The Origin of Species

by Charles Darwin

action of natural selection. For when a new insect first arrived on the

  island, the tendency of natural selection to enlarge or to reduce the

  wings, would depend on whether a greater number of individuals were saved

  by successfully battling with the winds, or by giving up the attempt and

  rarely or never flying. As with mariners shipwrecked near a coast, it

  would have been better for the good swimmers if they had been able to swim

  still further, whereas it would have been better for the bad swimmers if

  they had not been able to swim at all and had stuck to the wreck.

  The eyes of moles and of some burrowing rodents are rudimentary in size,

  and in some cases are quite covered up by skin and fur. This state of the

  eyes is probably due to gradual reduction from disuse, but aided perhaps by

  natural selection. In South America, a burrowing rodent, the tuco-tuco, or

  Ctenomys, is even more subterranean in its habits than the mole; and I was

  assured by a Spaniard, who had often caught them, that they were frequently

  blind; one which I kept alive was certainly in this condition, the cause,

  as appeared on dissection, having been inflammation of the nictitating

  membrane. As frequent inflammation of the eyes must be injurious to any

  animal, and as eyes are certainly not indispensable to animals with

  subterranean habits, a reduction in their size with the adhesion of the

  eyelids and growth of fur over them, might in such case be an advantage;

  and if so, natural selection would constantly aid the effects of disuse.

  It is well known that several animals, belonging to the most different

  classes, which inhabit the caves of Styria and of Kentucky, are blind. In

  some of the crabs the foot-stalk for the eye remains, though the eye is

  gone; the stand for the telescope is there, though the telescope with its

  glasses has been lost. As it is difficult to imagine that eyes, though

  useless, could be in any way injurious to animals living in darkness, I

  attribute their loss wholly to disuse. In one of the blind animals,

  namely, the cave-rat, the eyes are of immense size; and Professor Silliman

  thought that it regained, after living some days in the light, some slight

  power of vision. In the same manner as in Madeira the wings of some of the

  insects have been enlarged, and the wings of others have been reduced by

  natural selection aided by use and disuse, so in the case of the cave-rat

  natural selection seems to have struggled with the loss of light and to

  have increased the size of the eyes; whereas with all the other inhabitants

  of the caves, disuse by itself seems to have done its work.

  It is difficult to imagine conditions of life more similar than deep

  limestone caverns under a nearly similar climate; so that on the common

  view of the blind animals having been separately created for the American

  and European caverns, close similarity in their organisation and affinities

  might have been expected; but, as Schiodte and others have remarked, this

  is not the case, and the cave-insects of the two continents are not more

  closely allied than might have been anticipated from the general

  resemblance of the other inhabitants of North America and Europe. On my

  view we must suppose that American animals, having ordinary powers of

  vision, slowly migrated by successive generations from the outer world into

  the deeper and deeper recesses of the Kentucky caves, as did European

  animals into the caves of Europe. We have some evidence of this gradation

  of habit; for, as Schiodte remarks, 'animals not far remote from ordinary

  forms, prepare the transition from light to darkness. Next follow those

  that are constructed for twilight; and, last of all, those destined for

  total darkness.' By the time that an animal had reached, after numberless

  generations, the deepest recesses, disuse will on this view have more or

  less perfectly obliterated its eyes, and natural selection will often have

  effected other changes, such as an increase in the length of the antennae

  or palpi, as a compensation for blindness. Notwithstanding such

  modifications, we might expect still to see in the cave-animals of America,

  affinities to the other inhabitants of that continent, and in those of

  Europe, to the inhabitants of the European continent. And this is the case

  with some of the American cave-animals, as I hear from Professor Dana; and

  some of the European cave-insects are very closely allied to those of the

  surrounding country. It would be most difficult to give any rational

  explanation of the affinities of the blind cave-animals to the other

  inhabitants of the two continents on the ordinary view of their independent

  creation. That several of the inhabitants of the caves of the Old and New

  Worlds should be closely related, we might expect from the well-known

  relationship of most of their other productions. Far from feeling any

  surprise that some of the cave-animals should be very anomalous, as Agassiz

  has remarked in regard to the blind fish, the Amblyopsis, and as is the

  case with the blind Proteus with reference to the reptiles of Europe, I am

  only surprised that more wrecks of ancient life have not been preserved,

  owing to the less severe competition to which the inhabitants of these dark

  abodes will probably have been exposed.

