The Origin of Species

by Charles Darwin

difference between the descendants from a common parent, expressed by the

  terms genera, families, orders, &c., we can understand the rules which we

  are compelled to follow in our classification. We can understand why we

  value certain resemblances far more than others; why we are permitted to

  use rudimentary and useless organs, or others of trifling physiological

  importance; why, in comparing one group with a distinct group, we summarily

  reject analogical or adaptive characters, and yet use these same characters

  within the limits of the same group. We can clearly see how it is that all

  living and extinct forms can be grouped together in one great system; and

  how the several members of each class are connected together by the most

  complex and radiating lines of affinities. We shall never, probably,

  disentangle the inextricable web of affinities between the members of any

  one class; but when we have a distinct object in view, and do not look to

  some unknown plan of creation, we may hope to make sure but slow progress.

  Morphology. -- We have seen that the members of the same class,

  independently of their habits of life, resemble each other in the general

  plan of their organisation. This resemblance is often expressed by the

  term 'unity of type;' or by saying that the several parts and organs in the

  different species of the class are homologous. The whole subject is

  included under the general name of Morphology. This is the most

  interesting department of natural history, and may be said to be its very

  soul. What can be more curious than that the hand of a man, formed for

  grasping, that of a mole for digging, the leg of the horse, the paddle of

  the porpoise, and the wing of the bat, should all be constructed on the

  same pattern, and should include the same bones, in the same relative

  positions? Geoffroy St. Hilaire has insisted strongly on the high

  importance of relative connexion in homologous organs: the parts may

  change to almost any extent in form and size, and yet they always remain

  connected together in the same order. We never find, for instance, the

  bones of the arm and forearm, or of the thigh and leg, transposed. Hence

  the same names can be given to the homologous bones in widely different

  animals. We see the same great law in the construction of the mouths of

  insects: what can be more different than the immensely long spiral

  proboscis of a sphinx-moth, the curious folded one of a bee or bug, and the

  great jaws of a beetle?--yet all these organs, serving for such different

  purposes, are formed by infinitely numerous modifications of an upper lip,

  mandibles, and two pairs of maxillae. Analogous laws govern the

  construction of the mouths and limbs of crustaceans. So it is with the

  flowers of plants.

  Nothing can be more hopeless than to attempt to explain this similarity of

  pattern in members of the same class, by utility or by the doctrine of

  final causes. The hopelessness of the attempt has been expressly admitted

  by Owen in his most interesting work on the 'Nature of Limbs.' On the

  ordinary view of the independent creation of each being, we can only say

  that so it is;--that it has so pleased the Creator to construct each animal

  and plant.

  The explanation is manifest on the theory of the natural selection of

  successive slight modifications,--each modification being profitable in

  some way to the modified form, but often affecting by correlation of growth

  other parts of the organisation. In changes of this nature, there will be

  little or no tendency to modify the original pattern, or to transpose

  parts. The bones of a limb might be shortened and widened to any extent,

  and become gradually enveloped in thick membrane, so as to serve as a fin;

  or a webbed foot might have all its bones, or certain bones, lengthened to

  any extent, and the membrane connecting them increased to any extent, so as

  to serve as a wing: yet in all this great amount of modification there

  will be no tendency to alter the framework of bones or the relative

  connexion of the several parts. If we suppose that the ancient progenitor,

  the archetype as it may be called, of all mammals, had its limbs

  constructed on the existing general pattern, for whatever purpose they

  served, we can at once perceive the plain signification of the homologous

  construction of the limbs throughout the whole class. So with the mouths

  of insects, we have only to suppose that their common progenitor had an

  upper lip, mandibles, and two pair of maxillae, these parts being perhaps

  very simple in form; and then natural selection will account for the

  infinite diversity in structure and function of the mouths of insects.

  Nevertheless, it is conceivable that the general pattern of an organ might

  become so much obscured as to be finally lost, by the atrophy and

  ultimately by the complete abortion of certain parts, by the soldering

  together of other parts, and by the doubling or multiplication of

  others,--variations which we know to be within the limits of possibility.

  In the paddles of the extinct gigantic sea-lizards, and in the mouths of

  certain suctorial crustaceans, the general pattern seems to have been thus

  to a certain extent obscured.

  There is another and equally curious branch of the present subject; namely,

  the comparison not of the same part in different members of a class, but of

  the different parts or organs in the same individual. Most physiologists

  believe that the bones of the skull are homologous with--that is correspond

  in number and in relative connexion with--the elemental parts of a certain

  number of vertebrae. The anterior and posterior limbs in each member of

  the vertebrate and articulate classes are plainly homologous. We see the

  same law in comparing the wonderfully complex jaws and legs in crustaceans.

  It is familiar to almost every one, that in a flower the relative position

  of the sepals, petals, stamens, and pistils, as well as their intimate

  structure, are intelligible on the view that they consist of metamorphosed

  leaves, arranged in a spire. In monstrous plants, we often get direct

  evidence of the possibility of one organ being transformed into another;

  and we can actually see in embryonic crustaceans and in many other animals,

  and in flowers, that organs, which when mature become extremely different,

  are at an early stage of growth exactly alike.

