Chapter 22 Chapter 14 The Mutual Affinity of Organisms: Morphology, Embryology, Rudimentary Organs 2

Genesis and Embryology It is one of the most important disciplines in the whole of natural history.Everyone is familiar with the fact that insect metamorphosis is generally accomplished abruptly by a few stages; but in reality there are innumerable, gradual, though insidious, transformations.As Sir Lubbock has shown, a certain mayfly (Chloeon) molts more than twenty times in the course of its development, each moulting undergoing a definite amount of variation; It is done in a primitive, gradual manner.Many insects, and especially certain crustaceans, have shown us how wonderful the structural changes are accomplished in the process.Such changes, however, culminate in the so-called alternation of generations in some of the lower animals.For example, there is the curious fact that an exquisitely branched coral-shaped animal with polypi, and anchored to the rocks of the bottom of the sea, first buds and then divides laterally to give rise to floating Giant swarms of jellyfish; these jellyfish then lay eggs, from which hatch planktonic microscopic animals, which attach to rocks and develop into branched coral-like animals; and so on the cycle goes on endlessly.The belief that the process of alternation of generations and the usual process of metamorphosis is essentially identical has been greatly strengthened by Wagner's discovery that the larvae or maggots of one species of mosquito, Cecidomyia, asexually produce other larvae. , these other larvae at last develop into mature males and females, which reproduce their species by eggs in the usual manner.

It is worth noting that when Wagner's brilliant discovery was first announced, I was asked how to explain the ability of the larvae of this mosquito to acquire asexual reproduction?As long as this case is the only one, no solution can be proposed.But Grimm has shown that another species of mosquito, the Chironomus, reproduces in nearly the same manner, and he believes that this method is often found in this order.It is pupae, not larvae, that chironomids have this ability; Grimm further clarifies that this example somehow "connects gall midges to parthenogenesis in the family Coccidae"; — parthenogenesis The term reproductive means that mature females of the Scalidae can produce fertile eggs without mating with males.It is now known that certain animals of several classes possess the usual power of reproduction at an unusually early age; we have only to push parthenogenesis to earlier and earlier ages by gradual steps,--expressed by the chironomids. It is the middle stage, the pupal stage—that may explain the strange situation of the gall midge.

It has already been said that the different parts of the same individual, quite alike in the early embryonic stage, become quite different in the adult state, and serve very different purposes.In the same way it has been shown that the embryos of the most dissimilar species of the same class, which are generally closely alike, become widely dissimilar when fully developed.There is no better example of this last-mentioned fact than von Baer's statement: "The embryo of mammals, birds, lizards, snakes, and presumably turtles, too, in their earliest in the state of development, the whole and the manner of their parts, are very similar to each other; they are so similar that in fact we can only distinguish the embryos by their size. I have two small embryos soaked in alcohol , I forgot to paste their names, and now I can't say at all what class they belong to. They could be lizards or small birds, or very young mammals, the way the heads and bodies of these animals are formed is So completely similar. Yet these embryos have no limbs. But even in the earliest stages of development we know nothing if limbs exist, for the feet of lizards and mammals, the wings and feet of birds, and the hands and feet of man From the same basic form, as do the feet." The larvae of most crustaceans, in corresponding stages of development, closely resemble each other, however different the adults may become; and so do many other animals.The law of embryonic resemblance sometimes retains traces till a rather late age: birds of the same and allied genera, for example, often resemble each other in the plumage of their young; so.In the cat family, most species have stripes or spots as adults; lions and puma cubs also have clearly distinguishable stripes or spots.We may happen to see similar things in plants, too, though infrequently; for example, the first leaves of the gorse (ulex), and those of the Phyllodineous acacias, are like the usual leaves of the leguminous plants. , are pinnate or split.

The points at which the embryos of widely different animals of the same class resemble each other in structure are often not directly related to the conditions of their existence.In the vertebrate embryo, for example, the arteries near the gill-slits have a peculiar arc-like structure, which we cannot conceive to be any different from that of young mammals nourished in the womb, of bird eggs hatched in a nest, It is related to the similar living conditions of the frog eggs in the water.We have no reason to believe in such a relation, any more than we have reason to believe that similar bones in the human hand, in the wings of a bat, or in the fins of a dolphin, are related to similar conditions of life.No one imagines that the stripes of a young lion, or the spots of a young blackbird, are of any use to these animals.

