Charles Darwin · Public domain edition · 184 cards · 49 chapters
Charles Darwin's On the Origin of Species argues that species change over time through natural selection acting on inherited variation. This deck covers Darwin's preliminary abstract, his views on variation under domestication, and his pigeon-breeding evidence for common descent.
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Chapter 1 · Question
Why did Darwin publish this Abstract before completing his full work?
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Chapter 1 · Answer
Darwin says his work was nearly finished but would take two or three more years, and his health was far from strong. He was especially urged to publish because Mr. Wallace, studying natural history in the Malay archipelago, had arrived at almost exactly the same general conclusions on the origin of species.
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Introduction4 cards
Why did Darwin publish this Abstract before completing his full work?
Darwin says his work was nearly finished but would take two or three more years, and his health was far from strong. He was especially urged to publish because Mr. Wallace, studying natural history in the Malay archipelago, had arrived at almost exactly the same general conclusions on the origin of species.
Chapter 1
What does Darwin admit are the limitations of this Abstract?
Darwin states the Abstract must necessarily be imperfect: he cannot give references and authorities for his statements and must trust the reader to repose some confidence in his accuracy. He admits errors may have crept in, gives only general conclusions with a few illustrative facts, and notes that facts often apparently lead to opposite conclusions, which a fair result requires balancing on both sides.
Chapter 1
Why does Darwin reject external conditions alone as the cause of adaptation?
Darwin calls it preposterous to attribute structures like the woodpecker's feet, tail, beak, and tongue—admirably adapted to catch insects under bark—to mere external conditions. Likewise the misseltoe, nourished by certain trees, with seeds transported by certain birds and flowers requiring certain insects for pollen, cannot be explained by external conditions, habit, or the plant's volition.
Chapter 1
What does Darwin say about the 'Vestiges of Creation' explanation?
Darwin presumes the author of the 'Vestiges of Creation' would say that after an unknown number of generations some bird gave birth to a woodpecker and some plant to the misseltoe, produced perfect as now seen. Darwin finds this no explanation, because it leaves the coadaptations of organic beings to each other and to their physical conditions of life untouched and unexplained.
Chapter 1
CHAPTER I. (part 1)4 cards
Why does Darwin think domestic productions vary more than wild species?
The author argues domestic plants and animals generally differ more from each other than individuals of a wild species because they are raised under less uniform conditions of life, somewhat different from those the parent-species faced in nature. He also notes Andrew Knight's view that excess food may partly cause this variability, and that organisms must be exposed to new conditions for several generations before appreciable variation appears.
Chapter 2
What does Darwin say about the reproductive system and confinement?
The author argues the reproductive system is far more susceptible than any other part of the organisation to changes in conditions of life. He notes few things are harder than getting animals to breed freely under confinement, and that many vigorous cultivated plants rarely or never seed. He concludes variability may largely stem from the ovules or pollen being affected by the parent's treatment before conception.
Chapter 2
What is correlation of growth, and what examples does Darwin give?
Correlation of growth means any change in the embryo or larva almost certainly entails changes in the mature animal, so selecting one peculiarity unconsciously modifies other parts. Examples include breeders' belief that long limbs accompany elongated heads, cats with blue eyes being invariably deaf, hairless dogs having imperfect teeth, and pigeons with feathered feet having skin between their outer toes.
Chapter 2
How does Darwin view inheritance and its laws?
The author argues inheritance should be treated as the rule and non-inheritance as the anomaly, since no breeder doubts like produces like. He states the laws governing inheritance are quite unknown, and that a peculiarity tends to appear in offspring at the same age it first appeared in the parent. He confines this rule to the first appearance, not the primary cause.
Chapter 2
CHAPTER I. (part 2)4 cards
What does Darwin conclude about the origin of domestic dog breeds?
The author argues it is highly probable, from geographical and other considerations, that domestic dogs descended from several wild species. He fully admits dogs probably had multiple wild ancestors, yet insists there has also been an immense amount of inherited variation, since forms like the Italian greyhound, bloodhound, and bulldog are unlike any wild Canidae.
Chapter 3
Why does Darwin doubt that crossing alone can produce distinct domestic races?
Darwin argues crossing yields only forms intermediate between parents, so explaining races this way requires the extreme forms to have existed in the wild. He says the possibility of making distinct races by crossing has been greatly exaggerated: a race may be modified by occasional crosses aided by careful selection, but a permanent intermediate race cannot be formed without extreme care and long-continued selection. Sir J. Sebright experimented for this and failed.
Chapter 3
What evidence leads Darwin to believe all domestic pigeon breeds descend from the rock-pigeon?
Darwin argues that otherwise at least seven or eight aboriginal stocks would be needed, since crossing a lesser number cannot produce traits like the pouter's crop. The supposed stocks would all have to be rock-pigeons, yet only two or three other rock-pigeon species are known and none share the domestic characters. Their extermination is improbable, no breed has become feral, and the breeds share the rock-pigeon's distinctive colour marks.
Chapter 3
How does Darwin use pigeon colouring as evidence for common descent?
Darwin notes the rock-pigeon's slaty-blue colour, white rump, terminal dark tail-bar with white-edged outer feathers, and two black wing-bars—marks not occurring together in any other species of the family. In every well-bred domestic breed these marks sometimes concur perfectly, and mongrels from two distinct non-blue breeds can suddenly reacquire them, as when white fantails crossed with black barbs produced a blue grandchild.
Chapter 3
CHAPTER I. (part 3)4 cards
What does Darwin call the key to producing domestic breeds?
Darwin argues the key is man's power of accumulative selection: nature supplies successive slight variations, and man adds them up in directions useful to him. He insists this principle is not hypothetical, citing breeders who modified cattle and sheep greatly within a single lifetime, and quotes Youatt calling selection the agriculturist's 'magician's wand.'
Chapter 4
Why is unconscious selection more important to Darwin's argument than methodical selection?
Methodical selection aims deliberately at a new strain, but Darwin finds unconscious selection more important: everyone simply tries to own and breed from the best animals without intending to alter the breed permanently. Continued for centuries, this process improves and modifies breeds, as with Bakewell's cattle, King Charles's spaniel, and English racehorses surpassing the Arab stock.
Chapter 4
How does Darwin show selection predates modern practice?
Darwin denies selection is a modern discovery, citing acknowledgment in works of high antiquity: an ancient Chinese encyclopaedia gives the principle distinctly, Roman classical writers lay down explicit rules, and Genesis shows attention to animal colour. In rude periods of English history choice animals were imported and laws passed against their exportation, and the destruction of undersized horses is compared to nurserymen 'roguing' plants.
Chapter 4
What evidence shows selection's effects accumulate from differences too slight to see?
Darwin stresses that selection's importance lies in accumulating, over successive generations, differences 'absolutely inappreciable by an uneducated eye'—differences he himself vainly tried to appreciate. Fewer than one man in a thousand has the eye and judgment to be an eminent breeder. Youatt's two Leicester flocks, purely bred from Bakewell's stock for over fifty years, looked like distinct varieties.
Chapter 4
CHAPTER I. (part 4)4 cards
What conditions does Darwin say favour man's power of selection?
Darwin argues high variability supplies materials for selection, but the chance of useful variations appearing is much increased by keeping a large number of individuals, which requires favourable conditions for free breeding. Scanty stock prevents selection because all individuals breed. Most important is that the plant or animal be valued enough that close attention is paid to even slight deviations.
Chapter 5
Why does Darwin say distinct breeds of cats are rarely kept?
Darwin attributes the rarity or absence of distinct breeds of cats, donkeys, peacocks and geese mainly to selection not having been brought into play. In cats this is because their nocturnal rambling habits make pairing difficult; in donkeys because few are kept by poor people with little attention to breeding; in peacocks because they are not easily reared and large stocks are not kept.
Chapter 5
How does Darwin explain the recent rise of strawberry varieties?
Darwin notes the strawberry had always varied since cultivation, but slight varieties were neglected. Once gardeners picked out individual plants with slightly larger, earlier or better fruit, raised seedlings from them, and again selected the best seedlings, many admirable varieties appeared over thirty or forty years, aided by some crossing with distinct species. He mocks the idea that it was fortunate the strawberry began to vary just then.
Chapter 5
What does Darwin conclude about the causes of change in domestic races?
Darwin believes conditions of life act on the reproductive system and are highly important in causing variability, but he rejects variability as an inherent, necessary contingency in all organic beings. Effects are modified by inheritance and reversion, governed by unknown laws especially correlation of growth, with some contribution from direct conditions, use and disuse, and intercrossing. He insists the accumulative action of selection is by far the predominant power.
Chapter 5
CHAPTER II.3 cards
Which species vary most, according to Darwin's chapter on variation under nature?
Darwin states that wide-ranging, much diffused, and common species vary most. He also argues that species of larger genera in any country vary more than species of smaller genera, and that many species of larger genera resemble varieties in being closely but unequally related and in having restricted ranges.
Chapter 6
What are polymorphic genera, and how does Darwin explain them?
Polymorphic or protean genera are those in which species show inordinate variation, so hardly two naturalists agree which forms are species and which varieties; examples include Rubus, Rosa, and Hieracium among plants, plus some insects and Brachiopods. Darwin suspects these are variations in structures of no service or disservice to the species, not seized on and made definite by natural selection.
