Chapter 8 Chapter 5 The Law of Variation

Effects of Changed External Conditions—Use and Disuse Combined with Natural Selection; Organs of Fly and Sight—Climate Domestication—Associated Variation—Compensation and Economization of Growth—False Correlation—Repetition, Remnant Vulnerable to variation in structure of lower and lower systems - abnormally developed parts apt to be highly variable: characters of species are more variable than those of genus: secondary sexual characters are susceptible to variation - species of the same genus vary in a similar manner - Reappearance of long-lost traits—an abstract. I have sometimes spoken before of variation--so common and varied in organic beings under domestication, and to a lesser extent in those of nature--as to occur by chance.This is, of course, quite an incorrect statement, but it sufficiently shows our ignorance as to the causes of particular variations.Some writers believe that it is a function of the reproductive system that produces individual differences or slight deviations of structure, as does that which makes a child resemble his parents.But the fact that variation and deformity occur more frequently in domestic than in nature, and that the variability is greater in widely-distributed species than in narrowly-distributed species, leads to the conclusion that variability is generally related to that of life. conditions under which individual species have lived for several generations.In the first chapter I tried to show that changed external conditions act in two ways, directly on the whole system or only on certain parts of it, and indirectly through the reproductive system.In all cases two factors are involved, the nature of the living being, which is the most important of the two, and the nature of the external conditions.The direct action of changed external conditions produces definite or indeterminate results.In the latter case the institution seems to have become plastic, and we see a great deal of variability, in the former the nature of living beings is such that they yield easily under certain conditions. , and all individuals, or nearly all individuals, vary in the same way.

It is difficult to determine to what extent changes in external conditions, such as climate, food, etc., have worked in a definite way.We have reason to believe that, over time, their effect is greater than the obvious facts would suggest.We may safely assert, however, that the innumerable and complex mutual adaptations of structure, as we see among the various organic beings in nature, cannot be attributed to this action alone.In the following cases external conditions seem to have had some slight definite effect: E. Forbes asserts that shellfish which grow in the southern range, and if they live in shallow water, are more colorful than those which live in shallow water. The same kind of shellfish in the north or in deep water should be more distinct; but not necessarily so.Mr. Gould believes that birds of the same species live in brighter atmospheres than those that live by the sea or on islands; Wollaston believes that living by the sea affects Insect color.Moquin Tandon has given a list of plants which, when grown near the coast, are succulent in some degree, though not elsewhere.These slightly modified organisms are interesting because they exhibit characters similar to those possessed by the same species confined to the same external conditions.

When a variation is of the slightest usefulness to any being, we cannot tell how much of this variation is due to the cumulative action of natural selection, and how much is due to the definite action of the conditions of life.It is well known to furriers, for instance, that the further northerly they live, the thicker and finer the fur of the same species; but who can say how much of this difference is due to the fact that the individual with the warmest fur has, over many generations, How much of the benefit and preservation is due to the effect of the severe cold climate?For climate seems to have had some direct effect on the fur of our domestic animals.

Many instances may be given of the same species, under distinctly different external conditions, producing similar varieties; and, on the other hand, the same species under distinctly identical external conditions, producing dissimilar varieties.Again, countless instances of species which have remained pure, or not at all unchanged, though inhabiting very opposite climates, are familiar to every naturalist.This argument led me to consider the direct action of surrounding conditions to be of less importance than the tendency to vary from causes of which we are wholly ignorant. Conditions of life, in a sense, not only bring about variations, directly or indirectly, but also involve natural selection, for they determine whether this or that variety will survive or not.But when man is the agent of choice, it becomes evident that the two elements of change are distinct; variability is somehow activated, but it is man's will, which directs it toward a certain Directions add up; the latter effect corresponds to that of the survival of the fittest under natural conditions.

Effects of increased use and non-use of organs under natural selection From the facts given in the first chapter, I think it beyond doubt that in our domestic animals some organs are strengthened and enlarged by use, and others are reduced by disuse; and I The change is thought to be hereditary, in an unfettered state of nature, and since we do not know the type of progenitor, we have no standard of comparison by which to judge the effects of prolonged and continuous use and non-use; but there are many animals which have The construction of , is best explained in terms of the effects of disuse.As Professor Owen says, there is nothing more extraordinary in nature than the inability of a bird to fly; and yet there are a number of them.The 1oger-headed duck of South America can only flap its wings on the water, and its wings are nearly identical to those of the domestic Aylesbury duck; Mr. Ning'an (Mr. Cunningham) said that their young birds can fly, but they lose this ability when they grow up.As large ground-feeding birds seldom fly except to escape danger, the almost wingless state of the several species of birds that now inhabit, or not long ago inhabited, islands free of predators, is probably Due to non-use.The ostrich is indeed land-dwelling, and is exposed to dangers from which it cannot escape by flight, but it can defend itself by kicking its enemies as effectively as the quadruped.We may believe that the progenitor of the ostrich was originally like the wild goose in its habits, but as its body increased in size and weight in successive generations, it used its legs more and less Using its wings, it finally became unable to fly.

Kirbx has said (and I have seen the same fact) that many male dung-eating beetles often have their anterior phalanges, i.e., fore feet, broken off; None of them left the slightest trace, and the loss of the fore tarsus of the Onitesapelles is so frequent that the insect has been described as having no tarsus.In some other genera they have tarsus, but only in a rudimentary state.The Egyptians saw the sacred beetle Ateuchus, whose tarsus is completely absent.The question of whether accidental injuries are hereditary is still undecided; but Brown-Seqttuard's observation in the guinea-pig of a hereditary effect of surgery should lead us to Be careful.The complete absence of fore tarsus in the sacred beetle, and the mere remnants of tarsus in some other genera, therefore, is perhaps best regarded not as an inheritance of injury, but as a result of long-continued The result of disuse; as many dung-eating beetles generally lose the tarsus, which must have occurred early in their life; the tarsus, therefore, is not of great importance to these insects, or has not been much used by them. used by.

In some cases it is apt to attribute to disuse all or chiefly variations in structure which have been brought about by natural selection.Mr. Wollaston has found it a remarkable fact that, of the 550 species of beetles (more are now known) that inhabited Madeira, 200 had wings so incomplete that they could not fly; and This is the case with all the species of no less than twenty-three of the twenty-nine indigenous genera!There are several facts,--namely, that in many parts of the world beetles are frequently blown into the sea and drowned; in Madeira the beetle, as Wollaston observed, was well concealed, until a fair day. The time has just come out; the proportion of wingless beetles is greater in the unsheltered Desertas than in Madeira; and in particular there is an unusual fact, which Wollaston attaches special importance to, that Certain large groups of beetles, which absolutely require the use of wings, are elsewhere very numerous, but here are almost entirely absent; and these several observations lead us to believe that the chief cause of so many wingless Madeira beetles is probably related to the The combined action of natural selection is not employed, since in many successive generations some individuals of the beetle, either by virtue of their wings being somewhat less fully developed, or by their habit of indolence, who fly the least, will not be blown out to sea, and thus have the best chance of survival; on the contrary, those beetle individuals who are most fond of flying are most often blown to the sea by the wind, and are thus destroyed.

