The Origin of Species

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by Charles Darwin

3000 feet. Prof. Ramsay has published an account of a downthrow in

  Anglesea of 2300 feet; and he informs me that he fully believes there is

  one in Merionethshire of 12,000 feet; yet in these cases there is nothing

  on the surface to show such prodigious movements; the pile of rocks on the

  one or other side having been smoothly swept away. The consideration of

  these facts impresses my mind almost in the same manner as does the vain

  endeavour to grapple with the idea of eternity.

  I am tempted to give one other case, the well-known one of the denudation

  of the Weald. Though it must be admitted that the denudation of the Weald

  has been a mere trifle, in comparison with that which has removed masses of

  our palaeozoic strata, in parts ten thousand feet in thickness, as shown in

  Prof. Ramsay's masterly memoir on this subject. Yet it is an admirable

  lesson to stand on the North Downs and to look at the distant South Downs;

  for, remembering that at no great distance to the west the northern and

  southern escarpments meet and close, one can safely picture to oneself the

  great dome of rocks which must have covered up the Weald within so limited

  a period as since the latter part of the Chalk formation. The distance

  from the northern to the southern Downs is about 22 miles, and the

  thickness of the several formations is on an average about 1100 feet, as I

  am informed by Prof. Ramsay. But if, as some geologists suppose, a range

  of older rocks underlies the Weald, on the flanks of which the overlying

  sedimentary deposits might have accumulated in thinner masses than

  elsewhere, the above estimate would be erroneous; but this source of doubt

  probably would not greatly affect the estimate as applied to the western

  extremity of the district. If, then, we knew the rate at which the sea

  commonly wears away a line of cliff of any given height, we could measure

  the time requisite to have denuded the Weald. This, of course, cannot be

  done; but we may, in order to form some crude notion on the subject, assume

  that the sea would eat into cliffs 500 feet in height at the rate of one

  inch in a century. This will at first appear much too small an allowance;

  but it is the same as if we were to assume a cliff one yard in height to be

  eaten back along a whole line of coast at the rate of one yard in nearly

  every twenty-two years. I doubt whether any rock, even as soft as chalk,

  would yield at this rate excepting on the most exposed coasts; though no

  doubt the degradation of a lofty cliff would be more rapid from the

  breakage of the fallen fragments. On the other hand, I do not believe that

  any line of coast, ten or twenty miles in length, ever suffers degradation

  at the same time along its whole indented length; and we must remember that

  almost all strata contain harder layers or nodules, which from long

  resisting attrition form a breakwater at the base. Hence, under ordinary

  circumstances, I conclude that for a cliff 500 feet in height, a denudation

  of one inch per century for the whole length would be an ample allowance.

  At this rate, on the above data, the denudation of the Weald must have

  required 306,662,400 years; or say three hundred million years.

  The action of fresh water on the gently inclined Wealden district, when

  upraised, could hardly have been great, but it would somewhat reduce the

  above estimate. On the other hand, during oscillations of level, which we

  know this area has undergone, the surface may have existed for millions of

  years as land, and thus have escaped the action of the sea: when deeply

  submerged for perhaps equally long periods, it would, likewise, have

  escaped the action of the coast-waves. So that in all probability a far

  longer period than 300 million years has elapsed since the latter part of

  the Secondary period.

  I have made these few remarks because it is highly important for us to gain

  some notion, however imperfect, of the lapse of years. During each of

  these years, over the whole world, the land and the water has been peopled

  by hosts of living forms. What an infinite number of generations, which

  the mind cannot grasp, must have succeeded each other in the long roll of

  years! Now turn to our richest geological museums, and what a paltry

  display we behold!

  On the poorness of our Palaeontological collections. -- That our

  palaeontological collections are very imperfect, is admitted by every one.

  The remark of that admirable palaeontologist, the late Edward Forbes,

  should not be forgotten, namely, that numbers of our fossil species are

  known and named from single and often broken specimens, or from a few

  specimens collected on some one spot. Only a small portion of the surface

  of the earth has been geologically explored, and no part with sufficient

  care, as the important discoveries made every year in Europe prove. No

  organism wholly soft can be preserved. Shells and bones will decay and

  disappear when left on the bottom of the sea, where sediment is not

  accumulating. I believe we are continually taking a most erroneous view,

  when we tacitly admit to ourselves that sediment is being deposited over

  nearly the whole bed of the sea, at a rate sufficiently quick to embed and

  preserve fossil remains. Throughout an enormously large proportion of the

  ocean, the bright blue tint of the water bespeaks its purity. The many

  cases on record of a formation conformably covered, after an enormous

  interval of time, by another and later formation, without the underlying

  bed having suffered in the interval any wear and tear, seem explicable only

  on the view of the bottom of the sea not rarely lying for ages in an

  unaltered condition. The remains which do become embedded, if in sand or

  gravel, will when the beds are upraised generally be dissolved by the

  percolation of rain-water. I suspect that but few of the very many animals

  which live on the beach between high and low watermark are preserved. For

  instance, the several species of the Chthamalinae (a sub-family of sessile

  cirripedes) coat the rocks all over the world in infinite numbers: they

  are all strictly littoral, with the exception of a single Mediterranean

  species, which inhabits deep water and has been found fossil in Sicily,

  whereas not one other species has hitherto been found in any tertiary

  formation: yet it is now known that the genus Chthamalus existed during

  the chalk period. The molluscan genus Chiton offers a partially analogous

  case.

