The Origin of Species

by Charles Darwin

for we well know that many groups, formerly most extensively developed,

  have now become extinct. Looking still more remotely to the future, we may

  predict that, owing to the continued and steady increase of the larger

  groups, a multitude of smaller groups will become utterly extinct, and

  leave no modified descendants; and consequently that of the species living

  at any one period, extremely few will transmit descendants to a remote

  futurity. I shall have to return to this subject in the chapter on

  Classification, but I may add that on this view of extremely few of the

  more ancient species having transmitted descendants, and on the view of all

  the descendants of the same species making a class, we can understand how

  it is that there exist but very few classes in each main division of the

  animal and vegetable kingdoms. Although extremely few of the most ancient

  species may now have living and modified descendants, yet at the most

  remote geological period, the earth may have been as well peopled with many

  species of many genera, families, orders, and classes, as at the present

  day.

  Summary of Chapter -- If during the long course of ages and under varying

  conditions of life, organic beings vary at all in the several parts of

  their organisation, and I think this cannot be disputed; if there be, owing

  to the high geometrical powers of increase of each species, at some age,

  season, or year, a severe struggle for life, and this certainly cannot be

  disputed; then, considering the infinite complexity of the relations of all

  organic beings to each other and to their conditions of existence, causing

  an infinite diversity in structure, constitution, and habits, to be

  advantageous to them, I think it would be a most extraordinary fact if no

  variation ever had occurred useful to each being's own welfare, in the same

  way as so many variations have occurred useful to man. But if variations

  useful to any organic being do occur, assuredly individuals thus

  characterised will have the best chance of being preserved in the struggle

  for life; and from the strong principle of inheritance they will tend to

  produce offspring similarly characterised. This principle of preservation,

  I have called, for the sake of brevity, Natural Selection. Natural

  selection, on the principle of qualities being inherited at corresponding

  ages, can modify the egg, seed, or young, as easily as the adult. Amongst

  many animals, sexual selection will give its aid to ordinary selection, by

  assuring to the most vigorous and best adapted males the greatest number of

  offspring. Sexual selection will also give characters useful to the males

  alone, in their struggles with other males.

  Whether natural selection has really thus acted in nature, in modifying and

  adapting the various forms of life to their several conditions and

  stations, must be judged of by the general tenour and balance of evidence

  given in the following chapters. But we already see how it entails

  extinction; and how largely extinction has acted in the world's history,

  geology plainly declares. Natural selection, also, leads to divergence of

  character; for more living beings can be supported on the same area the

  more they diverge in structure, habits, and constitution, of which we see

  proof by looking at the inhabitants of any small spot or at naturalised

  productions. Therefore during the modification of the descendants of any

  one species, and during the incessant struggle of all species to increase

  in numbers, the more diversified these descendants become, the better will

  be their chance of succeeding in the battle of life. Thus the small

  differences distinguishing varieties of the same species, will steadily

  tend to increase till they come to equal the greater differences between

  species of the same genus, or even of distinct genera.

  We have seen that it is the common, the widely-diffused, and widely-ranging

  species, belonging to the larger genera, which vary most; and these will

  tend to transmit to their modified offspring that superiority which now

  makes them dominant in their own countries. Natural selection, as has just

  been remarked, leads to divergence of character and to much extinction of

  the less improved and intermediate forms of life. On these principles, I

  believe, the nature of the affinities of all organic beings may be

  explained. It is a truly wonderful fact--the wonder of which we are apt to

  overlook from familiarity--that all animals and all plants throughout all

  time and space should be related to each other in group subordinate to

  group, in the manner which we everywhere behold--namely, varieties of the

  same species most closely related together, species of the same genus less

  closely and unequally related together, forming sections and sub-genera,

  species of distinct genera much less closely related, and genera related in

  different degrees, forming sub-families, families, orders, sub-classes, and

  classes. The several subordinate groups in any class cannot be ranked in a

  single file, but seem rather to be clustered round points, and these round

  other points, and so on in almost endless cycles. On the view that each

  species has been independently created, I can see no explanation of this

  great fact in the classification of all organic beings; but, to the best of

  my judgment, it is explained through inheritance and the complex action of

  natural selection, entailing extinction and divergence of character, as we

  have seen illustrated in the diagram.

