The Origin of Species

might be selected, the thickness of the shell being known to vary like

  every other structure.

  Sexual Selection. -- Inasmuch as peculiarities often appear under

  domestication in one sex and become hereditarily attached to that sex, the

  same fact probably occurs under nature, and if so, natural selection will

  be able to modify one sex in its functional relations to the other sex, or

  in relation to wholly different habits of life in the two sexes, as is

  sometimes the case with insects. And this leads me to say a few words on

  what I call Sexual Selection. This depends, not on a struggle for

  existence, but on a struggle between the males for possession of the

  females; the result is not death to the unsuccessful competitor, but few or

  no offspring. Sexual selection is, therefore, less rigorous than natural

  selection. Generally, the most vigorous males, those which are best fitted

  for their places in nature, will leave most progeny. But in many cases,

  victory will depend not on general vigour, but on having special weapons,

  confined to the male sex. A hornless stag or spurless cock would have a

  poor chance of leaving offspring. Sexual selection by always allowing the

  victor to breed might surely give indomitable courage, length to the spur,

  and strength to the wing to strike in the spurred leg, as well as the

  brutal cock-fighter, who knows well that he can improve his breed by

  careful selection of the best cocks. How low in the scale of nature this

  law of battle descends, I know not; male alligators have been described as

  fighting, bellowing, and whirling round, like Indians in a war-dance, for

  the possession of the females; male salmons have been seen fighting all day

  long; male stag-beetles often bear wounds from the huge mandibles of other

  males. The war is, perhaps, severest between the males of polygamous

  animals, and these seem oftenest provided with special weapons. The males

  of carnivorous animals are already well armed; though to them and to

  others, special means of defence may be given through means of sexual

  selection, as the mane to the lion, the shoulder-pad to the boar, and the

  hooked jaw to the male salmon; for the shield may be as important for

  victory, as the sword or spear.

  Amongst birds, the contest is often of a more peaceful character. All

  those who have attended to the subject, believe that there is the severest

  rivalry between the males of many species to attract by singing the

  females. The rock-thrush of Guiana, birds of Paradise, and some others,

  congregate; and successive males display their gorgeous plumage and perform

  strange antics before the females, which standing by as spectators, at last

  choose the most attractive partner. Those who have closely attended to

  birds in confinement well know that they often take individual preferences

  and dislikes: thus Sir R. Heron has described how one pied peacock was

  eminently attractive to all his hen birds. It may appear childish to

  attribute any effect to such apparently weak means: I cannot here enter on

  the details necessary to support this view; but if man can in a short time

  give elegant carriage and beauty to his bantams, according to his standard

  of beauty, I can see no good reason to doubt that female birds, by

  selecting, during thousands of generations, the most melodious or beautiful

  males, according to their standard of beauty, might produce a marked

  effect. I strongly suspect that some well-known laws with respect to the

  plumage of male and female birds, in comparison with the plumage of the

  young, can be explained on the view of plumage having been chiefly modified

  by sexual selection, acting when the birds have come to the breeding age or

  during the breeding season; the modifications thus produced being inherited

  at corresponding ages or seasons, either by the males alone, or by the

  males and females; but I have not space here to enter on this subject.

  Thus it is, as I believe, that when the males and females of any animal

  have the same general habits of life, but differ in structure, colour, or

  ornament, such differences have been mainly caused by sexual selection;

  that is, individual males have had, in successive generations, some slight

  advantage over other males, in their weapons, means of defence, or charms;

  and have transmitted these advantages to their male offspring. Yet, I

  would not wish to attribute all such sexual differences to this agency:

  for we see peculiarities arising and becoming attached to the male sex in

  our domestic animals (as the wattle in male carriers, horn-like

  protuberances in the cocks of certain fowls, &c.), which we cannot believe

  to be either useful to the males in battle, or attractive to the females.

  We see analogous cases under nature, for instance, the tuft of hair on the

  breast of the turkey-cock, which can hardly be either useful or ornamental

  to this bird;--indeed, had the tuft appeared under domestication, it would

  have been called a monstrosity.

  Illustrations of the action of Natural Selection. -- In order to make it

  clear how, as I believe, natural selection acts, I must beg permission to

  give one or two imaginary illustrations. Let us take the case of a wolf,

  which preys on various animals, securing some by craft, some by strength,

  and some by fleetness; and let us suppose that the fleetest prey, a deer

  for instance, had from any change in the country increased in numbers, or

  that other prey had decreased in numbers, during that season of the year

  when the wolf is hardest pressed for food. I can under such circumstances

  see no reason to doubt that the swiftest and slimmest wolves would have the

  best chance of surviving, and so be preserved or selected,--provided always

  that they retained strength to master their prey at this or at some other

  period of the year, when they might be compelled to prey on other animals.

  I can see no more reason to doubt this, than that man can improve the

  fleetness of his greyhounds by careful and methodical selection, or by that

  unconscious selection which results from each man trying to keep the best

  dogs without any thought of modifying the breed.

