Chapter 10 Chapter 6 Difficulties of Doctrine 2

The swim bladder of a fish is a good illustration, for it clearly illustrates to us a highly important fact: that an organ originally constructed for one purpose—floating—is transformed into a very different purpose—breathing— organs.In some fishes, the swim bladder also serves as a supplement to the auditory organs.All physiologists agree that the swim bladder is homologous or ideally similar in position and structure to the lungs of the higher vertebrates: there is therefore no reason to doubt that the swim bladder has in fact become a lung, that is, into a kind of lung. Organs specialized in respiration.

From this point of view it can be deduced that all vertebrates with true lungs are descended from an ancient and unknown primitive type with a floating apparatus or swim bladder.Thus, as I deduce from Owen's interesting description of these organs, we can understand why every morsel of food and drink swallowed must pass through the small hole in the trachea, although there is a wonderful device there to keep the glottis closed , but they are still in danger of falling into the lungs.The higher vertebrates have completely lost their gills—but in their embryos the slits on either side of the neck and the arcuate arteries still mark the former position of the gills.But it is conceivable that the gills, which are now entirely lost, were gradually employed by natural selection for some different purpose; for example, Landois has shown that the wings of insects are developed from the trachea; so, in this In the greater class, the organs once used for respiration may in fact have been transformed into the organs of flight.

In considering the transition of organs, it is very important to remember the possibility of one function being transformed into another, so I will give another example.The peduncle cirripedes have two very small skinfolds, which I call the frenulum, which secrete mucus to hold the eggs together until they hatch in the pouch.This kind of cirripedes has no gills, and the epidermis of the whole body, the epidermis of the egg bag, and the small frenulum of the egg are all breathing.This is not the case with the barnacle family, that is, the sessile cirripedes. It has no egg-preserving frenulum, and the eggs are loosely placed at the bottom of the bag, covered with a tightly closed shell; but at the position equivalent to the frenulum, there are huge, The extremely pleated membrane, which communicates freely with the frenulum and the circulatory orifices of the body, is thought by all naturalists to function as gills.No one, I think, will now deny that the frenulum in this family is strictly homologous to the gills in other families; in fact they were gradually transformed into each other.There can therefore be no doubt that the two small folds of skin which formerly served as the frenulum, and at the same time very slightly aided in respiration, have been converted, by natural selection, into gills only by virtue of their enlargement and the disappearance of their mucous glands. up.If all stalked cirripedes were extinct (and stalked cirripedes were far more extinct than sessile cirripedes), who would have thought that the gills in sessile cirripedes were originally used to What about an organ that keeps eggs from being flushed out of the bag?

Another possible way of transition is through an earlier or later reproductive period.This is what has recently been advocated by American science professor (Prof. Cope) and others.Some animals are now known to be capable of reproduction at a very early stage, before the full character is acquired; if this power is fully developed in a species, the adult stage of development may sooner or later be lost; Especially when juveniles are significantly different from adults, the characters of this species will be greatly changed and degenerated.There are not a few animal characters which continue almost throughout their life until maturity.In mammals, for example, the shape of the skull often changes greatly with age, and Dr. Murie has given some charming examples of this with seals; The horns also have more branches, and the feathers of certain birds develop more beautifully as they grow older.Professor Cope said that the shape of the teeth of some lizards changes greatly with age. According to Fritz Müller's records, in crustaceans, not only many small parts, but also some important parts. Some of them show new traits after maturity.In all such cases--and there are many more--if the age of reproduction is delayed, the character of the species, at least that of adulthood, will vary; It is not impossible that the early and early stages of development will end quickly and eventually disappear.As to whether species have often or ever passed through this more abrupt mode of transition, I have no mature opinion; but if it has happened, the difference between young and adult, and between adult and old, Presumably initially it was obtained step by step.

