Staffordshire heath planted with Scotch fir, where birds, plants, and insect communities change after enclosure.
some cause, possibly in part through facility of diffusion amongst the crowded animals, been disproportionably favoured: and here comes in a sort of struggle between the parasite and its prey. On the other hand, in many cases, a large stock of individuals of the same species, relatively to the numbers of its enemies, is absolutely necessary for its preservation. Thus we can easily raise plenty of corn and rape-seed,…
He adds that cattle can determine Scotch fir survival in Staffordshire, but in places like Paraguay a fly limits feral herbivores, with insectivorous birds regulating the whole cascading system of interdependence.
effects of frequent intercrossing, and the ill effects of close interbreeding, probably come into play in some of these cases; but on this intricate subject I will not here enlarge. Many cases are on record showing how complex and unexpected are the checks and relations between organic beings, which have to struggle together in the same country. I will give only a single instance, which, though a simple one, has int…
Darwin argues that numerous ecological checks acting at different times determine species survival, using examples of plant competition, insect predation, and how closely related individuals often outcompete each other.
of a feline animal in large numbers in a district might determine, through the intervention first of mice and then of bees, the frequency of certain flowers in that district! In the case of every species, many different checks, acting at different periods of life, and during different seasons or years, probably come into play; some one check or some few being generally the most potent, but all concurring in determin…
He continues that competition is often strongest among species in the same genus, then states a key idea: organisms’ structures are related to competitors, prey, and escape, as shown by teeth, claws, and seed dispersal.
up for half a dozen generations, if they were allowed to struggle together, like beings in a state of nature, and if the seed or young were not annually sorted. As species of the same genus have usually, though by no means invariably, some similarity in habits and constitution, and always in structure, the struggle will generally be more severe between species of the same genus, when they come into competition with …
Darwin reasons that species do not expand unchecked within their ranges because competition limits them, and that in new environments their conditions shift, requiring advantages to persist; he emphasizes the ongoing war of nature.
I suspect that the chief use of the nutriment in the seed is to favour the growth of the young seedling, whilst struggling with other plants growing vigorously all around. Look at a plant in the midst of its range, why does it not double or quadruple its numbers? We know that it can perfectly well withstand a little more heat or cold, dampness or dryness, for elsewhere it ranges into slightly hotter or colder, dampe…
He introduces natural selection as the process that preserves beneficial variations and rejects harmful ones, compares its scope to human selection, and explains how changing conditions and isolation create “unoccupied places” for improvement.
NATURAL SELECTION. Natural Selection: its power compared with man’s selection, its power on characters of trifling importance, its power at all ages and on both sexes. Sexual Selection. On the generality of intercrosses between individuals of the same species. Circumstances favourable and unfavourable to Natural Selection, namely, intercrossing, isolation, number of individuals. Slow action. Extinction caused by Nat…
Darwin argues that extreme climate change is not required: tiny structural or behavioral shifts can tip balanced struggles, and since nature selects internally and continually, it can produce better adaptation than human selection.
any of the species, by better adapting them to their altered conditions, would tend to be preserved; and natural selection would thus have free scope for the work of improvement. We have reason to believe, as stated in the first chapter, that a change in the conditions of life, by specially acting on the reproductive system, causes or increases variability; and in the foregoing case the conditions of life are suppos…
Darwin states that natural selection continually scrutinizes variations across the world, and that even tiny differences—like bird and insect coloration or plant fruit down—can strongly affect survival under predation and disease.
“truer” in character than man’s productions; that they should be infinitely better adapted to the most complex conditions of life, and should plainly bear the stamp of far higher workmanship? 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 …
He explains how selection can act at different life stages because domestication-like stage-specific traits can reappear in offspring, and shows how modifying one part can drive further correlated changes without harming the species.
the organisation is modified through variation, and the modifications are accumulated by natural selection for the good of the being, will cause other modifications, often of the most unexpected nature. As we see that those variations which under domestication appear at any particular period of life, tend to reappear in the offspring at the same period;—for instance, in the seeds of the many varieties of our culinar…
Darwin defines sexual selection as a male-male struggle for females where unsuccessful males leave few or no offspring, arguing it is less rigorous than natural selection and can produce weapons and ornaments.
