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Book beginning: page 3

A note from Penman’s Guide: The following is a summarisation of Darwin’s Origin of Species, in bullet point form. The idea is to present his theory in as direct a manner as possible, bearing in mind its final use for literary purposes. Where possible, his own words have been maintained, but ideas and paragraphs condensed for ease of understanding. His first person approach has been kept, so all uses of ‘I’ are in reference to Darwin himself.

The main aim of including such a detailed version of the theory here is to help students understand the global meaning of this work, and how its mention in literature indicates a breaking away from religious beliefs into scientific ones, namely – creation vs. evolution. Remembering that humans and their intricate workings can also be explained by this book, the theory can be taken as a whole, or a specific point can be focused on, to better understand certain situations of life.

Ordered from larger to smaller, note: Genera – Species – Incipient species – Varieties.

Introduction

• Darwin came to the conclusion that each species had not been independently created, but had descended, like varieties, from other species (p4).
• It is, therefore, of the highest importance to gain a clear insight into the means of modification and coadaptation of organic beings (p5).
• We shall see that a large amount of hereditary modification is at least possible; and […] how great the power of man is in accumulating, by his Selection, successive slight variations in domesticated plants and animals (p5).
• Many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, will have a better chance of surviving, and thus be naturally selected (p6).
• The view which most naturalists entertain […] that each species has been independently created – is erroneous. I am fully convinced that species are not immutable but belong to lineal descendants of some other and generally extinct species. I am convinced that Natural Selection has been the main but not exclusive means of modification (p7).