The special difficulty of the theory of natural selection Though we must be extremely careful in asserting that any organ could not have arisen from successive, small, transitional stages, the theory of natural selection undoubtedly has serious difficulties. One of the most serious difficulties is that of neutral insects, whose structure is often different from that of males and fertile females; but as to this case will be discussed in the next chapter, the electrical organs of fishes provide another which is particularly difficult. An example of explanation; for it is impossible to conceive by what steps such strange organs were produced.But that's no surprise, because we don't even know what it's for.In the electric eel (Gymnotus) and electric whale (Torpedo), there is no doubt that these organs are used as a strong means of defense, or for catching food; but in the Ray (Ray), according to mad It has been observed by Matteucci that a similar organ in the tail, even when it is greatly stimulated, generates very little electricity; probably too little to be of any use for the above-mentioned purpose.Also, in the ray fish, in addition to the organs just mentioned, as Dr. McDonnell once explained, there is another organ near the head. Although it is known that it is not charged, it is Appears to be the true homologous organ of the electric genus.It is generally admitted that there is a close resemblance between these organs and common muscles in their internal structure, distribution of nerves, and state of response to various reagents.Furthermore, muscle contraction must be accompanied by electrical discharge, so special attention should be paid to it.And as Dr. Radcliffe maintains, "the electrical organs of the electric genus appear to be charged at rest in much the same way as the muscles and nerves are charged at rest, and the discharge of the electric genus is nothing special, probably It's just another form of electrical discharge from muscles and motor nerves when they're active."We have at present no other explanation than this; but as we know so little of the usefulness of this organ, and as we know nothing of the habits and structure of the progenitors of the electric fishes now living, it cannot be argued that there is It would be too bold to complete the gradual development of these organs by useful transitional stages.

At first it seems that these organs present another and more serious difficulty; for the electric generating organs are found in about a dozen species of fish, several of which are very distantly related.If the same organ is found in several members of the same class, especially when the members have very different habits of life, we can generally attribute the presence of the organ to inheritance from a common ancestor; This organ is due to loss incurred through non-use or natural selection.So, if the electrical organs were inherited from some ancient ancestor, we might expect that all electric fish should be specially related to each other; but this is far from the case.Nor is geology utterly convincing that most fishes had formerly possessed electric organs, and that their modified descendants have only now lost them.But when we look more closely at the matter, we find that in several fishes which have electric organs, they are located in different parts of the body—they are different in structure, e.g. The difference in arrangement, and, according to Pacini, also the process or method of generating electricity,—and finally, the source of the nerves leading to the generating organs, is perhaps the most important of all differences. .In several fishes, therefore, which possess electric organs, these organs cannot be regarded as homologous, but only as functionally equivalent.Consequently, there is no reason to suppose that they were descended from a common ancestor; for if they had a common ancestor, they should be closely alike in every respect.Thus disappearing the difficulty concerning apparently identical organs which have in fact been developed in several distantly allied species, there remains only a lesser, but still great, difficulty, namely, that in different groups In fishes, by what hierarchical steps does this organ develop.

The luminous organs, situated in different parts of the body, as seen in several insects belonging to quite different families, present us, in our state of want of knowledge, with a difficulty almost equal to that of the electric generating organs.There are other similar cases; for example, in plants, the pollen mass is produced on the stalk with the mucous glands, a very curious device, in the genus Anthropophyllum . . . As in the case of the same invention, in the cases just mentioned, natural selection works for the benefit of each organism, and utilizes all beneficial variations, so that in different organisms there arise organs which are functionally the same. , the common structure of these organs cannot be attributed to inheritance from a common ancestor.

Fritz Müller was careful to make almost the same argument in order to verify the conclusions reached in this book.Of the few species in several families of crustaceans, possessing air-breathing organs, adapted for life out of water, two of these families have been studied in particular detail by Miller, who are closely related, and whose species have everything. The important characters are all closely consistent: for example, their sensory organs, circulatory system, the location of the tufts in the complex stomach, and the structure of the gills for water breathing, and even the tiny hooks for cleaning the gills, are all closely consistent.It was to be expected, therefore, that in the few terrestrial species belonging to these two families the equally important organs of breathing the air should be the same; How can such an organ be made differently?

