Chapter 23 Chapter 15 Recap and Conclusion

Restatement of the Objections to the Theory of Natural Selection--Restatement of the General and Particular Cases in Support of the Theory of Natural Selection--Reasons for the General Belief in the Invariance of Species The impact of natural history research - concluding remarks. As the whole book is a long debate, it may be convenient for the reader to briefly restate the main facts and inferences. I do not deny that many serious objections may be raised against the theory of Descent with Variation, which is based on Variation and Natural Selection.I have endeavored to bring these objections to their full force, and the perfection of the more complex organs and instincts does not depend upon methods which transcend, or even resemble, human reason, but upon the accumulation of innumerable slight variations to the advantage of the individual, at first sight Come on, nothing is harder to believe.Nevertheless, although this may seem to our imagination a great and insurmountable difficulty, it is not a real difficulty if we admit the propositions that all parts and instincts of institutions exhibit at least individual differences. —The struggle for existence leads to the preservation of structurally or instinctively favorable deviations—Finally, in the state of perfection of each organ there are degrees, each of which is advantageous to its kind.The validity of these propositions, I think, is indisputable.

Doubtless it is extremely difficult to even conjecture, by what intermediate stages, many organs have been perfected, especially in the case of discontinuous, decaying groups of organisms which have been largely extinct; But we see so many strange gradations in nature, that extreme caution should be exercised when we say that any organ or instinct, or whole constitution, could not have been brought to its present state by many gradational steps.It must be admitted that there are cases of particular difficulty against the theory of natural selection, the most curious of which is the definite order of two or three species of workers, or sterile females, in the same colony; but I have attempted to show how these difficulties were overcome. of.

The almost universal sterility of species in the first cross, in contrast to the almost universal fertility of varieties in crosses, is so strikingly contrasted, that I must refer the reader to the facts presented at the end of Chapter IX. These facts, in my opinion, show conclusively that this sterility is not a special gift, any more than it is a special gift that trees of two different species cannot be grafted together; Differences in the reproductive system happen by chance.We see the validity of the above conclusions in the large differences obtained in the results of reciprocal crossings of the same two species, that is, where one species is used first as the male parent and then as the female parent.An analogy from the studies of plants of the second and third forms would clearly lead to the same conclusion, for when the forms are illegitimately combined, they produce few or no seeds, and their offspring are more or less sterile. and these forms are undoubtedly of the same species, differing from each other only in the reproductive organs and functions.

The fertility of the hybrids of varieties and of their hybrid offspring, though affirmed to be general by so many authors, cannot be regarded as such since the facts given by the high authorities Gartner and Colereuter It is quite correct.Most of the varieties tested have been produced under domestic conditions; and as domestic conditions (and I do not mean only captivity) almost certainly have a tendency to eliminate the sterility which, by analogy, occurs in the parent species. in crosses; so we should not expect that domestic conditions will likewise induce sterility in crosses of their modified offspring.This elimination of sterility evidently follows from the same cause which permits our domestic animals to reproduce freely under various circumstances; and this evidently results from their gradual adaptation to the constant changes in the conditions of life.

There are two parallel classes of facts which seem to suggest many accounts for the sterility of the first crosses of species, and of their hybrid offspring.On the one hand, there is good reason to believe that slight variations in the conditions of life will impart vigor and fertility to all organic beings.We also know that the crossing of different individuals of the same variety, and of different varieties, increases the number of their offspring, and certainly increases their size and vigor.This is chiefly due to how many different conditions of life the forms which have been crossed have been exposed to; for I have established by a series of painstaking experiments that, if all individuals of the same variety are exposed to the same conditions for several generations, the results from crossing The resulting benefits are often greatly diminished or eliminated altogether.This is one side of the truth.On the other hand, we know that species once long-term exposed to near-uniform conditions, when housed in captivity under vastly different new conditions, either die, or, alive, become sterile even if they remain perfectly healthy. up.This does not occur, or occurs only in a slight degree, in domesticated organisms long exposed to varying conditions.When, therefore, we see two distinct species interbreeding, the resulting hybrids, either dying shortly after conception or at a very early age, produce few hybrids, or, while alive, become more or less sterile, the result is most probable. It is because these hybrids seem to fuse two different institutions together, and have in fact been subjected to great changes in living conditions.Whoever can explain in a definite manner why, for example, an elephant or a fox is sterile when kept in captivity in its native land, and why a domestic pig or pig can reproduce in large numbers under the most dissimilar conditions, will then be able to understand A definite answer is given to the question, why two distinct species, when crossed, and their hybrid offspring, are generally more or less sterile, and why two domestic varieties, when crossed, and their hybrid offspring, are perfectly fertile. educated.

As regards geographical distribution, the difficulties encountered by the theory of descent with modification are extremely serious.All individuals of the same species, and all species of the same genus or even higher groups, are descended from a common ancestor; therefore, however remote and isolated they may be found on the globe, they must have been in succession. Migration from one place to all other places in the course of generations.How this happens is often quite impossible even to guess.However, since we have reason to believe that some species have maintained the form of the same species for an extremely long period (a period which is extremely long in terms of ages), the occasional wide spread of the same species should not be overemphasized; why That said, for there is always a good chance of extensive migrations, by many means, over long periods of time.Discontinuous or interrupted distributions can often be explained by extinctions of species in intermediate zones.It cannot be denied that we are still very ignorant of the full extent of the various climatic and geographical changes which have affected the earth during modern times; and these changes have often favored migration.As an illustration, I have attempted to show how effectually ice ages have been in affecting the distribution of the same and allied species over the earth.We are profoundly ignorant of many accidental delivery methods.In the case of different species of the same genus living in remote and isolated countries, as the process of modification must proceed slowly, all means of migration are made possible for a long period of time; is reduced to some extent.

According to the theory of natural selection, there must have been innumerable intermediate forms which united all the species in each group in fine gradations, like existing varieties, so that we may ask: Why don't we see these types of connections all around us?Why are not all living beings mingled into an insoluble disorder?With regard to existing forms, we should remember that we have no right to hope (except in rare cases) to discover a chain of direct connection between them, we can only compare existing forms with some extinct, displaced form. This linkage was found between.If a large area has been continuously maintained for a long period of time, and its climate and other conditions of life gradually and insensibly change from the area occupied by one species to that occupied by a closely allied species. , even in such regions, we have no just right to hope that intermediate varieties will often be found in intermediate regions.For we have reason to believe that only a few species of each genus have ever been modified; the others have become entirely extinct, leaving no modified descendants.Of the species which do vary, only a few occur simultaneously within the same country; and all the variations are accomplished gradually.I have also shown that the intermediate varieties originally present in the middle ground will probably be easily displaced by allied forms in any respect; The rate changes and improves; with the result that intermediate varieties are at last displaced and exterminated.

