The Origin of Species

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The Origin of Species Page 35

by Charles Darwin


  This great fact of the parallel succession of the forms of life throughout

  the world, is explicable on the theory of natural selection. New species

  are formed by new varieties arising, which have some advantage over older

  forms; and those forms, which are already dominant, or have some advantage

  over the other forms in their own country, would naturally oftenest give

  rise to new varieties or incipient species; for these latter must be

  victorious in a still higher degree in order to be preserved and to

  survive. We have distinct evidence on this head, in the plants which are

  dominant, that is, which are commonest in their own homes, and are most

  widely diffused, having produced the greatest number of new varieties. It

  is also natural that the dominant, varying, and far-spreading species,

  which already have invaded to a certain extent the territories of other

  species, should be those which would have the best chance of spreading

  still further, and of giving rise in new countries to new varieties and

  species. The process of diffusion may often be very slow, being dependent

  on climatal and geographical changes, or on strange accidents, but in the

  long run the dominant forms will generally succeed in spreading. The

  diffusion would, it is probable, be slower with the terrestrial inhabitants

  of distinct continents than with the marine inhabitants of the continuous

  sea. We might therefore expect to find, as we apparently do find, a less

  strict degree of parallel succession in the productions of the land than of

  the sea.

  Dominant species spreading from any region might encounter still more

  dominant species, and then their triumphant course, or even their

  existence, would cease. We know not at all precisely what are all the

  conditions most favourable for the multiplication of new and dominant

  species; but we can, I think, clearly see that a number of individuals,

  from giving a better chance of the appearance of favourable variations, and

  that severe competition with many already existing forms, would be highly

  favourable, as would be the power of spreading into new territories. A

  certain amount of isolation, recurring at long intervals of time, would

  probably be also favourable, as before explained. One quarter of the world

  may have been most favourable for the production of new and dominant

  species on the land, and another for those in the waters of the sea. If

  two great regions had been for a long period favourably circumstanced in an

  equal degree, whenever their inhabitants met, the battle would be prolonged

  and severe; and some from one birthplace and some from the other might be

  victorious. But in the course of time, the forms dominant in the highest

  degree, wherever produced, would tend everywhere to prevail. As they

  prevailed, they would cause the extinction of other and inferior forms; and

  as these inferior forms would be allied in groups by inheritance, whole

  groups would tend slowly to disappear; though here and there a single

  member might long be enabled to survive.

  Thus, as it seems to me, the parallel, and, taken in a large sense,

  simultaneous, succession of the same forms of life throughout the world,

  accords well with the principle of new species having been formed by

  dominant species spreading widely and varying; the new species thus

  produced being themselves dominant owing to inheritance, and to having

  already had some advantage over their parents or over other species; these

  again spreading, varying, and producing new species. The forms which are

  beaten and which yield their places to the new and victorious forms, will

  generally be allied in groups, from inheriting some inferiority in common;

  and therefore as new and improved groups spread throughout the world, old

  groups will disappear from the world; and the succession of forms in both

  ways will everywhere tend to correspond.

  There is one other remark connected with this subject worth making. I have

  given my reasons for believing that all our greater fossiliferous

  formations were deposited during periods of subsidence; and that blank

  intervals of vast duration occurred during the periods when the bed of the

  sea was either stationary or rising, and likewise when sediment was not

  thrown down quickly enough to embed and preserve organic remains. During

  these long and blank intervals I suppose that the inhabitants of each

  region underwent a considerable amount of modification and extinction, and

  that there was much migration from other parts of the world. As we have

  reason to believe that large areas are affected by the same movement, it is

  probable that strictly contemporaneous formations have often been

  accumulated over very wide spaces in the same quarter of the world; but we

  are far from having any right to conclude that this has invariably been the

  case, and that large areas have invariably been affected by the same

  movements. When two formations have been deposited in two regions during

  nearly, but not exactly the same period, we should find in both, from the

  causes explained in the foregoing paragraphs, the same general succession

  in the forms of life; but the species would not exactly correspond; for

  there will have been a little more time in the one region than in the other

  for modification, extinction, and immigration.

  I suspect that cases of this nature have occurred in Europe. Mr.