It is different, however, if at any stage in the embryonic life an animal is mobile and must find food for itself.The period of activity may occur earlier or later in life; but at whatever period it occurs, the adaptation of the young to the conditions of life will be as complete and beautiful as that of the adult animal.The important manner in which this is done has been well shown of late by Sir Lubbock, when he spoke of the close resemblance of the larvae of certain insects of very different "orders" and of the same "order" in terms of their habits of life. by the dissimilarity of the larvae of other insects of the order.Because of such adaptations, the resemblance of the larvae of parasitic animals is sometimes largely obscured; especially when divisions of labor occur at different stages of development; A place to attach must be found.Cases may even be given where the larvae of allied species or groups of species differ from each other more than the adults.In most cases, however, the motile larvae obey more or less closely the general laws of embryonic resemblance.The cirripedes furnish a good example of this kind; even the illustrious Cuvier did not see that a barnacle was a crustacean: but one only needs to look at the larva, and one will infallibly know that it is a crustacean.The same is true of the two principal divisions of the cirripedes, the stalked and sessile cirripedes, though very different in appearance, yet their larvae differ very little in all stages.

The constitution of the embryo also generally improves as it develops; and though I know that it is almost impossible to determine clearly what is a higher constitution and what is a lower constitution, I will still use this expression.Probably no one will object that butterflies are superior to caterpillars, but in some cases, however, adults must be considered inferior in rank to larvae, as is the case with certain parasitic crustaceans.A word about cirripedes again: the larvae in the first stage have three pairs of locomotor organs, a simple ocelli, and a snout, with which they prey in great numbers, since they have greatly increased in size.In the second stage, which corresponds to the chrysalis stage of butterflies, they have six pairs of finely constructed swimming legs, a pair of huge compound eyes, and extremely complex antennae; but they all have an incompletely closed mouth and cannot eat : Their duty at this stage is to use their well-developed sensory organs to find, use their lively swimming ability to reach a suitable place to attach to, and carry out their final metamorphosis. After the metamorphosis is completed, they Permanently settled and immovable: their legs are thus transformed into grasping organs; they regain a well-constructed mouth; but the antennae are gone, and their eyes are transformed into small, single, simple eye-spots.In this final state, the cirripedes may be regarded as having a higher or lower constitution than their larval state.In some genera, however, the larvae may develop into hermaphrodites of general structure, and into what I have called complementary males; A short-lived sac that lacks a mouth, stomach, and other vital organs except for reproductive organs.

We are so accustomed to seeing differences in structure between the embryo and the adult that we are apt to regard this difference as a necessity of growth; When parts can be distinguished, there is no reason why all their parts should not at once appear in proper proportion.This is the case in some whole groups of animals, and in some members of other groups, that the embryo does not differ much from the adult at any stage: Owen, for example, has remarked in the case of the cuttlefish, "There is no metamorphosis; cephalopods traits are manifested long before embryonic development is complete".Terrestrial molluscs and freshwater crustaceans are born with inherent shapes, while the marine members of both classes undergo considerable and often great changes in their generation.Also, the spider has barely undergone any metamorphosis.The larvae of most insects pass through a wormlike stage, whether they are mobile and adapted to various habits, or inactive because they are in suitable nourishment or are fed by their parent; But in some rare cases, such as the aphid, if we turn our attention to Professor Huxley's admirable drawings of the development of this insect, we can hardly see any trace of the worm-like stage.

Sometimes it's just the earlier developmental stages that don't appear.For instance, according to the remarkable discovery accomplished by Miller, certain shrimp-shaped crustaceans (similar to the genus Penoeus) first appear as simple nauplius-forms,1 followed by two or more Many times the zoea-stages, and then the mysis-stage, finally acquire their adult structure: in the whole great order malacostracan, to which these crustaceans belong, there are still No other members are known to have developed first via nauplii, though many arose as daphnia; Miller nevertheless gives some reasons in support of his belief that, without developmental inhibition, all such Crustaceans will first appear as nauplii.