Miller maintains, in my opinion, that such close resemblance in many respects of structure must be explained by inheritance from a common ancestor; are aquatic, so it is of course highly unlikely that their common ancestor was ever adapted to breathe air.Miller, therefore, examined this organ carefully in the air-breathing species; and he found that in each species this organ differs in several important points, as in the position of the pore, the method of opening and closing, and several other appurtenances. There are differences.That difference is intelligible, and perhaps even to be expected, provided that species belonging to different families slowly become more and more adapted to living out of water and breathing air.For these species, by belonging to different families, would differ in some degree, and on the principle that the nature of variation depends on two factors, namely, the nature of the organism and the nature of the environment, their variability must not exactly the same.Consequently, natural selection has had to work on different materials, or variations, in order to achieve functionally identical results; and the structures thus obtained will almost necessarily be different.On the hypothesis of separate creation the whole situation becomes incomprehensible.This line of discussion seems to carry a great deal of weight in making Miller accept the views I advocate in this book.

Another eminent zoologist, the late Prof. Claparede, made the same argument, and arrived at the same result.He elucidated that the parasitic mites (Acaridae) belonging to different subfamilies and families all had hairy hooks.These organs must have been developed separately, as they could not be descended from a common ancestor; and in some groups they are by variations of the forelegs, -- of the hind legs, -- of the jaw or lip, -- And the variation of the appendages below the back of the body, formed. From what has been said, we see in organic beings not all related, or only distantly allied, the same results achieved, and the same functions performed, by organs closely similar in appearance, though different in development.On the other hand, it is a common law throughout nature that the same result can be attained in a great variety of ways, sometimes even in closely allied beings.How different are the feathered wings of birds from the membranous wings of bats; still more so are the four wings of a butterfly, the two wings of a fly, and the two elytra of a beetle.The shell of the bivalve (Bivalvc) is constructed to open and close, but how many styles of two shells hinge from the long rows of intertwined teeth of the Nucula to the simple ligaments of the mussel (Mussel) !Seeds are dispersed by virtue of their small size,--by their capsules becoming light balloon-like membranes,--by burying them in various parts, containing nutrients, and having Within the brightly colored flesh, scattered to attract birds to eat them,--some with many kinds of hooks and anchors, and serrated awns, to cling to the furs of beasts--some with Wings and feathers of various shapes and structures are delicate and can be blown up and scattered by the slightest breeze.Let me give another example; for the problem of obtaining the same result by an enormous variety of methods is extremely remarkable.Certain authors maintain that living beings are formed, almost like toys in a store, for the sake of variety, by many means, but this view of nature is not credible.Dioecious plants, and plants that are monoecious but whose pollen does not naturally shed on the stigmas, require some assistance to accomplish fertilization.Several kinds of fertilization are accomplished in this way: the pollen-grains are light and loose, blown about by the wind, and are scattered on the stigma by chance alone; this is the simplest method imaginable.An almost equally simple but quite different method is found in many plants, where symmetrical flowers secrete a few drops of nectar, and thus attract the visits of insects; the insects carry pollen from the pistils to the stigma.