In Madeira there are also insects which forage not on the ground, as certain coleoptera and lepidoptera which forage in flowers, and which must frequently use their wings for food, which, Mr. Wollaston conjectures, The insect's wings not only do not shrink at all, they even increase in size.This is completely consistent with the role of natural selection.For when a new insect first arrives on the island, the tendency of natural selection to enlarge or shrink its wings will determine that most individuals will either fight victoriously against the wind and be preserved, or give up the attempt and fly less or less. It was saved without flying.For example, if a ship breaks near the coast, it would be better for the crew members who are good swimmers to swim as far as possible; for those who are not good swimmers, it would be better to cling to the broken ship.

The eyes of moles and some burrowing rodents are rudimentary, and in some cases completely covered by fur and fur.This state of the eye is probably due to the gradual shrinkage of the eye from disuse, but there may be some help here from natural selection.There is a burrowing rodent of South America called the tuco-tuco, or Ctenomys, which penetrates even more subterraneanly than the mole; a Spaniard who often catches them tells me that their eyes Mostly blind.I have kept a living one, and its eyes were indeed in this condition. After an autopsy, I found out that the cause was inflammation of the nictitating membrane.As often inflamed eyes must be injurious to any animal, and as eyes are certainly not necessary for animals with burrowing habits, they are in this case reduced in size, with the upper and lower eyelids glued together, and covered with hair. , may be advantageous; and if it is advantageous, natural selection will assist the effect of disuse.

It is well known that several animals, belonging to very different classes, inhabit the caves of Carniola and Kentucky, and are blind.Some crabs, although they have lost their eyes, still have their eye-stalks; as if the lens of the telescope has been lost, but the frame of the telescope still exists.As it is difficult to imagine that eyes could do any harm, though useless, to animals that live in the dark, their loss may be attributed to disuse.There is a blind animal called Neotoma. Prof. Silliman once caught two cave rats half a mile away from the entrance of the cave. It can be seen that they do not live in extremely deep places. Their two eyes Large and bright; this animal, I was told by Professor Silliman, when exposed to progressively increasing light, after about a month, was able to dimly recognize objects in front of it.

It is difficult to imagine that the conditions of life could be more alike than limestone caves in almost similar climates; so that, on the old view that the blind fauna created separately for the caves of America and Europe, their organization and affinities might be expected to be very similar.This is evidently not the case if we look at the whole fauna of these two places; with regard to insects alone, Sclhiodte says: The few similarities of type between Mammoth cave (in Kentucky) and Carniola's cave are but apparent manifestations of the similarities generally existing between the fauna of Europe and North America. "It seems to me that we must assume that the American animals, for the most part, had normal sight, and that they migrated slowly from generation to generation into deeper and deeper parts of the Kentucky caves, just as European animals migrated into European caves. .We have some evidence of this gradual change in habits; Hiatt says: "We therefore regard the subterranean fauna as small branches of geographically restricted fauna from adjacent places, which, once extended into darkness, adapt to the The animals that first pass from light to darkness are not far removed from the common types. Then, forms adapted to twilight light follow; finally, those adapted to total darkness, they The formation of it is very special." We must understand that these words of Hiatt do not apply to the same species, but to different species.By the time animals, through innumerable generations, have reached the deepest depths, their eyes have almost disappeared from disuse, and natural selection has often brought about other changes, such as the growth of antennae or vibrissae, as a blind compensation.Notwithstanding this variation we can see the affinities of the cave animals of America with other species of the American continent, and of the cave animals of Europe with those of the Continent.I have heard from Prof. Dana that this is true of some cave animals of America, and that certain cave insects of Europe closely resemble those of their surrounding countries.It is difficult to give a rational account of the affinities of the blind cave-fauna with the other animals of the two