Chapter 12 Chapter VII Objections to the Theory of Natural Selection 2

Mr. Mivart has suggested this situation, and said that a sudden and spontaneous change in the position of the eyes is unbelievable, with which I fully agree.He added, "It is incomprehensible how this transition, the very small segment in the journey of one eye to the other side of the head, can benefit the individual if the transition is gradual. The initial shift was more or less harmful than beneficial."An answer to this question can be found, however, in an excellent observation reported by Malm in 1867, that in the flatfishes, when very young and symmetrical, their eyes are divided on the sides of the head, but because The body is too high, the side fins are too small, and because there is no swim bladder, it cannot maintain an upright posture for a long time.After a while it got tired and fell on its side to the bottom.According to Mamm's observation, when they lie down in this way, they often turn the lower eyes upwards, looking up; and the eyes are turned so vigorously that the eyeballs are pressed tightly against the upper edge of the eye socket.The result is a temporary reduction in the width of the forehead between the eyes, which can be clearly seen.Once Mam saw a juvenile lift its lower eye and hold it down about a seventy-degree angle.

We must remember that the skull is cartilaginous at such an early stage and is flexible so that it readily obeys the pull of the muscles.And we know that in higher animals, even after early childhood, if their skin or muscles are chronically contracted by disease or some accident, the skull changes its shape accordingly.In long-eared rabbits, if one of their ears hangs forward and downward, its weight will pull all the skulls forward on that side, as I have drawn such a diagram.Mamm says that the newly hatched larvae of percbes, salmon, and several other symmetrical fishes also often have the habit of lying on one side at the bottom; The eyes look upwards; thus their skulls become slightly askew.However, these fish can maintain an upright position after a while, so no permanent effect will result from this.The flatfishes, on the other hand, grow larger as their bodies become more flattened, and their habit of lying on one side becomes deeper, thus producing a permanent effect on the shape of the head and the position of the eyes.Judging by analogy, this tendency to crooked bones is doubtless hereditary.Hiatt, contrary to certain other naturalists, believes that flatfishes are not quite symmetrical even in the embryonic stage, and if this is the case, we can understand why certain species of fish habitually lie on their backs when young. left, while some other species lie on the right.Mamm, confirming the above opinion, added that adults of the northern roughfin (Trachypterus arcticus), not belonging to the flatfish family, also lie on the left side of the water, and swim obliquely; Not alike.After describing Mamm's paper, Dr. Gunther, our great authority on ichthyology, comments: "The author proposes a very simple explanation for the unusual state of the flatfish family."

Thus we see that the initial phases of the eye moving from one side of the head to the other, which Mr. Mivart considers harmful, can be attributed to the effort of the eyes to look upwards while lying on their side at the bottom of the water. A habit, which is undoubtedly advantageous to the individual and to the species, some species of flounder have their bills bent downwards, and have no jaws of the head on the side of the eyes, as Dr. Traquair has shown. Supposedly, for the convenience of feeding at the bottom, the jaws are stronger and more powerful than those on the other side, a fact which we may ascribe to the hereditary effect of use.On the other hand, the less development of the whole lower body of the fish, including the lateral fins, may be accounted for by its lack of use; though Yarrell conjectures that the reduction of these fins would also be of advantage to flatfishes, because "more From the large upper fins, the lower fins have very little room to move."The plaice has four to seven teeth in the upper jaw, and twenty-five to thirty teeth in the lower jaw, and this ratio of the number of teeth may likewise be explained by the absence or use of them.From the absence of color on the ventral surfaces of most fishes, and of many other animals, we may reasonably suppose that the lack of color on the underside of flatfishes, whether right or left, is due to the absence of light.But we cannot assume that the distinctive spots on the upper body of the sole resemble the sandy bottom, or that some species, as Pouchet recently pointed out, have the ability to change color with the surrounding surface, or that the European turbot (turbot ) with bony nodules on the upper body, all due to the effect of light.Natural selection probably operates here, as it has adapted the general shape of the bodies of these fishes, and many other peculiarities, to their habits of life.We must remember, as I have argued before, that the genetic effects of the use of organ enhancements, or of their non-use, are enhanced by natural selection.For all spontaneous variation in the right direction will thus be preserved; just as those individuals of the greatest hereditary effect by enhanced and advantageous use of any part can be preserved.It seems impossible to determine how much is in each particular case attributable to the effects of use, and how much to natural selection.