Chapter 6
How does Darwin illustrate the difficulty of ranking doubtful forms with the primrose and cowslip?
Primula veris and elatior differ in appearance, flavour, odour, flowering period, station, altitude, and range, and Gärtner's many experiments showed they cross only with much difficulty, which Darwin says is hardly better evidence of being specifically distinct. Yet intermediate links unite them, and he finds overwhelming experimental evidence they descend from common parents, so they must be ranked as varieties.
Chapter 6
CHAPTER III.4 cards
How does Darwin define the 'struggle for existence' in this chapter?
Darwin uses the term in a large and metaphorical sense, including dependence of one being on another and success in leaving progeny, not just direct combat. A plant on a desert edge struggles against drought, though it is more properly dependent on moisture. He notes these senses pass into each other and he uses the general term for convenience.
Chapter 7
Why does a struggle for existence inevitably follow from organic increase?
Every being producing several eggs or seeds must suffer destruction at some period, or geometrical increase would make numbers too great for any country to support. More individuals are produced than can survive, so struggle occurs with the same species, distinct species, or physical conditions. Darwin calls this the doctrine of Malthus applied with manifold force to all animals and plants.
Chapter 7
What does Darwin say about checks to increase and their obscurity?
Darwin says what checks each species' tendency to increase is most obscure; we do not know exactly what the checks are in even one single instance. He notes our ignorance even regarding mankind, which is far better known. He compares nature to a yielding surface with ten thousand sharp wedges driven inwards by incessant blows.
Chapter 7
What evidence does Darwin give for rapid increase of naturalised species?
Darwin cites recorded cases of animals increasing astonishingly fast in nature when conditions favoured them for two or three seasons. Domestic animals run wild, like cattle and horses in South America and Australia, increased rapidly where statements were well authenticated. Introduced plants became common across whole islands in under ten years, and European plants now dominate wide plains of La Plata.
Chapter 7
CHAPTER IV. (part 1)4 cards
How does Darwin define Natural Selection?
Darwin defines Natural Selection as the preservation of favourable variations and the rejection of injurious variations. He argues that individuals with any advantage, however slight, have the best chance of surviving and procreating, while variations in the least degree injurious would be rigidly destroyed. Variations neither useful nor injurious are not affected and remain a fluctuating element, as perhaps seen in polymorphic species.
Chapter 8
How does Darwin compare nature's selection with man's selection?
The author argues nature's power far exceeds man's. Man acts only on external and visible characters, selects for his own good, keeps natives of many climates together, and does not exercise each selected character fittingly. Nature cares nothing for appearances, acts on every internal organ and constitutional difference, selects only for the good of the being tended, and has incomparably longer geological time, so her productions are 'truer' and better adapted.
Chapter 8
What circumstances favour natural selection according to Darwin?
Darwin argues favourable circumstances include changed conditions of life (which increase variability), isolation such as an island or barriers checking immigration, and a large number of individuals. Intercrossing and free immigration are unfavourable because intruders seize unoccupied places. He notes natural selection acts slowly, and that unless profitable variations occur, natural selection can do nothing.
Chapter 8
What does Darwin say natural selection cannot do?
Darwin states that what natural selection cannot do is modify the structure of one species, without giving it any advantage, for the good of another species. He notes such statements appear in works of natural history but he cannot find one case that bears investigation. He adds that a structure used only once, if highly important, might still be modified extensively.
Chapter 8
CHAPTER IV. (part 2)4 cards
How does Darwin explain the formation of two wolf varieties in different habitats?
Darwin suggests wolves in mountainous districts and lowlands would be forced to hunt different prey, and continued preservation of individuals best fitted for each site could slowly form two varieties. These varieties would cross and blend where they met. He adds that according to Mr. Pierce, two wolf varieties inhabit the Catskill Mountains: a light greyhound-like form pursuing deer and a bulkier, shorter-legged form attacking sheep.
Chapter 9
How does Darwin's imaginary nectar case explain flower-insect coadaptation?
Darwin supposes nectar excreted at petal bases attracts insects that become dusted with pollen and carry it between flowers, producing vigorous crossed seedlings. Flowers with larger nectaries are visited and crossed most often, gaining the upper hand; those with stamens and pistils positioned to favor pollen transport are likewise selected. He argues a flower and a bee might slowly become modified and adapted to each other by preservation of mutually favorable deviations.
Chapter 9
What does Darwin conclude about self-fertilization from breeders' experience?
Darwin says he collected a large body of facts, matching breeders' near-universal belief, that crosses between varieties or between individuals of another strain give vigor and fertility, while close interbreeding diminishes both. These facts incline him to believe it is a general law of nature that no organic being self-fertilizes for an eternity of generations, but that a cross with another individual is occasionally, perhaps at very long intervals, indispensable.
Chapter 9
What did Darwin find when examining the female holly tree's stigmas?
Darwin found a female holly tree sixty yards from a male tree. Examining stigmas of twenty flowers from different branches under the microscope, he found pollen grains on all without exception, some with a profusion. Since the wind had blown from female to male for days, wind could not have carried it. Despite cold, boisterous weather unfavorable to bees, every female flower examined had been fertilized by bees accidentally dusted with pollen.
Chapter 9
CHAPTER IV. (part 3)4 cards
What does Darwin conclude about large areas versus isolation for producing new species?
The author concludes that although isolation is of considerable importance, largeness of area matters more, especially for producing species able to endure long and spread widely. Large open areas support more individuals and more complex conditions, so favourable variations are likelier and competition is more severe. New forms that survive there have already beaten many competitors and will spread most widely.
Chapter 10
How does intercrossing affect the formation of varieties in birds and similar animals?
Intercrossing most affects animals that unite for each birth, wander much, and breed slowly. In such animals, Darwin states, varieties will generally be confined to separated countries, and he believes this is the case. Intercrossing also keeps individuals of a species true and uniform in character, acting more efficiently in animals that unite for each birth.
Chapter 10
Why does Darwin think fresh-water basins contain anomalous 'living fossils'?
Fresh-water basins together form a small area compared with sea or land, so competition between fresh-water productions has been less severe. New forms were formed more slowly and old forms exterminated more slowly. There Darwin finds seven genera of Ganoid fishes and anomalous forms like Ornithorhynchus and Lepidosiren, which he says may almost be called living fossils.
Chapter 10
What role does Darwin assign to isolation in natural selection?
In a confined or isolated area, conditions are largely uniform, so natural selection tends to modify all individuals similarly, and intercrossing with surrounding districts is prevented. Isolation more efficiently checks immigration of better-adapted organisms after physical changes, leaving open places for old inhabitants to adapt. It also gives time for new varieties to slowly improve, though very small areas support few individuals and retard new species.
Chapter 10
CHAPTER IV. (part 5)4 cards
According to Darwin, why do species of larger genera most often present varieties?
The author argues natural selection acts through one form having some advantage over others in the struggle for existence, so it chiefly acts on forms that already have an advantage. The largeness of a group shows its species inherited from a common ancestor some advantage in common. Hence the struggle for new and modified descendants mainly lies between the larger groups, all trying to increase in number.
Chapter 12
What does Darwin predict about the fate of small and broken groups?
Looking to the future, the author predicts that large, triumphant, least-broken groups will continue to increase for a long period. One large group will slowly conquer another, reducing its numbers and lessening its chance of further variation. Small and broken groups and sub-groups will finally tend to disappear, and extremely few species living at any one period will transmit descendants to a remote futurity.
Chapter 12
How does Darwin use the tree simile to represent affinities of organic beings?
The author believes the tree simile largely speaks the truth. Budding twigs represent existing species; former years' growth represents extinct species. Twigs branch out and overtop and kill surrounding twigs, as species overmaster others in the battle for life. Decayed, dropped limbs represent orders, families, and genera with no living representatives, known only from fossils.
Chapter 12
What does Darwin say must judge whether natural selection has acted in nature?
The author states that whether natural selection has really acted in nature, modifying and adapting forms of life to their conditions and stations, must be judged by the general tenour and balance of evidence given in the following chapters. He notes we already see how it entails extinction, and geology plainly declares how largely extinction has acted in the world's history.
Chapter 12
CHAPTER V. (part 1)4 cards
What does Darwin say causes most variation in offspring?
Darwin argues that variations are not due to chance but chiefly to the reproductive system being functionally disturbed in the parents by changes in the conditions of life. The male and female sexual elements seem affected before union. He admits profound ignorance of why particular parts vary, but insists some cause exists for each deviation, however slight.
Chapter 13
How does Darwin explain wingless beetles on Madeira?
Darwin believes the wingless condition of many Madeira beetles is mainly due to natural selection combined probably with disuse. Beetles that flew least survived by not being blown to sea, while frequent fliers perished. He notes 200 of 550 species cannot fly, and flower-feeding insects that must use wings keep or even enlarge them.
Chapter 13
How does Darwin explain blindness in cave animals?
Darwin attributes eye loss in cave animals wholly to disuse, since useless eyes are hard to imagine as injurious in darkness. But the cave-rat is an exception: its eyes are immense, and natural selection seems to have struggled against loss of light to enlarge them. He argues cave faunas of America and Europe were separately stocked by migration, not separate creation.