  With respect to the terrestrial productions which lived during the

  Secondary and Palaeozoic periods, it is superfluous to state that our

  evidence from fossil remains is fragmentary in an extreme degree. For

  instance, not a land shell is known belonging to either of these vast

  periods, with one exception discovered by Sir C. Lyell in the carboniferous

  strata of North America. In regard to mammiferous remains, a single glance

  at the historical table published in the Supplement to Lyell's Manual, will

  bring home the truth, how accidental and rare is their preservation, far

  better than pages of detail. Nor is their rarity surprising, when we

  remember how
large a proportion of the bones of tertiary mammals have been

  discovered either in caves or in lacustrine deposits; and that not a cave

  or true lacustrine bed is known belonging to the age of our secondary or

  palaeozoic formations.

  But the imperfection in the geological record mainly results from another

  and more important cause than any of the foregoing; namely, from the

  several formations being separated from each other by wide intervals of

  time. When we see the formations tabulated in written works, or when we

  follow them in nature, it is difficult to avoid believing that they are

  closely consecutive. But we know, for instance, from Sir R. Murchison's

  great work on Russia, what wide gaps there are in that country between the

  superimposed formations; so it is in North America, and in many other parts

  of the world. The most skilful geologist, if his attention had been

  exclusively confined to these large territories, would never have suspected

  that during the periods which were blank and barren in his own country,

  great piles of sediment, charged with new and peculiar forms of life, had

  elsewhere been accumulated. And if in each separate territory, hardly any

  idea can be formed of the length of time which has elapsed between the

  consecutive formations, we may infer that this could nowhere be

  ascertained. The frequent and great changes in the mineralogical

  composition of consecutive formations, generally implying great changes in

  the geography of the surrounding lands, whence the sediment has been

  derived, accords with the belief of vast intervals of time having elapsed

  between each formation.

  But we can, I think, see why the geological formations of each region are

  almost invariably intermittent; that is, have not followed each other in

  close sequence. Scarcely any fact struck me more when examining many

  hundred miles of the South American coasts, which have been upraised

  several hundred feet within the recent period, than the absence of any

  recent deposits sufficiently extensive to last for even a short geological

  period. Along the whole west coast, which is inhabited by a peculiar

  marine fauna, tertiary beds are so scantily developed, that no record of

  several successive and peculiar marine faunas will probably be preserved to

  a distant age. A little reflection will explain why along the rising coast

  of the western side of South America, no extensive formations with recent

  or tertiary remains can anywhere be found, though the supply of sediment

  must for ages have been great, from the enormous degradation of the

  coast-rocks and from muddy streams entering the sea. The explanation, no

  doubt, is, that the littoral and sub-littoral deposits are continually worn

  away, as soon as they are brought up by the slow and gradual rising of the

  land within the grinding action of the coast-waves.

  We may, I think, safely conclude that sediment must be accumulated in

  extremely thick, solid, or extensive masses, in order to withstand the

  incessant action of the waves, when first upraised and during subsequent

  oscillations of level. Such thick and extensive accumulations of sediment

  may be formed in two ways; either, in profound depths of the sea, in which

  case, judging from the researches of E. Forbes, we may conclude that the

  bottom will be inhabited by extremely few animals, and the mass when

  upraised will give a most imperfect record of the forms of life which then

  existed; or, sediment may be accumulated to any thickness and extent over a

  shallow bottom, if it continue slowly to subside. In this latter case, as

  long as the rate of subsidence and supply of sediment nearly balance each

  other, the sea will remain shallow and favourable for life, and thus a

  fossiliferous formation thick enough, when upraised, to resist any amount

  of degradation, may be formed.

  I am convinced that all our ancient formations, which are rich in fossils,

  have thus been formed during subsidence. Since publishing my views on this

  subject in 1845, I have watched the progress of Geology, and have been

  surprised to note how author after author, in treating of this or that

  great formation, has come to the conclusion that it was accumulated during

  subsidence. I may add, that the only ancient tertiary formation on the

  west coast of South America, which has been bulky enough to resist such

  degradation as it has as yet suffered, but which will hardly last to a

  distant geological age, was certainly deposited during a downward

  oscillation of level, and thus gained considerable thickness.

