The Origin of Species

by Charles Darwin

distinct progenitors. For instance, it is just possible, if our

  fantail-pigeons were all destroyed, that fanciers, by striving during long

  ages for the same object, might make a new breed hardly distinguishable

  from our present fantail; but if the parent rock-pigeon were also

  destroyed, and in nature we have every reason to believe that the

  parent-form will generally be supplanted and exterminated by its improved

  offspring, it is quite incredible that a fantail, identical with the

  existing breed, could be raised from any other species of pigeon, or even

  from the other well-established races of the domestic pigeon, for the

  newly-formed fantail would be almost sure to inherit from its new

  progenitor some slight characteristic differences.

  Groups of species, that is, genera and families, follow the same general

  rules in their appearance and disappearance as do single species, changing

  more or less quickly, and in a greater or lesser degree. A group does not

  reappear after it has once disappeared; or its existence, as long as it

  lasts, is continuous. I am aware that there are some apparent exceptions

  to this rule, but the exceptions are surprisingly few, so few, that E.

  Forbes, Pictet, and Woodward (though all strongly opposed to such views as

  I maintain) admit its truth; and the rule strictly accords with my theory.

  For as all the species of the same group have descended from some one

  species, it is clear that as long as any species of the group have appeared

  in the long succession of ages, so long must its members have continuously

  existed, in order to have generated either new and modified or the same old

  and unmodified forms. Species of the genus Lingula, for instance, must

  have continuously existed by an unbroken succession of generations, from

  the lowest Silurian stratum to the present day.

  We have seen in the last chapter that the species of a group sometimes

  falsely appear to have come in abruptly; and I have attempted to give an

  explanation of this fact, which if true would have been fatal to my views.

  But such cases are certainly exceptional; the general rule being a gradual

  increase in number, till the group reaches its maximum, and then, sooner or

  later, it gradually decreases. If the number of the species of a genus, or

  the number of the genera of a family, be represented by a vertical line of

  varying thickness, crossing the successive geological formations in which

  the species are found, the line will sometimes falsely appear to begin at

  its lower end, not in a sharp point, but abruptly; it then gradually

  thickens upwards, sometimes keeping for a space of equal thickness, and

  ultimately thins out in the upper beds, marking the decrease and final

  extinction of the species. This gradual increase in number of the species

  of a group is strictly conformable with my theory; as the species of the

  same genus, and the genera of the same family, can increase only slowly and

  progressively; for the process of modification and the production of a

  number of allied forms must be slow and gradual,--one species giving rise

  first to two or three varieties, these being slowly converted into species,

  which in their turn produce by equally slow steps other species, and so on,

  like the branching of a great tree from a single stem, till the group

  becomes large.

  On Extinction. -- We have as yet spoken only incidentally of the

  disappearance of species and of groups of species. On the theory of

  natural selection the extinction of old forms and the production of new and

  improved forms are intimately connected together. The old notion of all

  the inhabitants of the earth having been swept away at successive periods

  by catastrophes, is very generally given up, even by those geologists, as

  Elie de Beaumont, Murchison, Barrande, &c., whose general views would

  naturally lead them to this conclusion. On the contrary, we have every

  reason to believe, from the study of the tertiary formations, that species

  and groups of species gradually disappear, one after another, first from

  one spot, then from another, and finally from the world. Both single

  species and whole groups of species last for very unequal periods; some

  groups, as we have seen, having endured from the earliest known dawn of

  life to the present day; some having disappeared before the close of the

  palaeozoic period. No fixed law seems to determine the length of time

  during which any single species or any single genus endures. There is

  reason to believe that the complete extinction of the species of a group is

  generally a slower process than their production: if the appearance and

  disappearance of a group of species be represented, as before, by a

  vertical line of varying thickness, the line is found to taper more

  gradually at its upper end, which marks the progress of extermination, than

  at its lower end, which marks the first appearance and increase in numbers

  of the species. In some cases, however, the extermination of whole groups

  of beings, as of ammonites towards the close of the secondary period, has

  been wonderfully sudden.

  The whole subject of the extinction of species has been involved in the

  most gratuitous mystery. Some authors have even supposed that as the

  individual has a definite length of life, so have species a definite

  duration. No one I think can have marvelled more at the extinction of

  species, than I have done. When I found in La Plata the tooth of a horse

  embedded with the remains of Mastodon, Megatherium, Toxodon, and other

  extinct monsters, which all co-existed with still living shells at a very

  late geological period, I was filled with astonishment; for seeing that the

  horse, since its introduction by the Spaniards into South America, has run

  wild over the whole country and has increased in numbers at an unparalleled

  rate, I asked myself what could so recently have exterminated the former

  horse under conditions of life apparently so favourable. But how utterly

  groundless was my astonishment! Professor Owen soon perceived that the

  tooth, though so like that of the existing horse, belonged to an extinct

  species. Had this horse been still living, but in some degree rare, no

  naturalist would have felt the least surprise at its rarity; for rarity is

  the attribute of a vast number of species of all classes, in all countries.