From this simple stage we can see in sequence a myriad of devices, all serving the same purpose and functioning in essentially the same way, but which cause changes in the various parts of the flower.Nectar can be stored in receptacles of various shapes, and their stamens and pistils can change in many styles, sometimes forming trap-like devices, and sometimes performing clever adaptive movements due to stimulation or elasticity.From such a structure one may go down to such an extraordinary adaptation as that of the Cory-anthes recently described by Dr. Cruger.The lip, that is, part of the lower lip of this orchid is sunken inwards to form a large bucket, on which there are two horns that secrete almost pure drops of water, which constantly fall into the bucket; when the bucket is half When full, water overflows from the outlet on one side.The base of the lip, which fits over the bucket, is also sunken into a kind of cavity, with openings on either side; within this cavity there are singular fleshy ribs.Even the wisest man can never imagine the use of these parts if he has not seen what happens there.But Dr. Kruger saw swarms of large scale bees visiting the gigantic flowers of this orchid, not to drink the nectar, but to eat the fleshy ribs in the cavities above the buckets; Sometimes, they often bump into each other, so that they fall into the bucket, and their wings are soaked in water, so they can't fly, and they are forced to crawl out through the channel formed by the spout or overflow.Dr. Kruger has seen "linked ranks" of scumble-bees crawl out in this way after an involuntary bath.The passage is narrow and covered with the gynoecastilla, so that when the bee struggles to get out, it rubs its back first against the sticky stigma, and then against the mucilage of the pollen mass.Thus, when the scumble-bee crawls through the passage of a newly opened flower, it sticks the pollen mass to its back, and carries it away.Dr. Kruger sent me a flower soaked in alcohol and a bee which had been killed before it could quite get out, with the pollen clumps still stuck to its back.In this way, the pollen-carrying bee flies to another flower, or comes to the same flower for the second time, and is squeezed into the bucket by its companions, and then crawls out by that way. At this time, the pollen mass must first comes into contact with the gluey stigma, and sticks to it, and the flower is fertilized.We have now seen the full use of the parts of the flower, the use of the water-secreting horn, the use of the bucket half-filled—it is to prevent the bees from flying away, to force them to crawl out through the opening, and to make them rub against the Gluey pollen mass and gluey stigma in place. There is also a closely allied orchid, called Catasetum, whose flowers, although serving the same purpose, are quite different in structure, and the structure of the flowers is equally wonderful.The bees visit its flowers, as they do those of the scutellaria, for the purpose of gnawing on the lip, and when they do so they cannot help touching a long, pointed, sentient projection, I call these protrusions tentacles.As soon as the antenna is touched, it conveys a feeling that it vibrates to a membrane, which immediately splits; and an elastic force is released from this, so that the pollen mass is shot out like an arrow, in the direction just to make the glue stick. One end of the bee is glued to the back of the bee.This orchid is dioecious, and the pollen mass of the male plant is thus carried to the flower of the female plant, where it hits the stigma, which is sticky enough to break the elastic filaments and release the pollen. Stay, and fertilization will take place. It may be asked how, in the above-mentioned and countless other examples, we can understand this complex gradual step-by-step and the various methods used to achieve the same end?The answer, as has already been said, is undoubtedly, that two forms which already differ in some degree from each other, when they vary, will not be of exactly the same nature in their variability, so that, for the same general purpose, natural selection The results obtained will not be the same, and we should also remember that every highly developed organism has undergone many modifications, and each modified structure has a tendency to be inherited, so that each variation will not be the same. Lose easily, but will change further again and again.The constitution of every part of every species, whatever purpose it may serve, is therefore the composite of many hereditary variations, acquired by the species' successive adaptations from altered habits and conditions of life. Finally, although in many cases it is extremely difficult even to guess by what transitional forms the organs have attained their present state, yet considering the surviving and known forms as compared with the extinct and unknown forms, The number of the former strikes me at such a small number, but it is difficult to name an organ which does not form through a transitional stage.It is certainly true that new organs, as if created for a special purpose, seldom or never occur in any being;—as in the old but somewhat exaggerated adage in natural history, "Nature has no Leap" as pointed out.The maxim is admitted in the writings of nearly every experienced naturalist; or, as Milne Edwards once well put it, "Nature" is luxurious in