Between living and extinct organisms in the world, and between extinct species and older species in successive periods, there are countless chains of links that have become extinct.According to this theory, why are these interlocking types not filled in every geological layer?Why does not every collection of fossil remains furnish clear evidence of successive transitions and changes in the forms of life?While the study of geology undoubtedly reveals many chains that have existed before, bringing countless forms of life closer together, the infinite number of minute gradations and parallels it provides between past and present species does not satisfy the requirements of this doctrine; this is the most obvious of the many objections against it.Also, why do whole groups of allied species seem to appear suddenly in successive geological stages? (Although this is often an illusion.) Although we now know that life arose on this earth at an incalculably ancient time before the deposition of the lowest layers of the Cambrian, why are we under this system? Didn't huge formations contain the remains of our ancestors in Cambrian fossils?For, on this theory, such strata must have been deposited somewhere at such an ancient and wholly unknown age in the history of the world.

I can answer the above questions and objections only on the assumption that the geological record is more incomplete than most geologists believe.The number of specimens in all museums is by no means insignificant compared with the innumerable generations of innumerable species which must have existed.The parent-form of any two or more species is not directly intermediate in all its characters from its modified offspring, any more than the rock-pigeon is directly intermediate in crop and tail from its protruding offspring. Same between pigeons and fantails.If we study two organic beings, even if the study is well done, we cannot tell whether one species is the progenitor of another modified species unless we obtain most of the intermediate linkages; and from the incompleteness of the geological record, We have no legitimate right to hope to find so many chains.Had two or three, or even more, associated forms been found, they would have been simply listed by many naturalists as so many new species, had they been found in different geological sub-formations, however different they might be Minor, especially.Innumerable extant doubtful forms may be mentioned, all probably varieties; but who dares to say that so many chains of fossils will be found in the future that the naturalist will be able to decide whether these doubtful forms should be called varieties?Only a small part of the world has ever been geologically explored.Only certain classes of organisms have been preserved in the fossil state, at least in any great numbers.Many species, once formed, would have become extinct if they had never undergone any modification, and would have left no modified offspring; and the period during which species undergo modification, though long in years, is compared with the period during which species have remained of the same type. Up, probably still short.The dominant and widespread species are the most frequently and most varied, and the varieties are often endemic at first--both of these reasons it is less easy to find intermediate linkages in any one formation.Local varieties do not spread to other distant countries until they have been considerably modified and improved; and when they have dispersed and are found in a formation, they appear as if they had been created there suddenly. , and are thus simply classified as a new species.Most formations are intermittent in their deposition; their duration is presumably shorter than the average duration of species types.In most cases successive geological formations are separated by long blank intervals; for fossil-bearing formations thick enough to resist future engraving, as a general rule, can only descend on the seabed Only where a large amount of sediment is deposited can it be accumulated.There is generally no geological record of periods of alternating periods of rising and stationary water levels.In such latter periods there will probably be more variability in the forms of life; in periods of decline there will probably be more extinctions.

Concerning the absence of fossil-rich formations below the Cambrian formation, I can only return to the hypothesis advanced in Chapter X; But we have no reason to suppose that this will always be the case; so formations much older than any now known may still lie buried under the ocean.The objection that the time elapsed since the solidification of our planet has not been sufficient to allow living beings to effect the amount of change conceived is, as Sir Thompson vehemently maintains, perhaps one of the most serious objections ever raised concerning On this point I can only say: first, we do not know at what rate species change, measured in years, and second, many philosophers are not yet willing to admit that we know enough about the composition of the universe and the interior of the earth. knowledge, which can be used to safely infer the length of time in the earth's past.