I may give another example, where the origin of a construction is evidently wholly due to use or habit.In some American monkeys the end of the tail has been transformed into an extremely perfect grasping organ, which is used as a fifth hand.A commentator, who fully agrees with Mr. Mivart, says of this constitution, "It is impossible to believe that at any age the first slight inclination to grasp could have preserved the life of an individual so inclined, or could have benefited give them the opportunity to have offspring."But any such belief is unnecessary.Habit is probably sufficient for this kind of work, and habit almost means that some benefit, greater or less, can be derived from it.Brehm saw a young African monkey (Cer-copithecus) clinging to its mother's belly with its hands, while hooking its small tail to the mother's tail.Prof. Henslow (Prof. Henslow) kept several hamsters (Mus messorius), whose tails were not constructed to hold things; To aid their climbing, I have a similar report from Dr. Ginter, who once saw a rat suspend itself by its tail.If the hamster has a strictly arboreal habit, its tail may perhaps, as in the case of some members of the same order, be securely constructed.After examining this habit of African monkeys when they were young, it is difficult to explain why they did not do it later.The long tail of this monkey may be more useful to them as a balance organ than as a grasping organ during huge jumps.

The mammary glands are common to all classes of mammals, and are indispensable to their existence; so the mammary glands must have been developed at very remote times, of which we certainly know nothing.Mr. Mivart asked: "Can it be conceived that the young of any animal could have been prevented from dying by accidentally sucking a drop of less nourishing fluid from the swollen glands of its mother? What chance is there of perpetuating such a variation?" But the example is inappropriate.Most evolutionists admit that mammals are descended from marsupials; if this is the case, the mammary glands must have originally developed in the pouch.In the case of one fish (Hippocampus), the eggs are hatched in bags of this nature, and the young are for a time reared there; an American naturalist, Mr. Lockwood (Mr. Lockwood, on the basis of what he had seen of the development of juveniles, believed that they were nourished by secretions from the pouch endothelial glands.Is it at least probable, then, that with regard to the early progenitors of mammals, their young were reared in the same way almost before the name could be applied to them?And in this case those individuals who secrete milky juices, and in some degree or manner the most nutritious, will after all rear a greater number of well-nourished The offspring; therefore, this skin gland homologous to the mammary gland will be improved, or become more effective, and the glands distributed in a certain position in the bag will become more developed than the rest, which is consistent with the wide application in accordance with the principle of specialization; they then became breasts, but at first without teats, as we see in the platypus, the lowest of the mammals.By what means the glands distributed in certain places become more specialized than the rest, whether in part by compensatory effects of growth, effects of use, or natural selection, I cannot yet determine.

Unless the young are also able to feed on this secretion, the development of the mammary glands will be of no use, and will not be subject to natural selection.Understanding how young mammals instinctively learn to suck milk is no more difficult than understanding how unhatched chickens know how to gently crack open eggshells with specially adapted beaks, or how to peck at grains after leaving the eggshell for hours food, more difficult.In such cases the most probable explanation seems to be that the habit is first acquired by practice in older age, and is thereafter transmitted to younger offspring.But it is said that the young kangaroos do not suckle, but hold tightly to the teats of the mother, who shoots the milk into the mouth of her feeble, half-formed offspring.Regarding this problem, Mr. Mivart said, "If there is no special equipment, the wallaby will be suffocated because the milk invades the trachea, but there are special equipment. Its larynx is so long that it goes all the way to the The rear end of the nasal tube, so that air can freely enter the lungs, and the milk can pass harmlessly on both sides of this elongated larynx, and safely reach the esophagus at the back."How, Mr. Mivart asked, had natural selection removed from the adult kangaroo, and from most other mammals (supposedly descended from the marsupials), "this at least perfectly innocent and harmless structure?" It may be answered: Vocalization is indeed of high importance in many animals, and cannot be so vigorously so long as the larynx passes into the nasal tube; , is greatly hindered.

Let us now speak a little of the lower divisions of the animal kingdom.Echinoderms (starfish, sea urchins, etc.) have a remarkable organ called the pedicellaria, which, in a highly developed case, becomes a trident pincer—that is, consists of three serrated arms Formed, with three arms fitted closely together, at the tip of a flexible, muscularly moved shank.These pincers can firmly hold anything; Alexander Agassiz has seen a sea urchin (Echinus) quickly pass the fine particles of excrement from one pincer to the other, along certain lines of the body. The wiring falls down so as not to stain its case.But they no doubt have other uses besides the removal of various kinds of dirt; one of these is evidently defense.