  The affinities of all the beings of the same class have sometimes been

  represented by a great tree. I believe this simile largely speaks the

  truth. The green and budding twigs may represent existing species; and

  those produced during each former year may represent the long succession of

  extinct species. At each period of growth all the growing twigs have tried

  to branch out on all sides, and to overtop and kill the surrounding twigs

  and branches, in the same manner as species and groups of species have

  tried to overmaster other species in the great battle for life. The limbs

  divided into great branches, and these into lesser and lesser branches,

  were themselves once, when the tree was small, budding twigs; and this

  connexion of the former and present buds by ramifying branches may well

  represent the classification of all extinct and living species in groups

  subordinate to groups. Of the many twigs which flourished when the tree

  was a mere bush, only two or three, now grown into great branches, yet

  survive and bear all the other branches; so with the species which lived

  during long-past geological periods, very few now have living and modified

  descendants. From the first growth of the tree, many a limb and branch has

  decayed and dropped off; and these lost branches of various sizes may

  represent those whole orders, families, and genera which have now no living

  representatives, and which are known to us only from having been found in a

  fossil state. As we here and there see a thin straggling branch springing

  from a fork low down in a tree, and which by some chance has been favoured

  and is st
ill alive on its summit, so we occasionally see an animal like the

  Ornithorhynchus or Lepidosiren, which in some small degree connects by its

  affinities two large branches of life, and which has apparently been saved

  from fatal competition by having inhabited a protected station. As buds

  give rise by growth to fresh buds, and these, if vigorous, branch out and

  overtop on all sides many a feebler branch, so by generation I believe it

  has been with the great Tree of Life, which fills with its dead and broken

  branches the crust of the earth, and covers the surface with its ever

  branching and beautiful ramifications.

  Chapter V

 

  Laws of Variation

  Effects of external conditions -- Use and disuse, combined with natural

  selection; organs of flight and of vision -- Acclimatisation -- Correlation

  of growth -- Compensation and economy of growth -- False correlations --

  Multiple, rudimentary, and lowly organised structures variable -- Parts

  developed in an unusual manner are highly variable: specific characters

  more variable than generic: secondary sexual characters variable --

  Species of the same genus vary in an analogous manner -- Reversions to long

  lost characters -- Summary.

  I have hitherto sometimes spoken as if the variations--so common and

  multiform in organic beings under domestication, and in a lesser degree in

  those in a state of nature--had been due to chance. This, of course, is a

  wholly incorrect expression, but it serves to acknowledge plainly our

  ignorance of the cause of each particular variation. Some authors believe

  it to be as much the function of the reproductive system to produce

  individual differences, or very slight deviations of structure, as to make

  the child like its parents. But the much greater variability, as well as

  the greater frequency of monstrosities, under domestication or cultivation,

  than under nature, leads me to believe that deviations of structure are in

  some way due to the nature of the conditions of life, to which the parents

  and their more remote ancestors have been exposed during several

  generations. I have remarked in the first chapter--but a long catalogue of

  facts which cannot be here given would be necessary to show the truth of

  the remark--that the reproductive system is eminently susceptible to

  changes in the conditions of life; and to this system being functionally

  disturbed in the parents, I chiefly attribute the varying or plastic

  condition of the offspring. The male and female sexual elements seem to be

  affected before that union takes place which is to form a new being. In

  the case of 'sporting' plants, the bud, which in its earliest condition

  does not apparently differ essentially from an ovule, is alone affected.

  But why, because the reproductive system is disturbed, this or that part

  should vary more or less, we are profoundly ignorant. Nevertheless, we can

  here and there dimly catch a faint ray of light, and we may feel sure that

  there must be some cause for each deviation of structure, however slight.

  How much direct effect difference of climate, food, &c., produces on any

  being is extremely doubtful. My impression is, that the effect is

  extremely small in the case of animals, but perhaps rather more in that of

  plants. We may, at least, safely conclude that such influences cannot have

  produced the many striking and complex co-adaptations of structure between

  one organic being and another, which we see everywhere throughout nature.

  Some little influence may be attributed to climate, food, &c.: thus, E.

  Forbes speaks confidently that shells at their southern limit, and when

  living in shallow water, are more brightly coloured than those of the same

  species further north or from greater depths. Gould believes that birds of

  the same species are more brightly coloured under a clear atmosphere, than

  when living on islands or near the coast. So with insects, Wollaston is

  convinced that residence near the sea affects their colours. Moquin-Tandon

  gives a list of plants which when growing near the sea-shore have their

  leaves in some degree fleshy, though not elsewhere fleshy. Several other

  such cases could be given.

  The fact of varieties of one species, when they range into the zone of

  habitation of other species, often acquiring in a very slight degree some

  of the characters of such species, accords with our view that species of

  all kinds are only well-marked and permanent varieties. Thus the species

  of shells which are confined to tropical and shallow seas are generally

  brighter-coloured than those confined to cold and deeper seas. The birds

  which are confined to continents are, according to Mr. Gould,

  brighter-coloured than those of islands. The insect-species confined to

  sea-coasts, as every collector knows, are often brassy or lurid. Plants

  which live exclusively on the sea-side are very apt to have fleshy leaves.

  He who believes in the creation of each species, will have to say that this

  shell, for instance, was created with bright colours for a warm sea; but

  that this other shell became bright-coloured by variation when it ranged

  into warmer or shallower waters.