All admit that the geological record is incomplete; but few are willing to admit that it is so incomplete as our theory requires.If we observe long enough long intervals, the geological theory shows plainly that all species change; and that they change in the manner required by the theory, for they all change slowly and in a gradual manner. of.We see this clearly in the fossil remains of successive formations, which must be far more closely related to one another than those of distant formations. The foregoing has been a summary of the principal objections and difficulties which may justly be raised against this doctrine; I have now briefly reproduced my answers and explanations, as far as I know.For many years I have felt these difficulties to be so serious that their weight could not be doubted.But it is worth noting that the more important objections have to do with those matters of which we are admittedly ignorant; and we do not yet know how far we are ignorant.We do not yet know all possible transitional gradations between the simplest and most perfect organs; incomplete.Serious though these several objections are, they are by no means sufficient in my judgment to overthrow the theory of Descent with variation in descent. Let us now turn to the other side of the controversy. In domestication we see a great deal of variability caused, or at least excited, by changed conditions of life; but it often occurs in such an ambiguous manner that We are apt to regard variation as spontaneous, governed by many complex laws—by relative growth, by compensatory effects, by the increased use and disuse of organs, and by certain effects of surrounding conditions, which determine that our domesticated How much variation has occurred is with great difficulty; but we may safely infer that the amount of variation has been great, and that it has been inherited for a long time.So long as the conditions of life remain the same, we have reason to believe that variations which have been inherited through many generations may continue to be inherited through an almost infinite number of generations.On the other hand, we have evidence that variability, once operative, does not cease for long periods under domestication; we do not know when it ceased, for even the oldest domesticated Generate new variants. Variation is not actually caused by man; he merely unconsciously places the living being under new conditions of life, so that nature acts upon its constitution and causes it to vary.But man can and does select the variations nature has given him, and thus accumulate them in any desired manner.In this way he can adapt animals and plants to his own interests or inclinations.He may do this systematically, or he may do so unconsciously, the method of which is to preserve those individuals which are most useful to him or which suit his taste, without any attempt to alter the breed.He must be able to greatly influence the character of a breed by selecting in each successive generation those individual differences of the slightest degree which cannot be discerned except by the trained eye. have played a major part in useful domestic breeds.The fact that many of the breeds produced by man have to a large extent the condition of natural species is explained by the fact that many of the breeds are varieties or are originally different. The insoluble problem of species is illustrated. There is no reason why a principle which has worked so effectively under domestication should not work under nature, and in the continually recurring struggle for existence the advantageous individual or group survives, and in this we see a kind of A powerful and often active form of "selection".All living beings are highly multiplied according to a geometric progression, which inevitably leads to a struggle for existence.This rate of increase in height can be demonstrated by calculation--the rapid increase of many animals and plants in successive special seasons, and when they naturalize in new areas, will prove the rate of increase in height.More individuals are produced than are possible to survive.Slight differences in the balance can determine which individuals will survive and which will die—which varieties or species will increase in number, which will decrease in number or eventually become extinct.Individuals