Chapter 13
What role does Darwin assign to climate and food directly?
Darwin thinks the direct effect of climate and food is extremely doubtful, extremely small in animals and perhaps rather more in plants. He cites Forbes on brighter shells in warm shallow water, Gould on brighter continental birds, and Wollaston on coastal insects, but concludes such influences cannot produce complex co-adaptations; conditions act mainly indirectly by inducing variability.
Chapter 13
CHAPTER V. (part 2)4 cards
What does Darwin mean by 'correlation of growth'?
Darwin defines correlation of growth as the whole organisation being so tied together during growth and development that when slight variations in one part occur and are accumulated through natural selection, other parts also become modified. He calls the subject very important but most imperfectly understood, and notes the nature of the bond is frequently quite obscure.
Chapter 14
How does Darwin explain the rule that winged seeds never occur in fruits that do not open?
Alph. De Candolle remarked that winged seeds are never found in fruits which do not open. Darwin explains this not as correlation but as the manner in which natural selection can act: seeds could not gradually become winged through natural selection except in fruits that opened, since only then could plants with slightly better-fitted seeds gain an advantage in dispersal.
Chapter 14
What examples of correlation does Darwin cite involving cats, pigeons, and the Turkish dog?
Darwin cites the relation between blue eyes and deafness in cats, tortoise-shell colour with the female sex, feathered feet and skin between the outer toes in pigeons, down on young birds with future plumage colour, and the relation between hair and teeth in the naked Turkish dog—though he suspects homology may be involved in the last case.
Chapter 14
How does Darwin use the parasitic cirripede Proteolepas to argue against compensation?
Darwin argues natural selection continually tries to economise every part of the organisation. In the parasitic and protected Proteolepas, the whole anterior part of the head is reduced to a mere rudiment, saving nutriment wasted on a now-useless structure. This shows natural selection can reduce a superfluous part without requiring a corresponding enlargement of another part.
Chapter 14
CHAPTER V. (part 3)4 cards
Why does Darwin expect recently and greatly modified organs to remain more variable?
The author argues that an extraordinary amount of modification implies unusually large and long-continued variability, accumulated by natural selection. Because such change occurred within a period not extremely remote, selection has seldom yet fixed the variability or fully rejected individuals tending to revert. Hence generative variability should still be present in a high degree in such recently and greatly modified parts.
Chapter 15
Why are specific characters more variable than generic characters, according to Darwin?
The author attributes generic characters to inheritance from a remote common progenitor, since natural selection rarely modifies several species in exactly the same manner; having long remained constant, they are unlikely to vary now. Specific characters, however, varied and diverged within the period since the species branched off, so they are probably still somewhat variable, at least more so than long-constant parts.
Chapter 15
What two examples does Darwin give of secondary sexual differences matching specific differences?
Darwin cites beetles of the family Engidae, where Westwood noted the number of tarsal joints varies greatly and also differs between the sexes of the same species. Second, in fossorial hymenoptera, wing neuration is a character of high importance common to large groups, yet in certain genera it differs between species and likewise between the two sexes of the same species.
Chapter 15
What does Darwin mean by analogous variation, and what pigeon example illustrates it?
Distinct species or races may vary in similar ways because they inherited a common constitution and tendency to vary. Darwin notes that the most distinct pigeon breeds, in widely separated countries, produce sub-varieties with reversed head feathers and feathered feet, characters absent in the rock-pigeon. He also treats the pouter's extra tail feathers as representing the fantail's normal structure.
Chapter 15
CHAPTER V. (part 4)4 cards
What striping pattern appears in horse-genus hybrids, and what does Darwin conclude from it?
Hybrids of ass, zebra, quagga, and hemionus often show leg bars and shoulder stripes absent or faint in the parent species; Darwin cites Rollin, Martin, Lord Moreton's quagga hybrid, and Gray's ass-hemionus hybrid. He argues this tendency to revert to striping, strongest in hybrids and in dun horses, points to a common striped ancestor of the whole horse genus.
Chapter 16
How does Darwin use pigeon breeds to argue about horse species?
Darwin notes that pigeon breeds descended from a bluish barred ancestor reacquire the blue tint and bars whenever a breed varies to bluish, and mongrels of old true breeds tend to revert to blue. He says: call the pigeon breeds species, and the case exactly parallels the horse-genus species, supporting common descent rather than independent creation.
Chapter 16
What does Darwin say about the hypothesis that each equine species was independently created?
He says anyone holding that view must assert each species was created with a tendency to vary into stripes like other species, and, when crossed, to produce hybrids striped like other species rather than their own parents. Darwin argues this replaces a real cause with an unreal one, making the works of God a mockery and deception, like created fossil shells.
Chapter 16
According to the summary, why are some parts more variable than others?
Darwin says our ignorance of variation's laws is profound, but comparison shows the same laws act on varieties and species. Multiple parts vary because they are not closely specialised, so natural selection checks them less; lowly organisms vary more for the same reason; rudimentary organs, being useless, are disregarded by selection; and specific characters vary more than long-inherited generic ones.
Chapter 16
CHAPTER VI. (part 1)4 cards
Why does Darwin say transitional varieties are rarely found as fossils?
Darwin argues that extinction and natural selection go hand in hand: each new form tends to exterminate its less improved parent and transitional varieties. He also claims the geological record is far more imperfect than generally supposed, because fossil-bearing sediment accumulates only rarely, on shallow sea beds that slowly subside, so the earth's crust is like a vast museum whose collections were made at immensely remote intervals.
Chapter 17
Why are intermediate varieties between two forms usually rarer and short-lived?
Darwin argues intermediate varieties occupy narrow intermediate zones, so they exist in smaller numbers. Rarer forms face greater risk of extermination and are pressed by closely allied forms on both sides. More importantly, the two forms in larger areas have better chances of presenting favourable variations, so they beat and supplant the slower-modified intermediate form, which then disappears.
Chapter 17
What does Darwin's sheep illustration demonstrate about intermediate varieties?
Darwin imagines three sheep varieties: one on extensive mountains, one on a narrow hilly tract, one on wide plains, with breeders equally skilled. The large mountain and plain holders improve their breeds faster than the small hill holders, so the improved breeds soon replace the hill breed and come into close contact without the intermediate. He uses this to show why common forms supplant rarer intermediate ones.
Chapter 17
How does Darwin explain sharply defined species ranges instead of gradual blending?
Darwin argues ranges depend largely on other species — competitors, prey, and enemies — not solely on insensibly changing physical conditions. Since those species are already defined objects that do not blend, ranges tend to be sharply defined. On the confines, where numbers are low, fluctuations in enemies, prey, or seasons make utter extermination likely, sharpening the boundary further.
Chapter 17
CHAPTER VI. (part 2)4 cards
Why does Darwin say transitional forms of a perfected structure are rarely found today?
The author argues that animals with early transitional grades of a structure will seldom survive to the present, because they are supplanted by the very process of perfection through natural selection. Such intermediate forms also existed in smaller numbers, so the chance of finding them as fossils is always less than for species with fully developed structures.
Chapter 18
What examples of diversified habits in one species does Darwin give?
Darwin cites the South American tyrant flycatcher, which hovers like a kestrel, waits by water, and dives like a kingfisher; the British titmouse, which climbs like a creeper, kills small birds like a shrike, and hammers yew seeds like a nuthatch; and Hearne's black bear swimming with open mouth to catch insects like a whale.
Chapter 18
How does Darwin answer the objection that the eye seems too perfect for natural selection?
Darwin freely admits the eye's formation by natural selection seems absurd in the highest degree. Yet he argues that if numerous useful gradations from simple to complex eyes exist, if the eye varies slightly and the variations are inherited, and if any modification is useful under changing conditions, then the difficulty, though insuperable to imagination, is hardly real. Reason ought to conquer imagination.
Chapter 18
What would absolutely break down Darwin's theory of natural selection?
Darwin states that if it could be demonstrated that any complex organ existed which could not possibly have been formed by numerous, successive, slight modifications, his theory would absolutely break down. He adds that he can find no such case, though many organs exist whose transitional grades are unknown, especially in isolated species or organs shared by a whole class.
Chapter 18
CHAPTER VI. (part 3)3 cards
Why does Darwin invoke the canon 'Natura non facit saltum' against separate creation?
The author argues that transitional grades lead toward almost every named organ, and experienced naturalists widely admit the canon that nature makes no leap—as Milne Edwards put it, nature is prodigal in variety but niggard in innovation. On the theory of creation, why should independent beings be linked by graduated steps? Natural selection explains it: it acts only on slight successive variations and can never take a leap.
Chapter 19
How does Darwin answer the difficulty that organs of trifling importance could be shaped by natural selection?
He argues we are too ignorant of any being's whole economy to judge which slight modifications matter. Trifling characters like fruit down or flesh colour may determine insect attacks or correlate with constitution. The giraffe's tail as a fly-flapper seems trivial, yet cattle in South America depend on resisting insects; harassment reduces strength, leaving animals more subject to disease, dearth, or predators.
Chapter 19
What does Darwin say about structures of no direct use, and what would refute his theory?