continents, on the common view of their independent creation.The affinities of the several cave animals of the old and new worlds should be closely related, as we might expect from the well-known affinities of most other organisms of these two worlds.Since a blind species in the genus Batbysica is abundant under dark rocks far from the mouths of caves, the loss of vision in the cave species of this genus probably has no connection with their dark lives; it is quite natural, An insect which has lost its sense of sight is easily adapted to dark burrows.Another blind genus, Anophthalmus, also possesses this remarkable character; Mr. Murray has observed no such species anywhere but in caves; The species are different; and it is probable that the progenitors of these species, before losing their sight, were widespread on both continents, and then became extinct, except those who lived in caves.Some burrowing animals are very special, which is not surprising, such as the blind medaka (Amb1yopsis) mentioned by Agassiz, or the European reptile-the blind blind salamander (Proteus), they are all very strange Yes, I am only surprised that the remnants of ancient life are not more preserved, since the animals that dwell in dark places are rarer, and the competition is less intense. climate acclimatization The habits of plants are hereditary, such as the period of flowering, the period of dormancy, the amount of rainfall required for the germination of seeds, etc. I shall therefore say a little about domestication.If it were so common for plants of different species of the same genus to inhabit hot and cold regions, if it were true that all the species of the same genus were descended from a single parent-species, domestication must easily have worked its way through the long course of transmission. .It is well known that every species is adapted to its native climate: species brought from cold regions and even from temperate regions cannot tolerate tropical climates, and vice versa.There are also many succulents that cannot tolerate a humid climate.But the fitness of a species to the climate in which it lives is often overestimated.We may infer this from the fact that we often cannot predict whether an introduced plant will tolerate our climate, whereas many plants and animals introduced from different regions live here in perfect health.We have reason to believe that, in a state of nature, species are strictly limited in distribution, by competition with other organisms, in much the same way, or more so, than their adaptation to particular climates.But whether or not this adaptation to climate is close in most cases, we have evidence that certain few plants became to some extent naturally accustomed to different temperatures; that is to say, they became domesticated. : Dr. Hooker collected the seeds of the same species of pine and rhododendron from different heights in the Himalayas, cultivated them in England, and found them there to have different resistance to cold.Mr. Thwaites told me that he had seen the same fact in Ceylon; and that Mr. HC Watson had brought European plants from the Azores to England to make similar observations; I could give some other examples.With regard to animals, too, there are certain instances which can be cited of species greatly extending their ranges since recorded times, from warmer to cooler latitudes, and vice versa; Whether they were strictly adapted to their native climates, though we generally think they are; nor do we know whether they later became particularly domesticated to their new homes, and were better adapted to them than they were at first. We may infer that domestic animals were originally selected by savages for their usefulness, and for their ease of reproduction in confinement, and not for their ability to be transported to distant lands, so that the common From the extraordinary ability not only to resist very different climates, but to reproduce in that climate (this is a very severe test), it can be argued that the majority of animals living in a state of nature can easily Resistant to very different climates.We must not, however, carry this argument too far, for our domestic animals may have descended from several wild progenitors; for example, the blood of tropical wolves and boreal wolves may have been mixed in our domestic breeds.The rat and mouse cannot be regarded as domestic animals, but they have been carried by man to many parts of the world, and are now more widely distributed than any other rodent; in the north they live in Faroe ) in the cold climate of the South in the Falkland (Falkland), and also live on many islands in the tropics.Adaptability to any particular climate, therefore, may be regarded as a quality which can readily be transplanted into the wide flexibility of inner constitution which is common to most animals.According to this view, the