How, then, do we account for such facts in embryology? — that is, there are only very general, though not general, differences in structure between the embryo and the adult; — the various organs of the embryo in the same individual, which eventually become quite different and serve different purposes, are different in the early stages of growth. Similar;--the embryos or larvae of the most dissimilar species of the same class are generally, but not necessarily all, similar;--the embryo, while in the egg or womb, tends to preserve some of the features at that or a later period of life. Structures that are of no use to themselves; on the other hand, the larvae, which must feed for their own needs, are perfectly adapted to the surrounding conditions;--finally, some larvae are higher in institutional order than they are to develop. I believe that all these facts may be explained as follows.

Perhaps because deformities affect the embryo at a very early stage, it is generally supposed that slight variations or individual differences must also appear at an equally early stage.In this we have no evidence, and indeed all our evidence is on the contrary; for it is well known that breeders of cattle, horses, and fancy animals cannot, at some time after the animals are born, Be sure to point out what advantages or disadvantages their hatchlings will have.We see this clearly with our own children; we cannot tell whether a child will be tall or short, or what it will necessarily look like.The question is not at what period of life each variation occurs, but at what period the effect may be manifested.The causes of variation may act before the act of reproduction, and often, I believe, act on one or both parents.It is worth noting that so long as the very young animal remains in the mother's womb or in the egg, or so long as it is nourished and protected by the brood, most of its characters are acquired either earlier or later in life. , it doesn't matter to it.For example, with a bird which feeds by a very hooked beak, it does not matter whether it has a rafter of this shape when young, so long as it is nursed by its own brood.

In the first chapter I remarked that, at whatever age a variation first arose in the parents, it tended to reappear in the offspring at a corresponding age.Certain variations arise only at corresponding ages; for example, those of the silk-moth when in its larval, cocoon, or pupal state: or those of cattle when their horns are fully grown, and so it is.But, so far as we know, variations which first appear, whether early or late in life, have the same tendency to reappear in the offspring and in the corresponding ages of the parents.By no means do I mean that this is always the case, and I can cite several exceptions to variations (in the broadest sense of the word) which occur earlier in the offspring than in the parents. These two principles, that slight variations generally do not occur very early in life and are not inherited very early, explain, I believe, all the principal facts of embryology aforesaid.But first let us look at a few similar facts in the domestic varieties.Certain authors who have written a treatise on "dogs" have maintained that the greyhound and the bullhound, though so different, are in fact closely allied varieties, descended from the same wild species; Very curious to know how different their puppies are: I was told by the breeder that the differences between puppies are exactly the same as between the parents, which seems to be true by eye; When working with six-day-old puppies, I have found that puppies do not get the full amount of their proportional differences.Again, I have been told that the ponies of the cart-horse and the race-horse, a breed almost entirely by selection in domestic conditions, differ as much from the full-grown horse; Careful measurements of the cart mares and their three-day-old ponies showed that this was not the case. Since we have definite evidence that the breed of pigeons descended from a single wild breed, I compared young pigeons within twelve hours of hatching; The tail pigeon, the dwarf pigeon, the platoon pigeon, the dragon pigeon, the messenger pigeon, and