The author admits many structures are of no direct use, attributing them chiefly to inheritance, plus correlation of growth, reversion, sexual selection, and slight direct effects of physical conditions. He argues every detail was formerly useful to an ancestor or is now useful. He states that proof any structure was formed for the exclusive good of another species would annihilate his theory.
Chapter 19
CHAPTER VI. (part 4)4 cards
According to Darwin, what organ does he suggest was converted into an air-breathing lung?
Darwin states that a swim-bladder has apparently been converted into an air-breathing lung. He uses this as evidence that wonderful metamorphoses in function are at least possible, urging caution before concluding that no intermediate or transitional states could ever have existed, since the homologies of many organs and their intermediate states demonstrate such transformations are possible.
Chapter 20
What does Darwin mean by the canon 'Natura non facit saltum'?
The canon means 'nature makes no leap.' Darwin argues that if we look only at present inhabitants of the world, the canon is not strictly correct, but if we include all past times, it must by his theory be strictly true. He presents this as something his theory of natural selection lets us clearly understand the full meaning of.
Chapter 20
What are the two great laws Darwin says all organic beings were formed on?
Darwin says it is generally acknowledged that all organic beings have been formed on two great laws: Unity of Type and the Conditions of Existence. Unity of type means the fundamental agreement in structure among organic beings of the same class, independent of their habits of life. On his theory, unity of type is explained by unity of descent.
Chapter 20
Why does Darwin call the Conditions of Existence the higher law?
Darwin says the expression of conditions of existence, insisted on by Cuvier, is fully embraced by natural selection, which adapts varying parts to organic and inorganic conditions of life, aided by use and disuse and slightly affected by external conditions, all subject to laws of growth. Hence the law of Conditions of Existence is the higher law, since it includes, through inheritance of former adaptations, that of Unity of Type.
Chapter 20
CHAPTER VII. (part 1)4 cards
What does Darwin say about the origin of the primary mental powers?
Darwin states he has nothing to do with the origin of the primary mental powers, any more than with the origin of life itself. His concern is only with the diversities of instinct and other mental qualities of animals within the same class. He declines to define instinct, noting that the characters usually attributed to it are not universal, and that a little judgment or reason often comes into play even in low animals.
Chapter 21
What did P. Huber observe when he moved a caterpillar between hammocks at different stages?
Huber found that a caterpillar taken from a hammock completed to the sixth stage and placed in one completed only to the third simply re-performed the fourth, fifth, and sixth stages. Conversely, one taken from a third-stage hammock and placed in a finished one was embarrassed and seemed forced to start from the third stage, trying to complete already finished work. Darwin compares this to being interrupted while reciting something by rote.
Chapter 21
How did Darwin test whether aphides excrete for the ants' benefit?
Darwin removed all ants from about a dozen aphides on a dock-plant for several hours. Though he expected the aphides would want to excrete, none did, even when he stroked them with a hair as ants do with antennae. When an ant was allowed to visit, each aphis immediately lifted its abdomen and excreted a sweet drop. Young aphides behaved likewise, showing the action is instinctive. Darwin concludes the viscid excretion is probably a convenience to the aphides to have removed, so they do not excrete for the sole good of the ants.
Chapter 21
What examples of inherited domestic instincts does Darwin give in dogs?
Darwin cites young pointers that point and back other dogs the first time taken out, retrievers that inherit retrieving, and shepherd-dogs with an inherited tendency to run round rather than at a flock. He notes domestic instincts are less fixed than natural ones because they have been subject to less rigorous selection over a shorter period. Crosses show blending: a bull-dog cross affected greyhounds' courage for generations, and Le Roy describes a dog with a wolf great-grandfather that only failed to come straight when called.
Chapter 21
CHAPTER VII. (part 2)4 cards
What did Huber observe when he isolated thirty F. rufescens ants without slaves?
Huber shut up thirty F. rufescens workers with their favorite food, larvae, and pupae, yet they did nothing—they could not even feed themselves, and many perished of hunger. When he introduced a single slave of F. fusca, she instantly fed and saved the survivors, made cells, and tended the larvae. This showed the masters' absolute dependence on slaves.
Chapter 22
How does F. sanguinea's dependence on slaves differ from F. rufescens's?
F. rufescens is absolutely dependent: it builds no nest, determines no migrations, collects no food, and cannot even feed itself. F. sanguinea keeps far fewer slaves, and the masters decide when and where to build a new nest and carry the slaves during migration. In England the masters alone usually leave the nest to gather materials and food, so they receive less service than in Switzerland.
Chapter 22
What did Darwin observe when F. sanguinea encountered pupae of F. fusca versus F. flava?
Darwin found the ants at once distinguished the pupae: they eagerly and instantly seized F. fusca pupae, which they habitually enslave, but were terrified by F. flava pupae or even earth from their nest, and quickly ran away. About a quarter of an hour later, after the little yellow ants had crawled off, they took heart and carried off the pupae.
Chapter 22
What does Darwin propose as the origin of the slave-making instinct?
Darwin declines to conjecture the exact steps for F. sanguinea, but suggests that non-slave-making ants carry off other species' pupae as food; some pupae might develop and the reared ants follow their instincts and work. If capturing workers proved more advantageous than procreating them, natural selection could strengthen the pupa-collecting habit into permanent slave-making.
Chapter 22
CHAPTER VII. (part 3)4 cards
What experiment with vermilion wax tested how bees form flat cell bases?
Darwin placed a thin ridge of vermilion wax between opposed basins and let bees excavate from both sides. They stopped gnawing in time, leaving flat plates of ungnawed wax along the planes of imaginary intersection between the basins. He concluded bees can perceive when wax is thin enough and stop, working at nearly equal rates on opposite sides.
Chapter 23
How does Darwin describe the bees' building method, comparing it to masons?
Bees first build a rough wall ten to twenty times thicker than the finished cell wall, then gnaw it away equally on both sides, adding fresh wax on top. Darwin likens this to masons piling a broad ridge of cement, cutting it away near the ground, and adding more on the summit, leaving a thin growing wall crowned by a thick coping.
Chapter 23
Why does Darwin argue wax economy made complex bee cells advantageous?
Tegetmeier told Darwin that twelve to fifteen pounds of dry sugar are consumed per pound of wax secreted, and many bees idle for days during secretion. Since large honey stores support large colonies and hive security, saving wax saves honey. The author argues natural selection favored swarms wasting least honey on wax, transmitting economical instincts to new swarms.
Chapter 23
What difficulty with sterile neuter insects does Darwin call fatal to his theory?
Darwin says neuters or sterile females in insect communities often differ widely in instinct and structure from males and fertile females, yet being sterile they cannot propagate their kind. He calls this at first insuperable and actually fatal to his whole theory, and says the subject deserves lengthy discussion, which he postpones.
Chapter 23
CHAPTER VII. (part 4)4 cards
What ant example shows gradations among neuters within one nest?
Darwin cites Formica flava, whose larger workers have small but visible ocelli while smaller workers have rudimentary ocelli, with intermediate-sized workers connecting them. He dissected specimens and affirms the eyes are more rudimentary in smaller workers than size alone explains. He believes intermediate workers have exactly intermediate ocelli but dares not assert it positively.
Chapter 24
How does Darwin explain two distinct sterile worker castes in one nest?
The author argues natural selection acting on fertile parents first produced a graduated series, as in the driver ant Anomma. When extreme forms proved most useful to the community, parents generating them were selected until intermediates ceased to be produced. Thus two sharply defined sterile castes, differing from each other and their parents, arose. He calls this the most serious special difficulty his theory encountered.
Chapter 24
Why does Darwin say sterility is needed for division of labour in ants?
Ants work by inherited instincts and inherited tools or weapons, not acquired knowledge or manufactured instruments. A perfect division of labour could only be effected if workers are sterile; were they fertile, they would intercross and their instincts and structure would blend. Nature, the author believes, achieved this division of labour through natural selection.
Chapter 24
What does Darwin say the neuter-insect case demonstrates against Lamarck?
The author argues the case proves that, in animals as in plants, modification can be effected by accumulation of numerous slight accidental variations that are profitable, without exercise or habit. No exercise, habit, or volition in utterly sterile members could affect the fertile members, which alone leave descendants. He is surprised no one advanced this case against Lamarck's doctrine.
Chapter 24
CHAPTER VIII. (part 1)4 cards
What distinction does Darwin draw between sterility of first crosses and of hybrids?
Darwin separates two facts often confused: pure species crossed together produce few or no offspring though their sexual elements are perfect, while hybrids have structurally perfect reproductive organs that are functionally impotent, with male elements undeveloped or imperfect. In the first case the elements forming the embryo are perfect; in the second they are absent or imperfectly developed. He calls this distinction important for understanding the shared sterility.
Chapter 25
Why does Darwin doubt Gärtner's conclusion that species crosses are universally sterile?
Darwin argues Gärtner's method introduced error: hybridised plants must be castrated and secluded from insect-borne pollen, often kept potted indoors, and such treatment can itself impair fertility. Gärtner's own table shows about a score of self-fertilised castrated plants, half with reduced fertility. Gärtner also found the primrose and cowslip, likely varieties, only once or twice fertile, and the red and blue pimpernels absolutely sterile, leading Darwin to doubt many species are as sterile as claimed.