  Prestwich, in his admirable Memoirs on the eocene deposits of England and

  France, is able to draw a close general parallelism between the successive

  stages in the two countries; but when he compares certain stages in England

  with those in France, although he finds in both a curious accordance in the

  numbers of the species belonging to the same genera, yet the species

  themselves differ in a manner very difficult to account for, considering

  the proximity of the two areas,--unless, indeed, it be assumed that an

  isthmus separated two seas inhabited by distinct, but contemporaneous,

  faunas. Lyell has made similar observations on some of the later tertiary

  formations. Barrande, also, shows that there is a striking general

  parallelism in the successive Silurian deposits of Bohemia and Scandinavia;

  nevertheless he finds a surprising amount of difference in the species. If

  the several formations in these regions have not been deposited during the

  same exact periods,--a formation in one region often corresponding with a

  blank interval in the other,--and if in both regions the species have gone

  on slowly changing during the accumulation of the several formations and

  during the long intervals of time between them; in this case, the several

  formations in the two regions could be arranged in the same order, in

  accordance with the general succession of the form of life, and the order

  would falsely appear to be strictly parallel; nevertheless the species

  would not all be the same in the apparently corresponding stages in the two

  regions.

  On the Affinities of extinct Species to each other, and to living forms. --

  Let us now look to the mutual affinities of extinct and living species.

  They all fall
into one grand natural system; and this fact is at once

  explained on the principle of descent. The more ancient any form is, the

  more, as a general rule, it differs from living forms. But, as Buckland

  long ago remarked, all fossils can be classed either in still existing

  groups, or between them. That the extinct forms of life help to fill up

  the wide intervals between existing genera, families, and orders, cannot be

  disputed. For if we confine our attention either to the living or to the

  extinct alone, the series is far less perfect than if we combine both into

  one general system. With respect to the Vertebrata, whole pages could be

  filled with striking illustrations from our great palaeontologist, Owen,

  showing how extinct animals fall in between existing groups. Cuvier ranked

  the Ruminants and Pachyderms, as the two most distinct orders of mammals;

  but Owen has discovered so many fossil links, that he has had to alter the

  whole classification of these two orders; and has placed certain pachyderms

  in the same sub-order with ruminants: for example, he dissolves by fine

  gradations the apparently wide difference between the pig and the camel.

  In regard to the Invertebrata, Barrande, and a higher authority could not

  be named, asserts that he is every day taught that palaeozoic animals,

  though belonging to the same orders, families, or genera with those living

  at the present day, were not at this early epoch limited in such distinct

  groups as they now are.

  Some writers have objected to any extinct species or group of species being

  considered as intermediate between living species or groups. If by this

  term it is meant that an extinct form is directly intermediate in all its

  characters between two living forms, the objection is probably valid. But

  I apprehend that in a perfectly natural classification many fossil species

  would have to stand between living species, and some extinct genera between

  living genera, even between genera belonging to distinct families. The

  most common case, especially with respect to very distinct groups, such as

  fish and reptiles, seems to be, that supposing them to be distinguished at

  the present day from each other by a dozen characters, the ancient members

  of the same two groups would be distinguished by a somewhat lesser number

  of characters, so that the two groups, though formerly quite distinct, at

  that period made some small approach to each other.

  It is a common belief that the more ancient a form is, by so much the more

  it tends to connect by some of its characters groups now widely separated

  from each other. This remark no doubt must be restricted to those groups

  which have undergone much change in the course of geological ages; and it

  would be difficult to prove the truth of the proposition, for every now and

  then even a living animal, as the Lepidosiren, is discovered having

  affinities directed towards very distinct groups. Yet if we compare the

  older Reptiles and Batrachians, the older Fish, the older Cephalopods, and

  the eocene Mammals, with the more recent members of the same classes, we

  must admit that there is some truth in the remark.

  Let us see how far these several facts and inferences accord with the

  theory of descent with modification. As the subject is somewhat complex, I

  must request the reader to turn to the diagram in the fourth chapter. We

  may suppose that the numbered letters represent genera, and the dotted

  lines diverging from them the species in each genus. The diagram is much

  too simple, too few genera and too few species being given, but this is

  unimportant for us. The horizontal lines may represent successive

  geological formations, and all the forms beneath the uppermost line may be

  considered as extinct. The three existing genera, a14, q14, p14, will form

  a small family; b14 and f14 a closely allied family or sub-family; and o14,

  e14, m14, a third family. These three families, together with the many

  extinct genera on the several lines of descent diverging from the

  parent-form A, will form an order; for all will have inherited something in

  common from their ancient and common progenitor. On the principle of the

  continued tendency to divergence of character, which was formerly

  illustrated by this diagram, the more recent any form is, the more it will

  generally differ from its ancient progenitor. Hence we can understand the

  rule that the most ancient fossils differ most from existing forms. We

  must not, however, assume that divergence of character is a necessary

  contingency; it depends solely on the descendants from a species being thus

  enabled to seize on many and different places in the economy of nature.

  Therefore it is quite possible, as we have seen in the case of some

  Silurian forms, that a species might go on being slightly modified in

  relation to its slightly altered conditions of life, and yet retain

  throughout a vast period the same general characteristics. This is

  represented in the diagram by the letter F14.