  How inexplicable are these facts on the ordinary view of creation! Why

  should the brain be enclosed in a box composed of such numerous and such

  extraordinarily shaped pieces of bone? As Owen has remarked, the benefit

  derived from the yielding of the separate pieces in the act of parturition

  of mammals, will by no means explain the same construction in the skulls of

  birds. Why should similar bones have been created in the formation of the

  wing and leg of a bat, used as they are for such totally different

  purposes? Why should one crustacean, which has an extremely complex mouth

  formed of many parts, consequently always have fewer legs; or conversely,

  those with many legs have simpler mouths? Why should the sepals, petals,

  st
amens, and pistils in any individual flower, though fitted for such

  widely different purposes, be all constructed on the same pattern?

  On the theory of natural selection, we can satisfactorily answer these

  questions. In the vertebrata, we see a series of internal vertebrae

  bearing certain processes and appendages; in the articulata, we see the

  body divided into a series of segments, bearing external appendages; and in

  flowering plants, we see a series of successive spiral whorls of leaves.

  An indefinite repetition of the same part or organ is the common

  characteristic (as Owen has observed) of all low or little-modified forms;

  therefore we may readily believe that the unknown progenitor of the

  vertebrata possessed many vertebrae; the unknown progenitor of the

  articulata, many segments; and the unknown progenitor of flowering plants,

  many spiral whorls of leaves. We have formerly seen that parts many times

  repeated are eminently liable to vary in number and structure; consequently

  it is quite probable that natural selection, during a long-continued course

  of modification, should have seized on a certain number of the primordially

  similar elements, many times repeated, and have adapted them to the most

  diverse purposes. And as the whole amount of modification will have been

  effected by slight successive steps, we need not wonder at discovering in

  such parts or organs, a certain degree of fundamental resemblance, retained

  by the strong principle of inheritance.

  In the great class of molluscs, though we can homologise the parts of one

  species with those of another and distinct species, we can indicate but few

  serial homologies; that is, we are seldom enabled to say that one part or

  organ is homologous with another in the same individual. And we can

  understand this fact; for in molluscs, even in the lowest members of the

  class, we do not find nearly so much indefinite repetition of any one part,

  as we find in the other great classes of the animal and vegetable kingdoms.

  Naturalists frequently speak of the skull as formed of metamorphosed

  vertebrae: the jaws of crabs as metamorphosed legs; the stamens and

  pistils of flowers as metamorphosed leaves; but it would in these cases

  probably be more correct, as Professor Huxley has remarked, to speak of

  both skull and vertebrae, both jaws and legs, &c.,--as having been

  metamorphosed, not one from the other, but from some common element.

  Naturalists, however, use such language only in a metaphorical sense: they

  are far from meaning that during a long course of descent, primordial

  organs of any kind--vertebrae in the one case and legs in the other--have

  actually been modified into skulls or jaws. Yet so strong is the

  appearance of a modification of this nature having occurred, that

  naturalists can hardly avoid employing language having this plain

  signification. On my view these terms may be used literally; and the

  wonderful fact of the jaws, for instance, of a crab retaining numerous

  characters, which they would probably have retained through inheritance, if

  they had really been metamorphosed during a long course of descent from

  true legs, or from some simple appendage, is explained.

  Embryology. -- It has already been casually remarked that certain organs in

  the individual, which when mature become widely different and serve for

  different purposes, are in the embryo exactly alike. The embryos, also, of

  distinct animals within the same class are often strikingly similar: a

  better proof of this cannot be given, than a circumstance mentioned by

  Agassiz, namely, that having forgotten to ticket the embryo of some

  vertebrate animal, he cannot now tell whether it be that of a mammal, bird,

  or reptile. The vermiform larvae of moths, flies, beetles, &c., resemble

  each other much more closely than do the mature insects; but in the case of

  larvae, the embryos are active, and have been adapted for special lines of

  life. A trace of the law of embryonic resemblance, sometimes lasts till a

  rather late age: thus birds of the same genus, and of closely allied

  genera, often resemble each other in their first and second plumage; as we

  see in the spotted feathers in the thrush group. In the cat tribe, most of

  the species are striped or spotted in lines; and stripes can be plainly

  distinguished in the whelp of the lion. We occasionally though rarely see

  something of this kind in plants: thus the embryonic leaves of the ulex or

  furze, and the first leaves of the phyllodineous acaceas, are pinnate or

  divided like the ordinary leaves of the leguminosae.