Chapter 25
How does Darwin explain Gärtner's claim that hybrid fertility decreases over generations?
Darwin does not doubt fertility usually decreases, often suddenly in early generations, but attributes it to an independent cause: close interbreeding. Hybrids are seldom raised in numbers and are protected from insects, so each generation is fertilised by its own pollen. He notes Gärtner's own statement that artificially fertilising less fertile hybrids with hybrid pollen sometimes increases fertility, and argues such pollination often crosses distinct flowers, avoiding close interbreeding.
Chapter 25
What does Herbert's Crinum and Hippeastrum evidence show about crossing versus self-fertilisation?
Herbert found every ovule in a pod of Crinum capense fertilised by C. revolutum produced a plant, a case of perfect or more than normally perfect fertility in a first cross between distinct species. Darwin also cites Hippeastrum: some individual plants and all species of certain genera, such as Lobelia and Hippeastrum, can be more easily fertilised by pollen of a distinct species than by their own, though their own pollen is good for other species.
Chapter 25
CHAPTER VIII. (part 2)4 cards
According to Darwin, what governs the fertility of first crosses and hybrids?
Darwin states that the fertility of first crosses between species, and of the hybrids produced from them, is largely governed by their systematic affinity—their resemblance in structure and constitution, especially in physiologically important parts. However, he stresses the correspondence is by no means strict: very closely allied species may fail to unite, while very distinct species may cross with the utmost facility.
Chapter 26
What does Darwin mean by a reciprocal cross, and why does it matter?
A reciprocal cross means using each of two species as father and as mother in turn—for example, a stallion with a female ass, then a male ass with a mare. Darwin says the facility of reciprocal crosses often differs widely, as in Mirabilis jalappa and M. longiflora, where Kölreuter failed over two hundred times in one direction. This shows crossing capacity is independent of systematic affinity.
Chapter 26
What is Darwin's argument against sterility being specially endowed to prevent blending?
Darwin argues the complex rules do not indicate species were endowed with sterility to prevent confusion in nature. He asks why sterility varies so greatly in degree, why it is innately variable among individuals, why some species cross easily yet yield sterile hybrids, and why reciprocal crosses differ. He concludes sterility is incidental on unknown differences, chiefly in the reproductive systems.
Chapter 26
How does Darwin use grafting to illustrate incidental sterility?
Darwin compares crossing to grafting, which no one supposes is a specially endowed quality but is incidental on differences in the laws of growth. Grafting success is limited by systematic affinity—no one grafts trees of quite distinct families—yet not absolutely governed by it: the pear grafts more readily on the quince than on the apple. He concludes the complex laws of first crosses are likewise incidental on unknown differences.
Chapter 26
CHAPTER VIII. (part 3)3 cards
Why does Darwin doubt that fertility is a universal distinction between varieties and species?
The author argues that the very general fertility of varieties cannot be proved universal or a fundamental distinction. He cites the difficulty of judging sterility in nature, since any infertile variety would simply be ranked a species, and notes man selects only external characters in domestic varieties, not the reproductive system.
Chapter 27
What is the most important difference Gärtner found between hybrids and mongrels?
Gärtner held that in the first generation mongrels are more variable than hybrids, though he admitted hybrids from long-cultivated species are often variable and that hybrids of closely allied species vary more than those of distinct species. Darwin notes this difference graduates away rather than forming a sharp line.
Chapter 27
How does Darwin explain the greater variability of mongrels compared with hybrids?
The author argues mongrel parents are mostly domestic varieties, implying recent variability that may continue and be added to that from crossing. He also holds variability arises from the reproductive system being sensitive to changed conditions, so first-generation hybrids, from unaffected species, are not variable while their descendants are.
Chapter 27
CHAPTER IX. (part 1)4 cards
What is Darwin's explanation for the absence of finely graduated fossil links?
The author argues the main explanation lies in the extreme imperfection of the geological record. Although natural selection and extermination mean an enormous number of intermediate varieties must formerly have existed, geology does not reveal a finely graduated organic chain. Darwin calls this perhaps the most obvious and gravest objection to his theory.
Chapter 28
Between what forms should intermediate varieties be sought, according to Darwin?
Darwin argues we should not picture forms directly intermediate between two living species, but forms intermediate between each species and a common unknown progenitor. Using fantail and pouter pigeons descended from the rock-pigeon, he notes we would find no variety directly combining an expanded tail with an enlarged crop. The common parent may differ from both descendants more than they differ from each other.
Chapter 28
What evidence does Darwin give for the vastness of geological time?
Darwin cites Ramsay's measured British strata totaling 72,584 feet, about thirteen and three-quarter miles, with long blank periods between formations. He notes the Mississippi deposits sediment at only 600 feet per hundred thousand years. Denudation offers the best evidence: the Craven fault's displacement of 600 to 3000 feet, and the Weald's denudation, which he illustrates with a hypothetical sea-erosion rate.
Chapter 28
How does Darwin illustrate denudation using the Weald?
Darwin describes standing on the North Downs and viewing the South Downs, about 22 miles apart, picturing the great dome of rocks that once covered the Weald since the later Chalk. He admits the rate of marine erosion cannot be measured, so he assumes, as a crude notion, the sea eating 500-foot cliffs at one inch per century, while doubting any rock yields that fast except on exposed coasts.
Chapter 28
CHAPTER IX. (part 2)4 cards
According to Darwin, under what conditions can thick fossil-rich formations accumulate?
The author argues thick fossiliferous formations form during periods of subsidence, when the supply of sediment nearly balances the rate of subsidence, keeping the sea shallow and favourable for life. This allows remains to be embedded and preserved before decaying, and the mass to be thick enough to resist later degradation when upraised.
Chapter 29
Why does Darwin say the geological record is intermittent?
The author argues that during stationary or elevation periods, thick shallow-water deposits cannot accumulate, and any beds formed during elevation are destroyed by being upraised into the zone of coast-wave action. Only during subsidence, with sufficient sediment supply, do resistant fossil-rich formations form, so the record is almost necessarily rendered intermittent.
Chapter 29
Why may a single formation lack graduated links between its species?
The author argues each formation, though long, is likely short compared with the time needed to change one species into another. Species may appear mid-formation by immigration, disappear and reappear, and thick deposits are often barren except near their limits. Also, naturalists lack a golden rule for distinguishing species from varieties.
Chapter 29
What did Lyell and Dawson find in Nova Scotia carboniferous beds?
The author cites Messrs. Lyell and Dawson finding carboniferous beds 1400 feet thick in Nova Scotia, with ancient root-bearing strata at no fewer than sixty-eight different levels, one above the other. This shows long intervals of time and changes of level during deposition that would never have been suspected without the preserved trees.
Chapter 29
CHAPTER IX. (part 3)4 cards
What objection to transmutation did Agassiz, Pictet, and Sedgwick raise?
They urged the abrupt appearance of whole groups of allied species in certain formations as a fatal objection to the belief in transmutation of species. Darwin replies that we over-rate the perfection of the geological record and falsely infer that genera not found below a stage did not exist earlier; groups may have long existed and slowly multiplied elsewhere before invading Europe and the United States.
Chapter 30
How did the discovery of a sessile cirripede in Belgian chalk affect Darwin's argument?
Darwin had inferred sessile cirripedes were suddenly developed at the start of the tertiary, since none appeared in secondary beds despite excellent preservation prospects. Then M. Bosquet sent a drawing of an unmistakable sessile cirripede from the chalk of Belgium—a Chthamalus, a genus not found even in any tertiary stratum. This showed the group existed in the secondary period and might be progenitors of later species.
Chapter 30
Why does Darwin call the sudden appearance of species in the lowest fossiliferous strata a graver difficulty?
Numbers of species of the same group appear suddenly in the lowest known fossiliferous rocks. Darwin argues that if his theory is true, long periods elapsed before the lowest Silurian stratum was deposited—probably longer than the whole interval from the Silurian to the present—during which the world swarmed with living creatures. He can give no satisfactory answer why no records of these primordial periods are found, and calls the case at present inexplicable and a valid argument against his views.
Chapter 30
What hypothesis does Darwin offer about oceans and continents in the remotest periods?
Noting that no oceanic island yields even a remnant of palaeozoic or secondary formation, Darwin infers that during those periods neither continents nor continental islands existed where oceans now extend, and that where continents now exist, large tracts of land have existed since the earliest Silurian. He suggests areas of preponderant movement may have changed over ages, so continents may once have stood where oceans now spread.
Chapter 30
CHAPTER X. (part 1)4 cards
According to Darwin, what happens to a species or group once it has disappeared?
Darwin argues that when a species or group has once disappeared from the earth, the same identical form never reappears, even if the same organic and inorganic conditions of life recur. Offspring of another species might fill the same place in nature, but would inherit different characters from distinct progenitors, so would not be identical. He notes apparent exceptions like Barrande's "colonies," which Lyell explains as temporary migration from a distinct geographical province.
Chapter 31
How does Darwin explain differing rates of change among species?
Darwin argues there is no fixed law of development causing all inhabitants to change abruptly, simultaneously, or equally. Each species' variability is independent; whether natural selection accumulates it depends on complex contingencies: whether variation is beneficial, intercrossing, rate of breeding, changing physical conditions, and especially competition with other inhabitants. Thus one species may retain identical form much longer than others, or change less.
