The Origin of Species

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The Origin of Species Page 10

by Charles Darwin

beings, with which it comes into competition for food or residence, or from

  which it has to escape, or on which it preys. This is obvious in the

  structure of the teeth and talons of the tiger; and in that of the legs and

  claws of the parasite which clings to the hair on the tiger's body. But in

  the beautifully plumed seed of the dandelion, and in the flattened and

  fringed legs of the water-beetle, the relation seems at first confined to

  the elements of air and water. Yet the advantage of plumed seeds no doubt

  stands in the closest relation to the land being already thickly clothed by

  other plants; so that the seeds may be widely distributed and fall on

  unoccupied ground. In the water-beetle, the structure of its legs, so well

  adapted for diving, allows it to compete with other aquatic insects, to

  hunt for its own prey, and to escape serving as prey to other animals.

  The store of nutriment laid up within the seeds of many plants seems at

  first sight to have no sort of relation to other plants. But from the

  strong growth of young plants produced from such seeds (as peas and beans),

  when sown in the midst of long grass, I suspect that the chief use of the

  nutriment in the seed is to favour the growth of the young seedling, whilst

  struggling with other plants growing vigorously all around.

  Look at a plant in the midst of its range, why does it not double or

  quadruple its numbers? We know that it can perfectly well withstand a

  little more heat or cold, dampness or dryness, for elsewhere it ranges into

  slightly hotter or colder, damper or drier districts. In this case we can

  clearly see that if we wished in imagination to give the plant the power of

  increasing in number, we should have to give it some advantage over its

  competitors, or over the animals which preyed on it. On the confines of

  its geographical range, a change of constitution with respect to climate

  would clearly be an advantage to our plant; but we have reason to believe

  that only a few plants or animals range so far, that they are destroyed by

  the rigour of the climate alone. Not until we reach the extreme confines

  of life, in the arctic regions or on the borders of an utter desert, will

  competition cease. The land may be extremely cold or dry, yet there will

  be competition between some few species, or between the individuals of the

  same species, for the warmest or dampest spots.

  Hence, also, we can see that when a plant or animal is placed in a new

  country amongst new competitors, though the climate may be exactly the same

  as in its former home, yet the conditions of its life will generally be

  changed in an essential manner. If we wished to increase its average

  numbers in its new home, we should have to modify it in a different way to

  what we should have done in its native country; for we should have to give

  it some advantage over a different set of competitors or enemies.

  It is good thus to try in our imagination to give any form some advantage

  over another. Probably in no single instance should we know what to do, so

  as to succeed. It will convince us of our ignorance on the mutual

  relations of all organic beings; a conviction as necessary, as it seems to

  be difficult to acquire. All that we can do, is to keep steadily in mind

  that each organic being is striving to increase at a geometrical ratio;

  that each at some period of its life, during some season of the year,

  during each generation or at intervals, has to struggle for life, and to

  suffer great destruction. When we reflect on this struggle, we may console

  ourselves with the full belief, that the war of nature is not incessant,

  that no fear is felt, that death is generally prompt, and that the

  vigorous, the healthy, and the happy survive and multiply.

  Chapter IV

  Natural Selection

  Natural Selection -- its power compared with man's selection -- its power

  on characters of trifling importance -- its power at all ages and on both

  sexes -- Sexual Selection -- On the generality of intercrosses between

  individuals of the same species -- Circumstances favourable and

  unfavourable to Natural Selection, namely, intercrossing, isolation, number

  of individuals -- Slow action -- Extinction caused by Natural Selection --

  Divergence of Character, related to the diversity of inhabitants of any

  small area, and to naturalisation -- Action of Natural Selection, through

  Divergence of Character and Extinction, on the descendants from a common

  parent -- Explains the Grouping of all organic beings.

  How will the struggle for existence, discussed too briefly in the last

  chapter, act in regard to variation? Can the principle of selection, which

  we have seen is so potent in the hands of man, apply in nature? I think we

  shall see that it can act most effectually. Let it be borne in mind in

  what an endless number of strange peculiarities our domestic productions,

  and, in a lesser degree, those under nature, vary; and how strong the

  hereditary tendency is. Under domestication, it may be truly said that the

  whole organisation becomes in some degree plastic. Let it be borne in mind

  how infinitely complex and close-fitting are the mutual relations of all

  organic beings to each other and to their physical conditions of life. Can

  it, then, be thought improbable, seeing that variations useful to man have

  undoubtedly occurred, that other variations useful in some way to each

  being in the great and complex battle of life, should sometimes occur in

  the course of thousands of generations? If such do occur, can we doubt

  (remembering that many more individuals are born than can possibly survive)

  that individuals having any advantage, however slight, over others, would

  have the best chance of surviving and of procreating their kind? On the

  other hand, we may feel sure that any variation in the least degree

  injurious would be rigidly destroyed. This preservation of favourable

  variations and the rejection of injurious variations, I call Natural

  Selection. Variations neither useful nor injurious would not be affected

  by natural selection, and would be left a fluctuating element, as perhaps

  we see in the species called polymorphic.

