The Origin of Species

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by Charles Darwin

No naturalist pretends that all the species of a genus are equally distinct

  from each other; they may generally be divided into sub-genera, or

  sections, or lesser groups. As Fries has well remarked, little groups of

  species are generally clustered like satellites around certain other

  species. And what are varieties but groups of forms, unequally related to

  each other, and clustered round certain forms--that is, round their

  parent-species? Undoubtedly there is one most important point of

  difference between varieties and species; namely, that the amount of

  difference between varieties, when compared with each other or with their

  parent-species, is much less than that between the species of the same

  genus. But when we come to discuss the principle, as I call it, of

  Divergence of Character, we shall see how this may be explained, and how

  the lesser differences between varieties will tend to increase into the

  greater differences between species.

  There is one other point which seems to me worth notice. Varieties

  generally have much restricted ranges: this statement is indeed scarcely

  more than a truism, for if a variety were found to have a wider range than

  that of its supposed parent-species, their denominations ought to be

  reversed. But there is also reason to believe, that those species which

  are very closely allied to other species, and in so far resemble varieties,

  often have much restricted ranges. For instance, Mr. H. C. Watson has

  marked for me in the well-sifted London Catalogue of plants (4th edition)

  63 plants which are therein ranked as species, but which he considers as so

  closely allied to other species as to be of doubtful value: these 63

  reputed species range on an average over 6.9 of the provinces into which

  Mr. Watson has divided Great Britain. Now, in this same catalogue, 53

  acknowledged varieties are recorded, and these range over 7.7 provinces;

  whereas, the species to which these varieties belong range over 14.3

  provinces. So that the acknowledged varieties have very nearly the same

  restricted average range, as have those very closely allied forms, marked

  for me by Mr. Watson as doubtful species, but which are almost universally

  ranked by British botanists as good and true species.

  Finally, then, varieties have the same general characters as species, for

  they cannot be distinguished from species,--except, firstly, by the

  discovery of intermediate linking forms, and the occurrence of such links

  cannot affect the actual characters of the forms which they connect; and

  except, secondly, by a certain amount of difference, for two forms, if

  differing very little, are generally ranked as varieties, notwithstanding

  that intermediate linking forms have not been discovered; but the amount of

  difference considered necessary to give to two forms the rank of species is

  quite indefinite. In genera having more than the average number of species

  in any country, the species of these genera have more than the average

  number of varieties. In large genera the species are apt to be closely,

  but unequally, allied together, forming little clusters round certain

  species. Species very closely allied to other species apparently have

  restricted ranges. In all these several respects the species of large

  genera present a strong analogy with varieties. And we can clearly

  understand these analogies, if species have once existed as varieties, and

  have thus originated: whereas, these analogies are utterly inexplicable if

  each species has been independently created.

  We have, also, seen that it is the most flourishing and dominant species of

  the larger genera which on an average vary most; and varieties, as we shall

  hereafter see, tend to become converted into new and distinct species. The

  larger genera thus tend to become larger; and throughout nature the forms

  of life which are now dominant tend to become still more dominant by

  leaving many modified and dominant descendants. But by steps hereafter to

  be explained, the larger genera also tend to break up into smaller genera.

  And thus, the forms of life throughout the universe become divided into

  groups subordinate to groups.

  Chapter III

  Struggle for Existence

  Bears on natural selection -- The term used in a wide sense -- Geometrical

  powers of increase -- Rapid increase of naturalised animals and plants --

  Nature of the checks to increase -- Competition universal -- Effects of

  climate -- Protection from the number of individuals -- Complex relations

  of all animals and plants throughout nature -- Struggle for life most

  severe between individuals and varieties of the same species; often severe

  between species of the same genus -- The relation of organism to organism

  the most important of all relations.