Chapter 31
What does Darwin say about the extinction of species and groups?
Darwin argues extinction of old forms and production of new improved forms are intimately connected under natural selection. He rejects the old catastrophe view, holding that species and groups gradually disappear, first from one spot then another, finally from the world. Groups last unequal periods, with complete extinction generally slower than production, though some, like ammonites, were wonderfully sudden. He calls the subject needlessly mysterious.
Chapter 31
How does Darwin describe the appearance and disappearance of groups of species?
Darwin argues groups (genera and families) follow the same general rules as single species: a group does not reappear once disappeared, and its existence is continuous while it lasts. Groups generally increase gradually to a maximum, then decrease and go extinct. He compares this to a vertical line of varying thickness that thickens upward, holds steady, then thins out in upper beds.
Chapter 31
CHAPTER X. (part 2)4 cards
What does Darwin claim about the simultaneity of marine life changes worldwide?
Darwin argues that when marine forms are said to change simultaneously worldwide, this must not be taken as the same thousandth or hundred-thousandth year, nor in a very strict geological sense. He notes a skilled naturalist could hardly judge whether existing or pleistocene European shells most resemble those of the southern hemisphere, yet over a remote future epoch the upper pliocene, pleistocene, and modern beds of Europe, the Americas, and Australia would be correctly ranked as simultaneous in a geological sense.
Chapter 32
How does Darwin explain parallel succession of life forms via natural selection?
Darwin argues new species arise from new varieties with some advantage over older forms, and dominant, widely diffused species are most likely to produce new varieties and incipient species. These spread, vary, and produce further new species, while inferior forms are beaten and their allied groups slowly disappear. He notes dominant plants commonest at home and most widely diffused have produced the greatest number of new varieties, supporting this mechanism.
Chapter 32
Why does Darwin expect less strict parallel succession on land than in the sea?
Darwin reasons that diffusion would probably be slower for terrestrial inhabitants of distinct continents than for marine inhabitants of the continuous sea, since land populations are separated. He therefore expects, and claims apparently to find, a less strict degree of parallel succession in land productions than in sea productions. He also notes we lack sufficient data to judge whether land and freshwater productions change in parallel at distant points.
Chapter 32
What does Darwin say about extinct forms filling gaps between living groups?
Darwin argues all fossils fall into one grand natural system, explained by descent, and that extinct forms help fill wide intervals between existing genera, families, and orders. He cites Owen, who found so many fossil links between Ruminants and Pachyderms that he revised their classification, dissolving the apparent gap between pig and camel. Darwin adds that the most common case is ancient members of two groups differing by fewer characters than living members.
Chapter 32
CHAPTER X. (part 3)4 cards
Why do fossils from consecutive formations resemble each other more than those from remote formations?
All palæontologists insist that fossils from two consecutive formations are far more closely related than fossils from two remote formations; Pictet cites the general resemblance of organic remains across the several stages of the chalk, though the species are distinct in each stage. On the theory of descent this is expected, since each formation's accumulation was interrupted and long blank intervals intervened, so we find closely allied representative species rather than all intermediate varieties.
Chapter 33
In what sense does Darwin claim more recent forms are higher than ancient ones?
The author argues that naturalists have not defined high and low forms satisfactorily, but in one sense recent forms must be higher: each new species gained some advantage in the struggle for life over preceding forms. He suggests an eocene fauna would beat a palæozoic one, and British productions would likely exterminate many New Zealand natives, though he sees no way to test this progress.
Chapter 33
What is the 'law of the succession of types,' and how does Darwin explain it?
Fossil mammals in Australian caves are allied to living marsupials, and South American fossils to living armadillos and sloths; Owen extended this to Old World mammals. Darwin explains it by descent with modification: inhabitants of each quarter tend to leave closely allied modified descendants there, though the law is not immutable, since migration and dominant forms can replace feebler ones after great geographical changes.
Chapter 33
Does Darwin accept Agassiz's claim that ancient animals resemble embryos of recent animals?
The author argues the doctrine is very far from proved, following Pictet and Huxley, yet he fully expects it confirmed at least for subordinate groups branched off recently, since it accords with natural selection. He explains that embryos stay nearly unaltered while adults gain inherited differences, so the embryo is a picture of the ancient condition; but he notes the oldest known mammals, reptiles and fish already belong to their proper classes.
Chapter 33
CHAPTER XI. (part 1)4 cards
What first great fact about geographical distribution does Darwin identify?
Darwin states that neither the similarity nor the dissimilarity of inhabitants of various regions can be accounted for by their climatal and other physical conditions. He notes that almost every recent author who studied the subject reached this conclusion, and offers America—with its diverse climates yet distinct productions from the Old World—as near-sufficient proof.
Chapter 34
How does Darwin relate barriers to differences between regional productions?
Darwin argues barriers or obstacles to free migration are closely and importantly related to differences between productions of regions. He cites the distinct terrestrial productions of New and Old Worlds except where land nearly joins in the north, the isolation of Australia, Africa, and South America, and marine faunas separated by the Isthmus of Panama or open ocean.
Chapter 34
What third great fact does Darwin describe, and what cause does he assign?
Darwin describes the affinity of productions of the same continent or sea, though species differ at different points—successive related groups replacing each other, as with the two Rhea species or American-type rodents. He attributes this bond to inheritance, with dissimilarity due to modification through natural selection and, subordinately, direct influence of physical conditions.
Chapter 34
Why does Darwin favor each species originating in a single region?
Darwin argues that accepting a single region of origin preserves the vera causa of ordinary generation with subsequent migration, while rejecting it calls in a miracle. He notes mammals, with limited sea-crossing capacity, show no inexplicable cases of the same species on distant points, whereas some plants, with varied dispersal means, have crossed vast broken interspaces.
Chapter 34
CHAPTER XI. (part 2)4 cards
What did Darwin's seed immersion experiments show about resistance to sea-water?
Darwin, aided by Mr. Berkeley, found that out of 87 kinds of seeds, 64 germinated after 28 days' immersion in sea-water, and a few survived 137 days. He chiefly used small seeds without capsule or fruit, and all of these sank within a few days, so they could not float across wide sea spaces regardless of salt-water injury.
Chapter 35
How did drying affect the buoyancy of plants and fruits in sea-water?
Darwin dried stems and branches of 94 plants with ripe fruit and placed them on sea-water. Most sank quickly, but some that floated briefly while green floated much longer when dried: ripe hazel-nuts sank immediately but floated 90 days dried and then germinated; an asparagus plant with ripe berries floated 23 days green and 85 days dried, and its seeds germinated.
Chapter 35
What role did Darwin assign to birds in transporting seeds?
Darwin argued living birds are highly effective seed transporters. Seeds in a pigeon's crop survived 30 days on artificial salt-water and nearly all germinated; hard seeds pass uninjured through a turkey's digestive organs; and seeds from hawk and owl pellets germinated after 12 to 21 hours in birds' stomachs, with two beet seeds surviving two days and fourteen hours.
Chapter 35
Why did Darwin reject recent continuous land connections between continents?
Darwin cited the great difference in marine faunas on opposite sides of almost every continent, the close relation of tertiary inhabitants to present ones, and a relation between mammal distribution and sea depth. He also noted oceanic islands are almost universally volcanic, not granite or old fossiliferous rock, so they are not the wrecks of sunken continents.
Chapter 35
CHAPTER XI. (part 3)3 cards
Why does Darwin expect arctic forms to be less modified than Alpine forms after glacial retreat?
The author argues arctic forms migrated south and back north in a body, staying under nearly the same climate, so their mutual relations were little disturbed and they were not liable to much modification. Alpine productions, isolated on separate mountain summits and mingled with ancient Alpine species under different climates, had their relations disturbed and so were liable to modification—hence varieties and representative species among European mountain ranges.
Chapter 36
How does Darwin explain the pre-glacial uniformity of sub-arctic and northern temperate productions across the Old and New Worlds?
He invokes still earlier opposite climate changes: during the newer Pliocene, before the Glacial epoch, the climate was warmer, so organisms now at latitude 60° lived nearer the pole. Almost continuous circumpolar land from western Europe through Siberia to eastern America then allowed free intermigration, producing uniformity before the Glacial epoch. Later southward migration and separation favored much modification.
Chapter 36
What evidence does Darwin cite for former glacial action outside Europe?
He cites frozen mammals and mountain vegetation in Siberia; glaciers' marks along the Himalaya 900 miles apart, and Hooker seeing maize on ancient moraines in Sikkim; glacial evidence in New Zealand and possibly south-eastern Australia; in America, ice-borne rock fragments as far south as lat. 36°-37° east and 46° on the Pacific, erratic boulders on the Rocky Mountains, and a vast moraine in central Chile.
Chapter 36
CHAPTER XI. (part 4)4 cards
According to Darwin, why did northern forms beat southern forms on intertropical mountains?
The author argues northern forms were more dominant because they were generated in larger areas and more efficient workshops of the north. When commingled during the Glacial period, these northern forms largely replaced the endemic Alpine forms that had stocked the intertropical mountains before the Glacial period, much as European productions now beat natives in La Plata and Australia.
