The Origin of Species

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by Charles Darwin

our many tertiary and existing species.

  The case most frequently insisted on by palaeontologists of the apparently

  sudden appearance of a whole group of species, is that of the teleostean

  fishes, low down in the Chalk period. This group includes the large

  majority of existing species. Lately, Professor Pictet has carried their

  existence one sub-stage further back; and some palaeontologists believe

  that certain much older fishes, of which the affinities are as yet

  imperfectly known, are really teleostean. Assuming, however, that the

  whole of them did appear, as Agassiz believes, at the commencement of the

  chalk formation, the fact would certainly be highly remarkable; but I

  cannot see that it would be an insuperable difficulty on my theory, unless

  it could likewise be shown that the species of this group appeared suddenly

  and simultaneously throughout the world at this same period. It is almost

  superfluous to remark that hardly any fossil-fish are known from south of

  the equator; and by running through Pictet's Palaeontology it will be seen

  that very few species are known from several formations in Europe. Some

  few families of fish now have a confined range; the teleostean fish might

  formerly have had a similarly confined range, and after having been largely

  developed in some one sea, might have spread widely. Nor have we any right

  to suppose that the seas of the world have always been so freely open from

  south to north as they are at present. Even at this day, if the Malay

  Archipelago were converted into land, the tropical parts of the Indian

  Ocean would form a large and perfectly enclosed basin, in which any great

  group of marine animals might be multiplied; and here they would remain

  confined, until some of the species became adapted to a cooler climate, and

  were enabled to double the southern capes of Africa or Australia, and thus

  reach other and distant seas.

  From these and similar considerations, but chiefly from our ignorance of

  the geology of other countries beyond the confines of Europe and the United

  States; and from the revolution in our palaeontological ideas on many

  points, which the discoveries of even the last dozen years have effected,

  it seems to me to be about as rash in us to dogmatize on the succession of

  organic beings throughout the world, as it would be for a naturalist to

  land for five minutes on some one barren point in Australia, and then to

  discuss the number and range of its productions.

  On the sudden appearance of groups of Allied Species in the lowest known

  fossiliferous strata. -- There is another and allied difficulty, which is

  much graver. I allude to the manner in which numbers of species of the

  same group, suddenly appear in the lowest known fossiliferous rocks. Most

  of the arguments which have convinced me that all the existing species of

  the same group have descended from one progenitor, apply with nearly equal

  force to the earliest known species. For instance, I cannot doubt that all

  the Silurian trilobites have descended from some one crustacean, which must

  have lived long before the Silurian age, and which probably differed

  greatly from any known animal. Some of the most ancient Silurian animals,

  as the Nautilus, Lingula, &c., do not differ much from living species; and

  it cannot on my theory be supposed, that these old species were the

  progenitors of all the species of the orders to which they belong, for they

  do not present characters in any degree intermediate between them. If,

  moreover, they had been the progenitors of these orders, they would almost

  certainly have been long ago supplanted and exterminated by their numerous

  and improved descendants.

  Consequently, if my theory be true, it is indisputable that before the

  lowest Silurian stratum was deposited, long periods elapsed, as long as, or

  probably far longer than, the whole interval from the Silurian age to the

  present day; and that during these vast, yet quite unknown, periods of

  time, the world swarmed with living creatures.

  To the question why we do not find records of these vast primordial

  periods, I can give no satisfactory answer. Several of the most eminent

  geologists, with Sir R. Murchison at their head, are convinced that we see

  in the organic remains of the lowest Silurian stratum the dawn of life on

  this planet. Other highly competent judges, as Lyell and the late E.

  Forbes, dispute this conclusion. We should not forget that only a small

  portion of the world is known with accuracy. M. Barrande has lately added

  another and lower stage to the Silurian system, abounding with new and

  peculiar species. Traces of life have been detected in the Longmynd beds

  beneath Barrande's so-called primordial zone. The presence of phosphatic

  nodules and bituminous matter in some of the lowest azoic rocks, probably

  indicates the former existence of life at these periods. But the

  difficulty of understanding the absence of vast piles of fossiliferous

  strata, which on my theory no doubt were somewhere accumulated before the

  Silurian epoch, is very great. If these most ancient beds had been wholly

  worn away by denudation, or obliterated by metamorphic action, we ought to

  find only small remnants of the formations next succeeding them in age, and

  these ought to be very generally in a metamorphosed condition. But the

  descriptions which we now possess of the Silurian deposits over immense

  territories in Russia and in North America, do not support the view, that

  the older a formation is, the more it has suffered the extremity of

  denudation and metamorphism.

  The case at present must remain inexplicable; and may be truly urged as a

  valid argument against the views here entertained. To show that it may

  hereafter receive some explanation, I will give the following hypothesis.