With regard to these organs, M. Mivart again asked, as has been the case many times before: "What use could be made of the first sterile beginnings of this structure? And how could such incipient germs preserve the life of a sea urchin?" ?” He added: “Even if this clamping action were developed suddenly, it would not be beneficial without a free-moving shank; Nor would it have been of much use, but to deny that such complex mutual coordination of structure could not have evolved simultaneously by slight and indeterminate variations alone would seem to affirm an astonishingly paradoxical paradox. theory." Although this seems paradoxical to Mr. Mivart, it is understandable that three-pronged spines with fixed bases, but with a clamping action, do exist in some star fishes. Yes, if they use it at least in part as a means of defense, Mr. Agassiz, to whom I am very grateful, has informed me that there are other star fishes whose three pincer arms One has degenerated into the prop of the other two; and there are other genera whose third arm has been completely lost.According to Mr. Perrier's description, there are two kinds of fork spines on the shell of the oblique sea urchin (Ecbinoneus), one is like the fork spine of the thorn sea urchin, and the other is like the fork spine of the heart-shaped sea urchin (Spatangus). ; such cases are often interesting because they indicate a definite and sudden method of transition through the loss of one of the two states of an organ.

Concerning the evolutionary steps of these strange organs, Mr. Agassiz deduces from his own studies, as well as those of Miller, that the forked spines of star-fishes and sea-urchins should undoubtedly be regarded as modifications of common spines.This may be deduced from the manner of their individual development, and from a long and complete series of gradations in different species and genera, from the simple grain to the common spine to the perfect trident.This gradual evolution is seen even in the way in which the common spines and the forked spines with calcareous struts are connected to the shell.In certain genera of starfishes it is seen that "it is the connection which shows that the forked spines are nothing but modified branched spines."We can thus see fixed spines, with three equal-length, serrated, mobile branches joined near their base; and above, on the same spine, three more mobile branch.If the latter arise from the apex of a spine, a thick, trifurcated spine is in fact formed, as is seen on the same spine with three lower branches.There is no question that the pincer arm of the spine and the mobile branch of the spine are of the same nature.It is accepted that the common spine is for defense; if so, there is no reason to doubt that those with serrated and mobile branches serve the same purpose; They are all the more effective when they act on other instruments, so that every step from the ordinary fixed spine to the fixed forked spine is useful.

In some genera of star fishes these organs are not fixed, i.e. not on a fixed support, but on flexible, muscular short stalks; Here, in addition to defense, they probably also perform some additional functions.In sea urchins the steps from fixed spines to spines attached to the shell and thus mobile can be traced.It is a pity that there is no space here to give a more detailed summary of Mr. Agassiz's interesting observations on the development of the forkthorn.Every possible gradation, according to him, is also to be found between the fork-spines of star-fishes and the barb-spines of another group of echinoderms, the Ophiurians; This outline finds all possible gradations between the anchor-like spicules of the sea cucumber (Holothuria).