Chapter I: Variation under Domestication

• Organic beings must be exposed during several generations to the new conditions of life to cause any appreciable amount of variation; and when the organisation has once begun to vary, it generally continues to vary for many generations (p8).
• It has been disputed at what period of life the causes of variability, whatever they may be, generally act; whether during the early or late period of development of the embryo, or at the instant of conception. I am strongly inclined to suspect that the most frequent cause of variability may be attributed to the male and female reproductive elements having been affected prior to the act of conception. There is a remarkable effect which confinement or cultivation has on the functions of the reproductive system; […] nothing is easier than to tame an animal, and few things more difficult than to get it to breed freely under confinement. It has been found that very trifling changes, such as a little more or less water at some particular period of growth, will determine whether or not a plant sets a seed (p9).
• We can often see domesticated animals and plants, though weak and sickly, breeding quite freely under confinement; and then see individuals, though taken young from a state of nature, perfectly tamed, long-lived, and healthy, having their reproductive system so seriously affected by unperceived causes as to fail in acting not quite regularly, and producing offspring not perfectly like their parents (p9/10).
• Seedlings from the same fruit, and the young of the same litter, sometimes differ considerably from each other, though both the young and the parents have apparently been exposed to exactly the same conditions of life; and this shows how unimportant the direct effects of the conditions of life are in comparison with the laws of reproduction, growth, and inheritance; for had the action of the conditions been direct, if any of the young had varied, all would probably have varied in the same manner (p10).
• When all or nearly all the individuals exposed to certain conditions are affected in the same way, the change at first appears to be directly due to such conditions; but in some cases it can be shown that quite opposite conditions produce similar changes of structure. Nevertheless some slight amount of change may, I think, be attributed to the direct action of the conditions of life – as, in some cases, increased size from amount of food, colour from particular kinds of food and from light, and perhaps the thickness of fur from climate (p11).
• Habit also has a decided influence, as in the period of flowering with plants when transported from one climate to another. In animals it has a more marked effect: the domestic duck’s wing bones weigh less and leg bones weigh more in comparison to the wild-duck. I presume that this change may be safely attributed to the domestic duck flying much less, and walking more; and not a single domestic animal can be named which has not in some country drooping ears due to the disuse of the muscles of the ear, from the animals not being much alarmed by danger (p11).
• There are many laws regulating variation, Correlation of Growth being one of them. Any change in the embryo or larva will almost certainly lead to changes in the mature animal (p11).
• If man goes on selecting, and thus augmenting, any peculiarity, he will almost certainly unconsciously modify other parts of the structure, owing to the mysterious laws of the correlation of growth. The result of the various laws of variation is infinitely complex and diversified (p12).
• Any variation which is not inherited is unimportant for us. The number and diversity of inheritable deviations of structure is endless (p12).
• No breeder doubts how strong the tendency to inheritance is: like produces like in his fundamental belief. Doubts have been thrown on this principle by theoretical writers alone, even though deviation appears frequently. If strange and rare deviations of structure are truly inherited, less strange and commoner deviations may be freely admitted to be inherited. Perhaps the correct way of viewing the whole subject, would be, to look at the inheritance of every character, whatever it may be, as the rule, and non-inheritance as the anomaly (p12/3).
• The laws governing inheritance are quite unknown. A peculiarity is often transmitted from one sex to both sexes, or to one sex alone, more commonly but not exclusively to the same sex as the parent who passes it on (p13).
• A much more important rule, which I think may be trusted, is that, at whatever period of life a peculiarity first appears, it tends to appear in the offspring at a corresponding age, though sometimes earlier. For instance, inherited peculiarities in the horns of cattle could appear only in the offspring when nearly mature (p13).
• When under nature the conditions of life change, variations and reversions of character probably occur; but natural selection, as will hereafter be explained, will determine how far the new characters thus arising shall be preserved (p14).
• We generally perceive in each domestic race, less uniformity of character than in true species (p14).
• It has often been assumed that man has chosen for domestication animals and plants having an extraordinary inherent tendency to vary, and likewise to withstand diverse climates. I do not dispute that these capacities have added largely to the value of most of our domesticated productions; but how could a savage possibly know, when he first tamed an animal, whether it would vary in succeeding generation, and whether it would endure other climates? (p16).
• In the case of most of our anciently domesticated animals and plants, I do not think it is possible to come to any definite conclusion, whether they have descended from one or several species (p16).
• Some authors believe that every race which breeds true, has had its wild prototype. At this rate there must have existed at least a score of species of wild cattle, as many sheep, and several goats in Europe alone, a case which I can hardly believe (p17).
• However, by crossing we can get only forms in some degree intermediate between their parents; and if we account for our several domestic races by this process, we must admit the former existence of the most extreme forms in the wild state (p17).
• To demonstrate the last few points, in Darwin’s study of rock-pigeons he notes: if the several breeds are not varieties, and have not proceeded from the rock-pigeon, they must have descended from at least seven or eight aboriginal stocks; for it is impossible to make the present domestic breeds by the crossing of any lesser number. But only two or three species of rock-pigeon are known; and these have not any of the characters of the domestic breeds. Hence the supposed aboriginal stocks must either still exist in the countries where they were originally domesticated, and yet be unknown; and this considering their size, habits, and remarkable characters, seems very improbable; or they must have become extinct in the wild state. But birds breeding on precipices, and good fliers, are unlikely to be exterminated; and the common rock-pigeon, which has the same habits with the domestic breeds, has not been exterminated. Hence the supposed extermination of so many species having similar habits with the rock-pigeon seems to me a very rash assumption. […] Again, all recent experience shows that it is most difficult to get any wild animal to breed freely under domestication; yet on the hypothesis of the multiple origin of our pigeons, it must be assumed that at least seven or eight species were so thoroughly domesticated in ancient times by half-civilized man, as to be quite prolific under confinement. Hence it must be assumed not only that half-civilized man succeeded in thoroughly domesticating several species, but that he intentionally or by chance picked out extraordinary abnormal species; and further, that these very species have since all become extinct or unknown. To me, so many strange contingencies seem improbable in the highest degree (p20/1).
• Selection: let us now briefly consider the steps by which domestic races have been produced, either from one or from several allied species, although some little effect may be attributed to external conditions of life and habit. One of the most remarkable features in our domesticated races is that we see in them adaptations, not indeed to the animal’s or plant’s own good, but to man’s use or fancy (p25).
• We must, I think, look further than to mere variability. We cannot suppose that all the breeds were suddenly produced as perfect and as useful as we now see them; the key is man’s power of accumulative selection: nature gives successive variations and man adds them up in certain directions useful to him (p25/6).
• If selection consisted merely in separating some very distinct variety, and breeding from it, the principle would be so obvious as hardly to be worth notice; but its importance consists in the great effect produced by the accumulation in one direction, during successive generations (p27).
• When a race of plants is once pretty well established, the seed-raisers do not pick out the best plants, but merely go over their seed-beds, and pull up the ‘rogues’, as they call the plants that deviate from the proper standard (p27).
• It is not that the varieties which differ largely in some one point do not differ at all in other points (p28).
• Unconscious Selection: results from every one trying to possess and breed from the best individual animals. A man who intends on keeping pointers naturally tries to get as good dogs as he can, and afterwards breeds from his own best dogs, but has no wish or expectation of permanently altering the breed (p29).
• Slow and insensible changes could never be recognised unless actual measurements or careful drawings of the breeds in question had been made, which might serve for comparison (p29).
• Selection has consisted in always cultivating the best known variety (p31).
• Man can hardly select, or only with much difficulty, any deviation of structure excepting such as is externally visible; and indeed he rarely cares for what is internal. He can never act by selection, excepting on variations which are first given to him in some slight degree by nature (p32).
• We know nothing about the origin or history of any of our domestic breeds. But, in fact, a breed, like a dialect or language, can hardly be said to have had a definite origin. A man preserves and breeds from an individual with some slight deviation of structure, or takes more care than usual in matching his best animals and thus improves them. […] But as yet they will hardly have a distinct name, and from being only slightly valued, their history will be disregarded (p33).
• In semi-civilised countries, with little free communication, the spreading and knowledge of any new sub-breed will be a slow process. As soon as the points of value of the new sub-breed are once fully acknowledged, the principle of unconscious selection will always tend slowly to add to the characteristic features of the breed. But the chance will be infinitely small of any record having been preserved of such slow, varying, and insensible changes (p33).
• As variations manifestly useful or pleasing to man appear only occasionally, the chance of their appearance will be much increased by a large number of individuals being kept; hence this comes to be of the highest importance to success (p33).
• In the case of animals with separate sexes, facility in preventing crosses is an important element of success in the formation of new races – at least, in a country which is already stocked with other races. In this respect enclosure of the land plays a part. The inhabitants of open plains rarely possess more than one breed of the same species (p34).
• Summary of chapter I: to sum up on the origin of our Domestic Races of animals and plants, I believe that the conditions of life, from their action on the reproductive system, are so far of the highest importance as causing variability. The effects of variability are modified by various degrees of inheritance and of reversion. Variability is governed by many unknown laws, more especially by that of correlation of growth. Something may be attributed to the direct action of the conditions of life, and to use and disuse. […] The importance of the crossing of varieties has, I believe, been greatly exaggerated, both in regard to animals and to those plants which are propagated by seed. Selection, whether applied methodically and more quickly, or unconsciously and more slowly, but more efficiently, is by far the predominant Power (p35).

