Chapter 6 Chapter 4 Natural Selection: Survival of the Fittest 1

NATURAL SELECTION - ITS POWER COMPARED TO ARTIFICIAL SELECTION - ITS POWER OVER INVISIBLE CHARACTERS - ITS POWER OVER AGES AND SEXES - SEXUAL SELECTION - ON THE UNIVERSALITY OF CROSSING BETWEEN INDIVIDUALS OF THE SAME SPECIES - - Conditions favorable and unfavorable to the results of natural selection, i.e. crossing, segregation, number of individuals - slow action - extinction by natural selection - divergence of characters, connection with divergence of organisms in any small area, and Connexion with Naturalization - Action of Natural Selection, by divergence and extinction of characters, on the descendants of a common ancestor - Explanation of all taxonomy - Progression of organic systems - Preservation of inferior forms - Inheritance of characters Convergence—The Infinite Multiplication of Species—Summary.

How does the struggle for existence, briefly discussed in the previous chapter, work on variation?Can the principle of selection, which has played a huge role in the hands of human beings, be applied in nature?I think we will see that it can work extremely effectively.Let us bear in mind the infinite number of slight variations and individual differences in domesticated organisms, and to a lesser degree in those in a state of nature; and remember also the power of hereditary disposition.Under domestication, it may be said, the whole constitution of living beings becomes plastic to a certain extent.The variations we almost universally encounter in domesticated organisms are not, as Hooker and Asa Gray say, directly produced by human power; man cannot create varieties, nor prevent them; Variations are merely preserved and accumulated, and variations occur by man unintentionally placing organic beings under new and changing conditions of life; but similar changes in conditions of life can and do occur under natural conditions.We should also remember how complex and intimate are the relations of organic beings to one another and to the physical conditions of life; and thus infinitely divergent structures will always be of some use to living beings living under varying conditions.Since variations useful to man must have occurred, is it not possible that other variations, useful in some respects to every being, in the vast and complicated struggle for existence, would have occurred in the course of many successive generations?If such variations do occur (it must be remembered that more individuals are produced than are likely to survive), then those individuals which are superior (even if slightly) to others have the best chance of surviving and reproducing, What is there to doubt?On the other hand, we may be sure that any deleterious variation, even in the slightest degree, would be seriously destroyed.I call this preservation of favorable individual differences and variations, and the destruction of those harmful variations, "natural selection," or "survival of the fittest."Variations which are useless and innocuous, are not subject to natural selection, and either become erratic characters, as we see in some polymorphic species, or finally become fixed characters, as determined by the nature and nature of the organism. determined by external conditions.

Several authors have misunderstood or objected to the term "natural selection."Some have even imagined that natural selection can induce variations, when it can only preserve those variations which have already occurred and which are advantageous to the organism under its conditions of life.No one objects to the great effects of artificial selection, as the agronomist speaks; but in this case individual differences occurring in nature must precede man's selection for any purpose.Still others have objected to the term selection, arguing that it implies that altered animals are capable of conscious choice; and have even urged that, since plants have no volition, natural selection cannot be applied to them!Taken literally, there is no doubt that the term natural selection is inaccurate; yet who has objected to a chemist who speaks of a selective affinity for the elements?Strictly speaking it cannot be said that an acid chooses the base it wishes to combine.It is said that I speak of natural selection as a dynamic or "divine force"; but who has objected to a writer who says that gravitation controls the motion of the planets?Everyone knows what such metaphorical words imply; for the sake of simplicity, such nouns are almost necessary, and it is difficult to avoid the anthropomorphism of the word "Nature"; but what I call "Natural" "It just refers to the combined action and product of many natural laws, and the law is the causal relationship of various things we determine.With a little familiarity, these superficial objections will be forgotten.

We shall best understand the general process of natural selection by studying a place which is undergoing some slight physical change, such as a change of climate.As soon as the climate changes, the proportions of life there will change almost immediately, and some species will probably become extinct. From what we know of the close and complicated relationships of the living things in various places, it can be concluded that even leaving aside the climate Aside from changes in , any change in a creature's proportional numbers can seriously affect other creatures as well.If the frontiers of that country were open, new forms would necessarily migrate into it, and this would seriously disturb the relations of some of the original beings.Remember: How powerful is the influence of introducing a tree or a mammal from another country; it has been shown.But if, on an island, or a place partly surrounded by obstacles, new and adaptable forms cannot migrate freely, some place will be made in the natural composition of the place, and if some old Creatures have changed in some way, and they must have filled it; for if the area had been allowed to move freely, alien creatures would have taken its place long ago.Under such circumstances, slight variations, which in any respect benefit the individuals of any species, and enable them better to adapt to changed external conditions, will tend to be preserved; There is room for work.

As shown in the first chapter, there are good reasons to believe that changes in the conditions of life tend to increase variability; This is obviously a great benefit to natural selection.Natural selection cannot operate without favorable variation, and it must not be forgotten that only individual differences are covered by the term "variation."Just as the accumulation of individual differences in any given direction by man can produce great results in domesticated animals and plants, so natural selection can do this, and much more easily, because it has an incomparably long period to take place. effect.I do not believe that any great physical change, such as that of climate, or a high degree of isolation to hinder immigration, must have made some new openings before natural selection could improve some varying organisms to fill them.For all the organisms of each region struggle with one another in a fine balance of forces, and the slightest variation in the structure or habits of one species will often give it an advantage over the others; so long as the species continues to live under the same conditions of life. , and have gained advantage by the same means of subsistence and defense, the same variation will develop more and more, and often increase its advantage.There is no place where all the native organisms have now been so perfectly adapted to each other, and to the physical conditions in which they live, that none of them could not be better adapted or improved; For everywhere the aliens have often triumphed over the natives, and have firmly established themselves in the land.As alien beings thus prevail here and there over some of the natives, it may be safely asserted that the natives will also be advantageously modified to better resist those intruders.

