The Origin of Species

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The Origin of Species Page 45

by Charles Darwin

which were allied in the greatest number of points. In tumbler pigeons,

  though some sub-varieties differ from the others in the important character

  of having a longer beak, yet all are kept together from having the common

  habit of tumbling; but the short-faced breed has nearly or quite lost this

  habit; nevertheless, without any reasoning or thinking on the subject,

  these tumblers are kept in the same group, because allied in blood and

  alike in some other respects. If it could be proved that the Hottentot had

  descended from the Negro, I think he would be classed under the Negro

  group, however much he might differ in colour and other important

  characters from negroes.

  With species in a state of nature, every naturalist has in fact brought

  descent into his classification; for he includes in his lowest grade, or

  that of a species, the two sexes; and how enormously these sometimes differ

  in the most important characters, is known to every naturalist: scarcely a

  single fact can be predicated in common of the males and hermaphrodites of

  certain cirripedes, when adult, and yet no one dreams of separating them.

  The naturalist includes as one species the several larval stages of the

  same individual, however much they may differ from each other and from the

  adult; as he likewise includes the so-called alternate generations of

  Steenstrup, which can only in a technical sense be considered as the same

  individual. He includes monsters; he includes varieties, not solely

  because they closely resemble the parent-form, but because they are

  descended from it. He who believes that the cowslip is descended from the

  primrose, or conversely, ranks them together as a single species, and gives

  a single definition. As soon as three Orchidean forms (Monochanthus,

  Myanthus, and Catasetum), which had previously been ranked as three

  distinct genera, were known to be sometimes produced on the same spike,

  they were immediately included as a single species. But it may be asked,

  what ought we to do, if it could be proved that one species of kangaroo had

  been produced, by a long course of modification, from a bear? Ought we to

  rank this one species with bears, and what should we do with the other

  species? The supposition is of course preposterous; and I might answer by

  the argumentum ad hominem, and ask what should be done if a perfect

  kangaroo were seen to come out of the womb of a bear? According to all

  analogy, it would be ranked with bears; but then assuredly all the other

  species of the kangaroo family would have to be classed under the bear

  genus. The whole case is preposterous; for where there has been close

  descent in common, there will certainly be close resemblance or affinity.

  As descent has universally been used in classing together the individuals

  of the same species, though the males and females and larvae are sometimes

  extremely different; and as it has been used in classing varieties which

  have undergone a certain, and sometimes a considerable amount of

  modification, may not this same element of descent have been unconsciously

  used in grouping species under genera, and genera under higher groups,

  though in these cases the modification has been greater in degree, and has

  taken a longer time to complete? I believe it has thus been unconsciously

  used; and only thus can I understand the several rules and guides which

  have been followed by our best systematists. We have no written pedigrees;

  we have to make out community of descent by resemblances of any kind.

  Therefore we choose those characters which, as far as we can judge, are the

  least likely to have been modified in relation to the conditions of life to

  which each species has been recently exposed. Rudimentary structures on

  this view are as good as, or even sometimes better than, other parts of the

  organisation. We care not how trifling a character may be--let it be the

  mere inflection of the angle of the jaw, the manner in which an insect's

  wing is folded, whether the skin be covered by hair or feathers--if it

  prevail throughout many and different species, especially those having very

  different habits of life, it assumes high value; for we can account for its

  presence in so many forms with such different habits, only by its

  inheritance from a common parent. We may err in this respect in regard to

  single points of structure, but when several characters, let them be ever

  so trifling, occur together throughout a large group of beings having

  different habits, we may feel almost sure, on the theory of descent, that

  these characters have been inherited from a common ancestor. And we know

  that such correlated or aggregated characters have especial value in

  classification.

  We can understand why a species or a group of species may depart, in

  several of its most important characteristics, from its allies, and yet be

  safely classed with them. This may be safely done, and is often done, as

  long as a sufficient number of characters, let them be ever so unimportant,

  betrays the hidden bond of community of descent. Let two forms have not a

  single character in common, yet if these extreme forms are connected

  together by a chain of intermediate groups, we may at once infer their

  community of descent, and we put them all into the same class. As we find

  organs of high physiological importance--those which serve to preserve life

  under the most diverse conditions of existence--are generally the most

  constant, we attach especial value to them; but if these same organs, in

  another group or section of a group, are found to differ much, we at once

  value them less in our classification. We shall hereafter, I think,

  clearly see why embryological characters are of such high classificatory

  importance. Geographical distribution may sometimes be brought usefully

  into play in classing large and widely-distributed genera, because all the

  species of the same genus, inhabiting any distinct and isolated region,

  have in all probability descended from the same parents.