Chapter II: Variation under Nature

• Monstrosities: some considerable deviation of structure in one part, either injurious to or not useful to the species, and not generally propagated (p36).
• Parts which must be called important, whether viewed under a physiological or classificatory point of view, sometimes vary in the individuals of the same species (p37).
• Cases of great difficulty sometimes occur in deciding whether or not to rank one form as a variety of another. In determining whether a form should be ranked as a species or a variety, the opinion of naturalists having sound judgement and wide experience seems the only guide to follow (p38/9).
• Varieties: I look at individual differences as of high importance for us, as being the first step towards such slight varieties as are barely thought worth recording in works on natural history. And I look at varieties which are in any degree more distinct and permanent, as steps leading to more strongly marked and more permanent varieties, these then leading to sub-species, and to species (p42).
• I attribute the passage of a variety, from a state in which it differs very slightly from its parent to one in which it differs more, to the action of ‘natural selection’. I believe a well-marked variety may be justly called an incipient species (p42).
• It need not be supposed that all varieties or incipient species necessarily attain the rank of species. They may whilst in this incipient state become extinct (p42).
• If a variety were to flourish so as to exceed in numbers the parent species, it would then rank as the species, and the species as the variety; or it might come to supplant and exterminate the parent species; or both might co-exist, and both rank as independent species (p42).
• It will be seen that I look at the term species, as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other, and that it does not essentially differ from the term variety, which is given to less distinct and more fluctuating forms. The term variety, again, in comparison with mere individual differences, is also applied arbitrarily, and for mere convenience’ sake (p42).
• The dominant species are the most diffused in their own country, and are the most numerous in individuals – which oftenest produce well-marked varieties, or, as I consider them, incipient species. This, perhaps, might have been anticipated; for, as varieties, in order to become in any degree permanent, necessarily have to struggle with the other inhabitants of the country, and the species which are already dominant will be the most likely to yield offspring which, though in some slight degree modified, will still inherit those advantages that enabled their parents to become dominant over their compatriots (p43).
• In large genera the amount of difference between the species is often exceedingly small, therefore, the species of the larger genera resemble varieties, more than do the species of the smaller genera (p45/6).
• The species of the large genera are related to each other, in the same manner as the varieties of any one species are related to each other. No naturalist pretends that all the species of a genus are equally distinct from each other (p46).
• The characters which constitute the lesser differences between varieties will tend to increase into the greater differences between species (p46).
• Summary of chapter II: varieties have the same general characters as species, for they cannot be distinguished from them – except, firstly, by the discovery of intermediate linking forms; and secondly, by a certain amount of difference, because if two forms differ very little, they would generally be ranked as varieties. Throughout nature the forms of life which are now dominant tend to become still more so by leaving many modified and dominant descendants. (p47).

