The Origin of Species

by Charles Darwin

against my theory. Why, again, do whole groups of allied species appear,

  though certainly they often falsely appear, to have come in suddenly on the

  several geological stages? Why do we not find great piles of strata

  beneath the Silurian system, stored with the remains of the progenitors of

  the Silurian groups of fossils? For certainly on my theory such strata

  must somewhere have been deposited at these ancient and utterly unknown

  epochs in the world's history.

  I can answer these questions and grave objections only on the supposition

  that the geological record is far more imperfect than most geologists

  believe. It cannot be objected that there has not been time sufficient for

  any amount of organic change; for the lapse of time has been so great as to

  be utterly inappreciable by the human intellect. The number of specimens

  in all our museums is absolutely as nothing compared with the countless

  generations of countless species which certainly have existed. We should

  not be able to recognise a species as the parent of any one or more species

  if we were to examine them ever so closely, unless we likewise possessed

  many of the intermediate links between their past or parent and present

  states; and these many links we could hardly ever expect to discover, owing

  to the imperfection of the geological record. Numerous existing doubtful

  forms could be named which are probably varieties; but who will pretend

  that in future ages so many fossil links will be discovered, that

  naturalists will be able to decide, on the common view, whether or not

  these doubtful forms are varieties? As long as most of the links between

  any two species are unknown, if any one link or intermediate variety be

  discovered, it will simply be classed as another and distinct species.

  Only a small portion of the world has been geologically explored. Only

  organic beings of certain classes can be preserved in a fossil condition,

  at least in any great number. Widely ranging species vary most, and

  varieties are often at first local,--both causes rendering the discovery of

  intermediate links less likely. Local varieties will not spread into other

  and distant regions until they are considerably modified and improved; and

  when they do spread, if discovered in a geological formation, they will

  appear as if suddenly created there, and will be simply classed as new

  species. Most formations have been intermittent in their accumulation; and

  their duration, I am inclined to believe, has been shorter than the average

  duration of specific forms. Successive formations are separated from each

  other by enormous blank intervals of time; for fossiliferous formations,

  thick enough to resist future degradation, can be accumulated only where

  much sediment is deposited on the subsiding bed of the sea. During the

  alternate periods of elevation and of stationary level the record will be

  blank. During these latter periods there will probably be more variability

  in the forms of life; during periods of subsidence, more extinction.

  With respect to the absence of fossiliferous formations beneath the lowest

  Silurian strata, I can only recur to the hypothesis given in the ninth

  chapter. That the geological record is imperfect all will admit; but that

  it is imperfect to the degree which I require, few will be inclined to

  admit. If we look to long enough intervals of time, geology plainly

  declares that all species have changed; and they have changed in the manner

  which my theory requires, for they have changed slowly and in a graduated

  manner. We clearly see this in the fossil remains from consecutive

  formations invariably being much more closely related to each other, than

  are the fossils from formations distant from each other in time.

  Such is the sum of the several chief objections and difficulties which may

  justly be urged against my theory; and I have now briefly recapitulated the

  answers and explanations which can be given to them. I have felt these

  difficulties far too heavily during many years to doubt their weight. But

  it deserves especial notice that the more important objections relate to

  questions on which we are confessedly ignorant; nor do we know how ignorant

  we are. We do not know all the possible transitional gradations between

  the simplest and the most perfect organs; it cannot be pretended that we

  know all the varied means of Distribution during the long lapse of years,

  or that we know how imperfect the Geological Record is. Grave as these

  several difficulties are, in my judgment they do not overthrow the theory

  of descent with modification.

  Now let us turn to the other side of the argument. Under domestication we

  see much variability. This seems to be mainly due to the reproductive

  system being eminently susceptible to changes in the conditions of life; so

  that this system, when not rendered impotent, fails to reproduce offspring

  exactly like the parent-form. Variability is governed by many complex

  laws,--by correlation of growth, by use and disuse, and by the direct

  action of the physical conditions of life. There is much difficulty in

  ascertaining how much modification our domestic productions have undergone;

  but we may safely infer that the amount has been large, and that

  modifications can be inherited for long periods. As long as the conditions

  of life remain the same, we have reason to believe that a modification,

  which has already been inherited for many generations, may continue to be

  inherited for an almost infinite number of generations. On the other hand

  we have evidence that variability, when it has once come into play, does

  not wholly cease; for new varieties are still occasionally produced by our

  most anciently domesticated productions.

  Man does not actually produce variability; he only unintentionally exposes

  organic beings to new conditions of life, and then nature acts on the

  organisation, and causes variability. But man can and does select the

  variations given to him by nature, and thus accumulate them in any desired

  manner. He thus adapts animals and plants for his own benefit or pleasure.