of the same species are most closely in every way in competition with each other, and therefore generally fight most violently among themselves; varieties of the same species are almost equally violent, and next, species of the same genus.On the other hand, the struggle between organisms that are widely separated in natural systems is also often violent.The slightest advantage some individual has at any age or season over his rivals, or any slight degree of better adaptation to the surrounding physical conditions, will result in a shift in the balance. In dioecious animals there is in most cases a struggle between the males for the possession of the females.The strongest males, or those who struggle most successfully with the conditions of life, generally leave the greatest number of offspring.But success often depends on the male having special weapons, or means of defense, or charm; a slight advantage can lead to victory. Geology clearly shows that the various continents have undergone great physical changes, so that we might expect organic beings to have varied under nature as well as they have under domestication.If there is any variation under nature, it is an inexplicable fact that natural selection has not been at work.It has often been asserted that the quantity of variation is a strictly limited quantity in a state of nature, but this assertion cannot be proved.Man, though acting only on external characters and with unpredictable results, can produce enormous results in brief periods by accumulating individual differences in domesticated organisms; and every one admits that species exhibit individual differences.But, apart from individual differences, all naturalists admit the existence of natural varieties, which are considered sufficiently distinct to merit recording in taxonomic works.No one has ever drawn any sharp distinction between individual differences and slight varieties, or between more definite varieties and subspecies, or between subspecies and species.On separate continents, in distinct regions on the same continent separated by obstacles of any kind, and on remote islands, there exist a multitude of forms which some experienced naturalists rank as varieties, others Naturalists go so far as to rank them as geographical families or subspecies, and still others as distinct though closely allied species! Why, then, if animals and plants do undergo variation, however slight or slow, so long as such variation or individual differences are advantageous in any way, are not preserved and accumulated by natural selection, the survival of the fittest? ?Since man can patiently select the mutations that are useful to him, why don't the mutations that are beneficial to natural organisms often occur and be preserved, that is, selected under changing and complicated living conditions?Can there be a limit to this power which operates through ages and rigorously examines the whole constitution, constitution, and habits of every living being--encouraging the good and excluding the bad?I see no limit to this power of slowly and beautifully adapting each type to the most complex relations of life, and even the theory of natural selection seems highly plausible if we do not look further afield.I have repeated the difficulties and objections raised by the other side as fairly as possible: let us now turn to the particular facts and arguments in support of the doctrine. From the view that species are only strongly marked, stable varieties, and that each species exists first as a variety, we can understand the difference between species which are commonly supposed to have been produced by a particular act of creation and which are generally accepted to have been produced by laws of second nature. Why is there no boundary line between the variants produced.From this same point of view, we can also understand why many species of a genus should appear in many varieties where they have arisen and still flourish; It may be expected to continue; and this would be the case if the varieties were incipient species.Also, if the species of large genera furnish a greater number of varieties, i.e., incipient species, they will to some extent retain the character of varieties; for small.The closely allied species of large genera are evidently restricted in distribution, and they cluster in small groups around other species in affinities—both like varieties.From the view that each species was created independently, these relations are strange, but would be intelligible if each species existed first as varieties. Each species has a tendency to multiply excessively in geometrical progression; and the modified descendants of each species, by virtue of their greater variety in habits and structure, are