Human beings can and have produced great results by means of planned and unconscious selection. Why can't natural selection work?Man can act only on extrinsic and visible characters; "Nature" - if I may allow me to anthropomorphize nature preservation or survival of the fittest - is not concerned with appearances unless they are useful to the creature. "Nature" can act on every internal organ, every subtle difference of constitution, and the whole mechanism of life.Man chooses only for his own benefit: Nature chooses only for the benefit of the beings she protects.Selected characters are, as the very fact of their selection indicates, fully exercised by nature.Man keeps in the same place many organisms which grow in different climates; he seldom exercises each selected character in any particular and proper way; pigeons; he trained no particular method of long-backed or long-legged quadrupeds; he kept long-haired and short-haired sheep in the same climate.He does not allow the strongest males to fight for the female.He does not strictly destroy all animals of inferior qualities, but preserves all his creatures, so far as he can, in all seasons.He will often begin his selection on some semi-monstrous form; or at least on some modification so marked as to attract his attention, or to his obvious advantage.In a state of nature, the most minute difference in structure or constitution can alter the delicate balance of the struggle of life, and it is thus preserved.Human desire and effort are but a matter of a moment!How short is the life of man!How poor, then, are the results of man when compared with the cumulative results of "nature" throughout all geological epochs!In this way, the products of "nature" must have much more "real" characters than the products of human beings, be more infinitely adaptable to extremely complex living conditions, and manifest obviously higher skills, what is there for us? Surprised?

We may say figuratively that natural selection scrutinizes the slightest variation, rejecting the bad, preserving and accumulating the good, every day and every hour of the world; quietly, and very slowly, it works to improve the relation of every living being to the organic and inorganic conditions of life.This slow progression of change we cannot perceive unless there is a sign of the passage of time.But we know so little of the long geological past, and all we can see is that the types of life now are different from those that preceded them. To effect any great variation in a species, favorable variations or individual differences of the same nature must again occur, after a long period of time, after the varieties have once been formed; and these variations must again be preserved, and so, step by step. .Since individual differences of the same kind recur, this supposition should not be regarded as unfounded.But whether this assumption is correct, we can only judge by seeing whether it conforms to and can explain the general phenomena of nature.On the other hand, the general belief that the amount of variation is strictly limited is likewise an out-and-out assumption.

Though natural selection can act only through and for the benefit of individual organisms, it may also act in this way on those characters and structures which we tend to regard as of the least importance.When we see leaf-eaters green, bark-eaters dappled-gray; alpine grouse white in winter, red grouse heather-coloured, we must believe that this color is for the protection of these Birds and insects to avoid danger.The grouse, if not killed at a certain period of its life, must multiply to innumerable numbers; we know that they are infested in great numbers by birds of prey; and the hawk pursues its prey by its eyesight--sight so sharp that men in some parts of the Continent fear it. Do not keep white pigeons as they are extremely vulnerable.Natural selection, therefore, has acted in such a way as to give the species of grouse the proper colour, and, when they have once acquired it, to preserve it purely and permanently.Let us not think that the occasional removal of an animal of a particular color has little effect; let us remember how important it is to get rid of a slightly black lamb in a white flock.As mentioned earlier, Virginia pigs that eat "red roots" will live or die by their color.As regards plants, botanists regard the down and color of the fruit as very unimportant characters; yet we hear a good horticulturalist, Downing, say that in America a weevil (Curculio ) injure smooth-skinned fruits far more than fuzzy ones; purple plums are more injured by certain diseases than yellow ones; disease.If, by all means of artificial selection, such small differences would produce great differences in the cultivation of several varieties, then, in a state of nature, one tree would have to fight against another tree and a great number of predators, when the species would Differences in disease perception can powerfully determine which variety -- smooth or hairy-skinned, yellow or purple-fleshed -- succeeds.

In observing the many small differences between species, which, to judge by our limited knowledge, seem to be quite unimportant, we must not forget that climate, food, etc., undoubtedly have some direct effect upon them.It must also be remembered that, by virtue of the laws of correlation, if one part is modified, and this variation is accumulated through natural selection, other variations will follow, often of an unexpected character. We know that, under domestic conditions, those variations which arise at any particular period of life tend to recur in the offspring at the same period--for example, the shape, size, and flavor of the seeds of many varieties of vegetables and crops; As in the larval and pupal stages, in the color of the egg and undercoat of the chick in the hen, and in the horns of the sheep and the ox as it approaches maturity, so in a state of nature natural selection can act at any stage and alter living beings. , the reason for this is that natural selection can accumulate favorable variations in this period, and because these favorable variations can be inherited in the corresponding period.If a plant were to benefit by having its seeds carried far away by the wind, this would be effected by natural selection with no greater difficulty than by the cotton grower's selection to increase and improve the lint in the capsule.Natural selection can modify the larva of an insect to adapt itself to many accidents which the adult does not encounter; and these variations, through correlation, may affect the structure of the adult.The converse is also true, that variations in the adults will also affect the structure of the larvae; but in all cases natural selection will ensure that those variations are not injurious, because if they were injurious, the species would become extinct.

Natural selection can cause the structure of the daughter body to vary according to the parent body, and can also cause the structure of the parent body to vary according to the daughter body.In social animals, natural selection can adapt the structure of the individual to the good of the whole, if the selected variations benefit the whole.What natural selection cannot do: alter the constitution of one species, without conferring any benefit on it, but for the benefit of another species.Although this effect is mentioned in some works of natural history, I have not yet found a single case worth investigating where a structure used by an animal only once in its life, if it is of high importance in life, natural selection can make There are great variations of this structure; for example, the mandibles with which certain insects specialize in breaking cocoons, or the hard tip of the beak with which unhatched chicks pierce the shells of eggs, etc.It has been said that the best short-billed tumblers die in their shells more often than they hatch; so the fancier needs help in hatching.If, then, "nature" had, for the pigeon's own benefit, made the full-grown pigeon have an extremely short beak, the process of variation would have been very slow, and the young pigeons in the egg would have been strictly selected, and the selected Those youngsters with the strongest beaks will be selected, for all youngsters with weaker beaks must perish; or those with weaker and more easily broken shells will be selected, which, we know, are as thick as the others. Like all structures, they vary.