  He may do this methodically, or he may do it unconsciously by preserving

  the individuals most useful to him at the time, without any thought of

  altering the breed. It is certain that he can largely influence the

  character of a breed by selecting, in each successive generation,

  individual differences so slight as to be quite inappreciable by an

  uneducated eye. This process of selection has been the great agency in the

  production of the most distinct and useful domestic breeds. That many of

  the breeds produced by man have to a large extent the character of natural

  species, is shown by the inextricable doubts whether very many of them are

  varieties or aboriginal species.

  There is no obvious reason why the principles which have acted so

  efficiently under domestication should not have acted under nature. In the

  preservation of favoured individuals and races, during the

  constantly-recurrent Struggle for Existence, we see the most powerful and

  ever-acting means of selection. The struggle for existence inevit
ably

  follows from the high geometrical ratio of increase which is common to all

  organic beings. This high rate of increase is proved by calculation, by

  the effects of a succession of peculiar seasons, and by the results of

  naturalisation, as explained in the third chapter. More individuals are

  born than can possibly survive. A grain in the balance will determine

  which individual shall live and which shall die,--which variety or species

  shall increase in number, and which shall decrease, or finally become

  extinct. As the individuals of the same species come in all respects into

  the closest competition with each other, the struggle will generally be

  most severe between them; it will be almost equally severe between the

  varieties of the same species, and next in severity between the species of

  the same genus. But the struggle will often be very severe between beings

  most remote in the scale of nature. The slightest advantage in one being,

  at any age or during any season, over those with which it comes into

  competition, or better adaptation in however slight a degree to the

  surrounding physical conditions, will turn the balance.

  With animals having separated sexes there will in most cases be a struggle

  between the males for possession of the females. The most vigorous

  individuals, or those which have most successfully struggled with their

  conditions of life, will generally leave most progeny. But success will

  often depend on having special weapons or means of defence, or on the

  charms of the males; and the slightest advantage will lead to victory.

  As geology plainly proclaims that each land has undergone great physical

  changes, we might have expected that organic beings would have varied under

  nature, in the same way as they generally have varied under the changed

  conditions of domestication. And if there be any variability under nature,

  it would be an unaccountable fact if natural selection had not come into

  play. It has often been asserted, but the assertion is quite incapable of

  proof, that the amount of variation under nature is a strictly limited

  quantity. Man, though acting on external characters alone and often

  capriciously, can produce within a short period a great result by adding up

  mere individual differences in his domestic productions; and every one

  admits that there are at least individual differences in species under

  nature. But, besides such differences, all naturalists have admitted the

  existence of varieties, which they think sufficiently distinct to be worthy

  of record in systematic works. No one can draw any clear distinction

  between individual differences and slight varieties; or between more

  plainly marked varieties and sub-species, and species. Let it be observed

  how naturalists differ in the rank which they assign to the many

  representative forms in Europe and North America.

  If then we have under nature variability and a powerful agent always ready

  to act and select, why should we doubt that variations in any way useful to

  beings, under their excessively complex relations of life, would be

  preserved, accumulated, and inherited? Why, if man can by patience select

  variations most useful to himself, should nature fail in selecting

  variations useful, under changing conditions of life, to her living

  products? What limit can be put to this power, acting during long ages and

  rigidly scrutinising the whole constitution, structure, and habits of each

  creature,--favouring the good and rejecting the bad? I can see no limit to

  this power, in slowly and beautifully adapting each form to the most

  complex relations of life. The theory of natural selection, even if we

  looked no further than this, seems to me to be in itself probable. I have

  already recapitulated, as fairly as I could, the opposed difficulties and

  objections: now let us turn to the special facts and arguments in favour

  of the theory.

  On the view that species are only strongly marked and permanent varieties,

  and that each species first existed as a variety, we can see why it is that

  no line of demarcation can be drawn between species, commonly supposed to

  have been produced by special acts of creation, and varieties which are

  acknowledged to have been produced by secondary laws. On this same view we

  can understand how it is that in each region where many species of a genus

  have been produced, and where they now flourish, these same species should

  present many varieties; for where the manufactory of species has been

  active, we might expect, as a general rule, to find it still in action; and

  this is the case if varieties be incipient species. Moreover, the species

  of the large genera, which afford the greater number of varieties or

  incipient species, retain to a certain degree the character of varieties;

  for they differ from each other by a less amount of difference than do the

  species of smaller genera. The closely allied species also of the larger

  genera apparently have restricted ranges, and they are clustered in little

  groups round other species--in which respects they resemble varieties.