the tumbler pigeon carefully measured (but I will not list specific materials here) the proportion of the beak, the width of the mouth, the size of the nostrils and eyelids. length, foot size and leg length.Some of these pigeons, when grown, differ from one another in such an extraordinary manner in the length and shape of their beaks, and other characters, that if they were found in a state of nature, they would certainly be ranked in distinct genera.But when the young birds of these several species are lined up, though most of them can just be distinguished, there is incomparably less difference in the proportions mentioned above than in the full-grown birds.Certain features of the difference—such as the width of the beak—are barely perceptible in the fledglings.But there is one notable exception to this rule, for the chicks of the short-faced tumbler have almost exactly the same proportions as grown, and differ from the chicks of the wild rock-pigeon and other breeds. The above two principles illustrate these facts.Breeders select and breed dogs, horses, pigeons, etc. when they are near growth: they do not care whether the desired properties are acquired earlier or later in life, as long as the fully grown animal can have them .The examples just given, and especially that of the pigeon, illustrate that the differences in expressive characters which have accumulated by artificial selection and which have given value to his breed, do not generally appear very early in life, nor are these characters Inherited at a correspondingly very early age.But the case of the short-faced tumbler, which had its proper character as soon as twelve hours old, proves that this is not a general rule; Or if not, the difference must have been inherited not at the corresponding age, but at an earlier age. Let us now apply these two principles to account for species in their natural state.Let us consider a group of birds, which have been descended from some ancient form, and which have been modified by natural selection for different habits.Thus, as many of the slight, successive variations of several species do not occur at a very early age, but are inherited at corresponding ages, the young will vary little, and will be far more similar among themselves. There is a closer resemblance between the adults,--as we see in the breeds of pigeons.We may extend this view to very different structures and to whole classes.For example.The forelimbs, which were once used as legs by distant ancestors, may become suitable for use as hands in a certain type of offspring during the long process of variation; There will be no great variation among the species; though in each form the forelimbs of the adults differ greatly from each other.Whatever effect long continual use or non-use may have on altering limbs or other parts of any species, it occurs chiefly or only when it is so close to growth that it is compelled to use all its powers to earn a living. effect; the effect so produced will be transmitted to the offspring at a correspondingly close age to growth, so that the effect of the enhanced use or non-use of the various parts of the larva will vary little or little. In some animals successive variations may occur very early in life, or degrees of variation may be inherited at an earlier age than when they first appear.In any of these cases, as we have seen with the short-faced tumbler, the larva or embryo closely resembles the growing parent-type.This is the law of development in certain whole groups or only in certain subgroups, such as cuttlefish, terrestrial molluscs, freshwater crustaceans, arachnids, and some members of the great class of insects.As to the ultimate reason why these groups of young do not undergo any metamorphosis, we can see that this arises from the following; In this case they must vary in the same manner as their parents, and this is almost indispensable to their survival.Also, concerning the curious fact that many terrestrial and freshwater animals undergo no metamorphosis at all, while sea-dwelling members of the same group undergo various metamorphosis, Miller has pointed out an animal adapted to land. life in fresh or fresh