It may be in advantage here to state that the mass destruction which must happen by accident of all living beings has little or no influence upon the process of natural selection.For example, large numbers of eggs or seeds are eaten every year, and they can be changed by natural selection only if they undergo some mutation to avoid being devoured by their enemies.Yet if many of these eggs or seeds were not eaten and became individuals, they would perhaps be better adapted to the conditions of life than any individual who happened to survive.Moreover, the greater part of growing animals or plants, whether adroitly or not adapted to their conditions of life, must yearly be destroyed by accident; Good for the species, but this occasional death will not be mitigated.But even if the growing organisms were so much destroyed, if the number of individuals capable of surviving in each area were not all eliminated by such accidental causes--or even if the eggs or seeds were so much destroyed , but only a hundredth or a thousandth can develop,—then the most fit individual among those surviving beings, if varied in any favorable direction, will outnumber the less fit individual. able to reproduce more offspring.If all individuals were eliminated for the above reasons, as is often the case in practice, then natural selection would be powerless in some favorable directions.But nothing on this grounds is against the efficacy of natural selection at other times and in other respects; for there is really no reason for supposing that many species have been improved by variation at the same time and within the same regions. sexual selection In domestication there are traits which are frequently found in, and inherited from, only one sex; and no doubt so in natural states.Thus, as is sometimes seen, it may be possible to vary the male and female sexes by natural selection according to different habits of life, or, as commonly happens, to vary one sex according to the other.This leads me to a little elaboration of what I call "sexual selection," which takes the form not of one organism's struggle for survival against other organisms or external conditions, but between individuals of the same sex, which are usually males. The struggle for possession of females.The result is not that the defeated competitor dies, but that it leaves few or no offspring.So sexual selection is less drastic than natural selection.Generally speaking, the strongest males, the best suited to their place in nature, leave the greatest number of offspring.But in many cases, the victory is not so much due to general physical strength, but more due to the special weapons born by males.An antlerless stag or a spurless rooster rarely has a chance of leaving a large number of offspring.Sexual selection, which always permits the victors to breed, and thus does increase indomitable courage, length of the stalk, force of the beat of the wings against the foot of the stalk, is almost the same as that of the cruel cockfighter, who always Carefully select the most fighting cock.I don't know to what level in the natural world, there is no sexual selection; but some people describe that when a male crocodile (alligator) wants to possess a female, it fights, clamors, and walks around, just like the Indian war dance; Male salmon have been observed fighting all day long; male stag-beetles often bear scars which other males have bitten with their gigantic jaws; the incomparable observer M. Fabre It is not uncommon to see certain Hymenopteran males fighting exclusively for a single female, and she stays by, watching as if indifferently, and then walks away with the victor.The war is probably most violent among the males of polygamous animals, and these males are often equipped with special weapons.The male predators are already well armed; but they, and others, have been sexually selected for special defenses, as in the mane of the lion and the hooked jaws of the male salmon; for Shields are as important as swords and spears in winning victory. In birds this struggle is often of a milder nature.It is