Chapter 37
What analogy does Darwin use for intertropical mountains and their species?
Darwin calls a mountain an island on the land. Before the Glacial period, intertropical mountains were completely isolated, and he believes their productions yielded to those from the larger northern areas, just as real islands' productions have yielded to continental forms naturalised by man's agency.
Chapter 37
What does Darwin admit remains unexplained about migration and naturalisation?
He states he is far from supposing all difficulties are removed. Many remain: he does not pretend to indicate the exact lines and means of migration, why certain species rather than others migrated, why some were modified into new groups while others stayed unaltered, or why one species rather than another becomes naturalised by man's agency in a foreign land.
Chapter 37
How does Darwin explain distinct southern species at remote points like Kerguelen, New Zealand, and Fuegia?
For identical species, he believes icebergs, as suggested by Lyell, were largely concerned in dispersal toward the close of the Glacial period. For distinct species of exclusively southern genera, he suspects a former warmer period before the Glacial epoch, when antarctic lands now ice-covered supported a peculiar isolated flora, whose few forms were dispersed by occasional transport, sunken islands, and perhaps icebergs.
Chapter 37
CHAPTER XII. (part 1)4 cards
Why does Darwin think fresh-water productions range so widely despite land and sea barriers?
The author argues fresh-water species became fitted for short, frequent migrations from pond to pond or stream to stream, so wide dispersal follows almost necessarily. He notes fresh-water insects and shells in Brazil resembled British forms while surrounding land species differed. He also credits transport of seeds and eggs by water-birds, and for fish, slight recent changes in land level joining rivers.
Chapter 38
What experiment showed Darwin how fresh-water shells might travel on birds' feet?
Darwin suspended a duck's feet in an aquarium where minute fresh-water shell ova were hatching. Just-hatched shells crawled onto the feet and clung so firmly they could not be jarred off, though older ones would drop off voluntarily. They survived in damp air twelve to twenty hours, time enough for a duck or heron to fly six or seven hundred miles.
Chapter 38
What did Darwin's pond-mud experiment reveal about seed transport by birds?
He took three table-spoonfuls of mud from three points in a pond; dried, it weighed 6 3/4 ounces. Kept covered six months, it produced 537 plants of many kinds, all from mud fitting a breakfast cup. He argues wading birds with muddy feet, flushed from pond edges, could carry such seeds to distant islands, since they would not alight on the sea.
Chapter 38
What two features characterize inhabitants of oceanic islands on Darwin's view?
Their species are few compared with equal continental areas — New Zealand's 750 flowering plants versus richer continental floras, and Ascension originally under half-a-dozen. Yet the proportion of endemic species is often extremely large, as with Madeira's land-shells and the Galapagos birds. The author argues this fits descent with modification: occasional arrivals in isolation are eminently liable to modification.
Chapter 38
CHAPTER XII. (part 2)4 cards
Why does Darwin expect bats but not other mammals on remote islands?
The author argues bats can cross wide seas by flight, while no terrestrial mammal can be transported across such distances. He notes bats seen wandering by day over the Atlantic and two North American species reaching Bermuda, 600 miles from the mainland. Wandering bat species could then be modified by natural selection in their new homes, explaining endemic bats without terrestrial mammals.
Chapter 39
What relation does Darwin find between sea depth and island mammal affinities?
The author argues that islands separated by shallow channels tend to share identical or closely allied mammals with the mainland, while deep channels separate distinct faunas. Britain's shallow channel yields the same mammals as Europe; the West Indies' deeply submerged bank yields American forms but distinct species and genera. He cites Windsor Earl's Malay Archipelago observations and admits some anomalies.
Chapter 39
What is Darwin's key evidence from the Galapagos and Cape de Verde islands?
The author argues island inhabitants resemble those of the nearest mainland without being the same species. Galapagos species bear American affinity despite dissimilar conditions; Cape de Verde species are African. He claims independent creation cannot explain this, while colonization from the nearest continent followed by modification can. He notes Kerguelen and New Zealand plant anomalies and treats them as largely explicable.
Chapter 39
How does Darwin explain distinct species on separate Galapagos islands?
The author argues immigrants arrived by occasional transport, then faced different competitors and enemies on each island, so natural selection favored different varieties. He attributes his former difficulty to the error of overvaluing physical conditions over competition with other inhabitants. Deep channels and rapid currents, not mere visibility, keep the islands separated.
Chapter 39
CHAPTER XII. (part 3)4 cards
According to the chapter, what law regarding group rank and geographic range has long been observed?
The author states that the lower any group of organisms is in the scale, the more widely it is apt to range. He attributes this partly to low forms changing more slowly, giving them better chances to range widely while retaining specific character, and partly to their minute seeds and eggs being better fitted for distant transport. He notes Alph. de Candolle recently discussed this law for plants.
Chapter 40
What makes the difficulties of single-parent descent 'not insuperable' for Darwin?
The author argues the difficulties are not insuperable if we allow for ignorance of full effects of climatic and land-level changes, remember how little is known of occasional transport means (hardly ever properly experimentised on), and bear in mind that a species may have ranged continuously then gone extinct in intermediate tracts. He concedes the difficulties are often extremely grave for distinct species of the same genus.
Chapter 40
Why do oceanic islands have few inhabitants but many endemic species, on the author's view?
The author argues this follows from migration plus subsequent modification. Islands receive few immigrants, so have few inhabitants, but those colonists become modified and peculiar. He also explains the absence of batrachians and terrestrial mammals from oceanic islands, while isolated islands possess their own peculiar species of aerial mammals or bats, and notes a relation between mammals and sea depth between island and mainland.
Chapter 40
What parallelism does the author, following Edward Forbes, see between the laws of life in time and space?
The author argues the laws governing succession of forms in past times are nearly the same as those governing present differences in different areas. In both, species and groups have continuous endurance and continuous area as the general rule, with exceptions from migration or extinction in intermediate tracts; both show points of maximum development; and in both, lower members of each class generally change less than higher, with marked exceptions.
Chapter 40
CHAPTER XIII. (part 1)3 cards
On Darwin's view, what is the real bond underlying the Natural System of classification?
The author argues that the Natural System is not merely a scheme for grouping similar forms or revealing the Creator's plan. He believes propinquity of descent—the only known cause of similarity among organic beings—is the hidden bond partially revealed by our classifications, obscured by varying degrees of modification.
Chapter 41
What rule does Darwin give about parts concerned with special habits?
He states that the less a part of the organisation is concerned with special habits, the more important it becomes for classification. External resemblances tied to habits of life—mouse to shrew, dugong to whale, whale to fish—are ranked as merely adaptive or analogical characters, not as evidence of true affinity.
Chapter 41
Why does classification based on a single character always fail?
Because no part of the organisation is universally constant. Darwin notes that species may depart from allies in several characters, high and low importance alike, yet still be clearly placed, so the value of an aggregate of characters is evident—explaining Linnaeus's saying that the genus gives the characters, not the reverse.
Chapter 41
CHAPTER XIII. (part 2)4 cards
Why does Darwin think a genealogical arrangement is the only possible classification of languages?
Darwin argues that if we had a perfect pedigree of mankind, arranging races genealogically would give the best classification of languages. Including all extinct languages and slowly changing dialects, a genealogical arrangement would be the only possible one, since it connects all tongues by closest affinities and gives the filiation and origin of each.
Chapter 42
What distinction does Darwin credit Lamarck with calling attention to?
Darwin says Lamarck first called attention to the distinction between real affinities and analogical or adaptive resemblances, and Macleay and others ably followed. He illustrates with the dugong and whale, whose similar body shape and fin-like limbs are analogical, and with Linnaeus, who was misled by external appearance into classing a homopterous insect as a moth.
Chapter 42
Why are analogical or adaptive characters almost valueless to the systematist?
Darwin argues that characters matter for classification only insofar as they reveal descent. Animals from two distinct lines of descent may adapt to similar conditions and assume close external resemblance, but such resemblances conceal rather than reveal blood-relationship. The same characters can be analogical between classes yet show true affinity within a class, as with whale and fish body shapes.
Chapter 42
Why does Darwin say rudimentary structures are valuable for classification?
Darwin argues we have no written pedigrees and must infer community of descent from resemblances least likely to have been modified by recent conditions of life. On this view rudimentary structures are as good as, or sometimes better than, other parts. Trifling characters gain high value if they prevail across many species with very different habits, since only inheritance from a common parent explains them.
Chapter 42
CHAPTER XIII. (part 3)4 cards
What does Darwin mean by homology, and which scientist stressed relative connexion?
Homology is the resemblance in the general plan of organisation among members of the same class, independent of habits of life—the 'unity of type.' Darwin cites Geoffroy St. Hilaire, who insisted on the importance of relative connexion: parts may change greatly in form and size but always remain connected in the same order, so the same names can be given to homologous bones in widely different animals.
Chapter 43
How does Darwin explain homologous limbs and insect mouths via natural selection?
Darwin argues that on the theory of natural selection of successive slight modifications, each profitable to the modified form and often affecting other parts by correlation of growth, there is little tendency to alter the original pattern or transpose parts. If the ancient progenitor (archetype) of all mammals had limbs on the existing pattern, and insect progenitors had an upper lip, mandibles, and two pairs of maxillae, natural selection accounts for the diversity.