Chapter III: Struggle for Existence

• Groups of species constitute what are called ‘distinct genera’ (p49).
• Owing to the struggle of life, any variation, however slight and from whatever cause proceeding, if it be in any degree profitable to an individual of any species, will tend to the preservation of that individual, and will generally be inherited by its offspring. The offspring, also, will thus have a better chance of surviving, for, of the many individuals of any species which are periodically born, but a small number can survive. I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural Selection, in order to mark its relation to man’s power of selection. We have seen that man by selection can certainly produce great results, and can adapt organic beings to his own uses, through the accumulation of slight but useful variations, given to him by the hand of Nature. But Natural Selection, as we shall hereafter see, is a power incessantly ready for action, and is as immeasurably superior to man’s feeble efforts (p49).
• I use the term Struggle for Existence in a large and metaphorical sense, including dependence of one being on another, and including (which is more important) not only the life of the individual, but success in leaving progeny (p50).
• Every being, which during its natural lifetime produces several eggs or seeds, must suffer destruction during some period of its life, otherwise, on the principle of geometrical increase, its numbers would quickly become so inordinately great that no country could support the product. Hence, as more individuals are produced than can possibly survive, there must in every case be a struggle, for existence, either one individual with another of the same species, or with the individuals of distinct species, or with the physical conditions of life (p50/1).
• There is no exception to the rule that every organic being naturally increases at so high a rate, that if not destroyed, the earth would soon be covered by the progeny of a single pair (p51).
• The real importance of a large number of eggs or seeds is to make up for much destruction at some point in life; this period in the great majority of cases is an early one. If an animal can in any way protect its own eggs or young, a small number may be produced, and the average stock be fully kept up; but if many eggs or young are destroyed, many must be produced, or the species will become extinct. It would suffice to keep up the full number of tree, which lived on an average for a thousand years, if a single seed were produced once in a thousand years, supposing that this seed were never destroyed, and could be ensured to germinate in a fitting place. So that in all cases, the average number of any animal or plant depends only indirectly on the number of its eggs or seeds (p53).
• Never to forget that every single organic being around us may be said to be striving to the utmost to increase its numbers; that each lives by a struggle at some period of its life; that heavy destruction inevitably falls either on the young or old, during each generation or at recurrent intervals. Lighten the destruction ever so little, and the number of the species will almost instantaneously increase to any amount (p53).
• Eggs or very young animals seem generally to suffer most, but this is not invariably the case. With plants there is a vast destruction of seeds, but, I believe that it is the seedlings which germinate in ground already thickly stocked with other plants that suffer most; though, they are also destroyed in vast numbers by various enemies, chiefly slugs and insects. Furthermore, if turf be let to grow, the more vigorous plants gradually kill the less vigorous, even though they could be fully grown (p53/4).
• The amount of food for each species of course gives the extreme limit to which each can increase; but very frequently it is not the obtaining food, but the serving as prey to other animals, which determines the average numbers of species (p54).
• Climate plays an important part in determining the average numbers of a species as do periodical seasons of extreme cold or drought, which I believe to be the most effective of all checks (p54).
• The action of climate seems at first sight to be quite independent of the struggle for existence; but in so far as climate chiefly acts in reducing food, it brings on the most severe struggle between the individuals, whether of the same or of distinct species, which primarily subsist on the same food (p54).
• When we travel from south to north, or from a damp region to a dry, we invariably see some species gradually getting rarer and rarer, and finally disappearing; we are tempted to attribute the whole effect to the direct action of climate, but this is a very false view; we forget that each species, even where it most abounds, is constantly suffering enormous destruction at some period of its life, from enemies or from competitors for the same place and food; and if these enemies or competitors be in the least degree favoured by any slight change of climate, they will increase in numbers, and, as each area is already fully stocked with inhabitants, the other species will decrease (p54/5).
• Going northward, or in ascending a mountain, we far oftener meet with stunted forms, due to the directly injurious action of climate, than we do in proceeding southwards or in descending a mountain. When we reach the Arctic regions, or snow-capped summits, or absolute deserts, the struggle for life is almost exclusively with the elements (p55).
• When a species, owing to highly favourable circumstances, increases inordinately in numbers in a small tract, epidemics often ensue (for example, parasitic worms), and here we have a limiting check independent of the struggle for life (p55).
• A large stock of individuals of the same species, relatively to the number of its enemies, is absolutely necessary for its preservation. Thus we can easily raise plenty of corn in our fields, because the seeds are in great excess compared with the number of birds which feed on them; nor can the birds, though having a superabundance of food at this one season, increase in number proportionally to the supply of seed, as their numbers are checked during winter (p55/6).
• This view of the necessity of a large stock of the same species for its preservation, explains, I believe, some singular facts in nature, such as that of very rare plants being sometimes extremely abundant in the few spots where they do occur; and that of some social plants being social, that is abounding in individuals, even on the extreme confines of their range. For in such cases, we may believe, that a plant could exist only where the conditions of its life were so favourable that many could exist together, and thus save each other from utter destruction (p56).
• From experiments which I have tried, I have found that the visits of bees, if not indispensable, are at least highly beneficial to fertilisation; humble-bees alone visit the common red clover, as other bees cannot reach the nectar. Hence I have very little doubt, that if the whole genus of humble-bees became extinct or very rare in England, then in turn would the red clover become very rare, or wholly disappear (p58).
• In the case of every species, many different checks, acting at different periods of life, and during different seasons or years, probably come into play; some one check or some few being generally the most potent, but all concurring in determining the average number or even the existence of the species (p58/9).
• To keep up a mixed stock of extremely close varieties, they must each year be harvested separately, and the seed then mixed in due proportion, otherwise the weaker kinds will steadily decrease in numbers and disappear (p60).
• As species of the same genus usually have some similarity in habits and constitution, and always in structure, the struggle will generally be more severe between species of the same genus, when they come into competition with each other, than between species of distinct genera. Competition should be most severe between allied forms, which fill nearly the same place in the economy of nature (p60).
• We can see that when a plant or animal is placed in a new country amongst new competitors, though the climate may be exactly the same as in its former home, the conditions of its life will generally be changed in an essential manner. If we wished to increase its average numbers in its new home, we should have to modify it in a different way to what we should have done in its native country; for we should have to give it some advantage over a different set of competitors or enemies (p61/2).