believed by all who have studied the subject that the most violent competition among the males of many species of birds is to attract the female by song. One by one the birds spread out their beautiful feathers with the utmost care and display them with the best grace; Consorts, who have watched closely caged birds, know well that they often differ in their likes and dislikes towards individuals of the opposite sex: Sir R. Heron, for example, once described how eminently a spotted peacock attracted all his peacocks.I cannot here enter into some of the necessary details; but if man can for a short time, according to his standards of beauty, acquire the beauty and grace of his chicks, I have no good reason to doubt that the female birds conform to their standards. Among thousands of generations, the male bird with the best song or the most beautiful song is selected, which has a remarkable effect.Certain well-known laws concerning the difference in plumage of males and females from those of young birds may be obtained from the action of selection on variations which have occurred at different periods, and which have been inherited to the males alone or to both sexes for a considerable period of time. Partial explanation; but I have no room here to discuss the matter. Thus, if the male and female of any animal have the same general habits of life, but differ in structure, color, or ornamentation, I believe that this difference is mainly caused by sexual selection: this is due to some male individuals in their Arms, means of defense, or beauty have a slight advantage over other males, and these superior traits are passed on to male offspring in successive generations.I would, however, hesitate to attribute all sexual differences to this effect: for we see in domestic animals some traits arising and exclusively male, which have clearly not been increased by artificial selection.The tuft of hair on the breast of the wild turkey-cock is of no use, and it is doubtful whether it is an ornament in the eyes of the female;--yes, if it occurs in domestic conditions plexus, would be called deformed. An example of the role of natural selection, the survival of the fittest In order to see how natural selection works, allow me to give an imaginary example or two.Let us take the example of the wolf, which preys on all kinds of animals, some by cunning, some by physical strength, and some by quickness.We hypothesized that during the most difficult season for wolves, the most agile prey, such as deer, increased their numbers due to any change in that area, or that other prey decreased their numbers.Under such circumstances only the swiftest and slenderest wolves would have the best chance of surviving, and thus be preserved or selected--provided they had to prey on other animals at this or that season. , still retaining sufficient strength to subdue their prey.I see no reason to doubt this result, just as man, through careful and calculated selection, or through unconscious selection (people try to preserve the best dogs, without thinking at all to change the breed), just It is the same that can improve the agility of the greyhound.I will add that, according to Mr. Pierce, in the Catskill Mountains of America there are two varieties of wolves, one of which hunts deer like a lithe greyhound. , the other with a larger body and shorter legs, they often attack the shepherd's flock. It must be noted that in the cases given above I speak of the preservation of the most elongated individual wolf, and not of any single marked variation being preserved.In previous editions of this book, I have said that the latter situation also seems to occur frequently.I saw the high importance of individual differences, and this led me to a full discussion of the consequences of the unconscious selection of human beings, which consisted in preserving the more or less valuable individuals, and destroying the worst.I have also seen that, in a state of nature, the preservation of certain accidental deviations of structure, such as monstrosities, is a rare occurrence; and, if preserved