  Acclimatisation. -- Habit is hereditary with plants, as in the period of

  flowering, in the amount of rain requisite for seeds to germinate, in the

  time of sleep, &c., and this leads me to say a few words on

  acclimatisation. As it is extremely common for species of the same genus

  to inhabit very hot and very cold countries, and as I believe that all the

  species of the same genus have descended from a single parent, if this view

  be correct, acclimatisation must be readily effected during long-continued

  descent. It is notorious that each species is adapted to the climate of

  its own home: species from an arctic or even from a temperate region

  cannot endure a tropical climate, or conversely. So again, many succulent

  plants cannot endure a damp climate. But the degree of adaptation of

  species to the climates under which they live is often overrated. We may

  infer this from our frequent inability to predict whether or not an

  imported plant will endure our climate, and from the number of plants and

  animals brought from warmer countries which here enjoy good health. We

  have reason to believe that species in a state of nature are limited in

  their ranges by the competition of other organic beings quite as much as,

  or more than, by adaptation to particular climates. But whether or not the

  adaptation be generally very close, we have evidence, in the case of some

  few plants, of their becoming, to a certain extent, naturally habituated to

  different temperatures, or becoming acclimatised: thus the pines and

  rhododendrons, raised from seed collected by Dr. Hooker from trees growing

  at different heights on the Himalaya, were found in this country to possess

  different constitutional powers of resisting cold. Mr. Thwaites informs me

  that he has observed similar facts in Ceylon, and analogous observations

  have been made by Mr. H. C. Watson
on European species of plants brought

  from the Azores to England. In regard to animals, several authentic cases

  could be given of species within historical times having largely extended

  their range from warmer to cooler latitudes, and conversely; but we do not

  positively know that these animals were strictly adapted to their native

  climate, but in all ordinary cases we assume such to be the case; nor do we

  know that they have subsequently become acclimatised to their new homes.

  As I believe that our domestic animals were originally chosen by

  uncivilised man because they were useful and bred readily under

  confinement, and not because they were subsequently found capable of

  far-extended transportation, I think the common and extraordinary capacity

  in our domestic animals of not only withstanding the most different

  climates but of being perfectly fertile (a far severer test) under them,

  may be used as an argument that a large proportion of other animals, now in

  a state of nature, could easily be brought to bear widely different

  climates. We must not, however, push the foregoing argument too far, on

  account of the probable origin of some of our domestic animals from several

  wild stocks: the blood, for instance, of a tropical and arctic wolf or

  wild dog may perhaps be mingled in our domestic breeds. The rat and mouse

  cannot be considered as domestic animals, but they have been transported by

  man to many parts of the world, and now have a far wider range than any

  other rodent, living free under the cold climate of Faroe in the north and

  of the Falklands in the south, and on many islands in the torrid zones.

  Hence I am inclined to look at adaptation to any special climate as a

  quality readily grafted on an innate wide flexibility of constitution,

  which is common to most animals. On this view, the capacity of enduring

  the most different climates by man himself and by his domestic animals, and

  such facts as that former species of the elephant and rhinoceros were

  capable of enduring a glacial climate, whereas the living species are now

  all tropical or sub-tropical in their habits, ought not to be looked at as

  anomalies, but merely as examples of a very common flexibility of

  constitution, brought, under peculiar circumstances, into play.

  How much of the acclimatisation of species to any peculiar climate is due

  to mere habit, and how much to the natural selection of varieties having

  different innate constitutions, and how much to both means combined, is a

  very obscure question. That habit or custom has some influence I must

  believe, both from analogy, and from the incessant advice given in

  agricultural works, even in the ancient Encyclopaedias of China, to be very

  cautious in transposing animals from one district to another; for it is not

  likely that man should have succeeded in selecting so many breeds and

  sub-breeds with constitutions specially fitted for their own districts:

  the result must, I think, be due to habit. On the other hand, I can see no

  reason to doubt that natural selection will continually tend to preserve

  those individuals which are born with constitutions best adapted to their

  native countries. In treatises on many kinds of cultivated plants, certain

  varieties are said to withstand certain climates better than others: this

  is very strikingly shown in works on fruit trees published in the United

  States, in which certain varieties are habitually recommended for the

  northern, and others for the southern States; and as most of these

  varieties are of recent origin, they cannot owe their constitutional

  differences to habit. The case of the Jerusalem artichoke, which is never

  propagated by seed, and of which consequently new varieties have not been

  produced, has even been advanced--for it is now as tender as ever it

  was--as proving that acclimatisation cannot be effected! The case, also,

  of the kidney-bean has been often cited for a similar purpose, and with

  much greater weight; but until some one will sow, during a score of

  generations, his kidney-beans so early that a very large proportion are

  destroyed by frost, and then collect seed from the few survivors, with care

  to prevent accidental crosses, and then again get seed from these

  seedlings, with the same precautions, the experiment cannot be said to have

  been even tried. Nor let it be supposed that no differences in the

  constitution of seedling kidney-beans ever appear, for an account has been

  published how much more hardy some seedlings appeared to be than others.

  On the whole, I think we may conclude that habit, use, and disuse, have, in

  some cases, played a considerable part in the modification of the

  constitution, and of the structure of various organs; but that the effects

  of use and disuse have often been largely combined with, and sometimes

  overmastered by, the natural selection of innate differences.