Certain compound animals, formerly known as zoophytes, are now called Polyzoa, having wonderful organs called avicularia.The structure of these organs varies greatly in different species.In their most perfect state they closely resemble the vulture's head and beak in minute detail, being born on the neck and capable of movement, as are the lower jaws.I have observed a species in which the avicularia, born on the same branch, often move forward and backward in unison, with the mandibles wide open, at an angle of about ninety degrees, for five seconds; It made the entire swarm tremble.If a needle is touched to its jaws, they bite it so firmly that they shake the branch on which it rests. Mr. Mivart gives this example mainly because he thinks that the avicularia of the polyzoa and the fork-spine of the echinoderm are "essentially similar organs", and that these organs are far different from those in the animal kingdom. Development in sectors by natural selection is difficult.But in terms of structure alone, I see no resemblance between the trigeminal spines and the avicularia.The avicularia are rather similar to the pincers of crustaceans; Mr. Mivart may, with equal justice, point to this resemblance, and even their resemblance to the heads and beaks of birds, as a special difficulty.Mr. Busk, Dr. Smitt, and Dr. Nitsche--naturalists who have carefully studied this group--believed that the avicularia and monozoa (zooid) and the cells that make up the phytophytes are homologous; the movable lip, the lid of the cell, is the equivalent of the movable lower jaw of the avicularia, but Mr Basque is not aware of the present existence in single Any progression between the worm body and the avicularia.It is therefore impossible to conjecture by what useful gradation this can be changed into that; but it must not therefore be said that this gradation never existed. Since the pincers of crustaceans are to some extent similar to the avicularia of polyzoa, and both are used as pincers, it is worth pointing out that there is still a long series of useful grades on the pincers of crustaceans. Jin exists.In the first and simplest stage, the terminal segment of the limb, when closed, bears against the square top of the broad second segment, or against its entire side; thus clamping an object it touches; but the limb It is still used as a mobile organ.Next, a slightly protruding corner of the broad second segment, sometimes bearing irregular teeth, rests against these teeth when the distal segment closes; and improvement, the pliers will become more and more perfect until they become as efficient a tool as the lobster pliers, and virtually all such progress can be traced. In addition to the beak, polyzoa have a wonderful organ called vibra-cula.These vibrohairs generally consist of long setae that are mobile and easily irritated.I have examined a species whose vibraculae were slightly curved and serrated on the outer edges, and often all the vibraculae on the same colony moved simultaneously: they moved like long oars, causing a colony to move rapidly. passed under the objective lens of my microscope.If a branch of colony is placed face down, the vibracus become entangled, and they jerk themselves apart. The vibracus are supposed to have a defensive function, and they can be seen "slowly, as Mr. Sweeping silently over the surface of the colony, sweeping away things that would be harmful to the delicate inhabitants of the bug house as they reach out their tentacles."The avicularia, like vibraculae, presumably also served a defensive purpose, but they also captured and killed small animals, which were believed to have been swept by the currents to within reach of the tentacles of the individual worms.Some species have both avicularia and vibraculae, some species have only avicularia, and a few species have only vibraculae. Two bodies which differ more in appearance than the setae (i.e. vibraculae) and the bird-head-like avicularia are not easily conceived; yet they are almost certainly homologous, and descended from the same From a common root—that is, the single worm and its chamber—developed.We can therefore understand, as M. Basque told me, how the organs, in certain cases, gradually change from one form to another.Thus, in several species of the genus Lepralia, the movable jaw of the avicularia is so prominent, and so resembling setae, that the avicularia can only be determined from the upper fixed beak. nature.It is possible that the vibracula developed directly from the lip of the chamber, without passing through the avicularia stage; but it seems more probable that they passed through this stage, because