tolerance of man himself and his domestic animals to extremes of different climates, and the extinct elephant and rhinoceros have previously tolerated ice age climates, while their extant species have tropical and subtropical habits , these should not be regarded as abnormal events, but as some examples of the operation of very ordinary physical flexibility under special environmental conditions. How much of the domestication of species to any particular climate is due to habit alone, and how much to the natural selection of varieties having different internal constitutions, and how much to a combination of both, is a difficult question.By analogy, and by the constant advice in agricultural writings, and even in ancient Chinese encyclopedias, that great care must be exercised in transporting animals from here to there, I must believe that habit or custom has some influence.Since human beings have not necessarily succeeded in selecting so many breeds and sub-breeds, all with constitutions especially adapted to their region, I think that this result must be due to habit; on the other hand, natural selection must tend to Preserve those individuals who were born with the constitution best suited to their habitation.In treatises on many species of cultivated plants, it is written that certain varieties are more resistant to certain climates than others; and the American publications on fruit trees clearly state that certain varieties are often recommended for the North, and some for the South; Since most of these varieties are of recent origin, their differences in constitution cannot be attributed to habit.The Jerusalem artichoke, which has never been propagated by seed in England, and thus produced no new varieties, has been raised as an example of the futility of domestication, for it is as tender as ever!Again, the example of the kidey-bean is often cited for the same purpose, and with greater force; but if one sows the kidey-bean so early that a great part of it is destroyed by the frost, the few surviving Seeds were collected from these seedlings, taking care to prevent their accidental interbreeding, and he then took the same care to collect seeds from these seedlings, sowed them, and so continued for twenty generations before the experiment could be said to have been done. The constitution of the shoots never produced a difference, for it has been reported that some seedlings are indeed much more hardy than others, and I myself have seen striking cases of this. In conclusion, we may conclude that habit, or use and disuse, has in some cases played an important role in the variation of constitution and structure, but that this effect has often been largely related to the natural selection of internal variations. In combination, natural selection of intrinsic variation sometimes dominates this effect. correlated variation By correlative variation is meant that the whole system is so closely united in its growth and development that when slight variations in any one part are accumulated by natural selection, the other parts will also be modified.This is an extremely important question, one which is still very poorly understood, and here no doubt facts of quite different kinds are liable to be confused.As we shall shortly see, mere heredity often gives the false appearance of correlation.One of the most obvious real instances, is that variations in the structure of young animals, or larvae, naturally tend to affect the structure of mature animals.Homologous parts of the body, of equal structure in the early embryo, and necessarily under similar external conditions, have a marked tendency to vary in the same way: we see the right and left sides of the body, which vary in the same way. ; the front and rear feet, and even the jaws and limbs, were modified simultaneously, because some anatomists believe that the lower jaw and the limbs are homologous.I do not doubt that these tendencies are more or less entirely subject to natural selection; for instance, that a herd of stags with horns only on one side once existed, if this had been of any great use to the breed. , presumably natural selection will make it permanent. Certain authors have remarked that homologous parts have a tendency to join together; and this is often seen in deformed plants: the union of petals into tubes is one of the most common unions of homologous organs in a normal structure.The hard parts seem to influence the shape of the joined soft parts; and some authors believe that divergence in the shape of the pelvic discs in birds can cause marked divergence in the shape of their kidneys.Others believe that, in humans, the shape of the mother's pelvis affects the shape of the fetus' head due to pressure.In the case of snakes, according to Schlegel, the shape of the body and the state of ingestion determine the position and shape of the several most