with weak beaks would inevitably perish: or, more delicate and more easily broken shells might be selected, the thickness of the shell being known to vary like every other structure. *Sexual Selection*.—Inasmuch as peculiarities often appear under domestication in one sex and become hereditarily attached to that sex, the same fact probably occurs under nature, and if so, natural selection will be able to modify one …
He adds that sexual selection can explain differences in male and female traits, but not all male peculiarities must be useful or attractive, giving examples like turkey-cock tufts and tumbler-pigeon hatching.
produced being inherited at corresponding ages or seasons, either by the males alone, or by the males and females; but I have not space here to enter on this subject. Thus it is, as I believe, that when the males and females of any animal have the same general habits of life, but differ in structure, colour, or ornament, such differences have been mainly caused by sexual selection; that is, individual males have had…
Darwin illustrates natural selection with imagined wolf scenarios and real differences among cats’ hunting tendencies, then describes how plants’ nectar and insects’ pollination could favor cross-fertilization and nectar traits.
his greyhounds by careful and methodical selection, or by that unconscious selection which results from each man trying to keep the best dogs without any thought of modifying the breed. Even without any change in the proportional numbers of the animals on which our wolf preyed, a cub might be born with an innate tendency to pursue certain kinds of prey. Nor can this be thought very improbable; for we often observe g…
Darwin argues that plants with more pollen and larger anthers can be naturally selected, leading to regular insect-mediated pollen transfer and eventually greater separation of male and female sex functions in plants.
were destroyed, it might still be a great gain to the plant; and those individuals which produced more and more pollen, and had larger and larger anthers, would be selected. When our plant, by this process of the continued preservation or natural selection of more and more attractive flowers, had been rendered highly attractive to insects, they would, unintentionally on their part, regularly carry pollen from flower…
Using an imaginary coevolution, Darwin explains how increased nectar rewards and small changes in insect feeding anatomy can favor tighter plant–bee mutual adaptation and lead to reciprocal modifications.
the division of labour, individuals with this tendency more and more increased, would be continually favoured or selected, until at last a complete separation of the sexes would be effected. Let us now turn to the nectar-feeding insects in our imaginary case: we may suppose the plant of which we have been slowly increasing the nectar by continued selection, to be a common plant; and that certain insects depended in …
Darwin claims natural selection can only work through the preservation of tiny inherited changes over time, so it should replace ideas of sudden creation or large abrupt changes to organisms.
will natural selection, if it be a true principle, banish the belief of the continued creation of new organic beings, or of any great and sudden modification in their structure. *On the Intercrossing of Individuals*.—I must here introduce a short digression. In the case of animals and plants with separated sexes, it is of course obvious that two individuals must always unite for each birth; but in the case of hermap…
Darwin digresses on hermaphrodites, arguing that even when sexes are combined, occasional cross-fertilisation with another individual is likely necessary, aided by breeder observations of vigour and fertility in outcrosses.
of the meaning of the law) that no organic being self-fertilises itself for an eternity of generations; but that a cross with another individual is occasionally—perhaps at very long intervals—indispensable. On the belief that this is a law of nature, we can, I think, understand several large classes of facts, such as the following, which on any other view are inexplicable. Every hybridizer knows how unfavourable exp…
He supports occasional cross-fertilisation by citing flower structures that expose or enclose reproductive organs, bee-driven pollen transfer in pea-like flowers, and cases where close pollen can be prepotent yet foreign pollen is blocked.
from another species, the former will have such a prepotent effect, that it will invariably and completely destroy, as has been shown by Gärtner, any influence from the foreign pollen. When the stamens of a flower suddenly spring towards the pistil, or slowly move one after the other towards it, the contrivance seems adapted solely to ensure self-fertilisation; and no doubt it is useful for this end: but, the agency…
Darwin argues that many trees have separated male and female flowers, allowing pollen to move between flowers and trees, and that terrestrial animals rarely if ever self-fertilise, unlike aquatic animals aided by currents.
are crossed the case is directly the reverse, for a plant’s own pollen is always prepotent over foreign pollen; but to this subject we shall return in a future chapter. In the case of a gigantic tree covered with innumerable flowers, it may be objected that pollen could seldom be carried from tree to tree, and at most only from flower to flower on the same tree, and that flowers on the same tree can be considered as…
Darwin concludes it is a law-like tendency that hermaphrodites sometimes intercross, and outlines conditions that make natural selection more likely: abundant inherited variation, many individuals, and limited time for competitors to be outpaced.
them unisexual. But if, in fact, all hermaphrodites do occasionally intercross with other individuals, the difference between hermaphrodites and unisexual species, as far as function is concerned, becomes very small. From these several considerations and from the many special facts which I have collected, but which I am not here able to give, I am strongly inclined to suspect that, both in the vegetable and animal k…
He explains how intercrossing maintains uniformity within species but also allows new varieties through isolation and local selection, and he notes nurserymen prefer seed from large single-variety stands to reduce mixing.