able to capture many great differences in their natural composition. and thus increase their numbers by the best locales, so natural selection will often tend to preserve the most divergent offspring of any one species.In the long succession of modification, therefore, the slight differences peculiar to the varieties of the same species tend to increase into larger differences peculiar to the species of the same genus.New, improved varieties inevitably exclude and destroy old, less improved, and intermediate varieties; and thus the species becomes to a large extent definite and well-defined.The dominant species belonging to the larger groups in each class tend to give rise to new and dominant forms; consequently each large group tends to become larger, and at the same time more divergent in character.But all groups cannot continue to grow in this way, because the world cannot accommodate them, so the more dominant type will overthrow the less dominant type.This tendency of large groups to continue to increase and diverge in character, together with the inevitable mass extinction, explains the ordering of all organic forms in sub-groups, all of which were once included. Within the few outlines that dominate throughout.This great fact, which subsumes all living beings under the so-called "system of nature," is utterly inexplicable on the basis of special creation. Natural selection can act only by the accumulation of slight, successive, favorable variations, so that it cannot produce great or sudden changes; it can only act by short and slow steps.Thus the adage "There are no leaps in nature," which has been confirmed by every new increase in knowledge, is intelligible on this doctrine.We can understand the almost infinite variety of means available for the attainment of the same general end throughout nature, since every trait, once acquired, is perpetually inherited, and a structure which has been modified in many different respects must adapt Same general purpose.In short, we can understand why nature is wasteful in variation, though stingy in innovation.But if each species was created independently, no one can explain why this should be a law of nature. It seems to me that there are many other facts which can be explained on the basis of this doctrine.How strange it is: a woodpecker-like bird that hunts insects on the ground; an upland geese that rarely or never swims has webbed feet; a pigeon-like bird that dives and eats insects in the water; Learn about the habits and structure of puffins!There are endless other examples of this as well.But on the view that each species is constantly striving to increase in number, and that natural selection is always adapting the slowly varying offspring of each species to unoccupied or poorly occupied places in nature, the facts Not surprising, perhaps even expected. We can understand to some extent how there is so much beauty in all of nature; for it is largely due to selection.Beauty is not universal, according to our senses, and anyone who sees some poisonous snakes, some fish, some bats with faces as hideous as distorted people would admit it.Sexual selection has bestowed the most brilliant colours, graceful forms, and other ornaments on the males, and sometimes on the sexes of many birds, butterflies, and other animals.In birds, sexual selection has often been such that the song of the male pleases both the female and our hearing.The flowers and fruit stand out in their color against the green foliage, so the flowers are easily seen, visited and pollinated by insects, and the seeds are dispersed by birds.How certain colours, sounds, and shapes give pleasure to man and subhuman animals—that is, how the simplest sense of beauty was first acquired—we do not know, any more than we know certain tastes and aromas. How the original became the same as comfortable. As natural selection operates by competition, and adapts and improves the organisms of each locality, only with regard to its fellows; so the species of any one locality, though supposedly created for that country on the usual view, are We need not be surprised that those specially adapted to that country should be overthrown and displaced by naturalized organisms introduced from other lands.It is not surprising that all the designs in nature, even like the human eye, are not, so far as we can judge, absolutely perfect; or that they are somewhat incompatible with our notions of adaptation.The sting of a bee, when employed against an enemy, causes the death of the bee itself; the drones, produced in so many numbers for a single mating, are afterwards killed by their sterile sisters; waste; the instinctive hatred of the queen-bee for her fertile daughters; the horde-bee foraging in the living body