The Origin of Species

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by Charles Darwin


  We may often falsely attribute to correlation of growth, structures which

  are common to whole groups of species, and which in truth are simply due to

  inheritance; for an ancient progenitor may have acquired through natural

  selection some one modification in structure, and, after thousands of

  generations, some other and independent modification; and these two

  modifications, having been transmitted to a whole group of descendants with

  diverse habits, would naturally be thought to be correlated in some

  necessary manner. So, again, I do not doubt that some apparent

  correlations, occurring throughout whole orders, are entirely due to the

  manner alone in which natural selection can act. For instance, Alph. De

  Candolle has remarked that winged seeds are never found in fruits which do

  not open: I should explain the rule by the fact that seeds could not

  gradually become winged through natural selection, except in fruits which

  opened; so that the individual plants producing seeds which were a little

  better fitted to be wafted further, might get an advantage over those

  producing seed less fitted for dispersal; and this process could not

  possibly go on in fruit which did not open.

  The elder Geoffroy and Goethe propounded, at about the same period, their

  law of compensation or balancement of growth; or, as Goethe expressed it,

  'in order to spend on one side, nature is forced to economise on the other

  side.' I think this holds true to a certain extent with our domestic

  productions: if nourishment flows to one part or organ in excess, it

  rarely flows, at least in excess, to another part; thus it is difficult to

  get a cow to give much milk and to fatten readily. The same varieties of

  the cabbage do not yield abundant and nutritious foliage and a copious

  supply of oil-bearing seeds. When the seeds in our fruits become

  atrophied, the fruit itself gains largely in size and quality. In our

  poultry, a large tuft of feathers on the head is generally accompanied by a

  diminished comb, and a large beard by diminished wattles. With species in

  a state of nature it can hardly be maintained that the law is of universal

  application; but many good observers, more especially botanists, believe in

  its truth. I will not, however, here give any instances, for I see hardly

  any way of distinguishing between the effects, on the one hand, of a part

  being largely developed through natural selection and another and adjoining

  part being reduced by this same process or by disuse, and, on the other

  hand, the actual withdrawal of nutriment from one part owing to the excess

  of growth in another and adjoining part.

  I suspect, also, that some of the cases of compensation which have been

  advanced, and likewise some other facts, may be merged under a more general

  principle, namely, that natural selection is continually trying to

  economise in every part of the organisation. If under changed conditions

  of life a structure before useful becomes less useful, any diminution,

  however slight, in its development, will be seized on by natural selection,

  for it will profit the individual not to have its nutriment wasted in

  building up an useless structure. I can thus only understand a fact with

  which I was much struck when examining cirripedes, and of which many other

  instances could be given: namely, that when a cirripede is parasitic

  within another and is thus protected, it loses more or less completely its

  own shell or carapace. This is the case with the male Ibla, and in a truly

  extraordinary manner with the Proteolepas: for the carapace in all other

  cirripedes consists of the three highly-important anterior segments of the

  head enormously developed, and furnished with great nerves and muscles; but

  in the parasitic and protected Proteolepas, the whole anterior part of the

  head is reduced to the merest rudiment attached to the bases of the

  prehensile antennae. Now the saving of a large and complex structure, when

  rendered superfluous by the parasitic habits of the Proteolepas, though

  effected by slow steps, would be a decided advantage to each successive

  individual of the species; for in the struggle for life to which every

  animal is exposed, each individual Proteolepas would have a better chance

  of supporting itself, by less nutriment being wasted in developing a

  structure now become useless.

  Thus, as I believe, natural selection will always succeed in the long run

  in reducing and saving every part of the organisation, as soon as it is

  rendered superfluous, without by any means causing some other part to be

  largely developed in a corresponding degree. And, conversely, that natural

  selection may perfectly well succeed in largely developing any organ,

  without requiring as a necessary compensation the reduction of some

  adjoining part.

  It seems to be a rule, as remarked by Is. Geoffroy St. Hilaire, both in

  varieties and in species, that when any part or organ is repeated many

  times in the structure of the same individual (as the vertebrae in snakes,

  and the stamens in polyandrous flowers) the number is variable; whereas the

  number of the same part or organ, when it occurs in lesser numbers, is

  constant. The same author and some botanists have further remarked that

  multiple parts are also very liable to variation in structure. Inasmuch as

  this 'vegetative repetition,' to use Prof. Owen's expression, seems to be a

  sign of low organisation; the foregoing remark seems connected with the

  very general opinion of naturalists, that beings low in the scale of nature

  are more variable than those which are higher. I presume that lowness in

  this case means that the several parts of the organisation have been but

  little specialised for particular functions; and as long as the same part

  has to perform diversified work, we can perhaps see why it should remain

  variable, that is, why natural selection should have preserved or rejected

  each little deviation of form less carefully than when the part has to

  serve for one special purpose alone. In the same way that a knife which

  has to cut all sorts of things may be of almost any shape; whilst a tool

  for some particular object had better be of some particular shape. Natural

  selection, it should never be forgotten, can act on each part of each

  being, solely through and for its advantage.