  We can understand, on these views, the very important distinction between

  real affinities and analogical or adaptive resemblances. Lamarck first

  called attention to this distinction, and he has been ably followed by

  Macleay and others. The resemblance, in the shape of the body and in the

  fin-like anterior limbs, between the dugong, which is a pachydermatous

  animal, and the whale, and between both these mammals and fishes, is

  analogical. Amongst insects there are innumerable instances: thus

  Linnaeus, misled by external appearances, actually classed an homopterous

  insect as a moth. We see something of the same kind even in our domestic

  varieties, as in the thickened stems of the common and swedish turnip. The

  resemblance of the greyhound and racehorse is hardly more fanciful than the

  analogies which have been drawn by some authors between very distinct

  animals. On my view of characters being of real importance for

  classification, only in so far as they reveal descent, we can clearly

  understand why analogical or adaptive character, although of the utmost

  importance to the welfare of the being, are
almost valueless to the

  systematist. For animals, belonging to two most distinct lines of descent,

  may readily become adapted to similar conditions, and thus assume a close

  external resemblance; but such resemblances will not reveal--will rather

  tend to conceal their blood-relationship to their proper lines of descent.

  We can also understand the apparent paradox, that the very same characters

  are analogical when one class or order is compared with another, but give

  true affinities when the members of the same class or order are compared

  one with another: thus the shape of the body and fin-like limbs are only

  analogical when whales are compared with fishes, being adaptations in both

  classes for swimming through the water; but the shape of the body and

  fin-like limbs serve as characters exhibiting true affinity between the

  several members of the whale family; for these cetaceans agree in so many

  characters, great and small, that we cannot doubt that they have inherited

  their general shape of body and structure of limbs from a common ancestor.

  So it is with fishes.

  As members of distinct classes have often been adapted by successive slight

  modifications to live under nearly similar circumstances,--to inhabit for

  instance the three elements of land, air, and water,--we can perhaps

  understand how it is that a numerical parallelism has sometimes been

  observed between the sub-groups in distinct classes. A naturalist, struck

  by a parallelism of this nature in any one class, by arbitrarily raising or

  sinking the value of the groups in other classes (and all our experience

  shows that this valuation has hitherto been arbitrary), could easily extend

  the parallelism over a wide range; and thus the septenary, quinary,

  quaternary, and ternary classifications have probably arisen.

  As the modified descendants of dominant species, belonging to the larger

  genera, tend to inherit the advantages, which made the groups to which they

  belong large and their parents dominant, they are almost sure to spread

  widely, and to seize on more and more places in the economy of nature. The

  larger and more dominant groups thus tend to go on increasing in size; and

  they consequently supplant many smaller and feebler groups. Thus we can

  account for the fact that all organisms, recent and extinct, are included

  under a few great orders, under still fewer classes, and all in one great

  natural system. As showing how few the higher groups are in number, and

  how widely spread they are throughout the world, the fact is striking, that

  the discovery of Australia has not added a single insect belonging to a new

  order; and that in the vegetable kingdom, as I learn from Dr. Hooker, it

  has added only two or three orders of small size.

  In the chapter on geological succession I attempted to show, on the

  principle of each group having generally diverged much in character during

  the long-continued process of modification, how it is that the more ancient

  forms of life often present characters in some slight degree intermediate

  between existing groups. A few old and intermediate parent-forms having

  occasionally transmitted to the present day descendants but little

  modified, will give to us our so-called osculant or aberrant groups. The

  more aberrant any form is, the greater must be the number of connecting

  forms which on my theory have been exterminated and utterly lost. And we

  have some evidence of aberrant forms having suffered severely from

  extinction, for they are generally represented by extremely few species;

  and such species as do occur are generally very distinct from each other,

  which again implies extinction. The genera Ornithorhynchus and

  Lepidosiren, for example, would not have been less aberrant had each been

  represented by a dozen species instead of by a single one; but such

  richness in species, as I find after some investigation, does not commonly

  fall to the lot of aberrant genera. We can, I think, account for this fact

  only by looking at aberrant forms as failing groups conquered by more

  successful competitors, with a few members preserved by some unusual

  coincidence of favourable circumstances.

  Mr. Waterhouse has remarked that, when a member belonging to one group of

  animals exhibits an affinity to a quite distinct group, this affinity in

  most cases is general and not special: thus, according to Mr. Waterhouse,

  of all Rodents, the bizcacha is most nearly related to Marsupials; but in

  the points in which it approaches this order, its relations are general,

  and not to any one marsupial species more than to another. As the points

  of affinity of the bizcacha to Marsupials are believed to be real and not

  merely adaptive, they are due on my theory to inheritance in common.