  These are strange relations on the view of each species having been

  independently created, but are intelligible if all species first existed as

  varieties.

  As each species tends by its geometrical ratio of reproduction to increase

  inordinately in number; and as the modified descendants of each species

  will be enabled to increase by so much the more as they become more

  diversified in habits and structure, so as to be enabled to seize on many

  and widely different places in the economy of nature, there will be a

  constant tendency in natural selection to preserve the most divergent

  offspring of any one species. Hence during a long-continued course of

  modification, the slight differences, characteristic of varieties of the

  same species, tend to be augmented into the greater differences

  characteristic of species of the same genus. New and improved varieties

  will inevitably supplant and exterminate the older, less improved and

  intermediate varieties; and thus species are rendered to a large extent

  defined and distinct objects. Dominant species belonging to the larger

  groups tend to give birth to new and dominant forms; so that each large

  group tends to become still larger, and at the same time more divergent in

  character. But as all groups cannot thus succeed in increasing in size,

  for the world would not hold them, the more dominant groups beat the less

  dominant. This tendency in the large groups to go on increasing in size

  and diverging in character, together with the almost inevitable contingency

  of much extinction, explains the arrangement of all the forms of life, in

  groups subordinate to groups, all within a few great classes, which we now

  see everywhere around us, and which has prevailed throughout all time.

  This grand fact of the grouping of all organic beings seems to me utterly

  inexplicable on the theory of creation.

  As natural selection acts solely by accumulating slight, successive,

  favourable variations, it can produce no great or sudden modification; it

  can
act only by very short and slow steps. Hence the canon of 'Natura non

  facit saltum,' which every fresh addition to our knowledge tends to make

  more strictly correct, is on this theory simply intelligible. We can

  plainly see why nature is prodigal in variety, though niggard in

  innovation. But why this should be a law of nature if each species has

  been independently created, no man can explain.

  Many other facts are, as it seems to me, explicable on this theory. How

  strange it is that a bird, under the form of woodpecker, should have been

  created to prey on insects on the ground; that upland geese, which never or

  rarely swim, should have been created with webbed feet; that a thrush

  should have been created to dive and feed on sub-aquatic insects; and that

  a petrel should have been created with habits and structure fitting it for

  the life of an auk or grebe! and so on in endless other cases. But on the

  view of each species constantly trying to increase in number, with natural

  selection always ready to adapt the slowly varying descendants of each to

  any unoccupied or ill-occupied place in nature, these facts cease to be

  strange, or perhaps might even have been anticipated.

  As natural selection acts by competition, it adapts the inhabitants of each

  country only in relation to the degree of perfection of their associates;

  so that we need feel no surprise at the inhabitants of any one country,

  although on the ordinary view supposed to have been specially created and

  adapted for that country, being beaten and supplanted by the naturalised

  productions from another land. Nor ought we to marvel if all the

  contrivances in nature be not, as far as we can judge, absolutely perfect;

  and if some of them be abhorrent to our ideas of fitness. We need not

  marvel at the sting of the bee causing the bee's own death; at drones being

  produced in such vast numbers for one single act, and being then

  slaughtered by their sterile sisters; at the astonishing waste of pollen by

  our fir-trees; at the instinctive hatred of the queen bee for her own

  fertile daughters; at ichneumonidae feeding within the live bodies of

  caterpillars; and at other such cases. The wonder indeed is, on the theory

  of natural selection, that more cases of the want of absolute perfection

  have not been observed.

  The complex and little known laws governing variation are the same, as far

  as we can see, with the laws which have governed the production of

  so-called specific forms. In both cases physical conditions seem to have

  produced but little direct effect; yet when varieties enter any zone, they

  occasionally assume some of the characters of the species proper to that

  zone. In both varieties and species, use and disuse seem to have produced

  some effect; for it is difficult to resist this conclusion when we look,

  for instance, at the logger-headed duck, which has wings incapable of

  flight, in nearly the same condition as in the domestic duck; or when we

  look at the burrowing tucutucu, which is occasionally blind, and then at

  certain moles, which are habitually blind and have their eyes covered with

  skin; or when we look at the blind animals inhabiting the dark caves of

  America and Europe. In both varieties and species correlation of growth

  seems to have played a most important part, so that when one part has been

  modified other parts are necessarily modified. In both varieties and

  species reversions to long-lost characters occur. How inexplicable on the

  theory of creation is the occasional appearance of stripes on the shoulder

  and legs of the several species of the horse-genus and in their hybrids!

  How simply is this fact explained if we believe that these species have

  descended from a striped progenitor, in the same manner as the several

  domestic breeds of pigeon have descended from the blue and barred