  All the many forms, extinct and recent, descended from A, make, as before

  remarked, one order; and this order, from the continued effects of

  extinction and divergence of character, has become divided into several

  sub-families and families, some of which are supposed to have perished at

  different periods, and some to have endured to the present day.

  By looking at the diagram we can see that if many of the extinct forms,

  supposed to be embedded in the successive formations, were discovered at

  several points low down in the series, the three existing families on the

  uppermost line would be rendered less distinct from each other. If, for

  instance, the genera a1, a5, a10, f8, m3, m6, m9 were disinterred, these

  three families would be so closely linked together that they probably would

  have to be united into one great family, in nearly the same manner as has

  occurred with ruminants and pachyderms. Yet he who objected to call the

  extinct genera, which thus linked the living genera of three families

  together, intermediate in character, would be justified, as they are

  intermediate, not directly, but only by a long and circuitous course

  through many widely different forms. If many extinct forms were to be

  discovered above one of the middle horizontal lines or geological

  formations--for instance, above No. VI.--but none from beneath this line,

  then only the two families on the left hand (namely, a14, &c., and b14,

  &c.) would have to be united into one family; and the two other families

  (namely, a14 to f14 now including five genera, and o14 to m14) would yet

  remain distinct. These two families, however, would be less distinct from

  each other than they were before the discovery of the fossils. If, for

  instance, we suppose the existing genera of the two families to differ from

  each other by a dozen characters, in this case the genera, at the early

  period marked VI., would differ by a lesser number of characters; for at

  this early stage of descent they have not diverged in character from the

  common progenitor of the order, nearly so much as they subsequently

  diverged. Thus it comes that ancient and ex
tinct genera are often in some

  slight degree intermediate in character between their modified descendants,

  or between their collateral relations.

  In nature the case will be far more complicated than is represented in the

  diagram; for the groups will have been more numerous, they will have

  endured for extremely unequal lengths of time, and will have been modified

  in various degrees. As we possess only the last volume of the geological

  record, and that in a very broken condition, we have no right to expect,

  except in very rare cases, to fill up wide intervals in the natural system,

  and thus unite distinct families or orders. All that we have a right to

  expect, is that those groups, which have within known geological periods

  undergone much modification, should in the older formations make some

  slight approach to each other; so that the older members should differ less

  from each other in some of their characters than do the existing members of

  the same groups; and this by the concurrent evidence of our best

  palaeontologists seems frequently to be the case.

  Thus, on the theory of descent with modification, the main facts with

  respect to the mutual affinities of the extinct forms of life to each other

  and to living forms, seem to me explained in a satisfactory manner. And

  they are wholly inexplicable on any other view.

  On this same theory, it is evident that the fauna of any great period in

  the earth's history will be intermediate in general character between that

  which preceded and that which succeeded it. Thus, the species which lived

  at the sixth great stage of descent in the diagram are the modified

  offspring of those which lived at the fifth stage, and are the parents of

  those which became still more modified at the seventh stage; hence they

  could hardly fail to be nearly intermediate in character between the forms

  of life above and below. We must, however, allow for the entire extinction

  of some preceding forms, and for the coming in of quite new forms by

  immigration, and for a large amount of modification, during the long and

  blank intervals between the successive formations. Subject to these

  allowances, the fauna of each geological period undoubtedly is intermediate

  in character, between the preceding and succeeding faunas. I need give

  only one instance, namely, the manner in which the fossils of the Devonian

  system, when this system was first discovered, were at once recognised by

  palaeontologists as intermediate in character between those of the

  overlying carboniferous, and underlying Silurian system. But each fauna is

  not necessarily exactly intermediate, as unequal intervals of time have

  elapsed between consecutive formations.

  It is no real objection to the truth of the statement, that the fauna of

  each period as a whole is nearly intermediate in character between the

  preceding and succeeding faunas, that certain genera offer exceptions to

  the rule. For instance, mastodons and elephants, when arranged by Dr.

  Falconer in two series, first according to their mutual affinities and then

  according to their periods of existence, do not accord in arrangement. The

  species extreme in character are not the oldest, or the most recent; nor

  are those which are intermediate in character, intermediate in age. But

  supposing for an instant, in this and other such cases, that the record of

  the first appearance and disappearance of the species was perfect, we have

  no reason to believe that forms successively produced necessarily endure

  for corresponding lengths of time: a very ancient form might occasionally

  last much longer than a form elsewhere subsequently produced, especially in

  the case of terrestrial productions inhabiting separated districts. To

  compare small things with great: if the principal living and extinct races

  of the domestic pigeon were arranged as well as they could be in serial

  affinity, this arrangement would not closely accord with the order in time

 

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