The Origin of Species

by Charles Darwin

food, or to escape from beasts of prey.

  Organs now of trifling importance have probably in some cases been of high

  importance to an early progenitor, and, after having been slowly perfected

  at a former period, have been transmitted in nearly the same state,

  although now become of very slight use; and any actually injurious

  deviations in their structure will always have been checked by natural

  selection. Seeing how important an organ of locomotion the tail is in most

  aquatic animals, its general presence and use for many purposes in so many

  land animals, which in their lungs or modified swim-bladders betray their

  aquatic origin, may perhaps be thus accounted for. A well-developed tail

  having been formed in an aquatic animal, it might subsequently come to be

  worked in for all sorts of purposes, as a fly-flapper, an organ of

  prehension, or as an aid in turning, as with the dog, though the aid must

  be slight, for the hare, with hardly any tail, can double quickly enough.

  In the second place, we may sometimes attribute importance to characters

  which are really of very little importance, and which have originated from

  quite secondary causes, independently of natural selection. We should

  remember that climate, food, &c., probably have some little direct

  influence on the organisation; that characters reappear from the law of

  reversion; that correlation of growth will have had a most important

  influence in modifying various structures; and finally, that sexual

  selection will often have largely modified the external characters of

  animals having a will, to give one male an advantage in fighting with

  another or in charming the females. Moreover when a modification of

  structure has primarily arisen from the above or other unknown causes, it

  may at first have been of no advantage to the species, but may subsequently

  have been taken advantage of by the descendants of the species under new

  conditions of life and with newly acquired habits.

  To give a few instances to illustrate these latter remarks. If green

  woodpeckers alone had existed, and we did not know that there were many

  black and pied kinds, I dare say that we should have thought that the green

  colour was a beautiful adaptation to hide this tree-frequenting bird from

  its enemies; and consequently that it was a character of importance and

  might have been acquired through natural selection; as it is, I have no

  doubt that the colour is due to some quite distinct cause, probably to

  sexual selection. A trailing bamboo in the Malay Archipelago climbs the

  loftiest trees by the aid of exquisitely constructed hooks clustered around

  the ends of the branches, and this contrivance, no doubt, is of the highest

  service to the plant; but as we see nearly similar hooks on many trees

  which are not climbers, the hooks on the bamboo may have arisen from

  unknown laws of growth, and have been subsequently taken advantage of by

  the plant undergoing further modification and becoming a climber. The

  naked skin on the head of a vulture is generally looked at as a direct

  adaptation for wallowing in putridity; and so it may be, or it may possibly

  be due to the direct action of putrid matter; but we should be very

  cautious in drawing any such inference, when we see that the skin on the

  head of the clean-feeding male turkey is likewise naked. The sutures in

  the skulls of young mammals have been advanced as a beautiful adaptation

  for aiding parturition, and no doubt they facilitate, or may be

  indispensable for this act; but as sutures occur in the skulls of young

  birds and reptiles, which have only to escape from a broken egg, we may

  infer that this structure has arisen from the laws of growth, and has been

  taken advantage of in the parturition of the higher animals.

  We are profoundly ignorant of the causes producing slight and unimportant

  variations; and we are immediately made conscious of this by reflecting on

  the differences in the breeds of our domesticated animals in different

  countries,--more especially in the less civilized countries where there has

  been but little artificial selection. Careful observers are convinced that

  a damp climate affects the growth of the hair, and that with the hair the

  horns are correlated. Mountain breeds always differ from lowland breeds;

  and a mountainous country would probably affect the hind limbs from

  exercising them more, and possibly even the form of the pelvis; and then by

  the law of homologous variation, the front limbs and even the head would

  probably be affected. The shape, also, of the pelvis might affect by

  pressure the shape of the head of the young in the womb. The laborious

  breathing necessary in high regions would, we have some reason to believe,

  increase the size of the chest; and again correlation would come into play.

  Animals kept by savages in different countries often have to struggle for

  their own subsistence, and would be exposed to a certain extent to natural

  selection, and individuals with slightly different constitutions would

  succeed best under different climates; and there is reason to believe that

  constitution and colour are correlated. A good observer, also, states that

  in cattle susceptibility to the attacks of flies is correlated with colour,

  as is the liability to be poisoned by certain plants; so that colour would

  be thus subjected to the action of natural selection. But we are far too

  ignorant to speculate on the relative importance of the several known and

  unknown laws of variation; and I have here alluded to them only to show

  that, if we are unable to account for the characteristic differences of our

  domestic breeds, which nevertheless we generally admit to have arisen

  through ordinary generation, we ought not to lay too much stress on our

  ignorance of the precise cause of the slight analogous differences between

  species. I might have adduced for this same purpose the differences

  between the races of man, which are so strongly marked; I may add that some

  little light can apparently be thrown on the origin of these differences,

  chiefly through sexual selection of a particular kind, but without here

  entering on copious details my reasoning would appear frivolous.