  We shall best understand the probable course of natural selection by taking

  the case of a country undergoing some physical change, for instance, of

  climate. The proportional numbers of its inhabitants would almost

  immediately undergo a change, and some species might become extinct. We

  may conclude, from what we have seen of the intimate and complex manner in

  which the inhabitants of each country are bound together, that any change

  in the numerical proportions of some of the inhabitants, independently of

  the change of climate itself, would most seriously affect many of the

  others. If the country were open on its borders, new forms would certainly

  immigrate, and this also would seriously disturb the relations of some of

  the former inhabitants. Let it be remembered how powerful the influence of

  a single introduced tree or mammal has been shown to be. But in the case

  of an island, or of a country partly surrounded by barriers, into which new

  an
d better adapted forms could not freely enter, we should then have places

  in the economy of nature which would assuredly be better filled up, if some

  of the original inhabitants were in some manner modified; for, had the area

  been open to immigration, these same places would have been seized on by

  intruders. In such case, every slight modification, which in the course of

  ages chanced to arise, and which in any way favoured the individuals of any

  of the species, by better adapting them to their altered conditions, would

  tend to be preserved; and natural selection would thus have free scope for

  the work of improvement.

  We have reason to believe, as stated in the first chapter, that a change in

  the conditions of life, by specially acting on the reproductive system,

  causes or increases variability; and in the foregoing case the conditions

  of life are supposed to have undergone a change, and this would manifestly

  be favourable to natural selection, by giving a better chance of profitable

  variations occurring; and unless profitable variations do occur, natural

  selection can do nothing. Not that, as I believe, any extreme amount of

  variability is necessary; as man can certainly produce great results by

  adding up in any given direction mere individual differences, so could

  Nature, but far more easily, from having incomparably longer time at her

  disposal. Nor do I believe that any great physical change, as of climate,

  or any unusual degree of isolation to check immigration, is actually

  necessary to produce new and unoccupied places for natural selection to

  fill up by modifying and improving some of the varying inhabitants. For as

  all the inhabitants of each country are struggling together with nicely

  balanced forces, extremely slight modifications in the structure or habits

  of one inhabitant would often give it an advantage over others; and still

  further modifications of the same kind would often still further increase

  the advantage. No country can be named in which all the native inhabitants

  are now so perfectly adapted to each other and to the physical conditions

  under which they live, that none of them could anyhow be improved; for in

  all countries, the natives have been so far conquered by naturalised

  productions, that they have allowed foreigners to take firm possession of

  the land. And as foreigners have thus everywhere beaten some of the

  natives, we may safely conclude that the natives might have been modified

  with advantage, so as to have better resisted such intruders.

  As man can produce and certainly has produced a great result by his

  methodical and unconscious means of selection, what may not nature effect?

  Man can act only on external and visible characters: nature cares nothing

  for appearances, except in so far as they may be useful to any being. She

  can act on every internal organ, on every shade of constitutional

  difference, on the whole machinery of life. Man selects only for his own

  good; Nature only for that of the being which she tends. Every selected

  character is fully exercised by her; and the being is placed under

  well-suited conditions of life. Man keeps the natives of many climates in

  the same country; he seldom exercises each selected character in some

  peculiar and fitting manner; he feeds a long and a short beaked pigeon on

  the same food; he does not exercise a long-backed or long-legged quadruped

  in any peculiar manner; he exposes sheep with long and short wool to the

  same climate. He does not allow the most vigorous males to struggle for

  the females. He does not rigidly destroy all inferior animals, but

  protects during each varying season, as far as lies in his power, all his

  productions. He often begins his selection by some half-monstrous form; or

  at least by some modification prominent enough to catch his eye, or to be

  plainly useful to him. Under nature, the slightest difference of structure

  or constitution may well turn the nicely-balanced scale in the struggle for

  life, and so be preserved. How fleeting are the wishes and efforts of man!

  how short his time! and consequently how poor will his products be,

  compared with those accumulated by nature during whole geological periods.

  Can we wonder, then, that nature's productions should be far 'truer' in

  character than man's productions; that they should be infinitely better

  adapted to the most complex conditions of life, and should plainly bear the

  stamp of far higher workmanship?

  It may be said that natural selection is daily and hourly scrutinising,

  throughout the world, every variation, even the slightest; rejecting that

  which is bad, preserving and adding up all that is good; silently and

  insensibly working, whenever and wherever opportunity offers, at the

  improvement of each organic being in relation to its organic and inorganic

  conditions of life. We see nothing of these slow changes in progress,

  until the hand of time has marked the long lapse of ages, and then so

  imperfect is our view into long past geological ages, that we only see that

  the forms of life are now different from what they formerly were.