water instead of salt water, this slow process of change is greatly simplified by not passing through any larval stage; for under such a new and greatly changed habit it is difficult to find The larval stage is in turn suitable for the adult stage and not yet occupied or poorly occupied by other organisms.In this case natural selection will favor the gradual acquisition of the adult structure at younger and younger ages; and all traces of previous metamorphosis will at last disappear. If, on the other hand, it is advantageous for the larvae of an animal to follow a slightly different habit of life from the parent type, and thus have a slightly different structure, or if a larva which is already different from the parent is further modified, The larvae or larvae, then, may, by natural selection, become more and more different from the parent to any conceivable degree, according to the principles of heredity at corresponding ages.Differences in a larva may also relate to successive stages of its development; so a larva of the first stage may differ considerably from a larva of the second stage, as is the case with many animals.Adults may also become adapted to such places and habits - where the motor or sensory organs, etc., become useless; in this case, metamorphosis is regressed. From what has been said above, since the changes in structure of the larvae correspond to the modified habits of life, together with the inheritance in the corresponding age, we can understand how the stages of development passed by animals are different from the original state of their adult ancestors. completely different.Most of the best authorities are now convinced that the various larval and pupal stages of insects were thus acquired by adaptation, and not by inheritance of some ancient type.The curious case of the turnip genus (Sitaris), a beetle which has passed through certain abnormal stages of development, may perhaps illustrate how this happens.Its first larval form, described by Fabre, was a lively tiny insect with six legs, two long antennae, and four eyes.These larvae hatch in the hive; when the males emerge in the spring before the females, the larvae jump on them, and later on the females when the males and females mate.When the female bee lays her eggs on the honeycomb of the bee, the larvae of the turnip genus immediately jump on the eggs and eat them.Afterwards they undergo a complete transformation; their eyes disappear, their legs and antennae become rudimentary, and they feed on honey; so that they now more closely resemble the common larvae of insects; Further transformations finally emerge as the perfect beetle.Now, if there were an insect which transformed like the turnip, and became the progenitor of a whole new class of insects, the course of development of this new class would probably be quite different from that of our existing insects; And the first larval stage certainly does not represent any previous state of adult and archaic forms. On the other hand, it is highly probable that the embryonic or larval stages of many animals show us more or less completely the adult state of the ancestors of the whole group.In the great class of crustaceans, the forms which are very different from each other, viz. the parasitic species, the cirripedes, the entomostraca, and even the molluscs, first appear as larvae in the nauplii form. and because these larvae live and feed in the open ocean, and are not adapted to any particular habit of life, and for other reasons given by Miller, there probably existed, at some remote period, a sort of nauplii-like Separate adults once existed, and then along several divergent lines of descent, gave rise to the above-mentioned huge groups of crustaceans.Again, from what we know about the embryos of mammals, birds, fish, and reptiles, these animals are presumably the modified descendants of an ancient ancestor which, in the adult state, had been perfectly adapted to Aquatic living gills, a swim bladder, four flippers and a long tail. For all living beings that have ever lived, both extinct and modern, can be grouped in a few classes; and because all the members of each class are, according to our