  The foregoing remarks lead me to say a few words on the protest lately made

  by some naturalists, against the utilitarian doctrine that every detail of

  structure has been produced for the good of its possessor. They believe

  that very many structures have been created for beauty in the eyes of man,

  or for mere variety. This doctrine, if true, would be absolutely fatal to

  my theory. Yet I fully admit that many structures are of no direct use to

  their possessors. Physical conditions probably have had some little effect

  on structure, quite independently of any good thus gained. Correlation of

  growth has no doubt played a most important part, and a useful modification

  of one part will often have entailed on other parts diversified changes of

  no direct use. So again characters which formerly were useful, or which

  formerly had arisen from correlation of growth, or from other unknown

  cause, may reappear from the law of reversion, though now of no di
rect use.

  The effects of sexual selection, when displayed in beauty to charm the

  females, can be called useful only in rather a forced sense. But by far

  the most important consideration is that the chief part of the organisation

  of every being is simply due to inheritance; and consequently, though each

  being assuredly is well fitted for its place in nature, many structures now

  have no direct relation to the habits of life of each species. Thus, we

  can hardly believe that the webbed feet of the upland goose or of the

  frigate-bird are of special use to these birds; we cannot believe that the

  same bones in the arm of the monkey, in the fore leg of the horse, in the

  wing of the bat, and in the flipper of the seal, are of special use to

  these animals. We may safely attribute these structures to inheritance.

  But to the progenitor of the upland goose and of the frigate-bird, webbed

  feet no doubt were as useful as they now are to the most aquatic of

  existing birds. So we may believe that the progenitor of the seal had not

  a flipper, but a foot with five toes fitted for walking or grasping; and we

  may further venture to believe that the several bones in the limbs of the

  monkey, horse, and bat, which have been inherited from a common progenitor,

  were formerly of more special use to that progenitor, or its progenitors,

  than they now are to these animals having such widely diversified habits.

  Therefore we may infer that these several bones might have been acquired

  through natural selection, subjected formerly, as now, to the several laws

  of inheritance, reversion, correlation of growth, &c. Hence every detail

  of structure in every living creature (making some little allowance for the

  direct action of physical conditions) may be viewed, either as having been

  of special use to some ancestral form, or as being now of special use to

  the descendants of this form--either directly, or indirectly through the

  complex laws of growth.

  Natural selection cannot possibly produce any modification in any one

  species exclusively for the good of another species; though throughout

  nature one species incessantly takes advantage of, and profits by, the

  structure of another. But natural selection can and does often produce

  structures for the direct injury of other species, as we see in the fang of

  the adder, and in the ovipositor of the ichneumon, by which its eggs are

  deposited in the living bodies of other insects. If it could be proved

  that any part of the structure of any one species had been formed for the

  exclusive good of another species, it would annihilate my theory, for such

  could not have been produced through natural selection. Although many

  statements may be found in works on natural history to this effect, I

  cannot find even one which seems to me of any weight. It is admitted that

  the rattlesnake has a poison-fang for its own defence and for the

  destruction of its prey; but some authors suppose that at the same time

  this snake is furnished with a rattle for its own injury, namely, to warn

  its prey to escape. I would almost as soon believe that the cat curls the

  end of its tail when preparing to spring, in order to warn the doomed

  mouse. But I have not space here to enter on this and other such cases.

  Natural selection will never produce in a being anything injurious to

  itself, for natural selection acts solely by and for the good of each. No

  organ will be formed, as Paley has remarked, for the purpose of causing

  pain or for doing an injury to its possessor. If a fair balance be struck

  between the good and evil caused by each part, each will be found on the

  whole advantageous. After the lapse of time, under changing conditions of

  life, if any part comes to be injurious, it will be modified; or if it be

  not so, the being will become extinct, as myriads have become extinct.

  Natural selection tends only to make each organic being as perfect as, or

  slightly more perfect than, the other inhabitants of the same country with

  which it has to struggle for existence. And we see that this is the degree

  of perfection attained under nature. The endemic productions of New

  Zealand, for instance, are perfect one compared with another; but they are

  now rapidly yielding before the advancing legions of plants and animals

  introduced from Europe. Natural selection will not produce absolute

  perfection, nor do we always meet, as far as we can judge, with this high

  standard under nature. The correction for the aberration of light is said,

  on high authority, not to be perfect even in that most perfect organ, the

  eye. If our reason leads us to admire with enthusiasm a multitude of

  inimitable contrivances in nature, this same reason tells us, though we may

  easily err on both sides, that some other contrivances are less perfect.