early in the transformation, the worms harboring a single body other parts of the room, it is difficult to disappear immediately.In many cases the base of the vibrachoides has a grooved strut which seems to correspond to a fixed beak; though some species lack this strut at all.This view of the development of the vibracula, if reliable, is interesting; for, assuming that all species with avicularia had become extinct, it would never occur to the most imaginative mind that the vibracula was originally a bird-like species. A part of an organ in the form of a head, or part of an organ resembling an irregularly shaped box or hood.It is interesting to see two organs so different so developed from a common root; and as the mobile lips of the chamber serve to protect the individual insects, it is not difficult to believe that the lips first became The lower jaw of the avicularia, and then into long bristles, and all the gradations in between, can also play a protective role in different ways and under different environmental conditions. In the vegetable kingdom, Mr. Mivart mentions only two cases, namely, the flower structure of orchids and the movement of climbing plants.Concerning the orchids, he says: "The explanation of their origin is entirely unsatisfactory--very inadequate for the first, most subtle beginnings of structures. These structures are only It is effective." I have already dealt with this subject at length in another work, and therefore I shall here describe in some detail only the most remarkable characteristic of the flowers of the Orchidaceae, namely, their pollinia.The highly developed pollen mass is composed of a group of pollen grains, which grow on a flexible stalk, the pollen mass stalk, which is attached to a small piece of extremely sticky substance.In this way the pollen mass is transported by insects from one flower to the stigma of another.Some of the Orchidaceae have pollen masses without stalks, the pollen grains being held together by filaments only; but this case is not confined to the Orchidaceae, so it need not be discussed here; however I may mention that in the Orchid system The lowest genus, Cypripedium, from which we may see how these filaments were probably first developed.In other orchids, these filaments are attached to one end of the pollen-cube; this is the trace of the first occurrence of the pollen-cube peduncle.This is the origin of the stalk - even a fairly long and highly developed one - and we can find good evidence also from underdeveloped pollen grains sometimes buried in the central hard part. Concerning the second principal characteristic of the pollen-cube, the little sticky substance attached to the stalk-end, a long series of gradations can be enumerated, each of which is obviously of use to this plant, the other "orders" The stigmas of most of the flowers secrete very little sticky substance.Certain orchids secrete a similar viscous substance, but only one of the three stigmas secretes it in a particularly large amount: this one has probably become sterile as a result of the hypersecretion.When an insect visits a flower of this type, it wipes off some of this sticky substance, thus sticking away some of the pollen grains at the same time.From this simple case, which differs very little from most common flowers--to species in which the pollen-cube is attached to a very short and free stalk,--to species in which the pollen-cube is attached to a sticky substance, And other species in which the sterile stigmas vary greatly,—there are innumerable gradations.On the last occasion, the pollen mass is most developed and complete.Anyone who has carefully studied the flowers of orchids will not deny the existence of the above-mentioned series of gradations—the pollen grains of some orchids are only connected by filaments, and their stigmas are the same as those of ordinary flowers. The stigmas differ little from each other, and from this up to the highly complex pollen masses, which are well adapted for insect transport; nor would he deny that all the gradations in those few species are well adapted to the general structure of the various flowers by Pollinated by different insects.In