  From the nature of the organic remains, which do not appear to have

  inhabited profound depths, in the several formations of Europe and of the

  United States; and from the amount of sediment, miles in thickness, of

  which the formations are composed, we may infer that from first to last

  large islands or tracts of land, whence the sediment was derived, occurred

  in the neighbourhood of the existing continents of Europe and North

  America. But we do not know what was the state of things in the intervals

  between the successive formations; whether Europe and the United States

  during these intervals existed as dry land, or as a submarine surface near

  land, on which sediment was not deposited, or again as the bed of an open

  and unfathomable sea.

  Looking to the existing oceans, which are thrice as extensive as the land,

  we see them studded with many islands; but not one oceanic island is as yet

  known to afford even a remnant of any palaeozoic or secondary formation.

  Hence we may perhaps infer, that during the palaeozoic and secondary

  periods, neither continents nor continental islands existed where our

  oceans now extend; for had they existed there, palaeozoic and secondary

  formations would in all probability have been accumulated from sediment

  derived from their wear and tear; and would have been at least
partially

  upheaved by the oscillations of level, which we may fairly conclude must

  have intervened during these enormously long periods. If then we may infer

  anything from these facts, we may infer that where our oceans now extend,

  oceans have extended from the remotest period of which we have any record;

  and on the other hand, that where continents now exist, large tracts of

  land have existed, subjected no doubt to great oscillations of level, since

  the earliest silurian period. The coloured map appended to my volume on

  Coral Reefs, led me to conclude that the great oceans are still mainly

  areas of subsidence, the great archipelagoes still areas of oscillations of

  level, and the continents areas of elevation. But have we any right to

  assume that things have thus remained from eternity? Our continents seem

  to have been formed by a preponderance, during many oscillations of level,

  of the force of elevation; but may not the areas of preponderant movement

  have changed in the lapse of ages? At a period immeasurably antecedent to

  the silurian epoch, continents may have existed where oceans are now spread

  out; and clear and open oceans may have existed where our continents now

  stand. Nor should we be justified in assuming that if, for instance, the

  bed of the Pacific Ocean were now converted into a continent, we should

  there find formations older than the silurian strata, supposing such to

  have been formerly deposited; for it might well happen that strata which

  had subsided some miles nearer to the centre of the earth, and which had

  been pressed on by an enormous weight of superincumbent water, might have

  undergone far more metamorphic action than strata which have always

  remained nearer to the surface. The immense areas in some parts of the

  world, for instance in South America, of bare metamorphic rocks, which must

  have been heated under great pressure, have always seemed to me to require

  some special explanation; and we may perhaps believe that we see in these

  large areas, the many formations long anterior to the silurian epoch in a

  completely metamorphosed condition.

  The several difficulties here discussed, namely our not finding in the

  successive formations infinitely numerous transitional links between the

  many species which now exist or have existed; the sudden manner in which

  whole groups of species appear in our European formations; the almost

  entire absence, as at present known, of fossiliferous formations beneath

  the Silurian strata, are all undoubtedly of the gravest nature. We see

  this in the plainest manner by the fact that all the most eminent

  palaeontologists, namely Cuvier, Owen, Agassiz, Barrande, Falconer, E.

  Forbes, &c., and all our greatest geologists, as Lyell, Murchison,

  Sedgwick, &c., have unanimously, often vehemently, maintained the

  immutability of species. But I have reason to believe that one great

  authority, Sir Charles Lyell, from further reflexion entertains grave

  doubts on this subject. I feel how rash it is to differ from these great

  authorities, to whom, with others, we owe all our knowledge. Those who

  think the natural geological record in any degree perfect, and who do not

  attach much weight to the facts and arguments of other kinds given in this

  volume, will undoubtedly at once reject my theory. For my part, following

  out Lyell's metaphor, I look at the natural geological record, as a history

  of the world imperfectly kept, and written in a changing dialect; of this

  history we possess the last volume alone, relating only to two or three

  countries. Of this volume, only here and there a short chapter has been

  preserved; and of each page, only here and there a few lines. Each word of

  the slowly-changing language, in which the history is supposed to be

  written, being more or less different in the interrupted succession of

  chapters, may represent the apparently abruptly changed forms of life,

  entombed in our consecutive, but widely separated formations. On this

  view, the difficulties above discussed are greatly diminished, or even

  disappear.

  Chapter X

  On the Geological Succession of Organic Beings

  On the slow and successive appearance of new species -- On their different

  rates of change -- Species once lost do not reappear -- Groups of species

  follow the same general rules in their appearance and disappearance as do

  single species -- On Extinction -- On simultaneous changes in the forms of

  life throughout the world -- On the affinities of extinct species to each

  other and to living species -- On the state of development of ancient forms

  -- On the succession of the same types within the same areas -- Summary of

  preceding and present chapters.