at first, generally disappear thereafter by crossing with normal individuals.Having said that, I had no idea of ​​individual variations, either subtle or marked, until I read a strong and worthy paper in the North British Review (1867) , How rare is it that can be preserved for a long time.This author takes as an example a pair of animals which produced a total of two hundred offspring during their lifetime, most of which were destroyed from various causes, and an average of only two offspring survived to reproduce their kind.While this is an extremely high estimate for most of the higher animals, it is by no means the case for many of the lower ones.He then pointed out that if a single individual should arise, it would be mutated in some way, giving it twice as many chances of surviving as the others, while others would strongly prevent its survival because of the high mortality rate.Suppose it survives and reproduces, and that half of the offspring inherit the favorable variation; It continues to decrease in subsequent generations.I think the correctness of this argument is indisputable.Suppose, for example, that a bird of a certain species, by virtue of a hooked beak, can obtain food more easily, and suppose a bird is born with a very hooked beak, and thus flourishes, yet this individual excludes the common There is still very little chance of a form perpetuating its kind; but there can be no doubt, judging by what has happened to us under domestication, that if we preserve the majority of individuals with more or less hooked beaks in many generations , and also destroy a greater number of individuals with the straightest beaks, can lead to the above results. It must not be overlooked, however, that certain highly marked variations, which no one regards as mere individual differences, will frequently recur, owing to similar institutions acting alike.Of this fact we may cite many instances from our domesticated organisms.In this case, even if the modified individual does not at present transmit to his posterity the newly acquired character, it will doubtless, sooner or later, transmit to his posterity an even stronger tendency to vary in the same way, so long as the conditions of existence remain the same.Nor can there be any doubt that the tendency to vary in the same manner is often so great that all individuals of the same species, without the aid of any selection, will be likewise modified.Or that only one-third, or one-fifth, or one-tenth of the individuals are thus affected, and several instances may be given of the fact.Graba, for example, calculates that one-fifth of the guillemots in the Faroe Islands consist of a variety so well characterized that it was formerly listed as a separate species and called Uria lacrymans.In this case, if mutation is advantageous, the original type will soon be replaced by the mutated type by survival of the fittest. The effect of interbreeding on eliminating all the variations of species will be discussed hereafter; but here it may be shown that most animals and plants remain fixed in their native lands, and do not move out unnecessarily; Come back to where you came from.Thus, as seems to be a general rule with varieties in a state of nature, each newly formed variety is at first generally confined to one locality; Often breed together.If the new variety is victorious in the struggle for existence, it will slowly spread outward from the central area, constantly enlarging the circle, and fighting and overcoming the unchanged individuals on the borders. It is instructive to give another, more complicated instance of the action of natural selection.Some plants secrete a sweet sap, apparently for the removal of harmful substances from their body fluids: for example, the glands at the base of the stipules of some plants of the family Leguminos5 secrete this sap, and the abscents of the leaves of the common laurel (laurel). Glands also secrete this juice.This sap is small, but the insects seek it greedily; but the visit of the insects is of no benefit to the plant.Let us now suppose that, if any one