Chapter IV: Natural Selection

• Let it be borne in mind in what an endless number of strange peculiarities our domestic productions, and, in a lesser degree, those under nature, vary; and how strong the hereditary tendency is (p63).
• We may feel sure that any variation in the least degree injurious would be rigidly destroyed. This preservation of favourable variations and the rejection of injurious variations, I call Natural Selection. Variations neither useful nor injurious would not be affected by natural selection, and would be left a fluctuating element (p64).
• The intimate and complex manner in which the inhabitants of each country are bound together means that any change in the numerical proportions of some of the inhabitants, independently of the change of climate itself, would most seriously affect many of the others (p64).
• Every slight modification, which in the course of ages chanced to arise, and which in any way favoured the individuals of any of the species, by better adapting them to their altered conditions, would tend to be preserved (p64).
• We have reason to believe, as stated in the first chapter, that a change in the conditions of life, by specially acting on the reproductive system, causes or increases variability (p64).
• Unless profitable variations do occur, natural selection can do nothing. Not that, as I believe, any extreme amount of variability is necessary (p64/5).
• No country can be named in which all the native inhabitants are now so perfectly adapted to each other and to the physical conditions under which they live, that none of them could anyhow be improved (p65).
• Man can act only on external and visible characters; nature cares nothing for appearances, except in so far as they may be useful to any being. Nature can act on every internal organ, on every shade of constitutional difference, on the whole machinery of life. Man selects only for his own good; Nature only for that of the being which she tends (p65).
• Man keeps the natives of many climates in the same country: he feeds all the same, exposes them to the same climate, and he does not let the most vigorous males fight for the female (p65).
• Natural selection, on the other hand, is daily and hourly scrutinising, throughout the world, every variation, even the slightest (p66).
• We see nothing of these slow changes in progress, until the hand of time has marked the long lapse of ages, and then so imperfect is our view into long past geological ages, that we only see that the forms of life are now different from what they formerly were (p66).
• Bear in mind that there are many unknown laws of correlation of growth, which, when one part of the organisation is modified through variation, and the modifications are accumulated by natural selection for the good of the being, will cause other modifications (p67).
• Natural selection will modify the structure of the young in relation to the parent, and of the parent in relation to the young. In social animals it will adapt the structure of each individual for the benefit of the community; if each in consequence profits by the selected change. What natural selection cannot do is to modify the structure of one species, without giving it any advantage, for the good of another species. […] a structure used only once in an animal’s whole life, if of high importance to it, might be modified to any extent by natural selection, for example, the hard tip of a bird’s beak used for breaking the egg (p68).
• Sexual selection: this depends, not on a struggle for existence, but on a struggle between the males for possession of the females; the result is not death to the unsuccessful competitor, but few or no offspring. Sexual selection is, therefore, less rigorous than natural selection. In many cases, victory will depend not on general vigour, but on having special weapons, confined to the male sex. A hornless stag would have a poor chance of leaving offspring for example; on the other hand, the shield may also be as important for victory as the weapon itself, in the case of a lion’s mane for instance (p69).
• When the males and females of any animal have the same general habits of life, but differ in structure, colour, or ornament, such general differences have been mainly caused by sexual selection (p70).
• Natural selection banishes the belief of the continued creation of new organic beings, or of any great and sudden modifications in their structure (p74/5).
• I am strongly inclined to believe that with all hermaphrodites, two individuals, either occasionally or habitually, unite for the reproduction of their kind. There are many hermaphrodite animals which certainly do not habitually pair, and a vast majority of hermaphrodite plants. […] However, with animals and plants a cross between different varieties, or between individuals of the same variety but of another strain, gives vigour and fertility to the offspring; and on the other hand, close interbreeding diminishes said vigour and fertility. It is a general law of nature that no organic being self-fertilises itself for an eternity of generations; but that a cross with another individual is occasionally – perhaps at very long intervals – indispensable (p75).
• A large number of individuals, by giving a better chance for the appearance within any given period of profitable variations, will compensate for a lesser amount of variability in each individual, and is, I believe, an extremely important element of success (p79).
• Though nature grants vast periods of time for the work of natural selection, she does not grant an indefinite period, and if any one species does not become modified and improved in a corresponding degree with its competitors, it will soon be exterminated (p79).
• Natural selection will always tend to preserve all the individuals varying in the right direction, though in different degrees, so as better to fill up the unoccupied place (p80).
• A local variety when once formed might subsequently slowly spread to other districts (p80).
• Isolation, also, is an important element in the process of natural selection. In a confined or isolated area, if not very large, the organic and inorganic conditions of life will generally be in a great degree uniform; so that natural selection will tend to modify all the individuals of a varying species throughout the area in the same manner in relation to the same conditions (p81).
• Natural selection will always act very slowly, often only at long intervals of time, and generally on only a very few of the inhabitants of the same region at the same time (p84).
• As each selected and favoured form increases in number, so will the less favoured forms decrease and become rare. Rarity, as geology tells us, is the precursor to extinction (p85).
• Varieties are species in the process of formation, or are, as I have called them, incipient species; they are supposed prototypes and parents of future well-marked species, presenting slight and ill-defined differences (p86).
• Divergence of character: causing differences in breeds, at first barely appreciable, and then steadily increasing, diverging in character both from each other and from their common parent.
• The simple circumstance that the more diversified the descendants from any one species become in structure, constitution, and habits, by so much will they be better enabled to seize on many and widely diversified places in the polity of nature, and so be enabled to increase in numbers. But they can only do so by its varying descendants seizing on places at present occupied by other animals (p87).
• Species, which are the commonest and the most widely-diffused, vary more than rare species with restricted ranges (p90).
• Varieties or modified descendants, proceeding from the common parent, will generally go on increasing in number and diverging in character (p93).
• Natural selection acts through one form having some advantage over other forms in the struggle for existence, it will chiefly act on those which already have some advantage (p98).
• Looking to the future, we can predict that the groups of organic beings which are now large and triumphant, will for a long period continue to increase. But which groups will ultimately prevail, no man can predict (p98).
• Summary of chapter IV: if variations useful to any organic being do occur, assuredly individuals thus characterised will have the best chance of being preserved in the struggle for life; and from the strong principle of inheritance they will tend to produce offspring similarly characterised. This principle of preservation, I have called, Natural Selection (p99).