important internal organs. The nature of this combination is often not quite clear.Mr. Saint-Tirrell Jr. has emphatically observed that some deformities frequently coexist and others rarely coexist, and we cannot really give any reason for this.In cats, the relation of pure white coat and blue eyes to deafness, of the tortoise-colored cat to the female; The relations of the colors of the plumage in the future; and, again, the relations of the fur and tusks of the naked Turkish dog; and though homology no doubt plays a part here, could there be more curious relations than these?As to the last example of the above-mentioned correlation, it cannot, I think, be accidental that the two orders of mammals with the most abnormal epidermis, namely the cetaceans and the anodontids (the drosophila, the pangolin, etc.), also all have the most abnormal teeth. ; but there are so many exceptions to this rule, as Mr. Mivart has said, that it is of little value. No instance, so far as I know, of the importance of elucidating and employing the laws of correlation and variation which are independent, and therefore independent of natural selection, than the difference between the inner and outer flowers of certain Compositae and Umbelliferae up.It is well known, for example, that in daisies there is a difference in the central florets and erupting flowers, and this difference is often accompanied by a partial or total atrophy of the reproductive organs.But the seeds of some of these plants also vary in shape and carving.These differences are sometimes attributed to the pressure of the involucre on the florets, or to their mutual pressure, and the shape of the seeds of the erupted flowers of some Compositae is consistent with this idea; but in Umbelliferae, as Dr. Hooker has told me that the species which often differ most in their inner and outer flowers are by no means those which have the densest inflorescences.We may conceive that the development of the petals of the ejaculate depends on the absorption of nourishment from the reproductive organs, and this causes the underdevelopment of the reproductive organs; Different, but the seeds of inner and outer flowers are different.The difference between these seeds may be related to the different flow of nutrients to the central and peripheral flowers: at least we know that, of irregular flowers, those closest to the rachis are most liable to become peloria, i.e. abnormal symmetric flowers.To this fact I may add an instance, also as a striking example of the correlative action, that in many Pelargoniums the upper two petals of the central flower of the inflorescence frequently lose their dark spots; In this case, the nectary to which it is attached is very degenerated; thus the central flower becomes a straight flower, that is, a neat flower.If only one of the upper two petals loses color, the nectary is not very degenerated, but only greatly shortened. Concerning the development of the corolla, Sprengel's opinion is that the usefulness of the erupted flowers is to attract insects, and that the insect medium is highly advantageous or necessary for the fertilization of these plants, which is reasonable; Choices may already be in play.As regards the seeds, however, differences in their shape do not always correlate with any differences in the corolla, and so seem to be of no interest: in the Umbelliferae such differences are of such apparent importance—the species of the peripheral flowers The ovules of the seeds are sometimes orthotropic, but the seed ovules of the central flower are anatropous-so that Old De Candor mainly uses these characters to classify such plants.The variations in structure which taxonomists regard as highly valuable, therefore, may all be due to laws of variation and correlation, which, so far as we can judge, are of no use to species. Structures which are common to whole groups of species, and which are indeed due to mere heredity, are often falsely attributed to correlative variations; for an ancient progenitor may have acquired some variation of structure by natural selection, and After thousands of generations, another variation has been acquired, independent of the above-mentioned variation; and these two variations, if inherited to all descendants diverging in habits, naturally lead us to think that they must be related in some way.There are also other related cases, evidently due to the single action of natural selection.De Candolle, for example, has said that winged seeds are never found in undehiscent fruits; and of this law I may explain this: Unless the capsule is dehiscated, the seed cannot pass through natural selection. and gradually become winged; for seeds somewhat adapted to being blown by the wind, only when the capsule breaks open, have the advantage over those less adapted to widespread dispersal. Growth Compensation and Savings The old