  Even without any change in the proportional numbers of the animals on which

  our wolf preyed, a cub might be born with an innate tendency to pursue

  certain kinds of prey. Nor can this be thought very improbable; for we

  often observe great differences in the natural tendencies of our domestic

  animals; one cat, for instance, taking to catch rats, another mice; one

  cat, according to Mr. St. John, bringing home winged game, another hares or

  rabbits, and another hunting on marshy ground and almost nightly catching

  woodcocks or snipes. The tendency to catch rats rather than mice is known

  to be inherited. Now, if any slight innate change of habit or of structure

  benefited an individual wolf, it would have the best chance of surviving

  and of leaving offspring. Some of its young would probably inherit the

  same habits or structure, and by the repetition of this process, a new

  variety might be formed which would either supplant or coexist with the

  parent-form of wolf. Or, again
, the wolves inhabiting a mountainous

  district, and those frequenting the lowlands, would naturally be forced to

  hunt different prey; and from the continued preservation of the individuals

  best fitted for the two sites, two varieties might slowly be formed. These

  varieties would cross and blend where they met; but to this subject of

  intercrossing we shall soon have to return. I may add, that, according to

  Mr. Pierce, there are two varieties of the wolf inhabiting the Catskill

  Mountains in the United States, one with a light greyhound-like form, which

  pursues deer, and the other more bulky, with shorter legs, which more

  frequently attacks the shepherd's flocks.

  Let us now take a more complex case. Certain plants excrete a sweet juice,

  apparently for the sake of eliminating something injurious from their sap:

  this is effected by glands at the base of the stipules in some Leguminosae,

  and at the back of the leaf of the common laurel. This juice, though small

  in quantity, is greedily sought by insects. Let us now suppose a little

  sweet juice or nectar to be excreted by the inner bases of the petals of a

  flower. In this case insects in seeking the nectar would get dusted with

  pollen, and would certainly often transport the pollen from one flower to

  the stigma of another flower. The flowers of two distinct individuals of

  the same species would thus get crossed; and the act of crossing, we have

  good reason to believe (as will hereafter be more fully alluded to), would

  produce very vigorous seedlings, which consequently would have the best

  chance of flourishing and surviving. Some of these seedlings would

  probably inherit the nectar-excreting power. Those individual flowers

  which had the largest glands or nectaries, and which excreted most nectar,

  would be oftenest visited by insects, and would be oftenest crossed; and so

  in the long-run would gain the upper hand. Those flowers, also, which had

  their stamens and pistils placed, in relation to the size and habits of the

  particular insects which visited them, so as to favour in any degree the

  transportal of their pollen from flower to flower, would likewise be

  favoured or selected. We might have taken the case of insects visiting

  flowers for the sake of collecting pollen instead of nectar; and as pollen

  is formed for the sole object of fertilisation, its destruction appears a

  simple loss to the plant; yet if a little pollen were carried, at first

  occasionally and then habitually, by the pollen-devouring insects from

  flower to flower, and a cross thus effected, although nine-tenths of the

  pollen were destroyed, it might still be a great gain to the plant; and

  those individuals which produced more and more pollen, and had larger and

  larger anthers, would be selected.

  When our plant, by this process of the continued preservation or natural

  selection of more and more attractive flowers, had been rendered highly

  attractive to insects, they would, unintentionally on their part, regularly

  carry pollen from flower to flower; and that they can most effectually do

  this, I could easily show by many striking instances. I will give only

  one--not as a very striking case, but as likewise illustrating one step in

  the separation of the sexes of plants, presently to be alluded to. Some

  holly-trees bear only male flowers, which have four stamens producing

  rather a small quantity of pollen, and a rudimentary pistil; other

  holly-trees bear only female flowers; these have a full-sized pistil, and

  four stamens with shrivelled anthers, in which not a grain of pollen can be

  detected. Having found a female tree exactly sixty yards from a male tree,

  I put the stigmas of twenty flowers, taken from different branches, under

  the microscope, and on all, without exception, there were pollen-grains,

  and on some a profusion of pollen. As the wind had set for several days

  from the female to the male tree, the pollen could not thus have been

  carried. The weather had been cold and boisterous, and therefore not

  favourable to bees, nevertheless every female flower which I examined had

  been effectually fertilised by the bees, accidentally dusted with pollen,

  having flown from tree to tree in search of nectar. But to return to our

  imaginary case: as soon as the plant had been rendered so highly

  attractive to insects that pollen was regularly carried from flower to

  flower, another process might commence. No naturalist doubts the advantage

  of what has been called the 'physiological division of labour;' hence we

  may believe that it would be advantageous to a plant to produce stamens

  alone in one flower or on one whole plant, and pistils alone in another

  flower or on another plant. In plants under culture and placed under new

  conditions of life, sometimes the male organs and sometimes the female

  organs become more or less impotent; now if we suppose this to occur in

  ever so slight a degree under nature, then as pollen is already carried

  regularly from flower to flower, and as a more complete separation of the

  sexes of our plant would be advantageous on the principle of the division

  of labour, individuals with this tendency more and more increased, would be

  continually favoured or selected, until at last a complete separation of

  the sexes would be effected.