Chapter V: Laws of Variation

• Deviations of structure are in some way due to the nature of the conditions of life, to which the parents and their more remote ancestors have been exposed during several generations (p102).
• The reproductive system is eminently susceptible to changes in the conditions of life; and to this system being functionally disturbed in the parents, I chiefly attribute the varying of the offspring (p102).
• How much direct effect difference of climate, food etc., produces on any being is extremely doubtful. My impression is that the effect is extremely small in cases of animals, but perhaps rather more in that of plants. We may at least, safely conclude that such influences cannot have produced the many striking and complex co-adaptations of structure between one organic being and another (p103).
• Instances can be given of the same variety being produced under conditions of life which are as different as can be; and, on the other hand, of different varieties being produced from the same species under the same conditions. Such facts show how indirectly the conditions of life must act (p104).
• Effects of use and disuse: I think there can be little doubt that use in our domestic animals strengthens and enlarges certain parts, and disuse diminishes them; and that such modifications are inherited. In some cases we might easily put down to disuse modifications of structure which are wholly, or mainly, due to natural selection (p104/5).
• Acclimatisation: as it is extremely common for species of the same genus to inhabit very hot and very cold countries, and as I believe that all the species of the same genus have descended from a single parent, acclimatisation must be readily effected during long-continued descent. It is notorious that each species is adapted to the climate of its own home, but the degree of adaptation of the species to the climates under which they live is often overrated (p108).
• We have evidence, in the case of some few plants, of their becoming, to a certain extent, naturally habituated to different temperatures, or becoming acclimatised. Similarly, I think the common and extraordinary capacity in our domestic animals of not only withstanding the most different climates but of being perfectly fertile under them, may be used as an argument that a large proportion of other animals, now in a state of nature, could easily be brought to bear widely different climates (p109).
• How much of the acclimatisation of species to any peculiar climate is due to mere habit, and how much to the natural selection of varieties having different innate constitutions, and how much to both means combined, is a very obscure question (p110).
• Correlation of growth: the whole organisation is so tied together during its growth and development, that when slight variations in any one part occur, and are accumulated through natural selection, other parts become modified. Any modifications accumulated solely for the good of the young, will, it may safely be concluded, affect their structure as an adult; in the same manner as any malconformation affecting the early embryo, seriously affects the whole organisation of the adult (p111).
• Hard parts seem to affect the form of adjoining soft parts; it is believed by some authors that the diversity in the shape of the pelvis in birds causes the remarkable diversity in the shape of their kidneys (p111).
• When nourishment flows to one part or organ in excess, it rarely flows, at least in excess, to another part; thus it is difficult to get a cow to give much milk and to fatten readily (p114).
• Natural selection is continually trying to economise in every part of the organisation. If under changed conditions of life a structure before useful becomes less useful, any diminution, however slight, in its development, will be seized on by natural selection, for it will profit the individual not to have its nutriment wasted in building up a useless structure (p114).
• Conversely, natural selection may perfectly well succeed in largely developing any organ, without requiring as a necessary compensation the reduction of some adjoining part (p115).
• When any part or organ is repeated many times in the structure of the same individual (as the vertebrae in snakes) the number is variable; whereas the number of the same part or organ, when it occurs in lesser numbers, is constant. Multiple parts are also very liable to variation in structure which seems to be a sign of low organisation.
• Beings low in the scale of nature are more variable than those which are higher. I presume that lowness in this case means that the several parts of the organisation have been but little specialised for particular functions; and as long as the same part has to perform diversified work, we can perhaps see why it should remain variable, that is, why natural selection should have preserved or rejected each little deviation of form less carefully than when the part has to serve for one special purpose (p115).
• Rudimentary parts are apt to be highly variable. Their variability seems to be owing to their uselessness, and therefore to natural selection having no power to check deviations in their structure. Thus they are left to the free play of the various laws of growth, to the effects of long-continued disuse, and to the tendency of reversion (p116).
• In our domestic animals, if any part, or the whole animal, be neglected and no selection be applied, the part or the whole breed will cease to have a nearly uniform character. The breed will then be said to have degenerated (p117/8).
• Variable characters, produced by man’s selection, sometimes become attached, from causes quite unknown to us, more to one sex than to the other, generally the male sex (p118).
• A species very rarely endures for more than one geological period (p118).

Chapter VI: Difficulties on Theory

• The first paragraph of this chapter reads as follows: “Long before having arrived at this part of my work, a crowd of difficulties will have occurred to the reader […] but, to the best of my judgment, the greater number are only apparent, and those that are real are not, I think, fatal to my theory” (p132). This chapter then goes on to prove this point, through each major problem that could present this theory, except Instinct and Hybridism, which are explained in their own chapters. The points raised are somewhat repetitive throughout the rest of the book, and for this reason have not been presented here in more bullet points.
• Natural selection tends only to make each organic being as perfect as, or slightly more perfect than, the other inhabitants of the same country with which it has to struggle for existence (p155).
• Natural selection will produce nothing in one species for the exclusive good or injury of another: though it may well produce parts, organs, and excretions highly useful or injurious to another species, but in all cases at the same time useful to the owner (p157).
• The inhabitants of one country, generally the smaller one, will often yield, to the inhabitants of another. For in the larger country there will have existed more individuals, and more diversified forms, and the competition will have been severer, and thus the standard of perfection will have been rendered higher (p158).

Chapter VII: Instinct

• Under changed conditions of life, it is at least possible that slight modifications of instinct might be profitable to a species; and if it can be shown that instincts do vary ever so little, then I can see no difficulty in natural selection preserving and continually accumulating variations of instincts to any extent that may be profitable (p161).
• Natural instincts are lost under domestication (p165).
• There are common cases of closely allied, but certainly distinct species, when inhabiting distant parts of the world and living under considerably different conditions of life, often retaining nearly the same instincts (p186).