Saint-Tirrell and Goethe proposed at about the same time the compensation law of growth, the law of equilibrium; or as Goethe said: "In order to consume on one side, nature is forced to save on the other side." The same applies to our domestic animals to a certain extent: if too much nourishment goes to one part or organ, at least not so much goes to another; Cows are difficult.The same cabbage variety, which does not produce lush, nourishing leaves, bears profuse oil-seeds at the same time.While the seeds of our fruits shrivel, their fruits themselves are greatly improved in size and quality.Domestic chickens, with a large tuft of hair on the head, are generally accompanied by a reduced comb, and those with many beards are accompanied by a reduced wattle.It is difficult to apply this law generally to species in their natural state; but many good observers, especially botanists, are convinced of its truth.I shall not, however, prepare to give any examples here, since I find it difficult to discern in any way the effect of a part which, on the one hand, has been greatly developed by natural selection, and which part of the connection, by the same effect or by non-use On the other hand, a part of the nourishment is taken away, which is actually due to the overgrowth of another connecting part. I also conjecture that some of the cases of compensation which have been suggested, and certain other facts, may be reduced to a more general principle, namely, that natural selection has continually tried to economize every part of the system.If, under changed conditions of life, a structure which was useful before becomes less useful, it is advantageous to reduce the size of the structure, because it prevents the individual from wasting nourishment on building a useless structure. .I was impressed by the examination of cirripedes, and from this I understood the fact, and there are many similar cases, that when one cirriped is protected by parasitizing another, its The outer shell, the carapace, then disappears almost entirely.This is the case with the male Ibla, and indeed still more so with the parasitic Proteolepas: the carapace of all other cirripedes is extremely well developed, and is formed by a very well developed anterior end of the head. Composed of three highly important segments, and possessed of enormous nerves and muscles; but the parasitic and protected parasitic masonry, the whole anterior part of the head is so greatly degenerated that only a very small The remnants of the tentacles are attached to the base of the tentacles which have the function of capturing. If large and complex structures become superfluous, it is in the decisive interest of all generations of the species to omit them; for each animal is in a struggle for existence, and by reducing the waste of nourishment, it gains Better chance of sustaining yourself. I believe, therefore, that when any part of the body has been rendered superfluous by a change of habit, natural selection will eventually reduce it, without needing any corresponding degree of development or enlargement of some other part.Natural selection, on the other hand, may perfectly succeed in developing and enlarging an organ, without requiring the necessary compensation of a diminishment of some connecting part. Repeated, remnant, and low-level structures are prone to variation As the younger Saint-Tirrell remarked, whenever any part or organ of the same individual is repeated many times, whether in species or varieties (as the backbone of a snake, the stamens of a multistamenal flower), its number is liable to vary; It seems to be a rule, on the contrary, that the same parts or organs, if they are less numerous, remain stable.The author and some botanists further pointed out that any repetitive organs are extremely prone to variation in structure.In Professor Owen's terminology, this is called "vegetative repetition" (vegetative repetition), which is a sign of an inferior system, so what I said earlier, that in a natural system, lower organisms are more likely to mutate than higher organisms, is consistent with naturalism. The consensus of scholars is unanimous.我这里所谓低等的意思是指体制的若干部分很少专业化，以担任一些特殊机能；当同一器官势必担任多种工作时，我们大概能理解，它们为什么容易变异，因为自然选择对于这种器官形状上的偏差，无论保存或排斥，都比较宽松，不像对于专营一种功能的部分那样严格。这正如一把切割各种东西的刀子，差不多具有任何形状都可以；反之，专为某一特殊目的的工具，必须具有某一特殊的形状。永远不要忘记，自然选择只能通过和为了各生物的利益，才能发生作用。 正如一般所承认的，残迹器官高度容易变异。我们以后还要讲到这一问题；我在这里只补充一点，即它们的变异性似乎是由于它们毫无用处所引起的结果，因而也是由于自然选择无力抑制它们构造上的偏差所引起的结果。 任何一个物种的异常发达的部分，比起近似物种里的同一部分，有易于高度变异的倾向 数年前，我很被沃特豪斯的关于上面标题的论点所打动。欧文教授也似乎得出了近似的结论。要使人相信上述主张的真实性，如果不把我所搜集的一系列的事实举出来，是没有希望的，然而我不可能在这里把它们介绍出来。我只能说，我所相信的是一个极其普遍的规律。我考虑到可能发生错误的几种原因，但我希望我已对它们加以斟酌了。必须理解，这一规律决不能应用于任何身体部分，即使这是异常发达的部分，除非在它和许多密切近似物种的同一部分相比较下，显示出它在一个物种或少数物种里是异常发达时，才能应用这一规律。例如蝙蝠的翅膀，在哺乳动物纲中是一个最异常的构造，但在这里并不能应用这一规律，因为所有的蝙蝠都有翅膀；假如某一物种和同属的其他物种相比较，而具有显著发达的翅膀，那末只有在这种情况下，才能应用这一规律。在次级性征以任何异常方式出现的情况下，便可以大大地应用这一规律。