this case, and in nearly all others, it is possible to go still further down; it is possible to ask how the stigma of the common flower became sticky, but since we do not yet know the whole history of any group of organisms, It is therefore as useless to ask such questions as it is hopeless to attempt to answer them. We are now going to talk about climbing plants.A long series can be arranged from climbers that simply wrap around a support, to what I call leaf climbers and tendril-bearing climbers.The stems of the latter two classes have generally lost, if not often lost, the power of winding, though they still retain, and tendrils likewise possess, the power of rotation.The gradation from leaf-climbers to tendril-climbers is closely related, and there are certain plants which could be placed in either class at will.But in the ascent from mere twining plants to leaf-climbers, an important property is added, namely the responsiveness to touch, by means of which the petiole or pedicel, or which has become a tendril, , capable of being stimulantly bent around touching objects and wrapping them, anyone who has read my reports on these plants will, I think, admit that between mere twining plants and tendril climbers there is a difference in function All gradational changes in body and structure are each highly favorable to the species.For example, it would obviously be of great advantage for a twinering plant to become a leaf climber; a twinerer with a long petiole would probably develop into a leaf climber if the petiole possessed the requisite contact sensitivity. Winding is the easiest way of ascending the pillar, and is the lowest in the series, so it is natural to ask how plants first acquired this power, and have since been improved and increased by natural selection. The ability of the stem depends, first, on the extreme flexibility of the stem when young (a trait common to many non-climbing plants);The stem relies on this movement to rotate in all directions. Once the lower part of the stem hits any object and stops winding, its upper part can still continue to bend and rotate, so it must wind around the support and rise. In the early stages of each new shoot After growth, this rotational motion ceases.In many different families of plants, systematically distant, a single species and a single genus often possessed this power of turning, and thereby became twining plants, so they must have acquired it independently, and Not inherited from a common ancestor.This leads me, therefore, to predict that in non-climbing plants a slight propensity for such movements is not uncommon, and would provide a basis for the action and improvement of natural selection.When I made this prophecy, I knew only one partial example, of the young pedicel of Maurandia, which whirled slightly and irregularly, much like the stems of twining plants, but the habit was not at all Be exploited.Shortly thereafter Miller discovered that the young stems of an Alisma and a Linum—both not climbing plants, and very far apart in natural systems—although rotated Irregularly, but evidently capable of this; he said he had reason to suspect that it was also the case with some other species.Such slight movements do not seem to be of much use to that plant; at any rate, they are of no use to the climbing we are discussing.We can still see, however, that if the stems of such plants were originally flexible, and if the conditions in which they were placed favored their elevation, a slight and irregular habit of turning would Enlarged and exploited by natural selection.Until they transform into a well-developed entwining species. The sensibility of petioles, peduncles, and tendrils is almost equally accountable for the whirling motion of twining plants.Many species, belonging to very different groups, are endowed with this responsiveness, and it is so true that this property should also be seen in a nascent state in many species which have not yet become climbers. : I observed that the young flower stalks of the above-mentioned hairy grass can bend slightly to the side they touch.Morren found, in several species of Oxalis, that the leaves and petioles moved if