  Therefore we must suppose either that all Rodents, including the bizcacha,

  branched off from some very ancient Marsupial, which will have had a

  character in some degree intermediate with respect to all existing

  Marsupials; or that both Rodents and Marsupials branched off from a common

  progenitor, and that both groups have since undergone much modification in

  divergent directions. On either view we may suppose that the bizcacha has

  retained, by inheritance, more of the character of its ancient progenitor

  than have other Rodents; and therefore it will not be specially related to

  any one existing Marsupial, but indirectly to all or nearly all Marsupials,

  from having partially retained the character of their common progenitor, or

  of an early member of the group. On the other hand, of all Marsupials, as

  Mr. Waterhouse has remarked, the phascolomys resembles most nearly, not any

  one species, but the general order of Rodents. In this case, however, it

  may be strongly suspected that the resemblance is only analogical, owing to

  the phascolomys having become adapted to habits like those of a Rodent.

  The elder De Candolle has made nearly similar observations on the general

  nature of the affinities of distinct orders of plants.

  On the principle of the multiplication and gradual divergence in character

  of the species descended from a common parent, together with their

  retention by inheritance of some characters in common, we can understand

  the excessively complex and radiating affinities by which all the members

  of the same family or higher group are connected together. For the common

  parent of a whole family of species, now broken up by extinction into

  distinct groups and sub-groups, will have transmitted some of its

  characters, modified in various ways and degrees, to all; and the several

  species will consequently be related to each other by circuitous lines of

  affinity of various lengths (as may be seen in the diagram so often

  referred to), mounting up through many predecessors. As it is difficult to

  show the blood-relationship between the numerous kindred of any ancient and

  noble family, even by the aid of a genealogical tree, and almost impossible

  to do this without this aid, we can understand the extraordinary difficulty

  which naturalists have experienced in describing, without the aid of a

  diagram, the various affinities which they perceive be
tween the many living

  and extinct members of the same great natural class.

  Extinction, as we have seen in the fourth chapter, has played an important

  part in defining and widening the intervals between the several groups in

  each class. We may thus account even for the distinctness of whole classes

  from each other--for instance, of birds from all other vertebrate

  animals--by the belief that many ancient forms of life have been utterly

  lost, through which the early progenitors of birds were formerly connected

  with the early progenitors of the other vertebrate classes. There has been

  less entire extinction of the forms of life which once connected fishes

  with batrachians. There has been still less in some other classes, as in

  that of the Crustacea, for here the most wonderfully diverse forms are

  still tied together by a long, but broken, chain of affinities. Extinction

  has only separated groups: it has by no means made them; for if every form

  which has ever lived on this earth were suddenly to reappear, though it

  would be quite impossible to give definitions by which each group could be

  distinguished from other groups, as all would blend together by steps as

  fine as those between the finest existing varieties, nevertheless a natural

  classification, or at least a natural arrangement, would be possible. We

  shall see this by turning to the diagram: the letters, A to L, may

  represent eleven Silurian genera, some of which have produced large groups

  of modified descendants. Every intermediate link between these eleven

  genera and their primordial parent, and every intermediate link in each

  branch and sub-branch of their descendants, may be supposed to be still

  alive; and the links to be as fine as those between the finest varieties.

  In this case it would be quite impossible to give any definition by which

  the several members of the several groups could be distinguished from their

  more immediate parents; or these parents from their ancient and unknown

  progenitor. Yet the natural arrangement in the diagram would still hold

  good; and, on the principle of inheritance, all the forms descended from A,

  or from I, would have something in common. In a tree we can specify this

  or that branch, though at the actual fork the two unite and blend together.

  We could not, as I have said, define the several groups; but we could pick

  out types, or forms, representing most of the characters of each group,

  whether large or small, and thus give a general idea of the value of the

  differences between them. This is what we should be driven to, if we were

  ever to succeed in collecting all the forms in any class which have lived

  throughout all time and space. We shall certainly never succeed in making

  so perfect a collection: nevertheless, in certain classes, we are tending

  in this direction; and Milne Edwards has lately insisted, in an able paper,

  on the high importance of looking to types, whether or not we can separate

  and define the groups to which such types belong.

  Finally, we have seen that natural selection, which results from the

  struggle for existence, and which almost inevitably induces extinction and

  divergence of character in the many descendants from one dominant

  parent-species, explains that great and universal feature in the affinities

  of all organic beings, namely, their subordination in group under group.

  We use the element of descent in classing the individuals of both sexes and

  of all ages, although having few characters in common, under one species;

  we use descent in classing acknowledged varieties, however different they

  may be from their parent; and I believe this element of descent is the

  hidden bond of connexion which naturalists have sought under the term of

  the Natural System. On this idea of the natural system being, in so far as

  it has been perfected, genealogical in its arrangement, with the grades of

 

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