change, but stingy in innovation.If we go by creationism, then why are there so many variations and so few truly novel things?Why are all the parts and organs of many independent beings, created separately to fit their respective positions in nature, so generally connected together by gradually graded steps?Why doesn't "nature" take a sudden leap from one configuration to another?On the theory of natural selection, we can clearly understand why "nature" should not be so; for natural selection operates only by means of slight, continuous variations; she can never take great and sudden leaps, but must take Short, sure, though slow steps forward. apparently unimportant organs subjected to natural selection As natural selection operates by life and death—letting the fittest survive and the less fit perish—I have sometimes felt a great deal of difficulty in understanding the origin or formation of parts that are not very important. The difficulty is almost as great as that of understanding the most perfect and complex organs, though it is a very different kind of difficulty. First, Our knowledge of the whole organization of any one organic being is too scarce to tell what slight variations are important or unimportant.In a previous chapter I have given examples of minute characters, such as the hairiness of fruits, the color of the flesh, and the hide and coat of quadrupeds, as related to differences in constitution, or to determining whether insects are To attack related, can indeed be affected by natural selection.The giraffe's tail, like an artificial fly-whisk; that its fitness for present use has been through successive, slight modifications, each better adapted to such a trivial task as driving away flies, seems at first unbelievable. ; yet even in this case some consideration should be exercised before certainty, for we know that in South America the distribution and survival of cattle and other animals is entirely determined by resistance to insect attack; As long as individuals who can avoid such small enemies can spread to new pastures, they will gain a huge advantage.It is not that these large quadrupeds are actually destroyed by flies (with some rare exceptions), but that they are continually disturbed, and their strength is diminished, and, consequently, more prone to disease, or unable to survive when famine sets in. Then effectively find food, or escape the attack of carnivores. Organs which are now of little importance, were in some cases probably of high importance to early progenitors, and which, after being slowly perfected at a former period, are now of little use, are still nearly the same. will be transmitted to the existing species; but any actually injurious deviations in their structure will, of course, be checked by natural selection.Seeing what an important organ of locomotor the tail is in most aquatic animals, it may be thus explained its general presence and its common presence and in most terrestrial animals (whose aquatic origin is indicated by lungs or modified swim-bladders). multiple uses.A well-developed tail, as formed in an aquatic animal, may thereafter be used for various purposes--as a fly-whisk, as an organ of grip, or as an aid in turning like a dog's tail, though The tail is of little use in aiding turns, since the bare hare, which has almost no tail, can turn more quickly. Second, it is easy to mistake the importance of certain traits and to believe that they have developed by natural selection.We must never overlook: the effect of a definite action of changed conditions of life--of so-called spontaneous variation, which seems to have little relation to external conditions--of the tendency to reproduce long-lost characters. Effects,—effects produced by complex laws of growth, such as correlation, compensation, oppression of one part by another, etc.—and lastly those produced by sexual selection, by which a certain sexual preference is often obtained. useful characters, and can transmit them more or less completely to the other sex, though they are of no use to the other sex.But such indirectly acquired structures, though at first of no advantage to a species, will thereafter be utilized by its modified descendants under new conditions of life and newly acquired habits. If only the green woodpecker survived, and if we did not know that there were many species of black and variegated woodpeckers, I dare say we would have thought that green was a wonderful adaptation that made this frequent tree-to-tree bird to conceal itself from predators; it would turn out to be an important character, and to have been acquired by natural selection; when in fact the color was probably acquired chiefly by sexual selection.There is a trailing palm in the