  All geological facts tell us plainly that each area has undergone numerous

  slow oscillations of level, and apparently these oscillations have affected

  wide spaces. Consequently formations rich in fossils and sufficiently

  thick and extensive to resist subsequent degradation, may have been formed

  over wide spaces during periods of subsidence, but only where the supply of

  sediment was sufficient to keep the sea shallow and to embed and preserve

  the remains before they had time to decay. On the other hand, as long as

  the bed of the sea remained stationary, thick deposits could not have been

  accumulated in the shallow parts, which are the most favourable to life.

  Still less could this have happened during the alternate periods of

  elevation; or, to speak more accurately, the beds which were then

  accumulated will have been destroyed by being upraised and brought within

  the limits of the coast-action.

  Thus the geological record will almost necessarily be rendered

  intermittent. I feel much confidence in the truth of these views, for they

  are in strict accordance with the general principles inculcated by Sir C.

  Lyell; and E. Forbes independently arrived at a similar conclusion.

  One remark is here worth a passing notice. During periods of elevation the

  area of the land and of the adjoining shoal parts of the sea will be

  increased, and new stations will often be formed;--all circumstances most

  favourable, as previously explained, for the formation of new varieties and

  species; but during such periods there will generally be a blank in the

  geological record. On the other hand, during subsidence, the inhabited

  area and number of inhabitants will decrease (excepting the productions on

  the shores of a continent when first broken up into an archipelago), and

  consequently during subsidence, though there will be much extinction, fewer

  new varieties or species will be formed; and it is during these very

  periods of subsidence, that our great deposits rich in fossils have been

  accumulated. Nature may almost be said to have guarded against the

  frequent discovery of her transitional or linking forms.

  From the foregoing considerations it cannot be doubted that the geological

  record, viewed as a whole, is extremely imperfect; but if we confine our

  attention to any one formation, it becomes more difficult to understand,

  why we do not therein find closely graduated varieties between the allied

  species which lived at its commencement and at its close. Some cases are

  on record of the same species presenting distinct varieties in the upper<
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  and lower parts of the same formation, but, as they are rare, they may be

  here passed over. Although each formation has indisputably required a vast

  number of years for its deposition, I can see several reasons why each

  should not include a graduated series of links between the species which

  then lived; but I can by no means pretend to assign due proportional weight

  to the following considerations.

  Although each formation may mark a very long lapse of years, each perhaps

  is short compared with the period requisite to change one species into

  another. I am aware that two palaeontologists, whose opinions are worthy

  of much deference, namely Bronn and Woodward, have concluded that the

  average duration of each formation is twice or thrice as long as the

  average duration of specific forms. But insuperable difficulties, as it

  seems to me, prevent us coming to any just conclusion on this head. When

  we see a species first appearing in the middle of any formation, it would

  be rash in the extreme to infer that it had not elsewhere previously

  existed. So again when we find a species disappearing before the uppermost

  layers have been deposited, it would be equally rash to suppose that it

  then became wholly extinct. We forget how small the area of Europe is

  compared with the rest of the world; nor have the several stages of the

  same formation throughout Europe been correlated with perfect accuracy.

  With marine animals of all kinds, we may safely infer a large amount of

  migration during climatal and other changes; and when we see a species

  first appearing in any formation, the probability is that it only then

  first immigrated into that area. It is well known, for instance, that

  several species appeared somewhat earlier in the palaeozoic beds of North

  America than in those of Europe; time having apparently been required for

  their migration from the American to the European seas. In examining the

  latest deposits of various quarters of the world, it has everywhere been

  noted, that some few still existing species are common in the deposit, but

  have become extinct in the immediately surrounding sea; or, conversely,

  that some are now abundant in the neighbouring sea, but are rare or absent

  in this particular deposit. It is an excellent lesson to reflect on the

  ascertained amount of migration of the inhabitants of Europe during the

  Glacial period, which forms only a part of one whole geological period; and

  likewise to reflect on the great changes of level, on the inordinately

  great change of climate, on the prodigious lapse of time, all included

  within this same glacial period. Yet it may be doubted whether in any

  quarter of the world, sedimentary deposits, including fossil remains, have

  gone on accumulating within the same area during the whole of this period.

  It is not, for instance, probable that sediment was deposited during the

  whole of the glacial period near the mouth of the Mississippi, within that

  limit of depth at which marine animals can flourish; for we know what vast

  geographical changes occurred in other parts of America during this space

  of time. When such beds as were deposited in shallow water near the mouth

  of the Mississippi during some part of the glacial period shall have been

  upraised, organic remains will probably first appear and disappear at

  different levels, owing to the migration of species and to geographical

  changes. And in the distant future, a geologist examining these beds,

  might be tempted to conclude that the average duration of life of the

  embedded fossils had been less than that of the glacial period, instead of

  having been really far greater, that is extending from before the glacial

  epoch to the present day.

  In order to get a perfect gradation between two forms in the upper and

  lower parts of the same formation, the deposit must have gone on

  accumulating for a very long period, in order to have given sufficient time

 

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