  Let us now turn to the nectar-feeding insects in our imaginary case: we

  may suppose the plant of which we have been slowly increasing the nectar by

  continued selection, to be a common plant; and that certain insects

  depended in main part on its nectar for food. I could give many facts,

  showing how anxious bees are to save time; for instance, their habit of

  cutting holes and sucking the nectar at the bases of certain flowers, which

  they can, with a very little more trouble, enter by the mouth. Bearing

  such facts in mind, I can see no reason to doubt that an accidental

  deviation in the size and form of the body, or in the curvature and length

  of the proboscis, &c., far too slight to be appreciated by us, might profit

  a bee or other insect, so that an individual so characterised would be able

  to obtain its food more quickly, and so have a better chance of living and

  leaving descendants. Its descendants would probably inherit a tendency to

  a similar slight deviation of structure. The tubes of the corollas of the

  common red and incarnate clovers (Trifolium pratense and incarnatum) do not

  on a hasty glance appear to differ in length; yet the hive-bee can easily

  suck the nectar out of the incarnate clover, but not out of the common red

  clover, which is visited by humble-bees alone; so that whole fields of the

  red clover offer in vain an abundant supply of precious nectar to the

  hive-bee. Thus it might be a great advantage to the hive-bee to have a

  slightly longer or differently constructed proboscis. On the other hand, I

  have found by experiment that the fertility of clover greatly depends on

  bees visiting and moving parts of the corolla, so as to push the pollen on

  to the stigmatic surface. Hence, again, if humble-bees were to become rare

  in any country, it might be a great advantage to the red clover to have a

  sh
orter or more deeply divided tube to its corolla, so that the hive-bee

  could visit its flowers. Thus I can understand how a flower and a bee

  might slowly become, either simultaneously or one after the other, modified

  and adapted in the most perfect manner to each other, by the continued

  preservation of individuals presenting mutual and slightly favourable

  deviations of structure.

  I am well aware that this doctrine of natural selection, exemplified in the

  above imaginary instances, is open to the same objections which were at

  first urged against Sir Charles Lyell's noble views on 'the modern changes

  of the earth, as illustrative of geology;' but we now very seldom hear the

  action, for instance, of the coast-waves, called a trifling and

  insignificant cause, when applied to the excavation of gigantic valleys or

  to the formation of the longest lines of inland cliffs. Natural selection

  can act only by the preservation and accumulation of infinitesimally small

  inherited modifications, each profitable to the preserved being; and as

  modern geology has almost banished such views as the excavation of a great

  valley by a single diluvial wave, so will natural selection, if it be a

  true principle, banish the belief of the continued creation of new organic

  beings, or of any great and sudden modification in their structure.

  On the Intercrossing of Individuals. -- I must here introduce a short

  digression. In the case of animals and plants with separated sexes, it is

  of course obvious that two individuals must always unite for each birth;

  but in the case of hermaphrodites this is far from obvious. Nevertheless I

  am strongly inclined to believe that with all hermaphrodites two

  individuals, either occasionally or habitually, concur for the reproduction

  of their kind. This view, I may add, was first suggested by Andrew Knight.

  We shall presently see its importance; but I must here treat the subject

  with extreme brevity, though I have the materials prepared for an ample

  discussion. All vertebrate animals, all insects, and some other large

  groups of animals, pair for each birth. Modern research has much

  diminished the number of supposed hermaphrodites, and of real

  hermaphrodites a large number pair; that is, two individuals regularly

  unite for reproduction, which is all that concerns us. But still there are

  many hermaphrodite animals which certainly do not habitually pair, and a

  vast majority of plants are hermaphrodites. What reason, it may be asked,

  is there for supposing in these cases that two individuals ever concur in

  reproduction? As it is impossible here to enter on details, I must trust

  to some general considerations alone.

  In the first place, I have collected so large a body of facts, showing, in

  accordance with the almost universal belief of breeders, that with animals

  and plants a cross between different varieties, or between individuals of

  the same variety but of another strain, gives vigour and fertility to the

  offspring; and on the other hand, that close interbreeding diminishes

  vigour and fertility; that these facts alone incline me to believe that it

  is a general law of nature (utterly ignorant though we be of the meaning of

  the law) that no organic being self-fertilises itself for an eternity of

  generations; but that a cross with another individual is

  occasionally--perhaps at very long intervals--indispensable.

  On the belief that this is a law of nature, we can, I think, understand

  several large classes of facts, such as the following, which on any other

  view are inexplicable. Every hybridizer knows how unfavourable exposure to

  wet is to the fertilisation of a flower, yet what a multitude of flowers

  have their anthers and stigmas fully exposed to the weather! but if an

  occasional cross be indispensable, the fullest freedom for the entrance of

  pollen from another individual will explain this state of exposure, more