  Can we consider the sting of the wasp or of the bee as perfect, which, when

  used against many attacking animals, cannot be withdrawn, owing to the

  backward serratures, and so inevitably causes the death of the insect by

  tearing out its viscera?

  If we look at the sting of the bee, as having originally existed in a

  remote progenitor as a boring and serrated instrument, like that in so many

  members of the same great order, and which has been modified but not

  perfected for its present purpose, with the poison originally adapted to

  cause galls subsequently intensified, we can perhaps understand how it is

  that the use of the sting should so often cause the insect's own death:

  for if on the whole the power of stinging be useful to the community, it

  will fulfil all the requirements of natural selection, though it may cause

  the death of some few members. If we admire the truly wonderful power of

  scent by which the males of many insects find their females, can we admire

  the production for this single purpose of thousands of drones, which are

  utterly useless to the community for any other end, and which are

  ultimately slaughtered by their industrious and sterile sisters? It may be

  difficult, but we ought to admire the savage instinctive hatred of the

  queen-bee, which urges her instantly to destroy the young queens her

  daughters as soon as born, or to perish herself in the combat; for

  undoubtedly this is for the good of the community; and maternal love or

  maternal hatred, though the latter fortunately is most rare, is all the

  same to the inexorable principle of natural selection. If we admire the

  several ingenious contrivances, by which the flowers of the orchis and of

  many other plants are fertilised through insect agency, can we consider as

  equally perfect the elaboration by our fir-trees of dense clouds of pollen,

  in order that a few granules may be wafted by a chance breeze on to the

  ovules?

  Summary of Chapter -- We have in this chapter discussed some of the

  difficulties and objections which may be urged against my theory. Many of

  them are very grave; but I think that in the discussion light has been

  thrown on several facts, which on the theory of independent acts of

  creation are utterly obscure. We have seen that species at any one period

  are not indefinitely variable
, and are not linked together by a multitude

  of intermediate gradations, partly because the process of natural selection

  will always be very slow, and will act, at any one time, only on a very few

  forms; and partly because the very process of natural selection almost

  implies the continual supplanting and extinction of preceding and

  intermediate gradations. Closely allied species, now living on a

  continuous area, must often have been formed when the area was not

  continuous, and when the conditions of life did not insensibly graduate

  away from one part to another. When two varieties are formed in two

  districts of a continuous area, an intermediate variety will often be

  formed, fitted for an intermediate zone; but from reasons assigned, the

  intermediate variety will usually exist in lesser numbers than the two

  forms which it connects; consequently the two latter, during the course of

  further modification, from existing in greater numbers, will have a great

  advantage over the less numerous intermediate variety, and will thus

  generally succeed in supplanting and exterminating it.

  We have seen in this chapter how cautious we should be in concluding that

  the most different habits of life could not graduate into each other; that

  a bat, for instance, could not have been formed by natural selection from

  an animal which at first could only glide through the air.

  We have seen that a species may under new conditions of life change its

  habits, or have diversified habits, with some habits very unlike those of

  its nearest congeners. Hence we can understand, bearing in mind that each

  organic being is trying to live wherever it can live, how it has arisen

  that there are upland geese with webbed feet, ground woodpeckers, diving

  thrushes, and petrels with the habits of auks.

  Although the belief that an organ so perfect as the eye could have been

  formed by natural selection, is more than enough to stagger any one; yet in

  the case of any organ, if we know of a long series of gradations in

  complexity, each good for its possessor, then, under changing conditions of

  life, there is no logical impossibility in the acquirement of any

  conceivable degree of perfection through natural selection. In the cases

  in which we know of no intermediate or transitional states, we should be

  very cautious in concluding that none could have existed, for the

  homologies of many organs and their intermediate states show that wonderful

  metamorphoses in function are at least possible. For instance, a

  swim-bladder has apparently been converted into an air-breathing lung. The

  same organ having performed simultaneously very different functions, and

  then having been specialised for one function; and two very distinct organs

  having performed at the same time the same function, the one having been

  perfected whilst aided by the other, must often have largely facilitated

  transitions.

  We are far too ignorant, in almost every case, to be enabled to assert that

  any part or organ is so unimportant for the welfare of a species, that

  modifications in its structure could not have been slowly accumulated by

  means of natural selection. But we may confidently believe that many

  modifications, wholly due to the laws of growth, and at first in no way

  advantageous to a species, have been subsequently taken advantage of by the

  still further modified descendants of this species. We may, also, believe

  that a part formerly of high importance has often been retained (as the

  tail of an aquatic animal by its terrestrial descendants), though it has

  become of such small importance that it could not, in its present state,

  have been acquired by natural selection,--a power which acts solely by the

  preservation of profitable variations in the struggle for life.

  Natural selection will produce nothing in one species for the exclusive