species had a definite number of plants, from the insides of which secreted this sap, or nectar.Insects looking for nectar pick up the pollen and often carry it from one flower to another.The flowers of two different individuals of the same species are thus crossed; and this crossing, as has been well demonstrated, produces strong young plants, which are thus given the best chance of flourishing and surviving.The flowers of the plants which have the largest glands, the nectary glands, secrete the most nectar, are most frequently visited by insects, and are most frequently crossed; thus, in the long run, it occupies Dominance, and formed as a local variety.The flowers would also be advantageous if the stamens and pistils were placed in such a position as to suit the size and habits of the particular visiting insect, in any way facilitating the transport of pollen.Let us take the example of an insect that passes between flowers, collecting not nectar but pollen: the pollen is formed exclusively for fertilization, and its destruction is clearly a pure loss to the plant; If pollen is carried from one flower to another by pollen-eating insects, at first by accident, and later by habit, if interbreeding is thereby achieved, although nine out of ten pollen are destroyed, this is of great importance to the stolen De-pollinated plants are still beneficial, so those individuals that produce more and more pollen and have larger anther sacs are selected. When plants have long continued the above process, they become highly attractive to insects, who carry pollen from flower to flower unconsciously and regularly; and I can easily show from a number of remarkable facts that insects to do this work effectively.I will only give one example, which may at the same time illustrate a step in the differentiation of sexes in plants.Some bolly-trees bear only male flowers, which have four stamens, which produce only a very small amount of pollen, and which also has a rudimentary pistil; others have only female flowers, which have pistils of full size, But the pollen sacs on the four stamens were all shrunken, and not a single pollen could be found there.At a distance of well sixty yards from a male tree, I found a female tree, and I took twenty flowers from various branches, and examined their stigmas under a microscope, without exception, in all There are several grains of pollen on each stigma, and there are many pollen on several stigmas.For several days the wind had been blowing from the female to the male, so of course the pollen was not carried by the wind; it was cold and stormy, so it was not good for the bees.Even so, every female flower I examined was effectively fertilized by bees that passed between the trees in search of nectar.Now back to our imaginary situation: as soon as the plant becomes highly attractive to insects, the pollen will be passed by the insects from flower to flower in due time, and another process begins.No naturalist will doubt the benefit of what is called a "physiological division of labour"; so that we may believe that one flower or whole plant bears only stamens, and another flower or plant only pistils. , is beneficial to a plant.When plants are cultivated or placed under new conditions of life, sometimes the male organs, sometimes the female organs, become more or less impotent.Now if we suppose that this also happens, however slightly, in a state of nature, since the pollen has been passed from flower to flower in due time, and due to the more complete sex of the plant according to the division of labor Differentiation is beneficial, so more and more individuals with this tendency will continue to benefit and be selected, and finally achieve complete differentiation of the sexes.各种植物的性别分离依据二型性和其他途径现在显然正在进行中，不过要说明性别分离所采取的这等步骤，未免要浪费大多篇幅。我可以补充地说，北部美洲的某些冬青树，根据爱萨·葛雷所说的，正好处于一种中间状态，他说，这多少是杂性异株的。 现在让我们转来谈谈吃花蜜的昆虫，假定由于继续选择使得花蜜慢慢增多的植物是一种普通植物；并且假定某些昆虫主要是依靠它们的花蜜为食。我们可以举出许多事实，来说明蜂怎样急于节省时间：例如，它们有在某些花的基部咬一个洞来吸食花蜜的习性，虽然它们只要稍微麻烦一点就能从花的口部进去。记住这些事实，就可以相信，在某些环境条件下，如吻的曲度和长度等等个体差异，固然微细到我们不能觉察到的地步，但是对于蜂或其他昆虫可能是有利的，这样就使得某些个体比其他个体能够更快地得到食物；于是，它们所属的这一群就繁盛起来了，并且生出许多遗传有同样特性的类群。