they were lightly and repeatedly touched, or if the plant were shaken, especially Even more so after exposure to the scorching sun.I have repeated observations with several other species of Oxalis, with the same result; in some of them the movement is distinct, but is best seen in the young leaves; in several others the movement But it's extremely slight.According to Hofmeister, a high authority, it is a more important fact that the young stems and leaves of all plants, when shaken, are capable of motion; Early in growth, their petioles and tendrils are sensitive. In the young and growing organs of plants the slight movements caused by being touched or shaken seem seldom possible to them of any functional importance.But the ability of plants to move in response to various stimuli is of the utmost importance to them; tropism.The movements which occur when the nerves and muscles of animals are stimulated by an electric current, or by the absorption of strychnine, may be called accidental, because the nerves and muscles have no power for such stimuli. Very sensitive.The same is probably true of plants. Because they have the ability to move in response to certain stimuli, they will be excited in an accidental state when they are touched or shaken.It is not difficult, therefore, to admit that in the case of leaf-climbers and tendril plants it is this tendency which is exploited and increased by natural selection.However, for the reasons given in my reports, this probably occurs only in plants which have acquired the power of rotation, and which have thus become entangled plants. I have endeavored to explain how plants become entangled plants by the increased tendency of slight and irregular rotational movements, of no use to them at first; Is the accidental result of athletic ability and is acquired for other beneficial purposes.Whether natural selection has been assisted by hereditary effects employed in the gradual development of climbing plants, I do not yet know; but we know that certain periodic movements, such as the so-called sleeping movements of plants, are governed by habit. I have now dealt with enough, or too much, of the objections raised by a practiced naturalist who has carefully selected examples to demonstrate the inadequacy of natural selection in explaining the incipient stages of useful structure; , as I hope, there is no great difficulty in this matter, and this provides a good opportunity to discuss a little more about the gradual changes in structure which are often accompanied by changes in function— — This is an important question that has not been discussed in detail in previous editions of this book.Let me now briefly restate the above situation. With regard to the giraffe, among certain extinct ruminants capable of reaching high places, those individuals which have the longest neck and legs, etc., and which can bite leaves a little higher than the average height, will continue to be preserved. Therefore, those individuals who cannot feed at such heights will be constantly destroyed, and this will probably be enough for the production of this unusual quadruped.But the long use of all the parts, together with heredity, has probably greatly assisted the mutual harmony of the parts.With regard to the many insects which imitate various bodies, it may well be believed that the accidental resemblance to a common body, which in each case has been the basis for the operation of natural selection, has subsequently been brought about by the chance of slight variations which have brought this resemblance closer together. Save, so that the simulation gradually tends to perfection.This action will continue as long as the insect continues to vary, and as long as more and more perfect resemblances enable it to escape its sharp-sighted predators.In certain species of cetaceans there is a tendency for the jaws to have small irregular horny specks; and until these specks begin to change into pectin-like projections or teeth, as on the beaks of geese,— —then into short pectines, like those born on the beak of the domestic duck—then into pectines, like the beak of a plover-billed duck,—and finally into great pieces of baleen, like Green's As the blue whale puts it—the preservation of all these favorable variations seems to be entirely within the scope of natural selection.