  If we ask ourselves why this or that species is rare, we answer that

  something is unfavourable in its conditions of life; but what that

  something is, we can hardly ever tell. On the supposition of the fossil

  horse still existing as a rare species, we might have felt certain from the

  analogy of all other mammals, even of the slow-breeding elephant, and from

  the history of the naturalisation of the domestic horse in South America,

  that under more favourable conditions it would in a very few years have

  stocked the whole continent. But we could not have told what the

  unfavourable conditions were which checked its increase, whether some one

  or several contingencies, and at what period of the horse's life, and in

  what degree, they severally acted. If the conditions had gone on, however
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  slowly, becoming less and less favourable, we assuredly should not have

  perceived the fact, yet the fossil horse would certainly have become rarer

  and rarer, and finally extinct;--its place being seized on by some more

  successful competitor.

  It is most difficult always to remember that the increase of every living

  being is constantly being checked by unperceived injurious agencies; and

  that these same unperceived agencies are amply sufficient to cause rarity,

  and finally extinction. We see in many cases in the more recent tertiary

  formations, that rarity precedes extinction; and we know that this has been

  the progress of events with those animals which have been exterminated,

  either locally or wholly, through man's agency. I may repeat what I

  published in 1845, namely, that to admit that species generally become rare

  before they become extinct--to feel no surprise at the rarity of a species,

  and yet to marvel greatly when it ceases to exist, is much the same as to

  admit that sickness in the individual is the forerunner of death--to feel

  no surprise at sickness, but when the sick man dies, to wonder and to

  suspect that he died by some unknown deed of violence.

  The theory of natural selection is grounded on the belief that each new

  variety, and ultimately each new species, is produced and maintained by

  having some advantage over those with which it comes into competition; and

  the consequent extinction of less-favoured forms almost inevitably follows.

  It is the same with our domestic productions: when a new and slightly

  improved variety has been raised, it at first supplants the less improved

  varieties in the same neighbourhood; when much improved it is transported

  far and near, like our short-horn cattle, and takes the place of other

  breeds in other countries. Thus the appearance of new forms and the

  disappearance of old forms, both natural and artificial, are bound

  together. In certain flourishing groups, the number of new specific forms

  which have been produced within a given time is probably greater than that

  of the old forms which have been exterminated; but we know that the number

  of species has not gone on indefinitely increasing, at least during the

  later geological periods, so that looking to later times we may believe

  that the production of new forms has caused the extinction of about the

  same number of old forms.

  The competition will generally be most severe, as formerly explained and

  illustrated by examples, between the forms which are most like each other

  in all respects. Hence the improved and modified descendants of a species

  will generally cause the extermination of the parent-species; and if many

  new forms have been developed from any one species, the nearest allies of

  that species, i.e. the species of the same genus, will be the most liable

  to extermination. Thus, as I believe, a number of new species descended

  from one species, that is a new genus, comes to supplant an old genus,

  belonging to the same family. But it must often have happened that a new

  species belonging to some one group will have seized on the place occupied

  by a species belonging to a distinct group, and thus caused its

  extermination; and if many allied forms be developed from the successful

  intruder, many will have to yield their places; and it will generally be

  allied forms, which will suffer from some inherited inferiority in common.

  But whether it be species belonging to the same or to a distinct class,

  which yield their places to other species which have been modified and

  improved, a few of the sufferers may often long be preserved, from being

  fitted to some peculiar line of life, or from inhabiting some distant and

  isolated station, where they have escaped severe competition. For

  instance, a single species of Trigonia, a great genus of shells in the

  secondary formations, survives in the Australian seas; and a few members of

  the great and almost extinct group of Ganoid fishes still inhabit our fresh

  waters. Therefore the utter extinction of a group is generally, as we have

  seen, a slower process than its production.

  With respect to the apparently sudden extermination of whole families or

  orders, as of Trilobites at the close of the palaeozoic period and of

  Ammonites at the close of the secondary period, we must remember what has

  been already said on the probable wide intervals of time between our

  consecutive formations; and in these intervals there may have been much

  slow extermination. Moreover, when by sudden immigration or by unusually

  rapid development, many species of a new group have taken possession of a

  new area, they will have exterminated in a correspondingly rapid manner

  many of the old inhabitants; and the forms which thus yield their places

  will commonly be allied, for they will partake of some inferiority in

  common.