普通红三叶草和肉色三叶草（Tr. incarnatum）的管形花冠的长度，粗看起来并没有什么差异；然而蜜蜂能够容易地吸取肉色三叶草的花蜜，却不能吸取普通红三叶草的花蜜，只有土蜂才来访问红三叶草；所以红三叶草虽遍布整个田野，却不能把珍贵的花蜜丰富地供给蜜蜂。蜜蜂肯定是极喜欢这种花蜜的；因为我屡次看见，只有在秋季，才有许多蜜蜂从土蜂在花管基部所咬破的小孔里去吸食花蜜。这两种三叶草的花冠长度的差异，虽然决定了蜜蜂的来访，但相差为程度确是极其微细的；因为有人对我说过，当红三叶草被收割后，第二茬的花略略小些，于是就有许多蜜蜂来访问它们了。我不知道这种说法是否准确；也不知道另外发表的一种记载是否可靠——据说意大利种的蜜蜂（Ligurian bee）（一般被认为这只是普通蜜蜂种的一个变种，彼此可以自由交配），能够达到红三叶草的泌蜜处去吸食花蜜，因此富有这种红三叶草的一个地区，对于吻略长些的，即吻的构造略有差异的那些蜜蜂会大有利益。另一方面，这种三叶草的受精绝对要依靠蜂类来访问它的花，在任何地区里如果土蜂稀少了，就会使花管较短的或花管分裂较深的植物得到大的利益，因为这样，蜜蜂就能够去吸取它的花蜜了。这样，我就能理解，通过连续保存具有互利的微小构造偏差的一切个体，花和蜂怎样同时地或先后慢慢地发生了变异，并且以最完善的方式来互相适应。 我十分明了，用上述想像的例子来说明自然选择的学说，会遭到人们的反对，正如当初莱尔的“地球近代的变迁，可用作地质学的解说”这种宝贵意见所遭到的反对是一样的；不过在运用现今依然活动的各种作用，来解说深谷的凿成或内陆的长形崖壁的形成时，我很少听到有人说这是琐碎的或不重要的了。自然选择的作用，只是把每一个有利于生物的微小的遗传变异保存下来和累积起来；正如近代地质学差不多排除了一次洪水能凿成大山谷的观点那样，自然选择也将把连续创造新生物的信念、或生物的构造能发生任何巨大的或突然的变异的信念排除掉的。 论个体的杂交 我在这里必须稍微讲一些题外的话。雌雄异体的动物和植物每次生育，其两个个体都必须交配（除了奇特而且不十分理解的单性生殖），这当然是很明显的事；但在雌雄同体的情况下，这一点并不明显。然而有理由可以相信，一切雌雄同体的两个个体或偶然地或习惯地亦营接合以繁殖它们的种类。很久以前，斯普伦格尔（Sprengel ）、奈特及科尔路特就含糊地提出过这种观点了。不久我们就可以看到这种观点的重要性；但这里我必须把这个问题极简略地讲一下，虽然我有材料可作充分的讨论。一切脊椎动物，一切昆虫以及其他某些大类的动物，每次的生育都必须交配。近代的研究已经把从前认为是雌雄同体的数目大大减少了；大多数真的雌雄同体的生物也必须交配；这就是说，两个个体按时进行交配以营生殖，这就是我们所要讨论的一切，但是依然有许多雌雄同体的动物肯定不经常地进行交配，并且大多数植物是雌雄同株的。于是可以问：有什么理由可以假定在这等场合里，两个个体为了生殖而进行交配呢？在这里详细来论讨这一问题是不可能的，所以我只能作一般的考察。 第一，我曾搜集过大量事实，并且做过许多实验，表明动物和植物的变种间的杂交，或者同变种而不同品系的个体间的杂交，可以提高后代的强壮性和能育性；与此相反，近亲交配可以减小其强壮性和能育性，这和饲养家们的近乎普遍的信念是一致的。仅仅这等事实就使我相信，一种生物为了这一族的永存，就不自营受精，这是自然界的一般法则；和另一个体偶然地——或者相隔一个较长的期间一一进行交配，是必不可少的。 相信了这是自然法则。我想，我们才能理解下面所讲的几大类事实，这些事实，如用任何其他观点都下能得到解释。各个培养杂种的人都知道：暴露在雨下，对于花的受精是何等不利，然而花粉囊和柱头完全暴露的花是何等之多！尽管植物自己的花粉囊和雌蕊生的这么近，几乎可以保证自花受精，如果偶然的杂交是不可缺少的，那么从他花来的花粉可以充分自由地进入这一点，就可以解释上述雌雄蕊暴露的情况了。另一方面，有许多花却不同，它们的结籽器官是紧闭的，如蝶形花科即荚果科这一大科便是如此；但这些花对于来访的昆虫几乎必然具有美丽而奇妙的适应。蜂的来访对于许多蝶形花是如此必要，以致蜂的来访如果受到阻止，它们的能育性就会大大降低。昆虫从这花飞到那花，很少不带些花粉去的，这就给予植物以巨大利益。昆虫的作用有如一把驼毛刷子，这刷子只要先触着一花的花粉囊、随后再触到另一花的柱头，就足可以保证受精的完成了。但不能假定，这样，蜂就能产生出大量的种间杂种来；因为，假如植物自己的花粉和从另一物种带来的花粉落在同一个柱头上，前者的花粉占有的优势如此之大，以至它不可避免地要完全毁灭外来花粉的影响，该特纳就（Gartner）曾指出过这一点。 当一朵花的雄蕊突然向雌蕊弹跳，或者慢慢一枝一枝地向她弯曲，这种装置好像专门适应于自花受精；毫无疑问，这对于自花受精是有用处的。不过要使雄蕊向前弹跳，常常需要昆虫的助力，如科尔路特所阐明的小蘖（barberry）情形便是这样；在小蘖属里，似乎都有这种特别的装置以便利自花受精，如所周知，假如把密切近似的类型或变种栽培在近处，就很难得到纯粹的幼苗，这样看来，它们是大量自然进行杂交的。在许多其他事例里，自花受精就很不便利，它们有特别的装置，能够有效地阻止柱头接受自花的花粉，根据斯普伦格尔和别人的著作以及我自己的观察，我可以阐明这一点：例如，亮毛半边莲确有很美丽而精巧的装置，能够把花中相连的花粉囊里的无数花粉粒，在本花柱头还不能接受它们之前，全部扫除出去；因为从来没有昆虫来访这种花，至少在我的花园中是如此，所以它从不结籽。然而我把一花的花粉放在另一花的柱头上却能结籽，并由此培育成许多幼苗，我的花园中还有另一种半边莲，却有蜂来访问，它们就能够自由结籽。在很多其他场合里，虽然没有其他特别的机械装置，以阻止柱头接受同一朵花的花粉，然而如斯普伦格尔以及希尔德布兰德（Hildebrand ）和其他人最近指出的，和我所能证实的：花粉囊在柱头能受精以前便已裂开，或者柱头在花粉未成熟以前已经成熟，所以这些叫做两蕊异熟的植物（dichogamous plants），事实上是雌雄分化的，并且它们一定经常地进行杂交。上述二形性和三形性交替植物的情形与此相同。这些事实是何等奇异啊！同一花中的花粉位置和柱头位置是如此接近，好像专门为了自花受精似的，但在许多情形中，彼此并无用处，这又是何等奇异啊！如果我们用这种观点，即不同个体的偶然杂交是有利的或必需的，来解释此等事实，是何等简单啊！ 假如让甘蓝、萝卜、洋葱以及其他一些植物的几个变种在相互接近的地方进行结籽，那末由此培育出来的大多数实生苗，我发现都是杂种：例如，我把几个甘蓝的变种栽培在一起，由此培育出233 株实生苗，其中只有78株纯粹地保持了这一种类的性状，甚至在这78株中还有若干不是完全纯粹的。然而每一甘蓝花的雌蕊不但被自己的六个雄蕊所围绕，同时还被同株植物上的许多花的雄蕊所围绕；没有昆虫的助力各花的花粉也会容易地落在自己的柱头上；因为我曾发现，如果把花仔细保护起来，与昆虫隔离，它们也能结充分数量的籽，然而这许多变为杂种的幼苗是从哪里来的呢？这必定因为不同变种的花粉在作用上比自己的花粉更占优势的缘故；这是同种的不同个体互相杂交能够产生良好结果的一般法则的要素。如果不同的物种进行杂交，其情形正相反，因为这时植物自己的花粉几乎往往要比外来的花粉占优势；关于这一问题，我们在以后一章里还要讲到。 在一株大树满开无数花的情况下，我们可以反对地说，花粉很少能从这株树传送到那株树，充其量只能在同一株树上从这朵花传送到那朵花而已；而且在一株树上的花，只有从狭义来说，才可被认为是不同的个体。我相信这种反对是恰当的，但是自然对于这事已大大地有所准备，它给予树以一种强烈的倾向，使它们生有雌雄分化的花。当雌雄分化了，虽然雄花和雌花仍然生在一株树上，可是花粉必须按时从这花传到那花；这样花粉就有偶然从这树被传送到他树的较为良好的机会。属于一切“目”（Orders ）的树，在雌雄分化上较其他植物更为常见，我在英国所看到的情形就是这样；根据我的请求，胡克博士把新西兰的树列成了表，阿萨·葛雷把美国的树列成了表，其结果都不出我所料。另一方面，胡克博士告诉我说，这一规律不适用于澳洲；但是如果大多数的澳洲树木都是两蕊异熟的，那末，其结果就和它们具有雌雄分化的花的情形是一样的了。我对于树所作的这些简略叙述，仅仅为了引起对这一问题的注意而已。 现在略为谈谈动物方面：各式各样的陆栖种都是雌雄同体的，例如陆栖的软体动物和蚯蚓；但它们都需要交配。我还没有发现过一种陆栖动物能够自营受精。这种显著的事实，提供了与陆栖植物强烈不同的对照，采用偶然杂交是不可少的这一观点，它就是可以理解的了；因为，由于精子的性质，它不能像植物那样依靠昆虫或风作媒介，所以陆栖动物如果没有两个个体交配，偶然的杂交就不能完成。水栖动物中有许多种类是能自营受精的雌雄同体；水的流动显然可以给它们做偶然杂交的媒介。我同最高权威之一，即赫胥黎教授进行过讨论，希望能找到一种雌雄同体的动物，它的生殖器官如此完全地封闭在体内，以致没有通向外界的门径，而且不能接受不同个体的偶然影响，结果就像在花的场合中那样，我失败了。在这种观点指导之下，我以前长久觉得蔓足类是很难解释的一例；但是我遇到一个侥幸的机会，我竟能证明它们的两个个体，虽然都是自营受精的雌雄同体，确也有时进行杂交。 无论在动物或者植物里，同科中甚至同属中的物种，虽然在整个体制上彼此十分一致，却有些是雌雄同体的，有些是雌雄异体的，这种情形必会使大多数博物学者觉得很奇异。但是如果一切雌雄同体的生物事实上也偶然杂交，那末它们与雌雄异体的物种之间的差异，仅从机能上来讲，是很小的。 从这几项考察以及从许多我搜集的但不能在这里举出的一些特别事实看来，动物和植物的两个不同个体间的偶然杂交，即使不是普遍的、也是极其一般的自然法则。 