亨特（Hunter）所用的次级性征这一名词，是指属于雌雄一方的性状，但与生殖作用并无直接关系，这一规律可应用于雄性和雌性，但可应用于雌性的时候比较少，因为它们很少具有显著的次级性征。这一规律可以很明显地应用于次级性征，可能是由于这些性状不论是否以异常的方式而出现，总是具有巨大变异性的——我想这一事实很少值得怀疑。但是这一规律并不局限于次级性征，在雌雄同体的蔓足类里明白地表示了这种情形；我研究这一目时，特别注意了沃德豪斯的话，我十分相信，这一规律几乎常常是适用的。我将在未来的著作里，把一切较为显著的事例列成一个表；这里我只举出一个事例以说明这一规律的最大的应用性。无柄蔓足类（岩藤壶）的盖瓣，从各方面说，都是很重要的构造，甚至在不同的属里它们的差异也极小；但有一属，即在四甲藤壶属（Pyrgoma ）的若干物种里，这些瓣却呈现很大的分歧；这种同源的瓣的形状有时在异种之间竟完全不同；而且在同种个体里其变异量也非常之大，所以我们如果说这些重要器官在同种各变种间所表现的特性差异，大于异属间所表现的，并不算夸张。 关于鸟类，栖息在同一地方的同种个体，变异极小，我曾特别注意到它们；这一规律的确似乎是适用于这一纲的。我还不能发现这一规律可以应用于植物，假如不是植物的巨大变异性使得它们变异性的相对程度特别困难于比较，我对这一规律真实性的信赖就要发生严重的动摇。 当我们看到一个物种的任何部分或器官以显著的程度或显著的方式而发达时，正当的假定是，它对于那一物种是高度重要的；然而正是在这种情况下，它是显著易于变异的。为什么会如此呢？根据各个物种是被独立创造出来的观点，即它的所有部分都像我们今天所看到的那样，我就不能找出什么解释。但根据各个物种群是从其他某些物种传下来并且通过自然选择而发生了变异的观点，我想我们就能得到一些说明，首先让我说明几点。如果我们对于家养动物的任何部分或整体不予注意，而不施任何选择，那末这一部分（例如，多径鸡［Dorking fowl ］的肉冠），或整个品种，就不会再有一致的性状：可以说这一品种是退化了。在残迹器官方面，在对特殊目的很少专业化的器官方面，以及，大概在多形的类群方面，我们可以看到几乎同样的情形；因为在这些情形下，自然选择未曾或者不能发生充分的作用，因此体制便处于彷徨的状态。但是这里特别和我们有关系的是，在我们的家养动物里，那些由于连续的选择作用而现今正在迅速进行变化的构造也是显著于变异的，看一看鸽子的同一品种的一些个体吧，并且看一看翻飞鸽的嘴、传书鸽的嘴和肉垂、扇尾鸽的姿态及尾羽等等具有何等重大的差异量；这些正是目前英国养鸽家们主要注意的各点。甚至在同一个亚品种里，如短面翻飞鸽这个亚品种，要育成近乎完全标准的鸽子是极其困难的，多数都与标准距离甚远。因此可以确实地说，有一种经常的斗争在下述两方面之间进行着，一方面是回到较不完全的状态去的倾向，以及发生新变异的一种内在倾向，另一方面是保持品种纯真的不断选择的力量。最后还是选择获胜，因此我们不必担心会遭到如此失败，以致从优良的短面鸽品系里育出像普通翻飞鸽那样粗劣的鸽。在选择作用正在迅速进行的情况下，正在进行变异的部分具有巨大的变异性，是常常可以预料到的。 现在让我们转到自然界来。任何一个物种的一个部分如果比同属的其他物种异常发达，我们就可以断言，这一部分自那几个物种从该属的共同祖先分出的时期以来，已经进行了非常重大的变异。这一时期很少会十分久远，因为一个物种很少能延长到一个地质时代以上。所谓异常的变异量是指非常巨大的和长期连续的变异性而言，这种变异性是由自然选择为了物种的利益而被继续累积起来的。但是异常发达的部分或器官的变异性，既已如此巨大而且是在不很久远的时期内长久连续进行，所以按照一般规律，我们大概还可料想到，这些器官比在更长久时期内几乎保持稳定的体制的其他部分，具有更大的变异性。我相信事实就是这样。一方面是自然选择，另一方面是返祖和变异的倾向，二者之间的斗争经过一个时期会停止下来的；并且最异常发达的器官会成为稳定的，我觉得没有理由可以怀疑这一点。因此，一种器官，不管它怎样异常，既以近于同一状态传递给许多变异了的后代，如蝙蝠的翅膀，按照我们的理论来讲，它一定在很长久的时期内保持着差不多同样的状态；这样，它就不会比任何其他构造更易于变异。只有在变异是比较新近的、而且异常巨大的情况下，我们才能发现所谓发育的变异性（generative variability）依然高度存在。因为在这种情形下，由于对那些按照所要求的方式和程度发生变异的个体进行继续选择，以及由于对返归以前较少变异的状态进行继续排除，变异性很少被固定下来。 物种的性状比属的性状更易变异 前节所讨论的原理也可应用于现在这个问题。众所周知，物种的性状比属的性状更易变化。举一个简单的例子来说明：如果在一个大属的植物里，有些物种开蓝花，有些物种开红花，这颜色只是物种的一种性状；开蓝花的物种会变为开红花的物种，对此谁都不会感到惊奇，相反亦如是；但是，如果一切物种都是开蓝花的，这颜色就成为属的性状，而它的变异便是更异常的事情了。我选取这个例子的理由是因为多数博物学者所提出的解释不能在这里应用，他们认为物种的性状之所以比属的性状更易变异，是因为物种的分类所根据的那些部分，其生理重要性小于属的分类所根据的那些部分。我相信这种解释只是部分而间接地正确的；在《分类》一章里我还要讲到这一点。引用证据来支持物种的普通性状比属的性状更易变异的说法，几乎是多余的了；但关于重要的性状，我在博物学著作里一再注意到以下的事情，就是，当一位作者惊奇地谈到某一重要器官或部分在物种的大群中一般是极其固定的，但在亲缘密切的物种中差异却很大时，它在同种的个体中常常易于变异。这一事实指出，一般具有属的价值的性状，一经降低其价值而变为只有物种的价值时，虽然它的生理重要性还保持一样，但它却常常变为易于变异的了。同样的情形大概也可以应用于畸形：至少小圣·提雷尔无疑地相信，一种器官在同群的不同物种中，愈是正常地表现差异，在个体中就愈多受变态所支配。 按照各个物种是被独立创造的流俗观点来看，在独立创造的同属各物种之间，为什么构造上相异的部分比密切近似的部分更容易变异，我看对此无法做出任何说明。但是，按照物种只是特征显著的和固定的变种的观点来看，我们就可以预期常常看到，在比较近期内变异了的因而彼此有所差异的那些构造部分，还要继续变异。或者，可以用另一种方式来说明，凡是一个属的一切物种的构造彼此相似的、而与近缘属的构造相异的各点，就叫作属的性状。这些性状可以归因于共同祖先的遗传，因为自然选择很少能使若干不同的物种按照完全一样的方式进行变异，而这些不同的物种已经适于多少广泛不同的习性。所谓属的性状是在物种最初从共同祖先分出来以前就已经遗传下来了，此后它们没有发生什么变异，或者只出现了些许的差异，所以时至今日它们大概就不会变异了。另一方面，同属的某一物种与另一物种的不同各点就叫做物种的性状。因为这些物种的性状是在物种从一个共同祖先分出来以后，发生了变异并且出现了差异，所以它们大概还应在某种程度上常常发生变异，至少比那些长久保持稳定的那些体制的部分，更易变异。 第二性征易生变异。——我想无须详细讨论，博物学者们都会承认第二性征是高度变异的。他们还会承认，同群的物种彼此之间在第二性征上的差异，比在体制的其他部分上的差异更加广泛。例如，比较一下在第二性征方面有强烈表现的雄性鹑鸡类之间的差异量与雌性鹑鸡类之间的差异量，便可明了。这些性状的原始变异性的原因还不明显；但我们可以知道，为什么它们没有像其他性状那样地表现了固定性和一致性，因为它们是被性选择所积累起来的，而性选择的作用不及自然选择作用那样严格，它不致引起死亡，只是使较为不利的雄性少留一些后代而已。不管第二性征的变异性的原因是什么，因为它们是高度变异的，所以性选择就有了广阔的作用范围，因而也就能够成功地使同群的物种在第二性征方面比在其他性状方面表现较大的差异量。 同种的两性间第二性征的差异，一般都表现在同属各物种彼此差异所在的完全相同的那一部分，这是一个值得注意的事实。关于这一事实，我愿举出列在我的表中最前面的两个事例来说明；因为在这些事例中，差异具有非常的性质，所以它们的关系决不是偶然的。甲虫足部附节的同样的数目，是极大部分甲虫类所共有的一种性状；但是在木吸虫科（Engldx）6 里，如韦斯特伍得（Westwood）所说的，附节的数目变异很大；并且在同种的两性间，这个数目也有差异。还有，在掘地性膜翅类里，翅脉是大部分所共有的性状，所以是一种高度重要的性状；但是在某些属里，翅脉因物种不同而有差异，并且在同种的两性间也是如此。卢伯克爵士（Sir. J. Lubbock）近来指出，若干小形甲壳类动物极好地说明了这一法则。“例如，在角镖水蚤（Pontella）属里，第二性征主要是由它的触角和第五对脚表现出来的：同时物种的差异也主要表现在这些器官方面。”这种关系对于我的观点有明显的意义：我认为同属的一切物种之必然由一个共同祖先传下来与任何一个物种的两性由一个共同祖先传下来是一样的。因此，不管共同祖先或它的早期后代的哪一部分成为变异的，则这一部分的变异极其可能要被自然选择或性选择所利用，以使若干物种在自然组成中适于各自位置，而且使同一物种的两性彼此适合，或者使雄性在占有雌性方面适于和其他雄性进行斗争。 最后，我可以总结，物种的性状，即区别物种之间的性状，比属的性状，即一切物种所具有的性状，具有更大的变异性；——一个物种的任何部分与同属其他物种的同一部分相比较，表现异常发达时，这一部分常常具有高度的变异性；一个部分无论怎样异常发达，如果这是全部物种所共有的，则其变异性的程度是轻微的；——第二性征的变异性是大的，并且在亲缘密切的物种中其差异是大的；第二性征的差异和通常的物种差异，一般都表现在体制的同一部分，——这一切原理都是紧密关联在一起的。这主要是由于，同一群的物种都是一个共同祖先的后代，这个共同祖先遗传给它们许多共同的东西，——由于晚近发生大量变异的部分，比遗传已久而未曾变异的部分，可能继续变异下去，——由于随着时间的推移，自然选择能够或多或少地完全克服返祖倾向和进一步变异的倾向，——由于性选择不及自然选择那样严格，——更由于同一部分的变异，曾经被自然选择和性选择所积累，因此就使它适应了第二性征的目的以及一般的目的。 不同的物种呈现相似的变异，所以，一个物种的一个变种常常表现一个近似物种所固有的一种性状，或者复现一个早期祖代的某些性状。——观察一下我们的家养族，就会极其容易地理解这些主张。地区相隔辽远的一些极不相同的鸽的品种，呈现头生逆毛和脚生羽毛的亚变种——这是原来的岩鸽所不曾具有的一些性状；所以，这些就是两个或两个以上不同的族的相似变异。突胸鸽常有的十四枝或者甚至十六枝尾羽，可以被认为是一种变异，它代表了另一族即扇尾鸽的正常构造。我想不会有人怀疑，所有这些相似变异，系由于这几个鸽族都是在相似的未知影响下，从一个共同亲代遗传了相同的体质和变异倾向；在植物界里，我们也有一个相似变异的例子，见于“瑞典芜蔷”（Swedish turnip）和芜青甘蓝（Ruta baga）的肥大的茎（俗称根部）；若干植物学者把此等植物看作是从一个共同祖先培养出来的两个变种：如果不是这样，这个例子便成为在两个不同物种呈现相似变异的例子了；除此二者之外，还可加入第三者，即普通芜菁。按照每一物种是被独立创造的这一流俗观点，我们势必不能把这三种植物的肥大茎的相似性，都归因于共同来源的真实原因，也不能归因于按照同样方式进行变异的倾向，而势必归因于三种分离的而又密切关联的创造作用。诺丹曾在葫芦这一大科里、其他作家们曾在我们的谷类作物里观察到相似变异的同样事例。在自然状况下昆虫也发生同样的情形，最近曾被沃尔什先生很有才能地讨论过，他已经把它们归纳在他的“均等变异性”法则里去了。 