of the caterpillar; and other instances of this kind, are not surprising.According to the theory of natural selection.The strange thing is that no more examples of the lack of absolute perfection have actually been found. The complex and poorly understood laws which govern the production of varieties are, so far as we can judge, the same laws which govern the production of definite species.In both cases physical conditions seem to have produced some immediate and definite effect, but how great this effect we cannot say.Thus, when varieties enter any new country, they sometimes acquire some of the characters proper to the species there.For varieties and species, use and non-use seem to produce comparable effects; and it will be impossible to refute this conclusion if we see the following.For example, the duck-headed duck, which has flightless wings, is found in almost the same conditions as the domestic duck; the burrowing comb rat is sometimes blind, and some moles are usually blind, and their eyes are covered with skin; Many animals in the European Dark Hole are blind.With varieties and species correlative variations seem to play an important part, so that when one part varies, other parts must also vary.In varieties and species, long lost characters sometimes reappear in varieties and species.How, according to Tetsu, is to be explained the fact that several species of Equus, and their hybrids, occasionally have stripes on the shoulders and legs!How simple is the explanation of the foregoing facts if we believe that these species have descended from striped progenitors, just as several domestic breeds of the pigeon have descended from the striped blue rock-pigeon! On the usual view that each species was independently created, why should the characters of species, that is, the characters by which species of the same genus differ from one another, vary more than those of the genera they have in common?Why, for example, is the color of the flowers of any one species of a genus more readily variable when other species have flowers of different colors than when all species are of the same color?We can understand the fact that species are only well-characterized varieties, and that their characters have become highly stable; These are the characters by which these species are distinguished from one another; and these are therefore more easily varied than those of a genera which have been inherited for a long period of time without being varied.On the special theory, it cannot be explained why, in a single species of a genus, organs which have developed in such an unusual manner, and which we may naturally infer of great importance to that species, should vary with marked ease; but, according to In our view, since the divergence of several species from a common ancestor, this organ has undergone a great deal of variation and change, so that we may expect further variations of this organ generally.But an organ, like the wing of a bat, may be developed in the most extraordinary manner, but if the organ is common to many subordinate forms, that is, if it has been inherited over a long period of time. , this organ would not be more easily modified than other structures; for in this case long succession of natural selection would have rendered it stable.Look at instincts, some of which, though strange, present no more difficulty than the constitution of the body, on the theory of natural selection of successive, slight, and beneficial variations.We can thus understand why nature, in endowing several instincts to different animals of the same class, acts in graduated steps.I have attempted to show how important the principle of gradation is for the explanation of the admirable building power of the bee.Habit no doubt often plays a role in a change of instinct; but it is not necessarily indispensable, as we have seen in the case of neutral insects, which leave no offspring with a long succession of heredity. Habitual effect.根据同属的一切物种都是从一个共同祖先传下来的并且遗传了许多共同性状这一观点，我们便能了解近似物种当处在极不相同的条件之下时，怎么还具有几乎同样的本能；为什么南美洲热带和温带的鸫像不列颠的物种那样地用泥土涂抹它们的巢的内侧。根据本能是通过自然选择而缓慢获得的观点，我们对某些本能并不完全，容易发生错误，而且许多本能会使其他动物蒙受损失，就不必大惊小怪了。 如果物种只是特征很显著的、稳定的变种，我们便能立刻看出为什么它们的杂交后代在类似亲体的程度上和性质上——在由连续杂交而相互吸收方面以及在其他这等情形方面——就像公认的变种杂交后代那样地追随着同样的复杂法则，如果物种是独立创造的，并且变种是通过第二性法则产生出来的，这种类似就成为奇怪的事情了。 如果我们承认地质纪录不完全到极端的程度，那么地质纪录所提供的事实就强有力地支持了伴随着变异的生物由来学说。新的物种缓慢地在连续的间隔时间内出现；而不同的群经过相等的间隔时间之后所发生的变化量是大不相同的。物种和整个物群的绝灭，在有机世界的历史中起过非常显著的作用，这几乎不可避免地是自然选择原理的结果；因为旧的类型要被新而改进了的类型排挤掉。单独一个物种也好，整群的物种也好，当普通世代的链条一旦断绝时，就不再出现了。优势类型的逐渐散布，以及它们后代的缓慢变异，使得生物类型经过长久的间隔时间以后，看来好像是在整个世界范围内同时发生变化似的。各个地质层的化石遗骸的性状在某种程度上是介于上面地质层和下面地质层的化石遗骸之间的，这一事实可以简单地由它们在系统链条中处于中间地位来解释。一切绝灭生物都能与一切现存生物分类在一起，这一伟大事实是现存生物和绝灭生物都是共同祖先的后代的自然结果。因为物种在它们的由来和变化的悠久过程中一般已在性状上发生了分歧，所以我们便能理解为什么比较古代的类型，或每一群的早期祖先，如此经常地在某种程度上处于现存群之间的位置。