Chapter VIII: Hybridism

• First crosses between forms sufficiently distinct to be ranked as species, and their hybrids, are very generally, but not universally, sterile. This sterility is innately variable in individuals of the same species, and is eminently susceptible of favourable and unfavourable conditions (p210).
• The sterility of hybrids occurs from their imperfect reproductive systems, as they have had this system and their whole organisation disturbed by being compounded of two distinct species (p210).
• The capacity of one species or variety to take on another through crossing, is incidental on unknown differences in their reproductive systems (p210).
• It is well to remember, as previously stated, that the crossing of forms only slightly different is favourable to the vigour and fertility of their offspring (p210).
• First crosses between forms known to be varieties, and their mongrel offspring, are very generally, but not quite universally, fertile (p211).
• To begin with, Darwin says: The fertility of varieties, that is of the forms known or believed to have descended from common parents, when intercrossed, and likewise the fertility of their mongrel offspring, is, in my theory, of equal importance with the sterility of species; for it seems to make a broad and clear distinction between varieties and species (p188). However, in the conclusion of this chapter, Darwin goes on to say that “the facts briefly given in this chapter do not seem to me opposed to, but even rather to support the view, that there is no fundamental distinction between species and varieties” (p211).

Chapter IX: On the Imperfection of the Geological Record; Chapter X: On the Geological Succession of Organic Beings

• I have attempted to show that the geological record is extremely imperfect; that only a small portion of the globe has been geologically explored with care; and that only certain classes of organic beings have been largely preserved in a fossil state (p258).
• There has probably been more extinction during the periods of subsidence, and more variation during the periods of elevation, and during the latter, the record will have been least perfectly kept (p258).
• Migration has played an important part in the first appearance of new forms in any one area and formation; widely ranging species are those which have varied most, and have oftenest given rise to new species (p258).
• Species of different classes do not necessarily change together, or at the same rate, or in the same degree; yet in the long run, all undergo modification to some extent (p259).
• The extinction of old forms is the almost inevitable consequence of the production of new forms. We can understand why when a species has once disappeared, it never reappears. Groups of species increase in numbers slowly, and endure for unequal periods of time (p259).
• The dominant species of the larger dominant groups tend to leave many modified descendants, and thus new sub-groups and groups are formed. As these are formed, the species of the less vigorous groups, from their inferiority inherited from a common progenitor, tend to become extinct together, and to leave no modified offspring on the face of the earth. But the utter extinction of a whole group of species may often be a very slow process (p259).
• We can understand how it is that all the forms of life, ancient and recent, make together one grand system; for all are connected by generation (p260).
• We can understand, from the continued tendency to divergence of character, why the more ancient a form is, the more it generally differs from those now living (p260).
• The inhabitants of each successive period in the world’s history have beaten their predecessors in the race for life, and are, in so far, higher in the scale of nature; and this may account for the sentiment that organisation on the whole has progressed (p260).

Chapter XI: Geographical Distribution; Chapter XII: Geographical Distribution – continued

• Given that the mutual relations of organism to organism are of the highest importance, we can see why two areas having nearly the same physical conditions should often be inhabited by very different forms of life; for according to the length of time which has elapsed since new inhabitants entered one region; according to the nature of the communication which allowed certain forms and not others to enter, either in greater or lesser numbers; according or not, as those which entered happened to come in more or less direct competition with each other and with the aborigines; and according as the immigrants were capable of varying more or less rapidly, there would ensue in different regions, independently of their physical conditions, infinitely diversified conditions of life – there would be an almost endless amount of organic action and reaction – and we should find, as we do find, some groups of beings greatly, and some only slightly modified – some developed in great force, some existing in scanty numbers – in the different great geographical provinces of the world (p308/9).

Chapter XIII: Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Organs

• The natural system: it is genealogical in its attempted arrangement, with the grades of acquired difference marked by the terms varieties, species, genera, families, orders, and classes (p345).
• The several classes of facts which have been considered in this chapter, seem to me to proclaim so plainly, that the innumerable species, genera, and families of organic beings, with which this world is populated, have all descended, each within its own class or group, from common parents, and have all been modified in the course of descent (p345).