但是关于鸽子，还有另外一种情形，即在一切品种里会偶尔出石板蓝色的鸽子，它们的翅膀上有两条黑带，腰部白色，尾端有一条黑带，外羽近基部的外缘呈白色。因为这一切颜色都属于亲种岩鸽的特性，我假定这是一种返祖的情形，而不是在若干品种中所出现的新的相似变异，这是不会有人怀疑的。我想，我们可以有信心地作出这样的结论，因为，如我们已经看到的，此等颜色的标志非常容易在两个不同的、颜色各异的品种的杂交后代中出现；在这种情形下，这种石板蓝色以及几种色斑的重现并不是由于外界生活条件的作用，而仅是依据遗传法则的杂交作用的影响。 有些性状已经失去许多世代或者甚至数百世代还能重现，无疑是一件很令人惊奇的事实。但是，当一个品种和其他品种杂交，虽仅仅一次，它的后代在许多世代中还会有一种倾向，偶尔发生复现外来品种的性状，——有些人说大约是十二代或多至二十代。从一个祖先得来的血（用普通的说法），在十二世代后，其比例只为2048比1；然而，如我们所知道的，一般相信，返祖的倾向是被这种外来血液的残余部分所保持的。在一个未曾杂交过的、但是它的双亲已经失去了祖代的某种性状的一个品种里，重现这种失去了的性状的倾向，无论强或弱，如前面已经说过的，差不多可以传递给无数世代，即使我们可以看到相反的一面，也是如此。一个品种的已经亡失的一种性状，经过许多世代以后还重复出现，最近情理的假设是，并非一个个体突然又获得数百代以前的一个祖先所失去了的性状，而是这种性状在每个世代里都潜伏存在着，最后在未知的有利条件下发展起来了。例如，在很少产生一只蓝色鸽的排李鸽里，大概每一世代都有产生蓝色羽毛的潜在倾向。通过无数世代传递下来的这种倾向，比十分无用的器官即残迹器官同样传递下来的倾向在理论的不可能性上不会更大。产生残迹器官的倾向有时的确是这样遗传下去的。 同属的一切物种既然假定是从一个共同祖先传下来的，那就可以料想到，它们偶尔会按照相似的方式进行变异；所以两个物种或两个以上的物种的一些变种会彼此相似，或者某一物种的一个变种在某些性状上会与另一不同的物种相似，——这另一个物种，按照我们的观点，只是一个特征显著而固定的变种而已。但是单纯由于相似变异而发生的性状，其性质大概是不重要，因为一切机能上的重要性状的保存，须依照这个物种的不同习性，通过自然选择而决定的。我们可以进一步料想到，同属的物种偶尔会重现长久失去的性状。然而，因而我们不知道任何自然类群的共同祖先，所以也就不能把重现的性状与相似的性状区别开来。例如，如果我们不知道亲种岩鸽不具毛脚或倒冠毛，我们就不能说在家养品种中出现这样的性状究系返祖现象抑仅仅是相似变异；但我们从许多色斑可以推论出，蓝色是一种返祖的例子，因为色斑和蓝色是相关联的，而这许多色斑大概不会从一次简单的变异中一齐出现。特别是当颜色不同的品种进行杂交时，蓝色和若干色斑如此常常出现；由此我们尤其可以推论出上述一点。因此，在自然状况下，我们一般无法决定什么情形是先前存在的性状的重现，什么情形是新的而又相似的变异，然而，根据我们的理论，我们有时会发现一个物种的变异着的后代具有同群的其他个体已经具有的相似性状。这是无可怀疑的一点。 识别变异的物种的困难，主要在于变种好像模仿同属中的其他物种。还有，介于两个类型之间的类型不胜枚举，而这两端的类型本身是否可以列为物种也还有疑问；除非我们把一切这些密切近似类型都认为是分别创造的物种，不然的话，上述一点就阐明了，它们在变异中已经获得了其他类型的某些性状。但是相似变异的最好证据还在于性状一般不变的部分或器官，不过这些器官或部分偶尔也发生变异，以致在某种程度上与一个近似物种的同一部分或器官相似。我搜集了一系列的此种事例；但在这里，和以前一样，我很难把它们列举出来。我只能重复地说，这种情形的确存在，而且在我看来是很值得注意的。 然而，我要举出一个奇异而复杂的例子，这是一个任何重要性状完全不受影响的例子，但是它发生在同属的若干物种里——一部分是在家养状况下的，一部分是在自然状况下的。这个例子几乎可以肯定是返祖现象。驴的腿上有时有很明显的横条纹，和斑马腿上的相似：有人确定幼驴腿上的条纹最为明显，据我调查所得，我相信这是确实的。肩上的条纹有时是双重的，在长度和轮廓方面很易于变异。有一头白驴，这不是皮肤变白症，被描述为没有脊上和肩上的条纹，在深色的驴子里，此等条纹也很不明显或实际上完全失去了。据说，由帕拉斯命名的野驴（koulan of Pallas）的肩上有双重的条纹，布莱斯先生曾经看见过一头野驴的标本具有明显的肩条纹，虽然它本应是没有的；普尔上校（Col. Poole）告诉我说，这个物种的幼驹，一般在腿上都有条纹，而在肩上的条纹却很模糊。斑驴（quagga）虽然在体部有斑马状的明显条纹，但在腿上却没有；然而格雷博士（Dr. Grav）所绘制的一个标本，却在后脚踝关节处有极清楚的斑马状条纹。 关于马，我在英国搜集了许多极其不同品种的和各种颜色的马在脊上生有条纹的例子：暗褐色和鼠褐色的马在腿上生有横条纹的并不罕见，在栗色马中也有过一个这样的例子；暗褐色的马有时在肩上生有不明显的条纹，而且我在一匹赤褐色马的肩上也曾看到条纹的痕迹。我的儿子为我仔细检查了和描绘了双肩生有条纹的和腿部生有条纹的一匹暗褐色比利时驾车马；我亲自看见过一匹暗褐色的德文郡矮种马在肩上生有三条平行条纹，还有人向我仔细描述过一匹小形的韦尔什矮种马（Welsh pony）在肩上也生有三条平行的条纹。 在印度西北部，凯替华品种（Kattywar breed）的马，通常都生有条纹。我听普尔上校说，他曾为印度政府查验过这个品种，没有条纹的马被认为是非纯粹的品种。它们在脊上都生有条纹；腿上也通常生有条纹，肩上的条纹也很普通，有时候是双重的，有时候是三重的；还有，脸的侧面有时候也生有条纹。幼驹的条纹常常最明显；老马的条纹有时完全消失了。普尔上校见过初生的灰色和赤褐色的凯替华马都有条纹。从Ｗ·Ｗ·爱德华先生给我的材料中，我有理由推测，幼小的英国赛跑马在脊上的条纹比长成的马普遍得多。我自己近来饲养了一匹小马，它是由赤褐色雌马（是东土耳其雄马和佛兰德雌马的后代）和赤褐色英国赛跑马交配后产生的；这幼驹产下来一星期的时候，在它的臀部和前额生有许多极狭的、暗色的、斑马状的条纹，腿部也生有极轻微的条纹，但所有这些条纹不久就完全消失了。这里无须再详细地讲了。我可以说，我搜集了许多事例，表明不同地方的极其不同品种的马在腿上和肩上都生有条纹，从英国到中国东部，并且从北方的挪威到南方的马来群岛，都是如此。在世界各地，这种条纹最常见于暗褐色和鼠褐色的马；暗褐色这一名词，包括广大范围的颜色，从介于褐色和黑色中间的颜色起，一直到接近淡黄色止。 我知道曾就这个问题写过论文的史密斯上校（Col. H. Smith） 相信，马的若干品种是从若干原种传下来的，——其中一个原种是暗褐色的而且生有条纹；并且他相信上述的外貌都因为在古代与暗褐色的原种杂交所致。但我们可以稳妥地驳斥这种意见；因为那壮大的比利时驾车马，韦尔什杂种马，挪威矮脚马，细长的凯替华马等等，都栖息在世界上相隔甚远的地方，要说它们都必须曾经与一个假定的原种杂交过，则是十分不可能的。 现在让我们来讲一讲马属中几个物种的杂交效果。罗林（ Rollin ）断言驴和马杂交所产生的普通骡子，在腿上特别容易生有条纹；按照戈斯先生（Mr. Gosse ）的意见，美国某些地方的骡子，十分之九在腿上生有条纹。我有一次见过一匹骡子，腿上条纹如此之多，以致任何人都会想像它是斑马的杂种；Ｗ·Ｃ·马丁先生（ Mr. Martin ）在一篇有关马的优秀论文里，绘有一幅骡子图，与此相像。我曾见过四张驴和斑马的杂种彩色图，在它们的腿上所生的极明显条纹，远比身体其他部分为甚；并且其中有一匹在肩上生有双重条纹。莫顿爵士（Lord Morton ）有一个著名的杂种，是从栗色雌马和雄斑驴育成的，这杂种，以及后来这栗色雌马与黑色亚拉伯马所产生的纯种后代，在腿上都生有比纯种斑驴还要更加明显的横条纹，最后，还有另一个极其值得注意的事例，格雷博士曾绘制过驴子和野驴的一个杂种（并且他告诉我说，他还知道有第二个事例）；虽然驴只偶尔在腿上生有条纹，而野驴在腿上并没有条纹，甚至在肩上也没有条纹，但是这杂种在四条腿上仍然生有条纹，并且像暗褐色的德文郡马与韦尔什马的杂种一样，在肩上还生有三条短条纹，甚至在脸的两侧也生有一些斑马状的条纹。关于最后这一事实，我非常相信决不会有一条带色的条纹像普通所说的那样是偶然发生的，因此，驴和野驴的杂种在脸上生有条纹的事情便引导我去问普尔上校：是否条纹显著的凯替华品种的马在脸上也曾有过条纹，如上所述，他的回答是肯定的。 对于这些事实，我们现在怎样说明呢？我们看到马属的几个不同品种，通过简单的变异，就像斑马似的在腿上生有条纹，或者像驴似的在肩上生有条纹。至于马，我们看到，当暗褐色——这种颜色接近于该属其他物种的一般颜色——出现时，这种倾向便表现得强烈。条纹的出现，并不伴生形态上的任何变化或任何其他新性状。我们看到，这种条纹出现的倾向，以极不相同的物种之间所产生的杂种最为强烈。现在看一看几个鸽品种的情形：它们是从具有某些条纹和其他标志的一种浅蓝色的鸽子（包含两个或三个亚种或地方族）传下来的；如果任何品种由于简单的变异而具有浅蓝色时，此等条纹和其他标志必然会重新出现：但其形态或性状却不会有任何变化。当最古老的和最纯粹的各种不同颜色的品种进行杂交时，我们看到这些杂种就有重现蓝色和条纹以及其他标志的强烈倾向。我曾说过，解释这种古老性状重现的合理假设是，在每一连续世代的幼鸽里都有重现久已失去的性状的倾向，这种倾向，由于未知的原因，有时占优势。我们刚才谈到，在马属的若干物种里，幼马的条纹比老马更明显或表现得更普遍，如果把鸽的品种，其中有些是在若干世纪中纯正地繁殖下来的，称为物种，那末这种情形与马属的若干物种的情形是何等完全一致！至于我自己，我敢于自信地回顾到成千成万代以前，有一种动物具有斑马状的条纹，其构造大概很不相同，这就是家养马（不论它们是从一个或数个野生原种传下来的）、驴、亚洲野驴、斑驴以及斑马的共同祖先。 我推测那些相信马属的各个物种是独立创造出来的人会主张，每一个物种被创造出来就赋有一种倾向，在自然状况下和在家养状况下都按照这种特别方式进行变异，使得它常常像该属其他物种那样地变得具有条纹；同时每一个物种被创造出来就赋有一种强烈的倾向，当和栖息在世界上相隔甚远的地方的物种进行杂交时，所产生出的杂种在条纹方面不像它们自己的双亲，而像该属的其他物种。依我看来，接受这种观点，就是排斥了真实的原因，而代以不真实的或至少是不可知的原因。这种观点使得上帝的工作成为仅仅是模仿和欺骗的了；倘接受这一观点，我几乎就要与老朽而无知的天地创成论者们一起来相信贝类化石从来就不曾生活过，而只是在石头里被创造出来以模仿生活在海边的贝类的。 feed 关于变异法则，我们还是深深地无知的。我们能阐明这部分或那部分为什么发生变异的任何原因，在一百个例子中还不到一个。但是当我们使用比较的方法时，就可以看出同种的变种之间的较小差异，和同属的物种之间的较大差异，都受同样法则的支配。变化了的外界条件一般只会诱发彷徨变异，但有时也会引起直接的和一定的效果；这些效果随着时间的推移可以变成强烈显著的；关于这一点，我们还没有充分的证据。习性在产生体质的特性上；使用在器官的强化上，以及不使用在器官的削弱和缩小上，在许多场合里，都表现出强有力的效果。同源部分有按照同一方式进行变异的倾向，并且有合生的倾向。坚硬部分和外在部分的改变有时能影响较柔软的和内在的部分。当一部分特别发达时，大概它就有向邻近部分吸取养料的倾向；并且构造的每一部分如果被节约了而无损害，它就会被节约掉。早期构造的变化可以影响后来发育起来的部分；许多相关变异的例子，虽然我们还不能理解它们的性质，无疑是会发生的。重复