 

  Before entering on the subject of this chapter, I must make a few

  preliminary remarks, to show how the struggle for existence bears on

  Natural Selection. It has been seen in the last chapter that amongst

  organic beings in a state of nature there is some individual variability;

  indeed I am not aware that this has ever been disputed. It is immaterial

  for us whether a multitude of doubtful forms be called species or

  sub-species or varieties; what rank, for instance, the two or three hundred

  doubtful forms of British plants are entitled to hold, if the existence of

  any well-marked varieties be admitted. But the mere existence of

  individual variability and of some few well-marked varieties, though

  necessary as the foundation for the work, helps us but little in

  understanding how species arise in nature. How have all those exquisite

  adaptations of one part of the organisation to another part, and to the

  conditions of life, and of one distinct organic being to another being,

  been perfected? We see these beautiful co-adaptations most plainly in the

  woodpecker and missletoe; and only a little less plainly in the humblest

  parasite which clings to the hairs of a quadruped or feathers of a bird; in

  the structure of the beetle which dives through the water; in the plumed

  seed which is wafted by the gentlest breeze; in short, we see beautiful

  adaptations everywhere and in every part of the organic world.

  Again, it may be asked, how is it that varieties, which I have called

  incipient species, become ultimately converted into good and distinct

  species, which in most cases obviously differ from each other far more than

  do the varieties of the same species? How do those groups of species,

  which constitute what are called distinct genera, and which differ from

  each other more than do the species of the same genus, arise? All these

  results, as we shall more fully see in the next chapter, follow inevitably

  from the struggle for life. Owing to this struggle for life, any

  variation, however slight and from whatever cause proceeding, if it be in

  any degree profitable to an individual of any species, in its infinitely

  complex relations to other organic beings and to external nature, will tend

  to the preservation of that individual, and will generally be inherited by

  its offspring. 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, as the works of Nature are to those of Art.

  We will now discuss in a little more detail the struggle for existence. In

  my future work this subject shall be treated, as it well deserves, at much

  greater length. The elder De Candolle and Lyell have largely and

  philosophically shown that all organic beings are exposed to severe

  competition. In regard to plants, no one has treated this subject with

  more spirit and ability than W. Herbert, Dean of Manchester, evidently the

  result of his great horticultural knowledge. Nothing is easier than to

  admit in words the truth of the universal struggle for life, or more

  difficult--at least I have found it so--than constantly to bear this

  conclusion in mind. Yet unless it be thoroughly engrained in the mind, I

  am convinced that the whole economy of nature, with every fact on

  distribution, rarity, abundance, extinction, and variation, will be dimly

  seen or quite misunderstood. We behold the face of nature bright with

  gladness, we often see superabundance of food; we do not see, or we forget,

  that the birds which are idly singing round us mostly live on insects or

  seeds, and are thus constantly destroying life; or we forget how largely

  these songsters, or their eggs, or their nestlings, are destroyed by birds

  and beasts of prey; we do not always bear in mind, that though food may be

  now superabundant, it is not so at all seasons of each recurring year.

  I should premise that 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. Two canine animals in a time of dearth,

  may be truly said to struggle with each other which shall get food and

  live. But a plant on the edge of a desert is said to struggle for life

  against the drought, though more properly it should be said to be dependent

  on the moisture. A plant which annually produces a thousand seeds, of

  which on an average only one comes to maturity, may be more truly said to

  struggle with the plants of the same and other kinds which already clothe

  the ground. The missletoe is dependent on the apple and a few other trees,

  but can only in a far-fetched sense be said to struggle with these trees,

  for if too many of these parasites grow on the same tree, it will languish

  and die. But several seedling missletoes, growing close together on the

  same branch, may more truly be said to struggle with each other. As the

  missletoe is disseminated by birds, its existence depends on birds; and it

  may metaphorically be said to struggle with other fruit-bearing plants, in

  order to tempt birds to devour and thus disseminate its seeds rather than

  those of other plants. In these several senses, which pass into each

  other, I use for convenience sake the general term of struggle for

  existence.

  A struggle for existence inevitably follows from the high rate at which all

  organic beings tend to increase. Every being, which during its natural

  lifetime produces several eggs or seeds, must suffer destruction during

  some period of its life, and during some season or occasional year,

  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. It is the

  doctrine of Malthus applied with manifold force to the whole animal and

  vegetable kingdoms; for in this case there can be no artificial increase of

  food, and no prudential restraint from marriage. Although some species may

  be now increasing, more or less rapidly, in numbers, all cannot do so, for

  the world would not hold them.