  The points of structure, in which the embryos of widely different animals

  of the same class resemble each other, often have no direct relation to

  their conditions of existence. We cannot, for instance, suppose that in

  the embryos of the vertebrata the peculiar loop-like course of the arteries

  near the branchial slits are related to similar conditions,--in the young

  mammal which is nourished in the womb of its mother, in the egg of the bird

  which is hatched in a nest, and in the spawn of a frog under water. We

  have no more reason to believe in such a relation, than we have to believe

  that the same bones in the hand of a man, wing of a bat, and fin of a

  porpoise, are related to similar conditions of life. No one will suppose

  that the stripes on the whelp of a lion, or the spots on the young

  blackbird, are of any use to these animals, or are related to the

  conditions to which they are exposed.

  The case, however, is different when an animal during any part of its

  embryonic career is active, and has to provide for itself. The period of

  activity may come on earlier or later in life; but whenever it comes on,

  the adaptation of the larva to its conditions of life is just as perfect

  and as beautiful as in the adult animal. From such special adaptations,

  the similarity of the larvae or active embryos of allied animals is

  sometimes much obscured; and cases could be given of the larvae of two

  species, or of two groups of species, differing quite as much, or even

  more, from each other than do their adult parents. In most cases, however,

  the larvae, though active, still obey more or less closely the law of

  common embryonic resemblance. Cirripedes afford a good instance of this:

  even the illustrious Cuvier did not perceive that a barnacle was, as it

  certainly is, a crustacean; but a glance at the larva shows this to be the

  case in an unmistakeable manner. So again the two main divisions of

  cirripedes, the pedunculated and sessile, which differ widely in external

  appearance, have larvae in all their several stages barely distinguishable.

  The embryo in the course of development generally rises in organisation: I

  use this expression, though I am aware that it is hardly possible to define

  clearly what is meant by the organisation being higher or lower. But no

  one probably will dispute that the butterfly is higher than the

  caterpillar. In some cases, however, the mature animal is generally

  considered as lower in the scale than the larva, as with certain parasitic

  crustaceans. To refer once again to cirripedes: t
he larvae in the first

  stage have three pairs of legs, a very simple single eye, and a

  probosciformed mouth, with which they feed largely, for they increase much

  in size. In the second stage, answering to the chrysalis stage of

  butterflies, they have six pairs of beautifully constructed natatory legs,

  a pair of magnificent compound eyes, and extremely complex antennae; but

  they have a closed and imperfect mouth, and cannot feed: their function at

  this stage is, to search by their well-developed organs of sense, and to

  reach by their active powers of swimming, a proper place on which to become

  attached and to undergo their final metamorphosis. When this is completed

  they are fixed for life: their legs are now converted into prehensile

  organs; they again obtain a well-constructed mouth; but they have no

  antennae, and their two eyes are now reconverted into a minute, single, and

  very simple eye-spot. In this last and complete state, cirripedes may be

  considered as either more highly or more lowly organised than they were in

  the larval condition. But in some genera the larvae become developed

  either into hermaphrodites having the ordinary structure, or into what I

  have called complemental males: and in the latter, the development has

  assuredly been retrograde; for the male is a mere sack, which lives for a

  short time, and is destitute of mouth, stomach, or other organ of

  importance, excepting for reproduction.

  We are so much accustomed to see differences in structure between the

  embryo and the adult, and likewise a close similarity in the embryos of

  widely different animals within the same class, that we might be led to

  look at these facts as necessarily contingent in some manner on growth.

  But there is no obvious reason why, for instance, the wing of a bat, or the

  fin of a porpoise, should not have been sketched out with all the parts in

  proper proportion, as soon as any structure became visible in the embryo.

  And in some whole groups of animals and in certain members of other groups,

  the embryo does not at any period differ widely from the adult: thus Owen

  has remarked in regard to cuttle-fish, 'there is no metamorphosis; the

  cephalopodic character is manifested long before the parts of the embryo

  are completed;' and again in spiders, 'there is nothing worthy to be called

  a metamorphosis.' The larvae of insects, whether adapted to the most

  diverse and active habits, or quite inactive, being fed by their parents or

  placed in the midst of proper nutriment, yet nearly all pass through a

  similar worm-like stage of development; but in some few cases, as in that

  of Aphis, if we look to the admirable drawings by Professor Huxley of the

  development of this insect, we see no trace of the vermiform stage.

  How, then, can we explain these several facts in embryology,--namely the

  very general, but not universal difference in structure between the embryo

  and the adult;--of parts in the same individual embryo, which ultimately

  become very unlike and serve for diverse purposes, being at this early

  period of growth alike;--of embryos of different species within the same

  class, generally, but not universally, resembling each other;--of the

  structure of the embryo not being closely related to its conditions of

  existence, except when the embryo becomes at any period of life active and

  has to provide for itself;--of the embryo apparently having sometimes a

  higher organisation than the mature animal, into which it is developed. I

  believe that all these facts can be explained, as follows, on the view of

  descent with modification.

  It is commonly assumed, perhaps from monstrosities often affecting the

  embryo at a very early period, that slight variations necessarily appear at

  an equally early period. But we have little evidence on this head--indeed

  the evidence rather points the other way; for it is notorious that breeders