Malay Archipelago, which climbs the tallest trees by means of finely constructed hooks which cluster at the tips of its branches, a device which is undoubtedly of great use to this plant; Very similar hooks are also seen on climbing trees, and from the distribution of thorny species in Africa and South America, there is reason to believe that these hooks were originally used to defend against herbivores, so the thorns of vine palms may have been originally Developed for this purpose, the thorns were later improved and utilized when the plant was further modified and became a climbing plant.The bare hide on the vulture's head is commonly held to be a direct adaptation to indulging in putrefaction; perhaps it is, or perhaps it may be due to the direct action of putrefaction; but when we see When the male turkey's scalp is thus exposed, we must be very cautious in drawing any such inferences.The seams in the skulls of young mammals have been thought to be a wonderful adaptation to aid in birth, no doubt to make it easy, and perhaps necessary for it; They crawl out of their shells, and their skulls also have slits, so we can infer that this structure is due to the law of growth, but that higher animals use it for production. We are profoundly ignorant of the cause of every slight variation or individual difference; we have only to think of the differences among the breeds of domesticated animals in various places,--especially in less civilized countries, where very little systematic selection has been exercised. , - you will immediately realize this.The animals bred by savages everywhere also often had to struggle for their own existence, and they were exposed to some extent to the action of natural selection, while individuals with slightly different constitutions were best obtained in different climates. success.The susceptibility of cattle to the attack of flies, as to the poison of certain plants, is related to their colour; so even color is thus subject to the action of natural selection.Some observers believe that humid climates affect the growth of hairs, and that horns are associated with hairs.Alpine breeds often differ from lowland breeds; hilly places probably have an influence on the hind legs, as they use them more there, and the shape of the pelvis may even be affected thereby; so, by the law of homologous variation, Presumably the forelimbs and head are also affected.Also, the shape of the pelvic plate may affect the shape of certain parts of the calf in the womb due to pressure.The labor of breathing which is necessary in the high places, we have good reason to believe, tends to enlarge the breast; and here again the relevant effect is in effect.The effect of less exercise and more food on the whole system is probably more important; and H. von Nathusius has recently shown in his excellent paper that this is apparently one of the principal causes of the great variation in the breeds of pigs. reason.But we are too ignorant to give any thought to the relative importance of the several known and unknown causes of variation; but if we cannot explain the cause of their differences of character, we need not take too seriously the slight similar differences between true species, as yet unknown to the real cause. How much truth is the theory of utilitarianism: how is beauty obtained The arguments in the preceding section lead me to say a little more about this objection which some naturalists have lately objected to the doctrine of utility, which asserts that every minute point of construction arises for the benefit of its owner.Their belief that many structures were created for beauty, to please man or a "creator" (but "creator" is outside the scope of scientific discussion), or simply for variety has been discussed.If these theories were correct, my theory would have no room at all.I fully admit that there are many formations which are now of no immediate use to their proprietors, and perhaps never were of any use to their ancestors; but this does not prove that they were formed solely for beauty or fancy.There can be no doubt that the definite action of changed external conditions, and the various causes of variation which have been enumerated before, produce effects, perhaps great ones, whether or not any advantage is derived therefrom.But a still more important reason is that a major part of the constitution of each living being is inherited; and consequently, although each being is certainly suited to its place in nature, there are many constitutions and present habits of life. There is no very close and direct relationship.We find it difficult, therefore, to believe that the webbed feet of the upland geese and frigate-birds are of any special use to them; What