  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. Even slow-breeding man has

  doubled in twenty-five years, and at this rate, in a few thousand years,

  there would literally not be standing room for his progeny. Linnaeus has

  calculated that if an annual plant produced only two seeds--and there is no

  plant so unproductive as this--and their seedlings next year produced two,

  and so on, then in twenty years there would be a million plants. The

  elephant is reckoned to be the slowest breeder of all known animals, and I

  have taken some pains to estimate its probable minimum rate of natural

  increase: it will be under the mark to assume that it breeds when thirty

  years old, and goes on breeding till ninety years old, bringing forth three

  pair of young in this interval; if this be so, at the end of the fifth

  century there would be alive fifteen million elephants, descended from the

  first pair.

  But we have better evidence on this subject than mere theoretical

  calculations, namely, the numerous recorded cases of the astonishingly

  rapid increase of various animals in a state of nature, when circumstances

  have been favourable to them during two or three following seasons. Still

  more striking is the evidence from our domestic animals of many kinds which

  have run wild in several parts of the world: if the statements of the rate

  of increase of slow-breeding cattle and horses in South America, and

  latterly in Australia, had not been well authenticated, they would have

  been quite incredible. So it is with plants: cases could be given of

  introduced plants which have become common throughout whole islands in a

  period of less than ten years. Several of the plants now most numerous

  over the wide plains of La Plata, clothing square leagues of surface almost

  to the exclusion of all other plants, have been introduced from Europe; and

  there are plants which now range in India, as I hear from Dr. Falconer,

  from Cape Comorin to the Himalaya, which have been imported from America

  since its discovery. In such cases, and endless instances could be given,

  no one supposes that the fertility of these animals or plants has been

  suddenly and temporarily increased in any sensible degree. The obvious

  explanation is that the conditions of life have been very favourable, and

  that there has consequently been less destruction
of the old and young, and

  that nearly all the young have been enabled to breed. In such cases the

  geometrical ratio of increase, the result of which never fails to be

  surprising, simply explains the extraordinarily rapid increase and wide

  diffusion of naturalised productions in their new homes.

  In a state of nature almost every plant produces seed, and amongst animals

  there are very few which do not annually pair. Hence we may confidently

  assert, that all plants and animals are tending to increase at a

  geometrical ratio, that all would most rapidly stock every station in which

  they could any how exist, and that the geometrical tendency to increase

  must be checked by destruction at some period of life. Our familiarity

  with the larger domestic animals tends, I think, to mislead us: we see no

  great destruction falling on them, and we forget that thousands are

  annually slaughtered for food, and that in a state of nature an equal

  number would have somehow to be disposed of.

  The only difference between organisms which annually produce eggs or seeds

  by the thousand, and those which produce extremely few, is, that the

  slow-breeders would require a few more years to people, under favourable

  conditions, a whole district, let it be ever so large. The condor lays a

  couple of eggs and the ostrich a score, and yet in the same country the

  condor may be the more numerous of the two: the Fulmar petrel lays but one

  egg, yet it is believed to be the most numerous bird in the world. One fly

  deposits hundreds of eggs, and another, like the hippobosca, a single one;

  but this difference does not determine how many individuals of the two

  species can be supported in a district. A large number of eggs is of some

  importance to those species, which depend on a rapidly fluctuating amount

  of food, for it allows them rapidly to increase in number. But the real

  importance of a large number of eggs or seeds is to make up for much

  destruction at some period of life; and 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 yet 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 a 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.

  In looking at Nature, it is most necessary to keep the foregoing

  considerations always in mind--never to forget that every single organic

  being around us may be said to be striving to the utmost to increase in

  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 any check, mitigate the

  destruction ever so little, and the number of the species will almost

  instantaneously increase to any amount. The face of Nature may be compared

  to a yielding surface, with ten thousand sharp wedges packed close together

  and driven inwards by incessant blows, sometimes one wedge being struck,

  and then another with greater force.

  What checks the natural tendency of each species to increase in number is

  most obscure. Look at the most vigorous species; by as much as it swarms

  in numbers, by so much will its tendency to increase be still further

  increased. We know not exactly what the checks are in even one single

  instance. Nor will this surprise any one who reflects how ignorant we are

  on this head, even in regard to mankind, so incomparably better known than

  any other animal. This subject has been ably treated by several authors,

 

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