  Rudimentary parts, it has been stated by some authors, and I believe with

  truth, are apt to be highly variable. We shall have to recur to the

  general subject of rudimentary and aborted organs; and I will here only add

  that their variability seems to be owing to their uselessness, and

  therefore to natural selection having no power to check deviations in their

  structure. Thus rudimentary parts are left to the free play of the various

  laws of growth, to the effects of long-continued disuse, and to the

  tendency to reversion.

  A part developed in any species in an extraordinary degree or manner, in

  comparison with the same part in allied species, tends to be highly

  variable. -- Several years ago I was much struck with a remark, nearly to

  the above effect, published by Mr. Wat
erhouse. I infer also from an

  observation made by Professor Owen, with respect to the length of the arms

  of the ourang-outang, that he has come to a nearly similar conclusion. It

  is hopeless to attempt to convince any one of the truth of this proposition

  without giving the long array of facts which I have collected, and which

  cannot possibly be here introduced. I can only state my conviction that it

  is a rule of high generality. I am aware of several causes of error, but I

  hope that I have made due allowance for them. It should be understood that

  the rule by no means applies to any part, however unusually developed,

  unless it be unusually developed in comparison with the same part in

  closely allied species. Thus, the bat's wing is a most abnormal structure

  in the class mammalia; but the rule would not here apply, because there is

  a whole group of bats having wings; it would apply only if some one species

  of bat had its wings developed in some remarkable manner in comparison with

  the other species of the same genus. The rule applies very strongly in the

  case of secondary sexual characters, when displayed in any unusual manner.

  The term, secondary sexual characters, used by Hunter, applies to

  characters which are attached to one sex, but are not directly connected

  with the act of reproduction. The rule applies to males and females; but

  as females more rarely offer remarkable secondary sexual characters, it

  applies more rarely to them. The rule being so plainly applicable in the

  case of secondary sexual characters, may be due to the great variability of

  these characters, whether or not displayed in any unusual manner--of which

  fact I think there can be little doubt. But that our rule is not confined

  to secondary sexual characters is clearly shown in the case of

  hermaphrodite cirripedes; and I may here add, that I particularly attended

  to Mr. Waterhouse's remark, whilst investigating this Order, and I am fully

  convinced that the rule almost invariably holds good with cirripedes. I

  shall, in my future work, give a list of the more remarkable cases; I will

  here only briefly give one, as it illustrates the rule in its largest

  application. The opercular valves of sessile cirripedes (rock barnacles)

  are, in every sense of the word, very important structures, and they differ

  extremely little even in different genera; but in the several species of

  one genus, Pyrgoma, these valves present a marvellous amount of

  diversification: the homologous valves in the different species being

  sometimes wholly unlike in shape; and the amount of variation in the

  individuals of several of the species is so great, that it is no

  exaggeration to state that the varieties differ more from each other in the

  characters of these important valves than do other species of distinct

  genera.

  As birds within the same country vary in a remarkably small degree, I have

  particularly attended to them, and the rule seems to me certainly to hold

  good in this class. I cannot make out that it applies to plants, and this

  would seriously have shaken my belief in its truth, had not the great

  variability in plants made it particularly difficult to compare their

  relative degrees of variability.

  When we see any part or organ developed in a remarkable degree or manner in

  any species, the fair presumption is that it is of high importance to that

  species; nevertheless the part in this case is eminently liable to

  variation. Why should this be so? On the view that each species has been

  independently created, with all its parts as we now see them, I can see no

  explanation. But on the view that groups of species have descended from

  other species, and have been modified through natural selection, I think we

  can obtain some light. In our domestic animals, if any part, or the whole

  animal, be neglected and no selection be applied, that part (for instance,

  the comb in the Dorking fowl) or the whole breed will cease to have a

  nearly uniform character. The breed will then be said to have degenerated.