Chapter 43
What is serial homology, and why is it rare in molluscs?
Serial homology compares different parts or organs within the same individual, such as skull bones with vertebrae, or sepals, petals, stamens, and pistils as metamorphosed leaves. Darwin says molluscs show few serial homologies because even the lowest members of the class lack the indefinite repetition of a single part found in other great classes of animals and plants.
Chapter 43
Why do embryos of distinct animals within a class resemble each other?
Darwin notes embryos of distinct animals within a class are often strikingly similar—Agassiz forgot to ticket an embryo and could not tell whether it was mammal, bird, or reptile. Such embryonic resemblances often have no direct relation to conditions of existence, unlike active larvae adapted for special lines of life; cirripede larvae reveal barnacles are crustaceans, which Cuvier did not perceive.
Chapter 43
CHAPTER XIII. (part 4)3 cards
What did Darwin find when measuring six-day-old greyhound and bulldog puppies?
Darwin was told by breeders that greyhound and bulldog puppies differed as much as their parents, and by eye it seemed almost true. But on actually measuring the old dogs and their six-day-old puppies, he found the puppies had not nearly acquired their full amount of proportional difference. He reports the same result from measurements of a three-days-old colt of a race and a heavy cart-horse.
Chapter 44
Why does Darwin think embryonic structure matters so much for classification?
The author argues the embryo is the animal in its less modified state, so it partly reveals the structure of its progenitor. Groups that pass through the same or similar embryonic stages are therefore closely related, since community in embryonic structure reveals community of descent, however much adult structure has been modified. He notes cirripedes can be recognized by their larvae as crustaceans.
Chapter 44
What examples of rudimentary organs does Darwin give, and what do they show?
Darwin cites rudimentary mammae in male mammals, the bastard-wing in birds, one rudimentary lung lobe in many snakes, pelvic and hind-limb rudiments in others, teeth in foetal whales and unborn calves, and reduced flightless wings in insects. Some retain potentiality, as when male mammae secrete milk. An organ may abort for one function yet stay efficient for another, as in Composite male florets.
Chapter 44
CHAPTER XIII. (part 5)4 cards
What does Darwin say the 'natural system' of classification actually is?
The author argues the natural system is genealogical in its attempted arrangement, with grades of acquired difference marked by the terms varieties, species, genera, families, orders, and classes. He notes that descent has universally been used to rank together sexes, ages, and acknowledged varieties of the same species, however different in structure, and extending this element explains classification.
Chapter 45
On what principle does Darwin explain the leading facts of embryology?
Darwin explains embryology by successive slight variations not necessarily or generally supervening at a very early period of life, and being inherited at a corresponding period. This explains the resemblance of homologous parts in an individual embryo that later differ widely, and the resemblance of homologous parts across species of a class though fitted in adults for very different purposes.
Chapter 45
How does Darwin account for rudimentary organs and their abortion?
The author argues that when organs are reduced in size, either from disuse or selection, this generally occurs at that period of life when the being has to provide for its own wants; combined with the strength of inheritance, rudimentary organs and their final abortion present no inexplicable difficulties. On the contrary, their presence might even have been anticipated.
Chapter 45
What does Darwin say about the value of adaptive versus true-affinity characters?
The author argues there is a wide opposition in value between analogical or adaptive characters and characters of true affinity. He also notes that constant and prevalent characters are valued whether of high vital importance, of trifling importance, or, as in rudimentary organs, of no importance—and that these classification rules follow from common parentage with modification through natural selection, extinction, and divergence.
Chapter 45
CHAPTER XIV. (part 1)4 cards
What three propositions must be admitted to accept that complex organs evolved by natural selection?
Darwin argues the difficulty of complex organs being perfected by accumulating slight variations is not real if we admit: gradations in perfection of organs or instincts exist or could have existed, each good of its kind; all organs and instincts are variable in ever so slight a degree; and there is a struggle for existence leading to preservation of each profitable deviation. He says the truth of these propositions cannot be disputed.
Chapter 46
How does Darwin explain the sterility of species when first crossed?
Darwin argues this sterility is not a special endowment but incidental on constitutional differences in the reproductive systems of intercrossed species, comparable to two trees failing to graft. Evidence includes the vast difference in results when the same two species are crossed reciprocally. He contrasts this with hybrid sterility, where reproductive organs are functionally impotent, while in first crosses the organs on both sides are perfect.
Chapter 46
Why does Darwin say we should not expect to find intermediate linking forms all around us?
Darwin argues we have no right to expect direct connecting links between existing forms, only between each and some extinct supplanted form. Only a few species change at any one period, and slowly. Intermediate varieties in intermediate zones are liable to be supplanted by allied forms on either hand, which exist in greater numbers and are modified faster, so intermediates are eventually exterminated.
Chapter 46
What is Darwin's main answer to the absence of fossil evidence for gradual change?
Darwin answers the gravest objection — no fossil evidence of gradation and mutation — only on the supposition that the geological record is far more imperfect than most geologists believe. He notes only a small portion of the world is geologically explored, only certain classes of organisms can be preserved as fossils, formations are intermittent, and successive formations are separated by enormous blank intervals of time.
Chapter 46
CHAPTER XIV. (part 2)3 cards
Why does Darwin say nature is prodigal in variety but niggard in innovation?
Because natural selection acts solely by accumulating slight, successive, favourable variations, it can produce no great or sudden modification, only very short and slow steps. This makes the canon 'Natura non facit saltum' intelligible on his theory, while he argues it cannot be explained if each species were independently created.
Chapter 47
Why are specific characters more variable than generic characters on Darwin's view?
If species are only well-marked varieties whose characters became highly permanent, then specific characters have already varied since branching from a common progenitor, so they are more likely still to vary. Generic characters, inherited unchanged for an enormous period, are more constant. Darwin says this is inexplicable if species were independently created.
Chapter 47
Why does Darwin expect oceanic islands to have few species, many of them peculiar?
On migration with subsequent modification, islands receive few colonists, and those that arrive diverge into peculiar forms. Animals unable to cross wide ocean, like frogs and terrestrial mammals, should be absent, while bats, which can traverse the ocean, often appear as peculiar species on distant islands. Darwin says this is utterly inexplicable on independent acts of creation.
Chapter 47
CHAPTER XIV. (part 3)4 cards
What does Darwin say about naturalists who accept variation for some species but not others?
The author argues this is a strange, arbitrary position: naturalists admit many forms once thought special creations were produced by variation, yet refuse to extend the view to slightly different forms, without defining which are created and which by secondary laws. He predicts this will later be cited as an illustration of the blindness of preconceived opinion.
Chapter 48
How far does Darwin extend descent with modification, and what limits it?
He cannot doubt the theory embraces all members of the same class, citing chains of affinity, classification, fossil intermediates, rudimentary organs, common patterns and embryonic resemblance. He believes animals descended from at most four or five progenitors, plants from an equal or lesser number, and by analogy infers all organic beings descended from one primordial form, though he warns analogy may be a deceitful guide.
Chapter 48
What revolution in natural history does Darwin foresee once his views are admitted?
Systematists will no longer be haunted by doubt over whether a form is essentially a species; disputes over whether fifty British brambles are true species will cease. The only distinction between species and well-marked varieties will be that varieties are known or believed to be connected by intermediate gradations. Classifications will become genealogies, and terms like affinity and rudimentary organs will gain plain meaning.
Chapter 48
How does Darwin propose using organic change in fossils to measure time?
Since species arise and die out by slowly acting existing causes rather than miraculous creations or catastrophes, the amount of organic change between consecutive formations probably measures elapsed time. But he cautions some species may stay unchanged while others modify, so the measure must not be overrated; early life, being fewer and simpler, likely changed more slowly.
Chapter 48
CHAPTER XIV. (part 4)3 cards
What does Darwin's index entry for 'Disuse, effects of, under nature' point to?
The index entry 'Disuse, effects of, under nature, 134' directs readers to Darwin's discussion of how organs reduced through disuse can persist under natural conditions. It parallels his entry for habit under nature at the same page, showing he treats disuse and habit as related causes of modification, distinct from the effects of disuse under domestication listed separately.
Chapter 49
What does the index reveal about Darwin's treatment of rudimentary organs?
The index lists rudimentary organs under several headings: rudimentary ears (454), rudimentary horns (454), rudimentary nails (453), and rudimentary mammæ (451), plus a general entry 'Rudiments of organs, 450.' This clustering shows Darwin devoted sustained attention to vestigial structures as evidence bearing on descent, treating them across many different organisms rather than as an isolated curiosity.
Chapter 49
What does the index show about the relationship between lowness of structure and distribution?
Two linked entries appear: 'Lowness of structure connected with variability, 149' and 'Lowness, related to wide distribution, 406.' Together they indicate Darwin connected simple or low organisation both with greater variability and with broader geographical range. The index also lists De Candolle on low plants being widely dispersed (406), attributing that observation to him.
Chapter 49
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Text used: Project Gutenberg eBook #1228 (Public domain in the United States). Generated from the full text; cards paraphrase the source.
Version: Public domain edition. Retrieved 2026-10-18. Cards reviewed 2026-10-18.
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