通过自然选择有利于产生新类型的诸条件 这是一个极为错综的问题。大量的变异（这一名词通常包括个体差异在内）显然是有利的。个体数量大，如果在一定时期内发生有利变异的机会也较多，即使每一个体的变异量较少也可得到补偿；所以我相信，个体数量大乃是成功的高度重要因素。虽然大自然可以给予长久的时间让自然选择进行工作，但大自然并不能给予无限的时间；因为一切生物都努力在自然组成中夺取位置，任何一个物种，如果没有随着它的竞争者发生相应程度的变异和改进，便是绝灭。有利的变异至少由一部分后代所遗传，自然选择才能发挥作用。返祖倾向可能常常抑制或阻止自然选择的作用；但是这种倾向既不能阻止人类用选择方法来形成许多家养族，那么它怎么能胜过自然选择而不使它发挥作用呢？ 在有计划选择的情形下，饲养家为了一定的目的进行选择，如果允许个体自由杂交，他的工作就要完全归于失败，但是，有许多人，即使没有改变品种的意图，却有一个关于品种的近乎共同的完善标准，所有他们都试图用最优良的动物繁殖后代，这种无意识的选择，虽然没有把选择下来的个体分离开，肯定也会缓慢地使品种得到改进。在自然的状况下也是这样；因为在局限的区域内，其自然机构中还有若干地方未被完全占据，一切向正确方向变异的个体，虽然其程度有所不同，却都可以被保存下来。但如果地区辽阔，其中的几个区域几乎必然要呈现不同的生活条件；如果同一个物种在不同区域内发生了变异，那末这些新形成的变种就要在各个区域的边界上进行杂交。我们在第六章里将阐明，生活在中间区域的中间变种，在长久期间内通常会被邻近的诸变种之一所代替。凡是每次生育必须交配的、游动性很大的而且繁育不十分快的动物，特别会受到杂交的影响。所以具有这种本性的动物，例如鸟，其变种一般仅局限于隔离的地区内，我看到的情形正是如此。仅仅偶然进行杂交的雌雄同体的动物，还有每次生育必须交配但很少迁移而增殖甚快的动物，就能在任何一处地方迅速形成新的和改良的变种，并且常能在那里聚集成群，然后散布开去，所以这个新变种的个体常会互相交配。根据这一原理，艺园者常常喜欢从大群的植物中留存种籽，因其杂交的机会由是减少了。 甚至在每次生育必须交配而繁殖不快的动物里，我们也不能认为自由杂交常常会消除自然选择的效果；因为我可以举出很多的事实来说明，在同一地区内，同种动物的两个变种，经过长久的时间仍然区别分明，这是由于栖息的地点不同，由于繁殖的季节微有不同，或者由于每一变种的个体喜欢同各自变种的个体进行交配的缘故。 使同一物种或同一变种的个体在性状上保持纯粹和一致，杂交在自然界中起着很重要的作用。对于每次生育必须交配的动物，这等作用显然更为有效；但是前面已经说过，我们有理由相信，一切动物和植物都会偶然地进行杂交。即使只在间隔一个长时间后才进行一次杂交，这样生下来的幼体在强壮和能育性方面都远胜于长期连续自营受精生下来的后代，因而它们就会有更好的生存并繁殖其种类的机会；这样，即使间隔的时期很长，杂交的影响归根到底还是很大的。至于极低等的生物，它们不营有性生殖，也不行接合，根本不可能杂交，它们在同一生活条件下，只有通过遗传的原理以及通过自然选择，把那些离开固有模式的个体消灭掉，才能使性状保持一致。如果生活条件改变了，类型也发生变异了，那末只有依靠自然选择对于相似的有利变异的保存，变异了的后代才能获得性状的一致性。 隔离，在自然选择所引起的物种变异中，也是一种重要的因素。在一个局限的或者隔离的地区内，如果其范围不十分大，则有机的和无机的生活条件一般几乎是一致的；所以自然选择就趋向于使同种的一切个体按照同样方式进行变异，而与周围地区内生物的杂交也会由此受到阻止，瓦格纳（Moritz Wagner）最近曾发表过一篇关于这个问题的有趣论文，他指出，隔离在阻止新形成的变种间的杂交方面所起的作用，甚至比我设想的还要大。但是根据上述理由，我决不能同意这位博物学者所说的迁徙和隔离是形成新种的必要因素。当气候、陆地高度等外界条件发生了物理变化之后，隔离在阻止那些适应性较好的生物的移入方面，同样有很大重要性；因此这一区域的自然组成里的新场所就空出来了，并且由于旧有生物的变异而被填充起来。最后，隔离能为新变种的缓慢改进提供时间；这一点有时是非常重要的。但是，如果隔离的地区很小，或者周围有障碍物，或者物理条件很特别，生物的总数就会很小；这样，有利变异发生的机会便会减少，因而通过自然选择产生新种就要受到阻碍。 只是时间推移的本身并没有什么作用，这既不有利于自然选择，也不妨害它。我要说明这一点的原因，是因为有人误认为我曾假定时间这一因素在改变物种上有最重要的作用，好像一切生物类型由于某些内在法则必然要发生变化似的。时间的重要只在于：它使有利变异的发生、选择、累积和固定，有较好的机会，在这方面它的重要性是很大的。同样地，它也能增强物理的生活条件对于各生物体质的直接作用。 如果我们转向自然界来验证这等说明是否正确，并且我们所观察的只是任何一处被隔离的小区域，例如海洋岛，虽然生活在那里的物种数目很少，如我们在《地理分布》一章中所要讲到的；但是这些物种的极大部分是本地所专有的——就是说，它们仅仅产生在那里，而是世界别处所没有的。所以最初一看，好像海洋岛对于产生新种是大有利的。但这样我们可能欺骗了自己，因为我们如果要确定究竟是一个隔离的小地区，还是一个开放的大地区如一片大陆，最有利于产生生物新类型，我们就应当在相等的时间内来作比较；然而这是我们不可能做到的。虽然隔离对于新种的产生极为重要，但从全面看来，我都倾向于相信区域的广大更为重要，特别是在产生能够经历长久时间的而且能够广为分布的物种尤其如此。在广大而开放的地区内，不仅因为那里可以维持同种的大量个体生存，因而使发生有利变异有较好的机会，而且因为那里已经有许多物种存在，因而外界条件极其复杂；如果在这许多物种中有些已经变异或改进了，那么其他物种势必也要相应程度地来改进，否则就要被消灭。每一新类型，当它们得到大大的改进以后，就会向开放的、相连的地区扩展，因而就会与许多其他类型发生斗争。还有，广大的地区，虽然现在是连续的，却因为以前地面的变动，往往呈现着不连接状态；所以隔离的优良效果，在某种范围内一般是曾经发生的。最后，我可总结，虽然小的隔离地区在某些方面对于新种的产生是高度有利的，然而变异的过程一般在大地区内要快得多，并且更重要的是，在大地区内产生出来的而且已经战胜过许多竞争者的新类型，是那些分布得最广远而且产生出最多新变种和物种的类型。因此它们在生物界的变迁史中便占有比较重要的位置。 根据这种观点，我们对于在《地理分布》一章里还要讲到的某些事实，大概就可以理解了；例如，较小的大陆，如澳洲，它的生物，现在和较大的欧亚区域的生物比较起来，是有逊色的。这样正是大陆的生物，在各处岛屿上到处归化。在小岛上，生活竞争比较不剧烈，那里的变异较少，绝灭的情形也较少。因此，我们可以理解，为什么马得拉的植物区系，据O．希尔（Oswald Heer）说，在一定程度上很像欧洲的已经灭亡的第三纪植物区系。一切淡水盆地，总的来说，与海洋或陆地相比较，只是一个小小的地区。结果，淡水生物间的斗争也不像在他处那样剧烈；于是，新类型的产生就较缓慢，而且旧类型的灭亡也要缓慢些。硬鳞鱼类（Canoid fishes）以前是一个占有优势的目，我们在淡水盆地还可以找到它遗留下来的七个属；并且在淡水里我们还能找到现在世界上几种形状最奇怪的动物，口鸭嘴兽（Ornithorhynchus ）和肺鱼（Lepidosiren），它们像化石那样，与现今在自然等级上相离很远的一些目多少相联系着。这种形状奇怪的动物可以叫做活化石；由于它们居住在局限的地区内，并且由于变异较少，因而斗争也较不剧烈，所以它们能够一直存留到今天。 就这极复杂的问题所许可的范围内，现在对通过自然选择产生新种的有利条件和不利条件总起来说一说。我的结论是，对陆栖生物来说，地面经过多次变动的广大地区，最有利于产生许多新生物类型，它们既适于长期的生存，也适于广泛的分布。如果那地区是一片大陆，生物的种类和个体都会很多，因而就要陷入严厉的斗争。如果地面下陷，变为分离的大岛，每个岛上还会有许多同种的个体生存着；在各个新种分布的边界上的杂交就要受到抑制；在任何种类的物理变化之后，迁入也要受到妨碍，所以每一岛上的自然组成中的新场所，势必由于旧有生物的变异而被填充；时间也能允许各岛上的变种充分地变异和改进。如果地面又升高，再变为大陆，那里就会再发生剧烈的斗争；最有利的或最改进的变种，就能够分布开去，改进较少的类型就会大部绝灭，并且新连接的大陆上的各种生物的相对比例数便又发生变化；还有，这里又要成为自然选择的优美的活动场所，更进一步地来改进生物而产生出新种来。 我充分承认，自然选择的作用一般是极其缓慢的。只有在一个区域的自然组成中还留有一些地位，可以由现存生物在变异后而较好地占有，这时自然选择才能发生作用。这种地位的出现常决定于物理变化，这种变化一般是很缓慢的。此外还决定于较好适应的类型的迁入受到阻止。少数旧有生物一发生变异，其他生物的互相关系就常被打乱；这就会创造出新的地位，有待适应较好的类型填充进去；但这一切进行得极其缓慢，虽然同种的一切个体在某种微小程度上互有差异，但是要使生物体制的各部分发生适宜的变化，则常需很长时间。这种结果又往往受到自由杂交所显著延滞。许多人会说这数种原因已足够抵消自然选择的力量了，我不相信会如此。但我确相信自然选择的作用一般是极其缓慢的，须经过长久的时间，并且只能作用于同一地方的少数生物。我进一步相信此等缓慢的、断续的结果，和地质学告诉我们的这世界生物变化的速度和方式很相符合。 选择的过程虽然是缓慢的，如果力量薄弱的人类尚能在人工选择方面多有作为，那末，在很长的时间里，通过自然力量的选择，即通过最适者的生存，我觉得生物的变异量是没有止境的，一切生物彼此之间以及与它们的物理的生活条件之间互相适应的美妙而复杂的关系，也是没有止境的。