theory, connected together by minute gradations, If our collection is nearly complete, the best and only possible classification is probably that of genealogy; descent is thus the latent link of interconnection sought by naturalists under the term "natural systems."On this point of view we can understand why, in the eyes of most naturalists, the structure of the embryo is even more important in classification than that of the adult.In two or more groups of animals, however much their structure and habits may differ from each other in the adult state, we may be sure that they have all been descended from one parent-type if they pass through closely similar embryonic stages. , and thus are closely related to each other.Thus, a commonality in embryonic structure reveals a commonality of descent; but dissimilarity in embryonic development does not prove inconsistency of descent, since in one of the two populations a developmental stage may have been suppressed, or it may have Changed so much by adaptation to new habits that it is no longer recognizable.Even in groups in which the adults have undergone extreme variations, the commonality of origin is often revealed by the structure of the larvae; we see, for example, that cirripedes, though superficially resembling shellfish, are immediately capable of Know that they belong to the outline of crustaceans.As embryos often show us more or less clearly the structure of the less varied, ancient progenitors of a group, we can understand why the adult state of ancient, extinct forms often resembles the embryos of extant species of the same class. .Agassiz believed this to be a universal law of nature; we can expect to see this law proved to be true thereafter.However, it can only be shown to be true if the ancient progenitors of the group were not due to successive modifications very early in growth, nor were they due to variations earlier than when they first appeared The earlier ages are inherited and all obliterated.It must also be remembered that this law may be true, but because the geological record has not been extended far enough in time, it may remain unsubstantiated for a long time or never.If an ancient form adapted a particular mode of life in the larval state, and transmitted the same larval state to the offspring of the whole flock, then in this case, too, the law cannot strictly hold; Does not resemble any older type of adult state. Thus, it seems to me, these incomparably important facts of embryology are explained on the principle that variations among the many descendants of an ancient ancestor arose in the not very early stages of life. period, and was inherited in the corresponding period.The importance of embryology is greatly increased if we regard the embryo as a picture, somewhat blurred, of the ancestry of all members of the same class, either in its adult state or in its larval state. increased. rudimentary, atrophied and underdeveloped organs Organs or parts in this strange state, bearing the distinct stamp of disuse, are very common, and might even be said to be universal, throughout nature.It is impossible to name a higher animal, some part of which is not in a rudimentary state.Mammalian males, for example, have vestigial teats; snakes have a rudimentary lobe of their lungs; bird "bastard-wings" can safely be considered vestigial, and in some species the entire wing is rudimentary. So pronounced that it cannot be used to fly.What is more strange than that whale fetuses have teeth, and when they grow up have none; or that unborn calves have teeth in the upper jaw, but never penetrate the gums? The rudimentary organs clearly indicate their origin and significance in various ways.Beetles of closely allied species, or even of the same species, have either very large and complete wings, or only remnants of membranes, situated under firmly united elytra; The remnants represent the wings.Rudimentary organs sometimes retain their potency: this is occasionally seen in the teats of male mammals, which have been seen to be well developed and to produce milk.The same is true of the udders of the genus Bos, which normally have four well-developed teats and two rudimentary