  Correlation of Growth. -- I mean by this expression that the whole

  organisation is so tied together during its growth and development, that

  when slight variations in any one part occur, and are accumulated through

  natural selection, other parts become modified. This is a very important

  subject, most imperfectly understood. The most obvious case is, that

  modifications accumulated solely for the good of the young or larva, will,

  it may safely be concluded, affect the structure of the adult; in the same

  manner as any malconformation affecting the early embryo, seriously affects

  the whole organisation of the adult. The several parts of the body which

  are homologous, and which, at an early embryonic period, are alike, seem

  liable to vary in an allied manner: we see this in the right and left

  sides of the body varying in the same manner; in the front and hind legs,

  and even in the jaws and limbs, varying together, for the lower jaw is

  believed to be homologous with the limbs. These tendencies, I do not

  doubt, may be mastered more or less completely by natural selection: thus

  a family of stags once existed with an antler only on one side; and if this

  had been of any great use to the breed it might probably have been rendered

  permanent by natural selection.

  Homologous parts, as has been remarked by some authors, tend to cohere;

  this is often seen in monstrous plants; and nothing is more common than the

  union of homologous parts in normal structures, as the union of the petals

  of the corolla into a tube. Hard parts seem to affect the form of

  adjoining soft parts; it is believed by some authors that the diversity in

  the shape of the pelvis in birds causes the remarkable diversity in the

  shape of their kidneys. Others believe that the shape of the pelvis in the

  human mother influences by pressure the shape of the head of the child. In

  snakes, according to Schlegel, the shape of the body and the manner of

  swallowing determine the position of several of the most important viscera.

  The nature of the bond of correlation is very frequently quite obscure. M.

  Is. Geoffroy St. Hilaire has forcibly remarked, that certain

  malconformations very frequently, and that others rarely coexist, without

  our being able to assign an
y reason. What can be more singular than the

  relation between blue eyes and deafness in cats, and the tortoise-shell

  colour with the female sex; the feathered feet and skin between the outer

  toes in pigeons, and the presence of more or less down on the young birds

  when first hatched, with the future colour of their plumage; or, again, the

  relation between the hair and teeth in the naked Turkish dog, though here

  probably homology comes into play? With respect to this latter case of

  correlation, I think it can hardly be accidental, that if we pick out the

  two orders of mammalia which are most abnormal in their dermal coverings,

  viz. Cetacea (whales) and Edentata (armadilloes, scaly ant-eaters, &c.),

  that these are likewise the most abnormal in their teeth.

  I know of no case better adapted to show the importance of the laws of

  correlation in modifying important structures, independently of utility

  and, therefore, of natural selection, than that of the difference between

  the outer and inner flowers in some Compositous and Umbelliferous plants.

  Every one knows the difference in the ray and central florets of, for

  instance, the daisy, and this difference is often accompanied with the

  abortion of parts of the flower. But, in some Compositous plants, the

  seeds also differ in shape and sculpture; and even the ovary itself, with

  its accessory parts, differs, as has been described by Cassini. These

  differences have been attributed by some authors to pressure, and the shape

  of the seeds in the ray-florets in some Compositae countenances this idea;

  but, in the case of the corolla of the Umbelliferae, it is by no means, as

  Dr. Hooker informs me, in species with the densest heads that the inner and

  outer flowers most frequently differ. It might have been thought that the

  development of the ray-petals by drawing nourishment from certain other

  parts of the flower had caused their abortion; but in some Compositae there

  is a difference in the seeds of the outer and inner florets without any

  difference in the corolla. Possibly, these several differences may be

  connected with some difference in the flow of nutriment towards the central

  and external flowers: we know, at least, that in irregular flowers, those

  nearest to the axis are oftenest subject to peloria, and become regular. I

  may add, as an instance of this, and of a striking case of correlation,

  that I have recently observed in some garden pelargoniums, that the central

  flower of the truss often loses the patches of darker colour in the two

  upper petals; and that when this occurs, the adherent nectary is quite

  aborted; when the colour is absent from only one of the two upper petals,

  the nectary is only much shortened.

  With respect to the difference in the corolla of the central and exterior

  flowers of a head or umbel, I do not feel at all sure that C. C. Sprengel's

  idea that the ray-florets serve to attract insects, whose agency is highly

  advantageous in the fertilisation of plants of these two orders, is so

  far-fetched, as it may at first appear: and if it be advantageous, natural

  selection may have come into play. But in regard to the differences both

  in the internal and external structure of the seeds, which are not always

  correlated with any differences in the flowers, it seems impossible that

  they can be in any way advantageous to the plant: yet in the Umbelliferae

  these differences are of such apparent importance--the seeds being in some

  cases, according to Tausch, orthospermous in the exterior flowers and

  coelospermous in the central flowers,--that the elder De Candolle founded

  his main divisions of the order on analogous differences. Hence we see

  that modifications of structure, viewed by systematists as of high value,

  may be wholly due to unknown laws of correlated growth, and without being,

  as far as we can see, of the slightest service to the species.