  When a variation is of the slightest use to a being, we cannot tell how

  much of it to attribute to the accumulative action of natural selection,

  and how much to the conditions of life. Thus, it is well known to furriers

  that animals of the same species have thicker and better fur the more

  severe the climate is under which they have lived; but who can tell how

  much of this difference may be due to the warmest-clad individuals having

  been favoured and preserved during many generations, and how much to the

  direct action of the severe climate? for it would appear that climate has

  some direct action on the hair of our domestic quadrupeds.

  Instances could be given of the same variety being produced under

  conditions of life as different as can well be conceived; and, on the other

  hand, of different varieties being produced from the same species under the

  same conditions. Such facts show how indirectly the conditions of life

  must act. Again, innumerable instances are known to every naturalist of

  species keeping true, or not varying at all, although living under the most

  opposite climates. Such considerations as these incline me to lay very

  little weight on the direct action of the conditions of life. Indirectly,

  as already remarked, they seem to play an important part in affecting the

  reproductive system, and in thus inducing variability; and natural

  selection will then accumulate all profitable variations, however slight,

  until they become plainly developed and appreciable by us.

  Effects of Use and Disuse. -- From the facts alluded to in the first

  chapter, I think there can be little doubt that use in our domestic animals

  strengthens and enlarges certain parts, and disuse diminishes them; and

  that such modifications are inherited. Under free nature, we can have no

  standard of comparison, by which to judge of the effects of long-continued

  use or disuse, for we know not the parent-forms; but many animals have

&nb
sp; structures which can be explained by the effects of disuse. As Professor

  Owen has remarked, there is no greater anomaly in nature than a bird that

  cannot fly; yet there are several in this state. The logger-headed duck of

  South America can only flap along the surface of the water, and has its

  wings in nearly the same condition as the domestic Aylesbury duck. As the

  larger ground-feeding birds seldom take flight except to escape danger, I

  believe that the nearly wingless condition of several birds, which now

  inhabit or have lately inhabited several oceanic islands, tenanted by no

  beast of prey, has been caused by disuse. The ostrich indeed inhabits

  continents and is exposed to danger from which it cannot escape by flight,

  but by kicking it can defend itself from enemies, as well as any of the

  smaller quadrupeds. We may imagine that the early progenitor of the

  ostrich had habits like those of a bustard, and that as natural selection

  increased in successive generations the size and weight of its body, its

  legs were used more, and its wings less, until they became incapable of

  flight.

  Kirby has remarked (and I have observed the same fact) that the anterior

  tarsi, or feet, of many male dung-feeding beetles are very often broken

  off; he examined seventeen specimens in his own collection, and not one had

  even a relic left. In the Onites apelles the tarsi are so habitually lost,

  that the insect has been described as not having them. In some other

  genera they are present, but in a rudimentary condition. In the Ateuchus

  or sacred beetle of the Egyptians, they are totally deficient. There is

  not sufficient evidence to induce us to believe that mutilations are ever

  inherited; and I should prefer explaining the entire absence of the

  anterior tarsi in Ateuchus, and their rudimentary condition in some other

  genera, by the long-continued effects of disuse in their progenitors; for

  as the tarsi are almost always lost in many dung-feeding beetles, they must

  be lost early in life, and therefore cannot be much used by these insects.

  In some cases we might easily put down to disuse modifications of structure

  which are wholly, or mainly, due to natural selection. Mr. Wollaston has

  discovered the remarkable fact that 200 beetles, out of the 550 species

  inhabiting Madeira, are so far deficient in wings that they cannot fly; and

  that of the twenty-nine endemic genera, no less than twenty-three genera

  have all their species in this condition! Several facts, namely, that

  beetles in many parts of the world are very frequently blown to sea and

  perish; that the beetles in Madeira, as observed by Mr. Wollaston, lie much

  concealed, until the wind lulls and the sun shines; that the proportion of

  wingless beetles is larger on the exposed Dezertas than in Madeira itself;

  and especially the extraordinary fact, so strongly insisted on by Mr.

  Wollaston, of the almost entire absence of certain large groups of beetles,

  elsewhere excessively numerous, and which groups have habits of life almost

  necessitating frequent flight;--these several considerations have made me

  believe that the wingless condition of so many Madeira beetles is mainly

  due to the action of natural selection, but combined probably with disuse.

  For during thousands of successive generations each individual beetle which

  flew least, either from its wings having been ever so little less perfectly

  developed or from indolent habit, will have had the best chance of

  surviving from not being blown out to sea; and, on the other hand, those

  beetles which most readily took to flight will oftenest have been blown to

  sea and thus have been destroyed.

  The insects in Madeira which are not ground-feeders, and which, as the

  flower-feeding coleoptera and lepidoptera, must habitually use their wings

  to gain their subsistence, have, as Mr. Wollaston suspects, their wings not

  at all reduced, but even enlarged. This is quite compatible with the