teats; but the latter are sometimes well-developed and lactating in our domestic cows.With regard to plants, in individuals of the same species the petals are sometimes rudimentary and sometimes well developed.In certain dioecious plants, Korreuter found that when a species in which the male flowers had a rudimentary pistil was crossed with a hermaphrodite species naturally having a well-developed pistil, the rudimentary pistil was greatly increased in the offspring of the hybrid. larger; this clearly shows that the rudimentary pistil and the complete pistil are basically similar in nature.Parts of an animal may be in a complete state, and they may be rudimentary in a sense, because they are useless: e.g. the common salamander, the tadpole of the water salamander, as Lewis What Mr. said, "has gills and lives in the water; but the mountain salamander (Salamandra atra) lives in the high mountains, and both produce fully developed larvae. This animal has never lived in water. But if we cut open the fetus of the female of the newt, we shall find that the tadpoles in her have finely feathered gills; The future life of the animals is irrelevant, and it is not an adaptation to embryonic conditions; it is entirely a matter of ancestral adaptation, but a replay of a stage in their ancestral development." Organs which have both uses may become rudimentary or wholly undeveloped for the one, even the more important one, while being perfectly efficient for the other.In plants, for example, the function of the pistil is to carry the pollen tubes to the ovules in the ovary.The pistil has a stigma, which is supported by the style; but in some plants of the conglomerate the male florets, of course infertile, have a rudimentary pistil, since it has no stigma at the top; its style, however, is still well developed, and Hairy in usual manner, serving to brush off pollen in surrounding, adjoining anthers.Also, an organ may become rudimentary of its proper use, and be used for a different purpose: in some fishes the swim-bladder seems to be rudimentary of the proper function of flotation, but it becomes primitive. Respiratory organs or lungs.Many similar examples could be cited. Useful organs, however undeveloped they may be, should not be regarded as rudimentary, unless we have reason to suppose that they were formerly more highly developed, and that they may be in a rudimentary state on the way to further development. .Rudimentary organs, on the other hand, are either quite useless, such as teeth that never penetrate the gum, or barely useful, such as ostrich wings that serve only as windshields.As organs in this state were less developed, and even less useful than they are now, they could not have previously been produced by variation and natural selection, which operates only to preserve useful variations.They are preserved in part by the force of heredity, having a relation to former states of things.Nevertheless, it is often difficult to distinguish rudimentary organs from nascent organs; for we can only judge by analogy whether an organ is capable of further development, and only in so far as they are capable of further development should they be called nascent. of.Organs in this state are always rare; and as beings possessing such organs are generally displaced by successors having more perfect identical organs, they have long since become extinct.The penguin's wing is highly useful, it serves as a fin; so it may represent the nascent state of the wing: that's not to say I believe this to be true; it's more likely a shrunken organ, adapted to a new function Mutated, on the other hand, the kiwi's wings are quite useless, and indeed rudimentary.Owen regards the simple filamentous limbs of the lungfishes as "the beginning of the fully functionally developed organs in the higher vertebrates"; This fin shaft has underdeveloped rays or lateral branches.If the mammary gland of the platypus is compared with that of the cattle, it can be regarded as a nascent state.The egg bands of some cirripedes can no longer serve as egg attachments and are very underdeveloped. These are the gills in the primary state. Rudimentary organs are liable to vary in degree of development, as well as in other respects, among individuals of the same species.The degree of reduction of the same organ is sometimes very different in closely allied species.This latter fact is well exemplified by the condition of the wings of the female moths of the same family.Rudimentary organs may be completely shrunken; this means that in some animals or plants some organs are completely absent, though we might hope to find them by analogy, and they may indeed be occasionally seen in deformed individuals.