  Thus, as it seems to me, the manner in which single species and whole

  groups of species become extinct, accords well with the theory of natural

  selection. We need not marvel at extinction; if we must marvel, let it be

  at our presumption in imagining for a moment that we understand the many

  complex contingencies, on which the existence of each species depends. If

  we forget for an instant, that each species tends to increase inordinately,

  and that some check is always in action, yet seldom perceived by us, the

  whole economy of nature will be utterly obscured. Whenever we can

  precisely say why this species is more abundant in individuals than that;

  why this species and not another can be naturalised in a given country;

  then, and not till then, we may justly feel surprise why we cannot account

  for the extinction of this particular species or group of species.

  On the Forms of Life changing almost simultaneously throughout the World. -

  - Scarcely any palaeontological discovery is more striking than the fact,

  that the forms of life change almost simultaneously throughout the world.

  Thus our European Chalk formation can be recognised in many distant parts

  of the world, under the most different climates, where not a fragment of

  the mineral chalk itself can be found; namely, in North America, in

  equatorial South America, in Tierra del Fuego, at the Cape of Good Hope,

  and in the peninsula of India. For at these distant points, the organic

  remains in certain beds present an unmistakeable degree of resemblance to

  those of the Chalk. It is not that the same species are met with; for in

  some cases not one species is identically the same, but they belong to the

  same families, genera, and sections of genera, and sometimes are similarly

  characterised in such trifling points as mere superficial sculpture.

  Moreover other forms, which are not found in the Chalk of Europe, but which

  occur in the formations either above or below, are similarly absent at

  these distant points of the world. In the several successive palaeozoic

  formations of Russia, Western Europe and North America, a similar

  parallelism in the forms of life has been observed by several authors: so

  it is, according to Lyell, with the several European and North American

  tertia
ry deposits. Even if the few fossil species which are common to the

  Old and New Worlds be kept wholly out of view, the general parallelism in

  the successive forms of life, in the stages of the widely separated

  palaeozoic and tertiary periods, would still be manifest, and the several

  formations could be easily correlated.

  These observations, however, relate to the marine inhabitants of distant

  parts of the world: we have not sufficient data to judge whether the

  productions of the land and of fresh water change at distant points in the

  same parallel manner. We may doubt whether they have thus changed: if the

  Megatherium, Mylodon, Macrauchenia, and Toxodon had been brought to Europe

  from La Plata, without any information in regard to their geological

  position, no one would have suspected that they had coexisted with still

  living sea-shells; but as these anomalous monsters coexisted with the

  Mastodon and Horse, it might at least have been inferred that they had

  lived during one of the latter tertiary stages.

  When the marine forms of life are spoken of as having changed

  simultaneously throughout the world, it must not be supposed that this

  expression relates to the same thousandth or hundred-thousandth year, or

  even that it has a very strict geological sense; for if all the marine

  animals which live at the present day in Europe, and all those that lived

  in Europe during the pleistocene period (an enormously remote period as

  measured by years, including the whole glacial epoch), were to be compared

  with those now living in South America or in Australia, the most skilful

  naturalist would hardly be able to say whether the existing or the

  pleistocene inhabitants of Europe resembled most closely those of the

  southern hemisphere. So, again, several highly competent observers believe

  that the existing productions of the United States are more closely related

  to those which lived in Europe during certain later tertiary stages, than

  to those which now live here; and if this be so, it is evident that

  fossiliferous beds deposited at the present day on the shores of North

  America would hereafter be liable to be classed with somewhat older

  European beds. Nevertheless, looking to a remotely future epoch, there

  can, I think, be little doubt that all the more modern marine formations,

  namely, the upper pliocene, the pleistocene and strictly modern beds, of

  Europe, North and South America, and Australia, from containing fossil

  remains in some degree allied, and from not including those forms which are

  only found in the older underlying deposits, would be correctly ranked as

  simultaneous in a geological sense.

  The fact of the forms of life changing simultaneously, in the above large

  sense, at distant parts of the world, has greatly struck those admirable

  observers, MM. de Verneuil and d'Archiac. After referring to the

  parallelism of the palaeozoic forms of life in various parts of Europe,

  they add, 'If struck by this strange sequence, we turn our attention to

  North America, and there discover a series of analogous phenomena, it will

  appear certain that all these modifications of species, their extinction,

  and the introduction of new ones, cannot be owing to mere changes in marine

  currents or other causes more or less local and temporary, but depend on

  general laws which govern the whole animal kingdom.' M. Barrande has made

  forcible remarks to precisely the same effect. It is, indeed, quite futile

  to look to changes of currents, climate, or other physical conditions, as

  the cause of these great mutations in the forms of life throughout the

  world, under the most different climates. We must, as Barrande has

  remarked, look to some special law. We shall see this more clearly when we

  treat of the present distribution of organic beings, and find how slight is

  the relation between the physical conditions of various countries, and the

  nature of their inhabitants.