总之，现代类型在体制等级上一般被看做比古代类型为高；而且它们必须是较高级的，因为未来发生的、比较改进了的类型在生活斗争中战胜了较老的和改进较少的类型；它们的器官一般也更加专业化，以适于不同机能。这种事实与无数生物尚保存简单的而很少改进的适于简单生活条件的构造是完全一致的；同样地，这与某些类型在系统的各个阶段中为了更好的适于新的、退化的生活匀性而在体制上退化了的情形也是一致的。最后，同一大陆的近似类型——如澳洲的有袋类、美洲的贫齿类和其他这类例子——的长久延续的奇异法则也是可以理解的，因为在同一地区里，现存生物和绝灭生物由于系统的关系会是密切近似的。 看一看地理分布，如果我们承认，由于以前的气候变化和地理变化以及由于许多偶然的和未知的散布方法，在悠长的岁月中曾经有过从世界的某一部分到另一部分的大量迁徙，那么根据伴随着变异的生物由来学说，我们便能理解有关“分布”上的大多数主要事实。我们能够理解，为什么生物在整个空间内的分布和在整个时间内的地质演替会有这么动人的平行现象；因为在这两种情形里，生物通常都由世代的纽带所连结，而且变异的方法也是一样的。我们也体会了曾经引起每一个旅行家注意的奇异事实的全部意义，即在同一大陆上，在最不相同的条件下，在炎热和寒冷下，在高山和低地上，在沙漠和沼泽里，每一大纲里的生物大部分是显然相关联的；因为它们都是同一祖先和早期移住者的后代。根据以前迁徙的同一原理，在大多数情形里它与变异相结合，我们借冰期之助，便能理解在最遥远的高山上以及在北温带和南温带中的某些少数植物的同一性，以及许多其他生物的密切近似性；同样地还能理解，虽然被整个热带海洋隔开的北温带和南温带海里的某些生物的密切相似性。虽然两个地区呈现着同一物种所要求的密切相似的物理条件，如果这两个地区在长久时期内是彼此分开的，那么我们对于它们的生物的大不相同就不必大惊小怪；因为，由于生物和生物之间的关系是一切关系中的最重要关系，而且这两个地区在不同时期内会从其他地区或者彼此相互接受不同数量的移住者，所以这两个地区中的生物变异过程就必然是不同的。 依据谱系以后发生变化的这个迁徙的观点，我们便能理解为什么只有少数物种栖息在海洋岛上，而其中为什么有许多物种是特殊的即本地特有的类型。我们清楚的知道那些不能横渡广阔海面的动物群的物种，如蛙类和陆栖哺乳类，为什么不栖息在海洋岛上；另一方面，还可理解，像蝙蝠这些能够横渡海洋的动物，其新而特殊的物种为什么往往见于离开大陆很远的岛上。海洋岛上有蝙蝠的特殊物种存在，却没有一切其他陆栖哺乳类，根据独立创造的学说，这等情形就完全不能得到解释了。 任何两个地区有密切近似的或代表的物种存在，从伴随着变异的生物由来学说的观点看来，是意味着同一亲类型以前曾经在这两个地区栖息过；并且，无论什么地方，如果那里有许多密切近似物种栖息在两个地区，我们必然还会在那里发现两个地区所共有的某些同一物种。无论在什么地方，如果那里有许多密切近似的而区别分明的物种发生，那么同一群的可疑类型和变种也会同样地在那里发生。各个地区的生物必与移入者的最近根源地的生物有关联，这是具有高度一般性的法则。在加拉帕戈斯群岛、胡安·斐尔南德斯群岛（Juan Fernandez）以及其他美洲岛屿上的几乎所有的植物和动物与邻近的美洲大陆的植物和动物的动人关系中，我们看到这一点；也在佛得角群岛以及其他非洲岛屿上的生物与非洲大陆生物的关系中看到这一点。必须承认，根据特创说，这些事实是得不到解释的。 我们已经看到，一切过去的和现代的生物都可群下分群，而且绝灭的群往往介于现代诸群之间，在这等情形下，它们都可以归入少数的大纲内，这一事实，根据自然选择及其所引起的绝灭和性状分歧的学说，是可以理解的。根据这些同样的原理，我们便能理解，每一纲里的类型的相互亲缘关系为什么是如此复杂和曲折的。我们还能理解，为什么某些性状比其他性状在分类上更加有用；——为什么适应的性状虽然对于生物具有高度的重要性，可是在分类上却几乎没有任何重要性；为什么从残迹器官而来的性状，虽然对于生物没有什么用处，可是往往在分类上具有高度的价值；还有，胚胎的性状为什么往往是最有价值的。一切生物的真实的亲缘关系，与它们的适应性的类似相反，是可以归因于遗传或系统的共同性的。“自然系统”是一种依照谱系的排列，依所获得的差异诸级，用变种、物种、属、科等术语来表示的；我们必须由最稳定的性状，不管它们是什么，也不管在生活上多么不重要，去发现系统线。 人的手、蝙蝠的翅膀、海豚的鳍和马的腿都由相似的骨骼构成，——长颈鹿颈和象颈的脊椎数目相同，——以及无数其他的这类事实，依据伴随着缓慢的、微小而连续的变异的生物由来学说，立刻可以得到解释，蝙蝠的翅膀和腿，——螃蟹的颚和腿，——花的花瓣、雄蕊和雌蕊，虽然用于极其不同的目的，但它们的结构样式都相似。这些器官或部分在各个纲的早期祖先中原来是相似的，但以后逐渐发生了变异，根据这样观点，上述的相似性在很大程度上还是可以得到解释的。连续变异不总是在早期年龄中发生，并且它的遗传是在相应的而不是在更早的生活时期；依据这一原理，我们更可清楚地理解，为什么哺乳类、鸟类、爬行类和鱼类的胚胎会如此密切相似，而在成体类型中又如此不相似。呼吸空气的哺乳类或鸟类的胚胎就像必须依靠很发达的鳃来呼吸溶解在水中的空气的鱼类那样地具有鳃裂和弧状动脉，对此我们用不到大惊小怪。 不使用，有时借自然选择之助，往往会使在改变了的生活习性或生活条件下变成无用的器官而缩小；根据这一观点，我们便能理解残迹器官的意义。但是不使用和选择一般是在每一生物到达成熟期并且必须在生存斗争中发挥充分作用的时期，才能对每一生物发生作用，所以对于在早期生活中的器官没有什么影响力；因此那器官在这早期年龄里不会被缩小或成为残迹的。比方说，小牛从一个具有很发达牙齿的早期祖先遗传了牙齿，而它们的牙齿从来不穿出上颚牙床肉；我们可以相信，由于舌和颚或唇通过自然选择变得非常适于吃草，而无需牙齿的帮助，所以成长动物的牙齿在以前就由于不使用而缩小了；可是在小牛中，牙齿却没有受到影响，并且依据遗传在相应年龄的原理，它们从遥远的时期一直遗传到今天。带着毫无用处的鲜明印记的器官，例如小牛胚胎的牙齿或许多甲虫的连合鞘翅下的萎缩翅，竟会如此经常发生，根据每一生物以及它的一切不同部分都是被特别创造出来的观点，这将是多么完全不可理解的事情。可以说“自然”曾经煞费苦心地利用残迹器官、胚胎的以及同原的构造来泄露她的变化的设计，只是我们太盲目了，以致不能理解她的意义。 上述事实和论据使我完全相信，物种在系统的悠久过程中曾经发生变化，对此我已做了复述。这主要是通过对无数连续的、轻微的、有利的变异进行自然选择而实现的；并且以重要的方式借助于器官的使用和不使用的遗传效果；还有不重要的方式，即同不论过去或现在的适应性构造有关，它们的发生依赖外界条件的直接影响，也依赖我们似乎无知的自发变异。看来我以前是低估了在自然选择以外导致构造上永久变化的这种自发变异的频率和价值。但是因为我的结论最近曾被严重地歪曲，并且说我把物种的变异完全归因于自然选择，所以请让我指出，在本书的第一版中，以及在以后的几版中，我曾把下面的话放在最显著的地位——即《绪论》的结尾处：“我相信'自然选择'是变异的最主要的但不是独一无二的手段。”这话并没有发生什么效果。根深柢固的误解力量是大的；但是科学的历史示明，这种力量幸而不会长久延续。 几乎不能设想，一种虚假的学说会像自然选择学说那样地以如此令人满意的方式解释上述若干大类的事实。最近有人反对说，这是一种不妥当的讨论方法；但是，这是用来判断普通生活事件的方法，并且是最伟大的自然哲学者们所经常使用的方法。光的波动理论就是这样得来的；而地球环绕中轴旋转的信念，直到最近还没有直接的证据。要说科学对于生命的本质或起源这个更高深的问题还没有提出解释，这并不是有力的异议。谁能够解释什么是地心吸力的本质呢？现在没有人会反对遵循地心吸力这个未知因素所得出的结果；尽管列不尼兹（Leibnitz）以前曾经责难牛顿，说他引进了“玄妙的性质和奇迹到哲学里来”。 本书所提出的观点为什么会震动任何人的宗教感情，我看不出有什么好的理由。要想指出这种印象是如何短暂，记住以下情形就够了：人类曾有过最伟大发现，即地心吸力法则，也被列不尼兹抨击为“自然宗教的覆灭，因而推理地也是启示宗教的覆灭”。一位著名的作者兼神学者写信给我说，“他已逐渐觉得，相信'神'创造出一些少数原始类型，它们能够自己发展成其他必要类型，与相信'神'需要一种新的创造作用以补充'神'的法则作用所引起的空虚，同样都是崇高的'神'的观念”。 可以质问，为什么直到最近差不多所有在世的最卓越的博物学者和地质学者都不相信物种的可变性呢。不能主张生物在自然状况下不发生变异；不能证明变异量在悠久年代的过程中是一种有限的量；在物种和特征显著的变种之间未曾有、或者也不能有清楚的界限。不能主张物种杂交必然是不育的，而变种杂交必然是能育的；或者主张不育性是创造的一种特殊禀赋和标志。只要把地球的历史想成是短暂的，几乎不可避免地就要相信物种是不变的产物；而现在我们对于时间的推移已经获得某种概念，我们就不可没有根据地去假定地质的纪录是这样完全，以致如果物种曾经有过变异，地质就会向我们提供有关物种变异的明显证据。 但是，我们天然地不愿意承认一个物种会产生其他不同物种的主要原因，在于我们总是不能立即承认巨大变化所经过的步骤，而这些步骤又是我们不知道的。这和下述情形一样：当莱尔最初主张长行的内陆岩壁的形成和巨大山谷的凹下都是由我们现在看到的依然发生作用的因素所致，对此许多地质学者都感到难于承认。思想大概不能掌握即便是一百万年这用语的充分意义；而对于经过几乎无限世代所累积的许多轻微变异，其全部效果如何更是不能综合领会的了。 虽然我完全相信本书在提要的形式下提出来的观点是正确的；但是，富有经验的博物学者的思想在岁月的悠久过程中装满了大量事实，其观点与我的观点直接相反，我并不期望说服他们。在“创造的计划”、“设计的一致”之类的说法下，我们的无知多么容易被荫蔽起来，而且还会只把事实复述一遍就想像自己已经给予了一种解释。无论何人，只要他的性情偏重尚未解释的难点，而不重视许多事实的解释他就必然要反对这个学说。在思想上被赋有很大适应性的并且已经开始怀疑物种不变性的少数博物学者可以受到本书的影响；但是我满怀信心地看着将来，——看着年轻的、后起的博物学者，他们将会没有偏见地去看这个问题的两方面。已被引导到相信物种是可变的人们，无论是谁，如果自觉地去表示他的确信，他就做了好事；因为只有这样，才能把这一问题所深深受到的偏见的重负移去。 几位卓越的博物学者最近发表他们的信念，认为每一属中都有许多公认的物种并不是真实的物种；而认为其他物种才是真实的，就是说，被独立创造出来的。依我看来，这是一个奇怪的结论。他们承认，直到最近还被他们自己认为是特别创造出来的、并且大多数博物学者也是这样看待它们的、因而具有真实物种的一切外部特征的许多类型，是由变异产生的，但是他们拒绝把这同一观点引伸到其他稍微不同的类型。虽然如此，他们并不冒充他们能够确定，或者甚至猜测，哪些是被创造出来的生物类型，哪些是由第二性法则产生出来的生物类型。他们在某一种情形下承认变异是真实原因，而在另一种情形下却又断然否认它，而又不指明这两种情形有何区别。总有一天这会被当做奇怪的例子来说明先人之见的盲目性。这些作者对奇迹般的创造行为并不比对通常的生殖感到更大的惊奇。但是他们是否真地相信，在地球历史的无数时期中，某些元素的原子会突然被命令骤然变成活的组织呢？