  Let us now see whether the several facts and rules relating to the

  geological succession of organic beings, better accord with the common view

  of the immutability of species, or with that of their slow and gradual

  modification, through descent and natural selection.

  New species have appeared very slowly, one after another, both on the land

  and in the waters. Lyell has shown that it is hardly possible to resist

  the evidence on this head in the case of the several tertiary stages; and

  every year tends to fill up the blanks between them, and to make the

  percentage system of lost and new forms more gradual. In some of the most

  recent beds, though undoubtedly of high antiquity if measured by years,

  only one or two species are lost forms, and only one or two are new forms,

  having here appeared for the first time, either locally, or, as far as we

  know, on the face of the earth. If we may trust the observations of

  Philippi in Sicily, the successive changes in the marine inhabitants of

  that island have been many and most gradual. The secondary formations are

  more broken; but, as Bronn has remarked, neither the appearance nor

  disappearance of their many now extinct species has been simultaneous in

  each separate formation.

  Species of different genera and classes have not changed at the same rate,

  or in the same degree. In the oldest tertiary beds a few living shells may

  still be found in the midst of a multitude of extinct forms. Falconer has

  given a striking instance of a similar fact, in an existing crocodile

  associated with many strange and lost mammals and reptiles in the

  sub-Himalayan deposits. The Silurian Lingula differs but little from the

  living species of this genus; whereas most of the other Silurian Molluscs

  and all the Crustaceans have changed greatly. The productions of the land

  seem to change at a quicker rate than those of the sea, of which a striking

  instance has lately been observed in Switzerland. There is some reason to

  believe that organisms, considered high in the scale of nature, change more

  quickly than those that are low: though there are exceptions to this rule.

  The amount of organic change, as Pictet has remarked, does not strictly

  correspond with the succession of our geological formations; so that

  between each two consecutive formations, the forms of life have seldom

  changed in exactly the same degree. Yet if we compare any but the most
>
  closely related formations, all the species will be found to have undergone

  some change. When a species has once disappeared from the face of the

  earth, we have reason to believe that the same identical form never

  reappears. The strongest apparent exception to this latter rule, is that

  of the so-called 'colonies' of M. Barrande, which intrude for a period in

  the midst of an older formation, and then allow the pre-existing fauna to

  reappear; but Lyell's explanation, namely, that it is a case of temporary

  migration from a distinct geographical province, seems to me satisfactory.

  These several facts accord well with my theory. I believe in no fixed law

  of development, causing all the inhabitants of a country to change

  abruptly, or simultaneously, or to an equal degree. The process of

  modification must be extremely slow. The variability of each species is

  quite independent of that of all others. Whether such variability be taken

  advantage of by natural selection, and whether the variations be

  accumulated to a greater or lesser amount, thus causing a greater or lesser

  amount of modification in the varying species, depends on many complex

  contingencies,--on the variability being of a beneficial nature, on the

  power of intercrossing, on the rate of breeding, on the slowly changing

  physical conditions of the country, and more especially on the nature of

  the other inhabitants with which the varying species comes into

  competition. Hence it is by no means surprising that one species should

  retain the same identical form much longer than others; or, if changing,

  that it should change less. We see the same fact in geographical

  distribution; for instance, in the land-shells and coleopterous insects of

  Madeira having come to differ considerably from their nearest allies on the

  continent of Europe, whereas the marine shells and birds have remained

  unaltered. We can perhaps understand the apparently quicker rate of change

  in terrestrial and in more highly organised productions compared with

  marine and lower productions, by the more complex relations of the higher

  beings to their organic and inorganic conditions of life, as explained in a

  former chapter. When many of the inhabitants of a country have become

  modified and improved, we can understand, on the principle of competition,

  and on that of the many all-important relations of organism to organism,

  that any form which does not become in some degree modified and improved,

  will be liable to be exterminated. Hence we can see why all the species in

  the same region do at last, if we look to wide enough intervals of time,

  become modified; for those which do not change will become extinct.

  In members of the same class the average amount of change, during long and

  equal periods of time, may, perhaps, be nearly the same; but as the

  accumulation of long-enduring fossiliferous formations depends on great

  masses of sediment having been deposited on areas whilst subsiding, our

  formations have been almost necessarily accumulated at wide and irregularly

  intermittent intervals; consequently the amount of organic change exhibited

  by the fossils embedded in consecutive formations is not equal. Each

  formation, on this view, does not mark a new and complete act of creation,

  but only an occasional scene, taken almost at hazard, in a slowly changing

  drama.

  We can clearly understand why a species when once lost should never

  reappear, even if the very same conditions of life, organic and inorganic,

  should recur. For though the offspring of one species might be adapted

  (and no doubt this has occurred in innumerable instances) to fill the exact

  place of another species in the economy of nature, and thus supplant it;

  yet the two forms--the old and the new--would not be identically the same;

  for both would almost certainly inherit different characters from their

 

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