On the above principle, nurserymen always prefer getting seed from a large body of plants of the same variety, as the chance of intercrossing with other varieties is thus lessened. Even in the case of slow-breeding animals, which unite for each birth, we must not overrate the effects of intercrosses in retarding natural selection; for I can bring a considerable catalogue of facts, showing that within the same area, …
Darwin argues that large areas tend to generate and spread new species faster than small isolated ones by combining severe competition, complex conditions, and repeated cycles of land fragmentation and reconnection over long periods.
like a continent, has been most favourable for the production of new organic forms, we ought to make the comparison within equal times; and this we are incapable of doing. Although I do not doubt that isolation is of considerable importance in the production of new species, on the whole I am inclined to believe that largeness of area is of more importance, more especially in the production of species, which will pro…
He concludes by sketching a long-term cycle: competition on continents, isolation in islands, inhibited immigration and local modification, then renewed competition after uplift, with natural selection acting extremely slowly but producing extensive evolutionary change.
forms may almost be called living fossils; they have endured to the present day, from having inhabited a confined area, and from having thus been exposed to less severe competition. To sum up the circumstances favourable and unfavourable to natural selection, as far as the extreme intricacy of the subject permits. I conclude, looking to the future, that for terrestrial productions a large continental area, which wil…
Darwin argues that natural selection preserves advantageous variations while over time rare forms decline, become extinct, and that closely competing, allied species—often via domesticated examples like cattle breeds—can be displaced.
coadaptations between all organic beings, one with another and with their physical conditions of life, which may be effected in the long course of time by nature’s power of selection. *Extinction*.—This subject will be more fully discussed in our chapter on Geology; but it must be here alluded to from being intimately connected with natural selection. Natural selection acts solely through the preservation of variati…
Darwin introduces divergence of character, claiming that small differences among varieties can, through selection by different “fanciers,” grow into larger species differences, aided in nature by better occupation of diverse ecological niches.
black cattle were displaced by the long-horns, and that these “were swept away by the short-horns” (I quote the words of an agricultural writer) “as if by some murderous pestilence.” *Divergence of Character*.—The principle, which I have designated by this term, is of high importance on my theory, and explains, as I believe, several important facts. In the first place, varieties, even strongly-marked ones, though ha…
Darwin explains that diversification lets descendants of a lineage exploit new prey, habitats, and diets, and he generalizes with grass and wheat experiments, plus small-area ecology where intense competition favors diverse forms.
habits, by so much will they be better enabled to seize on many and widely diversified places in the polity of nature, and so be enabled to increase in numbers. We can clearly see this in the case of animals with simple habits. Take the case of a carnivorous quadruped, of which the number that can be supported in any country has long ago arrived at its full average. If its natural powers of increase be allowed to ac…
He argues that naturalised plants often become more diverse and less like the native flora, citing Alph. De Candolle and Dr. Asa Gray, and interprets this as evidence that diversification improves success against competitors.
differences of habit and constitution, determine that the inhabitants, which thus jostle each other most closely, shall, as a general rule, belong to what we call different genera and orders. The same principle is seen in the naturalisation of plants through man’s agency in foreign lands. It might have been expected that the plants which have succeeded in becoming naturalised in any land would generally have been cl…
Using a branching diagram, Darwin models how slight, heritable variations are preserved by selection, divergence accumulates over thousands of generations, and recorded varieties/species arise as different branches succeed or die out.
Now let us see how this principle of great benefit being derived from divergence of character, combined with the principles of natural selection and of extinction, will tend to act. The accompanying diagram will aid us in understanding this rather perplexing subject. Let A to L represent the species of a genus large in its own country; these species are supposed to resemble each other in unequal degrees, as is so ge…