  Although natural selection can act only through and for the good of each

  being, yet characters and structures, which we are apt to consider as of

  very trifling importance, may thus be acted on. When we see leaf-eating

  insects green, and bark-feeders mottled-grey; the alpine ptarmigan white in

  winter, the red-grouse the colour of heather, and the black-grouse that of

  peaty earth, we must believe that these tints are of service to these birds

  and insects in preserving them from danger. Grouse, if not destroyed at

  some period of their lives, would increase in countless numbers; they are

  known to suffer largely from birds of prey; and hawks are guided by

  eyesight to their prey,--so much so, that on parts of the Continent persons

  are warned not to keep white pigeons, as being the most liable to

  destruction. Hence I can see no reason to doubt that natural selection

  might be most effective in giving the proper colour to each kind of grouse,

  and in keeping that colour, when once acquired, true and constant. Nor

  ought we to think that the occasional destruction of an animal of any

  particular colour would produce little effect: we should remember how

  essential it is in a flock of white sheep to destroy every lamb with the

  faintest trace of black. In plants the down on the fruit and the colour of

  the flesh are considered by botanists as characters of the most trifling

  importance: yet we hear from an excellent horticulturist, Downing, that in

  the United States smooth-skinned fruits suffer far more from a beetle, a

  curculio, than those with down; that purple plums suffer far more from a

  certain disease than yellow plums; whereas another disease attacks

  yellow-fleshed peaches far more than those with other coloured flesh. If,

  with all the aids of art, these slight differences make a great difference

  in cultivating the several varieties, assuredly, in a state of nature,

  where the trees would have to struggle with other trees and with a host of

  enemies, such differences would effectually settle which variety, whether
a

  smooth or downy, a yellow or purple fleshed fruit, should succeed.

  In looking at many small points of difference between species, which, as

  far as our ignorance permits us to judge, seem to be quite unimportant, we

  must not forget that climate, food, &c., probably produce some slight and

  direct effect. It is, however, far more necessary to bear in mind that

  there are many unknown laws of correlation of growth, which, when one part

  of the organisation is modified through variation, and the modifications

  are accumulated by natural selection for the good of the being, will cause

  other modifications, often of the most unexpected nature.

  As we see that those variations which under domestication appear at any

  particular period of life, tend to reappear in the offspring at the same

  period;--for instance, in the seeds of the many varieties of our culinary

  and agricultural plants; in the caterpillar and cocoon stages of the

  varieties of the silkworm; in the eggs of poultry, and in the colour of the

  down of their chickens; in the horns of our sheep and cattle when nearly

  adult;--so in a state of nature, natural selection will be enabled to act

  on and modify organic beings at any age, by the accumulation of profitable

  variations at that age, and by their inheritance at a corresponding age.

  If it profit a plant to have its seeds more and more widely disseminated by

  the wind, I can see no greater difficulty in this being effected through

  natural selection, than in the cotton-planter increasing and improving by

  selection the down in the pods on his cotton-trees. Natural selection may

  modify and adapt the larva of an insect to a score of contingencies, wholly

  different from those which concern the mature insect. These modifications

  will no doubt affect, through the laws of correlation, the structure of the

  adult; and probably in the case of those insects which live only for a few

  hours, and which never feed, a large part of their structure is merely the

  correlated result of successive changes in the structure of their larvae.

  So, conversely, modifications in the adult will probably often affect the

  structure of the larva; but in all cases natural selection will ensure that

  modifications consequent on other modifications at a different period of

  life, shall not be in the least degree injurious: for if they became so,

  they would cause the extinction of the species.

  Natural selection will modify the structure of the young in relation to the

  parent, and of the parent in relation to the young. In social animals it

  will adapt the structure of each individual for the benefit of the

  community; if each in consequence profits by the selected change. What

  natural selection cannot do, is to modify the structure of one species,

  without giving it any advantage, for the good of another species; and

  though statements to this effect may be found in works of natural history,

  I cannot find one case which will bear investigation. A structure used

  only once in an animal's whole life, if of high importance to it, might be

  modified to any extent by natural selection; for instance, the great jaws

  possessed by certain insects, and used exclusively for opening the

  cocoon--or the hard tip to the beak of nestling birds, used for breaking

  the egg. It has been asserted, that of the best short-beaked

  tumbler-pigeons more perish in the egg than are able to get out of it; so

  that fanciers assist in the act of hatching. Now, if nature had to make

  the beak of a full-grown pigeon very short for the bird's own advantage,

  the process of modification would be very slow, and there would be

  simultaneously the most rigorous selection of the young birds within the

  egg, which had the most powerful and hardest beaks, for all with weak beaks

  would inevitably perish: or, more delicate and more easily broken shells

 

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