  In rudimentary organs, and in those which have been but little specialised

  for any particular purpose, and perhaps in polymorphic groups, we see a

  nearly parallel natural case; for in such cases natural selection either

  has not or cannot come into full play, and thus the organisation is left in

  a fluctuating condition. But what here more especially concerns us is,

  that in our domestic animals those points, which at the present time are

  undergoing rapid change by continued selection, are also eminently liable

  to variation. Look at the breeds of the pigeon; see what a prodigious

  amount of difference there is in the beak of the different tumblers, in the

  beak and wattle of the different carriers, in the carriage and tail of our

  fantails, &c., these being the points now mainly attended to by English

  fanciers. Even in the sub-breeds, as in the short-faced tumbler, it is

  notoriously difficult to breed them nearly to perfection, and frequently

  individuals are born which depart widely from the standard. There may be

  truly said to be a constant struggle going on between, on the one hand, the

  tendency to reversion to a less modified state, as well as an innate

  tendency to further variability of all kinds, and, on the other hand, the

  power of steady selection to keep the breed true. In the long run

  selection gains the day, and we do not expect to fail so far as to breed a

  bird as coarse as a common tumbler from a good short-faced strain. But as

  long as selection is rapidly going on, there may always be expected to be

  much variability in the structure undergoing modification. It further

  deserves notice that these variable characters, produced by man's

  selection, sometimes become attached, from causes quite unknown to us, more

  to one sex than to the other, generally to the male sex, as with the wattle

  of carriers and the enlarged crop of pouters.

  Now let us turn to nature. When a part has been developed in an

  extraordinary manner in any one species, compared with the other species of

  the same genus, we may conclude that this part has undergone an

  extraordinary amount of modification, since the period when the species

  branched off from the common progenitor of the genus. This period will

  seldom be remote in any extreme degree, as species very rarely endure for

  more than one geological period. An extraordinary amount of modification

  implies an unusually large and long-continued amount of variability, which

  has continually been accumulated by natural selection for the benefit of

  the species. But as the variability of the extraordinarily-developed part

  or organ has been so great and long-continued within a period not

  excessively remote, we might, as a general rule, expect still to find more

  variability in such parts than in other parts of the organisation, which

  have remained for a much longer period nearly constant. And this, I am

  convinced, is the case. That the struggle between natural selection on the

  one hand, and the tendency to reversion and variability on the other hand,

  will in the course of time cease; and that the most abnormally
developed

  organs may be made constant, I can see no reason to doubt. Hence when an

  organ, however abnormal it may be, has been transmitted in approximately

  the same condition to many modified descendants, as in the case of the wing

  of the bat, it must have existed, according to my theory, for an immense

  period in nearly the same state; and thus it comes to be no more variable

  than any other structure. It is only in those cases in which the

  modification has been comparatively recent and extraordinarily great that

  we ought to find the generative variability, as it may be called, still

  present in a high degree. For in this case the variability will seldom as

  yet have been fixed by the continued selection of the individuals varying

  in the required manner and degree, and by the continued rejection of those

  tending to revert to a former and less modified condition.

  The principle included in these remarks may be extended. It is notorious

  that specific characters are more variable than generic. To explain by a

  simple example what is meant. If some species in a large genus of plants

  had blue flowers and some had red, the colour would be only a specific

  character, and no one would be surprised at one of the blue species varying

  into red, or conversely; but if all the species had blue flowers, the

  colour would become a generic character, and its variation would be a more

  unusual circumstance. I have chosen this example because an explanation is

  not in this case applicable, which most naturalists would advance, namely,

  that specific characters are more variable than generic, because they are

  taken from parts of less physiological importance than those commonly used

  for classing genera. I believe this explanation is partly, yet only

  indirectly, true; I shall, however, have to return to this subject in our

  chapter on Classification. It would be almost superfluous to adduce

  evidence in support of the above statement, that specific characters are

  more variable than generic; but I have repeatedly noticed in works on

  natural history, that when an author has remarked with surprise that some

  important organ or part, which is generally very constant throughout large

  groups of species, has differed considerably in closely-allied species,

  that it has, also, been variable in the individuals of some of the species.

  And this fact shows that a character, which is generally of generic value,

  when it sinks in value and becomes only of specific value, often becomes

  variable, though its physiological importance may remain the same.

  Something of the same kind applies to monstrosities: at least Is. Geoffroy

  St. Hilaire seems to entertain no doubt, that the more an organ normally

  differs in the different species of the same group, the more subject it is

  to individual anomalies.

  On the ordinary view of each species having been independently created, why

  should that part of the structure, which differs from the same part in

  other independently-created species of the same genus, be more variable

  than those parts which are closely alike in the several species? I do not

  see that any explanation can be given. But on the view of species being

  only strongly marked and fixed varieties, we might surely expect to find

  them still often continuing to vary in those parts of their structure which

  have varied within a moderately recent period, and which have thus come to

  differ. Or to state the case in another manner:--the points in which all

  the species of a genus resemble each other, and in which they differ from

  the species of some other genus, are called generic characters; and these

  characters in common I attribute to inheritance from a common progenitor,

  for it can rarely have happened that natural selection will have modified

  several species, fitted to more or less widely-different habits, in exactly

  the same manner: and as these so-called generic characters have been

 

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