例如玄参科（Scrophulariaceae）的大多数植物，其第五条雄蕊已完全萎缩；可是我们可以断定第五条雄蕊曾经存在过，因为可以在这一科的许多物种中找到它的残迹物，并且这一残迹物有时会完全发育，就像有时我们在普通的金鱼草（snap- dragon）里所看到的那样。当在同一纲的不同成员中追寻任何器官的同原作用时，没有比发现残迹物更为常见的了，或者为了充分理解诸器官的关系，没有比残迹物的发现更为有用的了。欧文所绘的马、黄牛和犀牛的腿骨图很好地示明了这一点。 这是一个重要的事实，即残迹器官，如鲸鱼和反刍类上颚的牙齿，往往见于胚胎，但以后又完全消失了。我相信，这也是一条普遍的法则，即残迹器官，如用相邻器官来比较，则在胚胎里比在成体里要大一些；所以这种器官早期的残迹状态是较不显著的，甚至在任何程度上都不能说是残迹的，因此，成体的残迹器官往往被说成还保留胚胎的状态。 刚才我已举出了有关残迹器官的一些主要事实。当仔细考虑到它们时，无论何人都会感到惊奇；因为它告诉我们大多数部分和器官巧妙地适应于某种用处的同一推理能力，也同等明晰地告诉我们这些残迹的或萎缩的器官是不完全的，无用的。在博物学著作中，一般把残迹器官说成是“为了对称的缘故”或者是为了要“完成自然的设计”而被创造出来的。但这并不是一种解释，而只是事实的复述。这本身就有矛盾：例如王蛇（boa-constrictor）有后肢和骨盘的残迹物，如果说这些骨的保存是为了“完成自然的设计”，那末正如魏斯曼教授所发问的，为什么其他的蛇不保存这些骨，它们甚至连这些骨的残迹都没有呢？如果认为卫星“为了对称的缘故”循着椭圆形轨道绕着行星运行，因为行星是这样绕着太阳运行的，那末对于具有这样主张的天文学者，将作何感想呢？有一位著名的生理学者假定残迹器官是用来排除过剩的或对于系统有害的物质的，他用这个假定来解释残迹器官的存在；但是我们能假定那微小的乳头（papilla）——它往往代表雄花中的雌蕊并且只由细胞组织组成——能够发生这样作用吗？我们能假定以后要消失的、残迹的牙齿把像磷酸钙这样贵重的物质移去可以对于迅速生长的牛胚胎有利益吗？当人的指头被截断时，我们知道在断指上会出现不完全的指甲，如果我相信这些指甲的残迹是为了排除角状物质而发育的，那么就得相信海牛的鳍上的残迹指甲也是为了同样的目的而发育的。 按照伴随着变异的生物由来的观点，残迹器官的起源是比较简单的；并且我们能够在很大程度上理解控制它们不完全发育的法则。在我们的家养生物中，我们有许多残迹器官的例子，——如无尾绵羊品种的尾的残基，——无耳绵羊品种的耳的残迹，——无角牛的品种，据尤亚特说，特别是小牛的下垂的小角的重新出现，——以及花椰菜（cauliflower）的完全花的状态。我们在畸形生物中常常看到各种部分的残迹；但是我怀疑任何这种例子除了示明残迹器官能够产生出来以外，是否能够说明自然状况下的残迹器官的起源；因为衡量证据，可以清楚地示明自然状况下的物种并不发生巨大的、突然的变化。但是我们从我们家养生物的研究中得知，器官的不使用导致了它们的缩小；而且这种结果是遗传的。 不使用大概是器官退化的主要因素。它起初以缓慢的步骤使器官愈来愈完全地缩小，一直到最后成为残迹的器官，——像栖息在暗洞里的动物眼睛，以及栖息在海洋岛上的鸟类翅膀，就是这样。还有，一种器官在某种条件下是有用的，在其他条件下可能是有害的，例如栖息在开阔小岛上的甲虫的翅膀就是这样；在这种情形下，自然选择将会帮助那种器官缩小，直到它成为无害的和残迹的器官。 在构造上和机能上任何能够由细小阶段完成的变化都在自然选择的势力范围之内；所以一种器官由于生活习性的变化而对于某种目的成为无用或有害时，大概可以被改变而用于另一目的。一种器官大概还可以只保存它的以前的机能之一。原来通过自然选择的帮助而被形成的器官，当变成无用时，可以发生很多变异，因为它们的变异已不再受自然选择的抑制了。所有这些都与我们在自然状况下看到的很相符合。还有，不管在生活的哪一个时期，不使用或选择可以使一种器官缩小，这一般都发生在生物到达成熟期而势必发挥它的全部活动力量的时候，而在相应年龄中发生作用的遗传原理就有一种倾向，使缩小状态的器官在同一成熟年龄中重新出现，但是这一原理对于胚胎状态的器官却很少发生影响。这样我们就能理解，在胚胎期内的残迹器官如与邻接器官相比，前者比较大，而在成体状态中前者就比较小。例如，如果一种成长动物的指在许多世代中由于习性的某种变化而使用得愈来愈少，或者如果一种器官或腺体在机能上使用得愈来愈少，那么我们便可以推论，它在这种动物的成体后代中就要缩小，但是在胚胎中却几乎仍保持它原来的发育标准。 可是还存在着以下的难点。在一种器官已经停止使用因而大大缩小以后，它怎么能够进一步地缩小，一直到只剩下一点残迹呢？最后它怎么能够完全消失呢？那器官一旦在机能上变成为无用的以后，“不使用”几乎不可能继续产生任何进一步的影响。某种补充的解释在这里是必要的，但我不能提出。比方说，如果能够证明体制的每一部分有这样一种倾向：它向着缩小方面比向着增大方面可以发生更大程度的变异，那么我们就能理解已经变成为无用的一种器官为什么还受不使用的影响而成为残迹的，以至最后完全消失；因为向着缩小方面发生的变异不再受自然选择的抑制。在以前一章里解释过的生长的经济的原理，对于一种无用器官变成为残迹的，或者有作用；根据这一原理，形成任何器官的物质，如果对于所有者没有用处，就要尽可能地被节省。但是这一原理几乎一定只能应用于缩小过程的较早阶段；因为我们无法设想，比方说在雄花中代表雌花雌蕊的并且只由细胞组织形成的一种微小突起，为了节省养料的缘故，能够进一步地缩小或吸收。 最后，不管残迹器官由什么步骤退化到它们现在那样的无用状态，因为它们都是事物先前状态的记录并且完全由遗传的力量被保存下来，——根据分类的系统观点，我们就能理解分类学者在把生物放在自然系统中的适宜地位时，怎么会常常发见残迹器官与生理上高度重要的器官同等地有用。残迹器官可以与一个字中的字母相比，它在发音上已无用，而在拼音上仍旧保存着，但这些字母还可以用作那个字的起源的线索。根据伴随着变异的生物由来的观点，我们可以断言，残迹的、不完全的、无用的或者十分萎缩的器官的存在，对于旧的生物特创说来说，必定是一个难点，但按照本书说明的观点来说，这不仅不是一个特殊的难点甚至是可以预料到的。 feed 在这一章里我曾企图示明：在一切时期里，一切生物在群之下还分成群的这样排列，——一切现存生物和绝灭生物被复杂的、放射状的、曲折的亲缘线连结起来而成为少数大纲的这种关系的性质，——博物学者在分类中所遵循的法则和遇到的困难，——那些性状，不管它们具有高度重要性或最少重要性，或像残迹器官那样毫无重要性，如果是稳定的、普遍的，对于它们所给予的评价，——同功的即适应的性状和具有真实亲缘关系的性状之间在价值上的广泛对立；以及其他这类法则；——如果我们承认近似类型有共同的祖先，并且它们通过变异和自然选择而发生变化因而引起绝灭以及性状的分歧，那么，上述一切就是自然的了。在考虑这种分类观点时，应该记住血统这个因素曾被普遍地用来把同一物种的性别、龄期、二型类型以及公认变种分类在一起，不管它们在构造上彼此有多大不同。如果把血统这因素——这是生物相似的一个确知原因，——扩大使用，我们将会理解什么叫做“自然系统”：它是力图按谱系进行排列，用变种、物种、属、科、目和纲等术语来表示所获得的差异诸级。 根据同样的伴随着变异的生物由来学说，“形态学”中的大多数大事就成为可以理解的了，——无论我们去观察同一纲的不同物种在不管有什么用处的同原器官中所表现的同一形式；或者去观察同一个体动物和个体植物中的系列同源和左右同源，都可以得到理解。 根据连续的、微小的变异不一定在或一般不在生活的很早时期发生并且遗传在相应时期的原理，我们就能理解“胚胎学”中的主要事实；即当成熟时在构造上和机能上变得大不相同的同原器官在个体胚胎中是密切类似的；在近似的而显明不同的物种中那些虽然在成体状态中适合于尽可能不同的习性的同原部分或器官是类似的。幼虫是活动的胚胎，它们随着生活习性的变化而多少发生了特殊的变异，并且把它们的变异在相应的很早龄期遗传下去。根据这些同样的原理——并且记住，器官由于不使用或由于自然选择的缩小，一般发生在生物必须解决自己需要的生活时期，同时还要记住，遗传的力量是多么强大，——那么，残迹器官的发生甚至是可以预料的了。根据自然的分类必须按照谱系的观点，就可理解胚胎的性状和残迹器官在分类中的重要性。 最后，这一章中已经讨论过的若干类事实，依我看来，是这样清楚地示明了，栖息在这个世界上的无数的物种、属和科，在它们各自的纲或群的范围之内，都是从共同祖先传下来的，并且都在生物由来的进程中发生了变异，这样，即使没有其他事实或论证的支持，我也会毫不踌躇地采取这个观点。