在鸭科里，这栉片最初是当牙齿用的，随后部分当牙齿用，部分当滤器用，最后，就几乎完全当滤器用了。 关于上述的角质栉片或鲸须的这等构造，据我们所能判断的来说，习性或使用对于它们的发展，很少或者没有作用。相反地，比目鱼下侧的眼睛向头的上侧转移，以及一个具有把握性的尾的形成，几乎完全可以归因于连续的使用以及伴随着的遗传作用。关于高等动物的乳房，最可能的设想是，最初有袋动物的袋内全表面的皮腺都分泌出一种营养的液体；后来这等皮腺通过自然选择，在机能上得到改进，并且集中在一定的部位，于是形成了乳房。要理解某些古代棘皮动物的作防御用的分枝棘刺，怎样通过自然选择而发展成三叉棘，比起理解甲壳动物的钳是通过最初专作行动用的肢的末端二节的微细的、有用的变异而得到发展，并没有更多的困难。在群栖虫类的鸟嘴体和震毛里，我们看到从同一根源发展成外观上大不相同的器官；并且关于震毛，我们能够理解那些连续的级进变化可能有什么用处。关于兰科植物的花粉块，可以从原本用来把花粉粒结合在一起的细丝，追踪出逐渐粘合成花粉块的柄；还有，如普通花的柱头所分泌的粘性物质，可以供作虽不十分一样的、但大致相同的目的之用，这种粘性物质附着在花粉块柄的游离末端上所经过的步骤也是可以追踪出来的；——所有这等级进变化对于各该植物都是显著有利的。至于攀缘植物，我不必重复刚才已经讲过的那些了。 经常有人问道，自然选择既然如此有力量，为什么对于某些物种显然有利的这种或那种构造，没有被它们获得呢？但是考虑到我们对于各种生物的过去历史以及对于今日决定它们的数量和分布范围的条件是无知的，要想对于这样的问题给予确切的回答，是不合理的。在许多情形里，仅能举出一般的理由，只有在少数情形里，才可以举出具体的理由。这样，要使一个物种去适应新的生活习性，许多协调的变异几乎是不可少的，并且常常可以遇到以下的情形，即那些必要的部分不按照正当的方式或正当的程度进行变异。许多物种一定由于破坏作用，而阻止了它们增加数量，这种作用和某些构造在我们看来对物种有利，因此便想像它们是通过自然选择而被获得的，但并无关系。在这种情形里，生存斗争并不依存于这等构造，所以这等构造不会通过自然选择而被获得。在许多情形里，一种构造的发展需要复杂的、长久持续的而且常常具有特殊性质的条件；而遇到这种所需要的条件的时候大概是很少的。我们所想像的、并且所往往错误想像的对于物种有利的任何一种构造，在一切环境条件下都是通过自然选择而被获得的，这种信念与我们所能理解的自然选择的活动方式是相反对的。米伐特先生并不否认自然选择有一些效力，但是他认为，我用它的作用来解说这等现象，“例证还不够充分”。他的主要论点刚才已被讨论过了，其他的论点以后还要讨论到。依我看来，这些论点似乎很少有例证的性质，其分量远不及我们的论点，我们认为自然选择是有力量的，而且常常受到其他作用的帮助。我必须补充一点，我在这里所用的事实和论点，有些已在最近出版的《医学外科评论》 （Medico Chirurgical Review）的一篇优秀的论文里，为了同样的目的而被提出过了。 今日，几乎所有的博物学者都承认有某种形式的进化。米伐特先生相信物种是通过“内在的力量或倾向”而变化的，这种内在的力量究竟是什么，实在全无所知。所有进化论者都承认物种有变化的能力；但是，依我看来，在普通变异性的倾向之外，似乎没有主张任何内在力量的必要；普通变异性通过人工选择的帮助，曾经产生了许多适应性良好的家养族；而且它通过自然选择的帮助，将会同等好地、一步一步地产生出自然的族，即物种。最后的结果，如已经说过的那样，一般是体制的进步，但在某些少数例子里是体制的退化。 米伐特先生进而相信新种“是突然出现的，而且是由突然变异而成”，还有一些博物学者附和他的这种观点。例如，他假定已经绝灭了的三趾马（Hipparion ）和马之间的差异是突然发生的。他认为，鸟类的翘膀“除了由于具有显著而重要性质的、比较突然的变异而发展起来的以外，其他方法都是难于相信的”；并且显然他把这种观点推广到蝙蝠和翼手龙（pterodactyles）的翅膀。这意味着进化系列里存在着巨大的断裂或不连续性，这结论，依我看来，是极端不可能的。 任何人如果相信进化是缓慢而逐渐的，当然也会承认物种的变化可以是突然的和巨大的，有如我们在自然状况下，或者甚至在家养状况下所看到的任何单独变异那样。但是如果物种受到饲养或栽培，它就比在自然状况下更容易变异，所以，像在家养状况下常常发生的那样巨大而突然的变异，不可能在自然状况下常常发生。家养状况下的变异，有若干可以归因于返祖遗传，这样重新出现的性状，在许多情形里，大概最初是逐渐获得的。还有更多的情形，必定叫作畸形，如六指的人、多毛的人、安康羊、尼亚太牛等等：因为它们在性状上与自然的物种大不相同，所以它们对于我们的问题所能提供的解释是很少的，除了这些突然的变异之外，少数剩下来的变异，如果在自然状况下发生，充其量只能构成与亲种类型仍有密切相联的可疑物种。 我怀疑自然的物种会像家养族那样也突然发生变化，并且我完全不相信米伐特先生所说的自然的物种以奇特的方式发生变化，理由如下。根据我们的经验，突然而显著的变异，是单独地、并且间隔较长的时间，在家养生物里发生的。如果这种变异在自然状况下发生，如前面所说的，将会由于偶然的毁灭以及后来的相互杂交而容易失去；在家养状况下，除非这类突然变异由人的照顾被隔离并被特别保存起来，我们所知道的情况也是那样的。因此，如果新种像米伐特先生所假定的那种方式而突然出现，那么，几乎有必要来相信若干奇异变化了的个体会同时出现在同一个地区内， 但这是和一切推理相违背的。就像在人类的无意识选择的场合中那样，这种难点只有根据逐渐进化的学说才可以避免；所谓逐渐进化，是通过多少朝着任何有利方向变化的大多数个体的保存，和朝相反方向变化的大多数个体的毁灭来实现的。 许多物种以极其逐渐的方式而进化，几乎是无可怀疑的。许多自然的大科里的物种甚至属，彼此是这样地密切近似，以致难以分别的不在少数。在各个大陆上，从北到南，从低地到高地等等，我们可以看到许多密切相似的或代表的物种；在不同的大陆上，我们有理由相信它们先前曾经是连续的，也可以看到同样的情形。但是，在作这些和以下的叙述时，我不得不先提一提以后还要讨论的问题，看一看环绕一个大陆的许多岛屿，那里的生物有多少只能升到可疑物种的地位。如果我们观察过去的时代，拿刚刚消逝的物种与今日还在同一个地域内生存的物种相比较；或者拿埋存在同一地质层的各亚层内的化石物种相比较，情形也是这样。显然，许许多多的物种与现今依然生存的或近代曾经生存过的其他物种的关系，是极其密切的；很难说这等物种是以突然的方式发展起来的。同时不要忘记，当我们观察近似物种的、而不是不同物种的特殊部分时，有极其微细的无数级进可以被追踪出来，这等微细的级进可以把大不相同的构造连接起来。 许多事实，只有根据物种由极微细的步骤发展起来的原理，才可以得到解释。例如，大属的物种比小属的物种在彼此关系上更密切，而且变种的数目也较多。大属的物种又像变种环绕着物种那样地集成小群；它们还有类似变种的其他方面，我在第二章里已经说明过了。根据同一个原则，我们能够理解，为什么物种的性状比属的性状更多变异；以及为什么以异常的程度或方式发展起来的部分比同一物种的其他部分更多变异。在这方面还可以举出许多类似的事实。 虽然产生许多物种所经过的步骤，几乎肯定不比产生那些分别微小变种的步骤为大；但是还可以主张，有些物种是以不同的和突然的方式发展起来的。不过要作这样承认，不可没有坚强的证据。昌西·赖特先生曾举出一些模糊的而且在若干方面有错误的类比来支持突然进化的观点，如说无机物质的突然结晶，或具有小顶的椭圆体从一小面陷落至另一小面；这些类比几乎是没有讨论的价值的。然而有一类事实，如在地层里突然出现新而不同的生物类型，最初一看，好像能支持突然发展的信念。但是这种证据的价值全然决定于与地球史的辽远时代有关的地质记录是否完全。如果那记录像许多地质学者所坚决主张的那样，是片断的话，那末，新类型好像是突然出现的说法，就不值得奇怪了。 除非我们承认转变就像米伐特先生所主张的那样巨大，如鸟类或蝙蝠的翅膀是突然发展的，或者三趾马会突然变成马，那末，突然变异的信念，对于地层里相接连锁的缺乏，不会提供任何说明。但是对于这种突然变化的信念，胚胎学却提出了强有力的反对。众所周知，鸟类和蝙蝠的翅膀，以及马和别种走兽的腿，在胚胎的早期是没有区别的，它们后来以不可觉察的微细步骤分化了。如以后还要说到的，胚胎学上一切种类的相似性可作如下的解释，即现存物种的祖先在幼小的早期以后，发生了变异，并且把新获得的性状传递给相当年龄的后代。这样，胚胎几乎是不受影响的，并且可作为那个物种的过去情况的一种纪录。因此，现存物种在发育的最初阶段里，与属于同一纲的古代的、绝灭的类型常常十分相似。按照这种胚胎相似的观点，事实上按照任何观点，都不能相信一种动物会经过上述那样巨大而突然的转变；何况在它的胚胎的状态下，一点也找不到任何突然变异的痕迹；它的构造的每一个微细之点，都是以不可觉察的微细步骤发展起来的。 如果相信某种古代生物类型通过一种内在力量或内在倾向而突然转变为，例如，有翅膀的动物，那么他就几乎要被迫来假设许多个体都同时发生变异，这是与一切类比的推论相违背的。不能否认，这等构造上的突然而巨大的变化，与大多数物种所明显进行的变化是大不相同的。进而他还要被迫来相信，与同一生物的其他一切部分美妙地相适应的、以及与周围条件美妙地相适应的许多构造都是突然产生的；并且对于这样复杂而奇异的相互适应，他就不能举出丝毫的解释来了。他还要被迫来承认，这等巨大而突然的转变在胚胎上不曾留下一点痕迹。依我看来，承认这些，就是走进了奇迹的领域，而离开科学的领域了。