Chapter XIV: Recapitulation and Conclusion

• Complex organs and instincts should have been perfected by the accumulation of innumerable slight variations, each good for the individual possessor (p346).
• There is a struggle for existence leading to the preservation of each profitable deviation of structure or instinct (p346).
• The almost universal sterility of species when first crossed, which forms so remarkable a contrast with the almost universal fertility of varieties when crossed […] is incidental on constitutional differences in the reproductive systems of the intercrossed species (p347).
• Geographical distribution: all the individuals of the same species must have descended from common parents; and therefore, in however distant and isolated parts of the world they are now found, they must in the course of successive generations have passed from some one part to the others (p348).
• It cannot be denied that we are as yet very ignorant of the full extent of the various climatal and geographical changes which have affected the earth during modern periods; where such changes will obviously have greatly facilitated migration (p348).
• With respect to existing forms, we should remember that we have no right to expect to discover directly connecting links between them, but only between each and some extinct form (p349).
• Only a small portion of the world has been geographically explored; and only organic beings of certain classes can be preserved in a fossil condition, at least in any great number (p350).
• It is widely ranging species that vary most, and varieties that are often at first local – both of these causes render the discovery of intermediate links less likely. Local varieties will not spread into other and distant regions until they are considerably modified and improved; and when they do spread, if discovered in a geological formation, they will appear as if suddenly created there, and will be simply classed as new species (p350).
• The more important objections against my theory, relate to questions on which we are confessedly ignorant; nor do we know how ignorant we are (p351).
• Under domestication we see much variability. This seems to be mainly due to the reproductive system being eminently susceptible to changes in the conditions of life; so that this system, when not rendered impotent, fails to reproduce offspring exactly like the parent-form. Variability is governed by many complex laws – by correlation of growth, by use and disuse, and by the direct action of the physical conditions of life (p351).
• We have evidence that variability, when it has once come into play, does not wholly cease; for new varieties are still occasionally produced by our most anciently domesticated productions (p351).
• Man does not actually produce variability; he only unintentionally exposes organic beings to new conditions of life, and then nature acts on the organisation, and causes variability. But man can and does select the variations given to him by nature. He thus adapts animals and plants for his own benefit or pleasure. He may do this methodically, or he may do it unconsciously by preserving the individuals most useful to him at the time, without any thought of altering the breed (p351/2).
• There is no obvious reason why the principles which have acted so efficiently under domestication should not have acted under nature (p352).
• More individuals are born than can possibly survive (p352).
• As the individuals of the same species come into the closest competition with each other, the struggle will generally be most severe between them; the slightest advantage in one being, at any age or during any season, over those with which it comes into competition, or better adaptation in however slight a degree to the surrounding physical conditions, will turn the balance (p352).
• With animals having separate sexes there will in most cases be a struggle between the males for possession of the females. The most vigorous individual, or those which have most successfully struggled with their conditions of life, will generally leave most progeny. But success will often depend on having special weapons or means of defence, or on the charms of the males (p352/3).
• Species are strongly marked and permanent varieties, and each species first existed as a variety (p354).
• As all groups cannot succeed in increasing in size, for the world would not hold them, the more dominant groups beat the less dominant (p355).
• As natural selection acts solely by accumulating slight, successive, favourable variations, it can produce no great or sudden modification; it can act only by very short and slow steps (p355).
• As natural selection acts by competition, it adapts the inhabitants of each country only in relation to the degree of perfection of their associates (p355).
• In both varieties and species, use and disuse seem to have produced some effect (p356).
• In both varieties and species correlation of growth seems to have played a most important part, so that when one part has been modified other parts are necessarily modified. In both varieties and species reversions to long-lost characters occur (p356).
• Instincts offer no greater difficulty than corporeal structure do on the theory of the natural selection of successive, slight, but profitable modifications; and also habits, no doubt, sometimes come into play in modifying instincts (p357).
• Neither single species nor groups of species reappear when the chain of ordinary generation has once been broken (p358).
• Disuse, aided sometimes by natural selection, will often tend to reduce an organ, when it has become useless by changed habits or under changed conditions of life; but disuse and selection will generally act on each creature, when it has come to maturity and has to play its full part in the struggle for existence (p361).
• The belief that species were immutable productions was almost unavoidable as long as the history of the world was thought to be of short duration; and now that we have acquired some idea of the lapse of time, we are too apt to assume, without proof, that the geological record is so perfect that it would have afforded us plain evidence of the mutation of species, if they had undergone said change (p362).
• It is so easy to hide our ignorance under such an expression as the ‘plan of creation’, and to think that we give an explanation when we only restate a fact. Any one whose disposition leads them to attach more weight to unexplained difficulties than to the explanation of a certain number of facts will certainly reject my theory. A few naturalists, endowed with much flexibility of mind, and who have already begun to doubt on the immutability of species, may be influenced by this volume; but I look with confidence to the future, to young and rising naturalists, who will be able to view both sides of the question with impartiality (p363).
• Sometimes, fossil remains can fill up very wide intervals between existing orders, as do organs in a rudimentary condition, which plainly show that an earlier progenitor had the organ in a fully developed state (p364).
• I believe that animals have descended from at most only four or five progenitors, and plants from an equal or lesser number (p364).
• All living things have much in common, in their chemical composition, their germinal vesicles, their cellular structure, and their laws of growth and reproduction. We see these even in so trifling as circumstance as that the same poison often similarly affects both plants and animals; therefore I should infer from analogy that probably all the organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed (p364).
• Hereafter we shall be compelled to acknowledge that the only distinction between species and well-marked varieties is, that the latter are known, or believed, to be connected at the present day by intermediate gradations, whereas species were formally thus connected. We shall be led, from here on after, to weigh more carefully and to value higher the actual amount of difference between them. It is quite possible that forms now generally acknowledged to be merely varieties may hereafter be thought worthy of specific names (p365).
• The amount of organic change in the fossils of consecutive formations probably serves as a fair measure of the lapse of natural time. A number of species, however, keeping in a body might remain for a long period unchanged, whilst within these same period, several of these species, by migrating into new countries and coming into competition with foreign associates, might become modified; so that we must not overrate the accuracy of organic change as a measure of time. During early periods of the earth’s history, when the forms of life were probably fewer and simpler, the rate of change was probably slower; and at the first dawn of life, when very few forms of the simplest structure existed, the rate of change may have been slow in an extreme degree. The whole history of the world, as at present known, although of a length quite incomprehensible by us, will hereafter be recognised as a mere fragment of time, compared with the ages which have elapsed since the first creature, the progenitor of innumerable extinct and living descendants, was created (p367).
• Elaborately constructed forms, so different from each other, and dependant on each other in so complex a manner, have all been produced by the laws acting around us. These laws, taken in the largest sense, are Growth with Reproduction; Inheritance which is almost implied by Reproduction; Variability from the indirect and direct action of the external conditions of life, and from use and disuse; a Ratio of Increase so high as to lead to a Struggle of life, and as a consequence to Natural Selection, necessitating Divergence of Character and the Extinction of less-improved forms (p369).

Darwin, Charles. The Origin of Species. Wordsworth Editions Limited, 1998.