他们相信在每次假定的创造行为中都有一个个体或许多个体产生出来吗？所有无限繁多种类的动物和植物在被创造出来时究竟是卵或种籽或充分长成的成体吗？在哺乳类的情形下，它们是带着营养的虚假印记从母体子宫内被创造出来的吗？毫无疑问，相信只有少数生物类型或只有某一生物类型的出现或被创造的人并不能解答这类问题的。几位作者曾主张，相信创造成百万生物与创造一种生物是同样容易的；但是莫波丢伊（Maupertuis）的“最小行为”的哲学格言会引导思想更愿意接受较少的数目；但是肯定地我们不应相信，每一大纲里的无数生物在创造出来时就具有从单独一个祖先传下来的明显的、欺人的印记。 作为事物以前状态的纪录，我在以上诸节和其他地方记下了博物学者们相信每一物种都是分别创造的若干语句；我因为这样表达意见而大受责难。但是，毫无疑问，在本书第一版出现时，这是当时一般的信念。我以前向很多博物学者谈论过进化的问题，但从来没有一次遇到过任何同情的赞成。在那个时候大概有某些博物学者的确相信进化，但是他们或者沉默无言，或者叙述得这么模糊以致不容易理解他们所说的意义。现在的情形就完全不同了，几乎每一博物学者都承认伟大的进化原理。尽管如此，还有一些人，他们认为物种曾经通过十分不能解释的方法而突然产生出新的、完全不同的类型：但是，如我力求示明的，大量的证据可以提出来反对承认巨大而突然的变化。就科学的观点而论，为进一步研究着想，相信新的类型以不能理解的方法从旧的、十分不同的类型突然发展出来，比相信物种从尘土创造出来的旧信念，并没有什么优越之处。 可以问，我要把物种变异的学说扩展到多远。这个问题是难于回答的，因为我们所讨论的类型愈是不同，有利于系统一致性的论点的数量就愈少，其说服力也愈弱。但是最有力的论点可以扩展到很远。整个纲的一切成员被一条亲缘关系的连锁连结在一起，一切都能够按群下分群的同一原理来分类。化石遗骸有时有一种倾向，会把现存诸目之间的巨大空隙填充起来。 残迹状态下的器官清楚地示明了，一种早期祖先的这种器官是充分发达的；在某些情形里这意味着它的后代已发生过大量变异。在整个纲里，各种构造都是在同一样式下形成的，而且早期的胚胎彼此密切相似。所以我不能怀疑伴随着变异的生物由来学说把同一大纲或同一界的一切成员都包括在内。我相信动物至多是从四种或五种祖先传下来的，植物是从同样数目或较少数目的祖先传下来的。 类比方法引导我更进一步相信，一切动物和植物都是从某一种原始类型传下来的。但是类比方法可能把我们导入迷途。虽然如此，一切生物在它们的化学成分上、它们的细胞构造上、它们的生长法则上、它们对于有害影响的易感性上都有许多共同之点。我们甚至在以下那样不重要的事实里也能看到这一点，即同一毒质常常同样地影响各种植物和动物；瘿蜂所分泌的毒质能引起野蔷薇或橡树产生畸形。在一切生物中，或者某些最低等的除外，有性生殖似乎在本质上都是相似的。在一切生物中，就现在所知道的来说，最初的胚胞是相同的；所以一切生物都是从共同的根源开始的。如果当我们甚至看一看这两个主要部分——即看一看动物界和植物界——某些低等类型如此具有中间的性质，以致博物学者们争论它们究竟应该属于哪一界。正如阿萨·格雷教授所指出的，“许多低等藻类的孢子和其他生殖体可以说起初在特性上具有动物的生活，以后无可怀疑地具有植物的生活”。所以，依据伴随着性状分歧的自然选择原理，动物和植物从这些低等的中间类型发展出来，并不是不可信的；而且，如果我们承认了这一点，我们必须同样地承认曾经在这地球上生活过的一切生物都是从某一原始类型传下来的。但是这推论主要是以类比方法为根据的，它是否被接受并无关紧要。正如刘易斯先生所主张的，毫无疑问，在生命的黎明期可能就有许多本同的类型发生；但是，倘真如此，则我们便可断定，只有很少数类型曾经遗留下变异了的后代。因为，正如我最近关于每一大界、如“脊椎动物”，“关节动物”等的成员所说的，在它们的胚胎上、同原构造上、残迹构造上，我们都有明显的证据可以证明每一界里的一切成员都是从单独一个祖先传下来的。 我在本书所提出的以及华莱斯先生所提出的观点，或者有关物种起源的类似的观点，一旦被普遍接受以后，我们就能够隐约地预见到在博物学中将会发生重大革命。分类学者将能和现在一样地从事劳动，但是他们不会再受到这个或那个类型是否为真实物种这一可怕疑问的不断搅扰。这，我确信并且我根据经验来说，对于各种难点将不是微不足道的解脱。有关的五十个物种的不列颠树莓类（bramble）是否为真实物种这一无休止的争论将会结束。分类学者所做的只是决定（这点并不容易）任何类型是否充分稳定并且能否与其他类型有所区别，而给它下一个定义；如果能够给它下一定义，那就要决定那些差异是否充分重要，值得给以物种的名称。后述一点将远比它现在的情形更加重要；因为任何两个类型的差异，不管如何轻微，如果不被中间诸级把它们混合在一起，大多数博物学者就会认为这两个类型都足以提升到物种的地位。 从此以后，我们将不得不承认物种和特征显著的变种之间的唯一区别是：变种已被知道或被相信现在被中间级进联结起来，而物种却是在以前被这样联结起来的。因此，在不拒绝考虑任何两个类型之间目前存在着中间级进的情况下，我们将被引导更加仔细地去衡量、更加高度地去评价它们之间的实际差异量。十分可能，现在一般被认为只是变种的类型，今后可能被相信值得给以物种的名称；在这种情形下，科学的语言和普通的语言就一致了。总而言之，我们必须用博物学者对待属那样的态度来对待物种，他们承认属只不过是为了方便而做出的人为组合。这或者不是一个愉快的展望；但是，对于物种这一术语的没有发现的、不可能发现的本质，我们至少不会再做徒劳的探索。 博物学的其他更加一般的部门将会大大地引起兴趣。博物学者所用的术语如亲缘关系、关系、模式的同一性、父性、形态学、适应的性状、残迹的和萎缩的器官等等，将不再是隐喻的，而会有它的鲜明的意义。当我们不再像未开化人把船看做是完全不可理解的东西那样地来看生物的时候；当我们把自然界的每一产品看成是都具有悠久历史的时候；当我们把每一种复杂的构造和本能看成是各各对于所有者都有用处的设计的综合，有如任何伟大的机械发明是无数工人的劳动、经验、理性以及甚至错误的综合的时候；当我们这样观察每一生物的时候，博物学的研究将变得——我根据经验来说——多么更加有趣呀！ 在变异的原因和法则、相关法则、使用和不使用的效果、外界条件的直接作用等等方面，将会开辟一片广大的、几乎未经前人踏过的研究领域。家养生物的研究在价值上将大大提高。人类培育出来一个新品种，比起在已经记载下来的无数物种中增添一个物种，将会成为一个更加重要、更加有趣的研究课题。我们的分类，就它们所能被安排的来说，将是按谱系进行的；那时它们才能真地显示出所谓“创造的计划”。当我们有一确定目标的时候，分类的规则无疑会变得更加简单。我们没有得到任何谱系或族徽；我们必须依据各种长久遗传下来的性状去发现和追踪自然谱系中的许多分歧的系统线。残迹器官将会确实无误地表明长久亡失的构造的性质。称做异常的、又可以富于幻想地称做活化石的物种和物种群，将帮助我们构成一张古代生物类型的图画。胚胎学往往会给我们揭露出每一大纲内原始类型的构造，不过多少有点模糊而已。 如果我们能够确定同一物种的一切个体以及大多数属的一切密切近似物种，曾经在不很遥远的时期内从第一个祖先传下来，并且从某一诞生地迁移出来；如果我们更好地知道迁移的许多方法，而且依据地质学现在对于以前的气候变化和地平面变化所提出的解释以及今后继续提出的解释，那么我们就确能以令人赞叹的方式追踪出全世界生物的过去迁移情况。甚至在现在，如果把大陆相对两边的海栖生物之间的差异加以比较，而且把大陆上各种生物与其迁移方法显然有关的性质加以比较，那么我们就能对古代的地理状况多少提出一些说明。 地质学这门高尚的科学，由于地质纪录的极端不完全而损失了光辉。埋藏着生物遗骸的地壳不应被看做是一个很充实的博物馆，它所收藏的只是偶然的、片段的、贫乏的物品而已。每一含有化石的巨大地质层的堆积应该被看做是由不常遇的有利条件来决定的，并且连续阶段之间的空白间隔应该被看做是极长久的。但是通过以前的和以后的生物类型的比较，我们就能多少可靠地测出这些间隔的持续时间。当我们试图依据生物类型的一般演替，把两个并不含有许多相同物种的地质层看做严格属于同一时期时，必须谨慎。因为物种的产生和绝灭是由于缓慢发生作用的、现今依然存在的原因，而不是由于创造的奇迹行为；并且因为生物变化的一切原因中最重要的原因是一种几乎与变化的或者突然变化的物理条件无关的原因，即生物和生物之间的相互关系，——一种生物的改进会引起其他生物的改进或绝灭；所以，连续地质层的化石中的生物变化量虽不能作为一种尺度来测定实际的时间过程，但大概可以作为一种尺度来测定相对的时间过程。可是，许多物种在集体中可能长时期保持不变，然而在同一时期里，其中若干物种，由于迁徙到新的地区并与外地的同住者进行竞争，可能发生变异；所以我们对于把生物变化作为时间尺度的准确性，不必有过高的评价。 我看到了将来更加重要得多的广阔研究领域。心理学将稳固地建筑在赫伯特·斯潘塞先生所已良好奠定的基础上，即每一智力和智能都是由级进而必然获得的。人类的起源及其历史也将由此得到大量说明。 最卓越的作者们对于每一物种曾被独立创造的观点似乎感到十分满足。依我看来，世界上过去的和现在的生物之产生和绝灭就像决定个体的出生和死亡的原因一样地是由于第二性的原因，这与我们所知道的“造物主”在物质上打下印记的法则更相符合。当我把一切生物不看作是特别的创造物，而看作是远在寒武系第一层沉积下来以前就生活着的某些少数生物的直系后代，依我看来，它们是变得尊贵了。从过去的事实来判断，我们可以稳妥地推想，没有一个现存物种会把它的没有改变的外貌传递到遥远的未来。并且在现今生活的物种很少把任何种类的后代传到极遥远的未来；因为依据一切生物分类的方式看来，每一属的大多数物种以及许多属的一切物种都没有留下后代，而是已经完全绝灭了。展望未来，我们可以预言，最后胜利的并且产生占有优势的新物种的，将是各个纲中较大的优势群的普通的、广泛分布的物种。既然一切现存生物类型都是远在寒武纪以前生存过的生物的直系后代，我们便可肯定，通常的世代演替从来没有一度中断过，而且还可确定，从来没有任何灾变曾使全世界变成荒芜。因此我们可以多少安心地去眺望一个长久的、稳定的未来。因为自然选择只是根据并且为了每一生物的利益而工作，所以一切肉体的和精神的禀赋都有向着完善化前进的倾向。 凝视树木交错的河岸，许多种类的无数植物覆盖其上，群鸟鸣于灌木丛中，各种昆虫飞来飞去，蚯蚓在湿土里爬过，并且默想一下，这些构造精巧的类型，彼此这样相异，并以这样复杂的方式相互依存，而它们都是由于在我们周围发生作用的法则产生出来的，这岂非有趣之事。这些法则就其最广泛的意义来说，就是伴随着“生殖”的“生长”；几乎包含在生殖以内的“遗传”；由于生活条件的间接作用和直接作用以及由于使用和不使用所引起的变异：生殖率如此之高以致引起“生存斗争”，因而导致“自然选择”、并引起“性状分歧”和较少改进的类型的“绝灭”。这样，从自然界的战争里，从饥饿和死亡里，我们便能体会到最可赞美的目的，即高级动物的产生，直接随之而至。认为生命及其若干能力原来是由“造物主”注入到少数类型或一个类型中去的，而且认为在这个行星按照引力的既定法则继续运行的时候，最美丽的和最奇异的类型从如此简单的始端，过去，曾经而且现今还在进化着；这种观点是极其壮丽的。