special use do bones have for these animals.We may very safely attribute these structures to heredity.But webbed feet were no doubt as useful to the ancestors of the upland geese and frigate birds, as they are to most extant water-birds.We may therefore believe that the ancestors of the seal were not born with flippers, but with five-toed feet, adapted for walking or grasping; The bones, based on the principle of utilitarianism, probably developed from the reduction of most bones in the fins of some ancient fish-shaped ancestors of this whole class, but the reasons for the following changes, such as certain effects of external conditions, the so-called spontaneous It is almost impossible to decide how much weight should be given to variation, and to the complex laws of growth, etc.; but with these important exceptions we may assert that the constitution of every living being is or was always to its owner be of some direct or indirect use. Concerning the belief that living beings were made beautiful in order to please man—a belief which has been declared to overthrow all my theories—I may begin by pointing out that the sense of beauty is clearly determined by psychological Any real quality of the object of appreciation is irrelevant, and aesthetic notions are not innate or inalterable.For example, we see that men of different races have completely different aesthetic standards for women.If beautiful things were created solely for the enjoyment of man, it should be pointed out that before the appearance of human beings, the beauty on the earth should not be as beautiful as after they appeared on the stage.The beautiful spiral and conical shells of the Eoceneepoch, and the finely carved nautilus fossils of the Secondary Period, were created so that man could admire them in the chamber many ages later. did it come out?Few things are more beautiful than the tiny silica shells of diatoms; were they created to be viewed and admired under a powerful microscope?The beauty of diatoms, as well as many other things, is apparently due entirely to the symmetry of growth.Flowers are nature's most beautiful creations: they stand out against green leaves and at the same time make them beautiful, so they can be easily seen by insects.I have come to this conclusion from seeing the invariable rule, viz., that the anemophilus never has a showy corolla.Several plants habitually have two flowers, one open and colored, so as to attract insects, and the other closed, uncolored, devoid of nectar, and never visited by insects.We may therefore affirm that, if there had not been the development of insects on the surface of the earth, our plants would not have been adorned with beautiful flowers, but only unbeautiful flowers, as we have in the chrysanthemum, twig, walnut, As seen in ash, thatch, spinach, sorrel, nettles, all fertilized with the aid of the wind.The same argument is quite applicable to fruit; ripe strawberries or cherries are both pleasant to the eye and palatable,--the richly colored fruit of the Spindlewood tree and the scarlet berries of the holly are both beautiful. stuff,—that's what anyone admits.But this beauty serves only to attract birds and beasts, and to scatter the seeds which, when the fruit is eaten, are scattered with dung; and I deduce this to be true, because no exception to the following rule has ever been found. ：即，埋藏在任何种类的果实里（即生在肉质的或柔软的瓤囊里）的种籽，如果果实有任何鲜明的颜色或者由于黑色或白色而惹起注目，总是这样散布的。 另一方面，我愿意承认大多数的雄性动物，如一切最美丽的鸟类，某些鱼类、爬行类和哺乳类，以及许多华丽彩色的蝴蝶，都是为着美而变得美的；但这是通过性选择所获得的成果，就是说，由于比较美的雄体曾经继续被雌体所选中，而不是为了取悦于人。鸟类的鸣声也是这样。我们可以从一切这等情形来推论：动物界的大部分在爱好美丽的颜色和音乐的音响方面，都有相似嗜好。当雌体具有像雄体那样的美丽颜色时，——这种情形在鸟类和蝴蝶里并不罕见，其原因显然在于通过性选择所获得的颜色，不只遗传于雄体，而且遗传于两性。最简单形态的美的感觉，——即是从某种颜色、形态和声音所得到一种独特的快乐，——在人类和低于人类的动物的心理里是怎样发展起来的呢，这实在是一个很难解的问题。如果我们追究为什么某种香和味可以给与快感，而别的却给与不快感，这时我们就会遇到同样的困难。在一切这等情形里，习性似乎有某种程度的作用；但是在每个物种的神经系统的构造里，一定还存在着某种基本的原因。 自然选择不可能使一个物种产生出全然对另一个物种有利的任何变异；虽然在整个自然界中，一个物种经常利用其他物种的构造而得到利益。但是自然选择能够而且的确常常产生出直接对别种动物有害的构造，如我们所看到的蝮蛇的毒牙，姬蜂的产卵管——依靠它就能够把卵产在别种活昆虫的身体里。假如能够证明任何一个物种的构造的任何一部分全然为了另一物种的利益而形成，那就要推翻我的学说了，因为这些构造是不能通过自然选择而产生的。虽然在博物学的著作里有许多关于这种成果的叙述，但我不能找到一个这样的叙述是有意义的。人们认为响尾蛇的毒牙系用以自卫和杀害猎物；但某些作者假定它同时具有于自己不利的响器，这种响器会预先发出警告，使猎物警戒起来。这样，我差不多也可相信猫准备纵跳时卷动尾端是为了使命运已经被决定的鼠警戒起来。但更可信的观点是，响尾蛇用它的响器，眼镜蛇膨胀它的颈部皱皮，蝮蛇在发出很响而粗糙的嘶声时把身体胀大，都是为了恐吓许多甚至对于最毒的蛇也会进行攻击的鸟和兽。蛇的后这种行为和母鸡看见狗走近她的小鸡时便把羽毛竖起、两翼张开的原理是一样的。动物设法把它们的敌害吓走，有许多方法，但这里限于篇幅，无法详述。 自然选择从来不使一种生物产生对于自己害多利少的任何构造，因为自然选择完全根据各种生物的利益并且为了它们的利益而起作用。正如帕利（Paley）曾经说过的，没有一种器官的形成是为了给予它的所有者以苦痛或损害。如果公平地衡量由各个部分所引起的利和害，那末可以看到，从整体来说，各个部分都是有利的。经过时间的推移，生活条件的改变，如果任何部分变为有害的，那么它就要改变；倘不如此，则这种生物就要绝灭，如无数的生物已经绝灭了的一样。 自然选择只是倾向于使每一种生物与栖息于同一地方的、和它竞争的别种生物一样地完善，或者使它稍微更加完善一些。我们可以看到，这就是在自然状况下所得到的完善化的标准。例如，新西兰的土著生物彼此相比较都是同样完善的；但是在从欧洲引进的植物和动物的前进队伍面前，它们迅速地屈服了。自然选择不会产生绝对的完善，并且就我们所能判断的来说，我们也不曾在自然界里遇见过这样高的标准。米勒曾经说过，光线偏差的校正，甚至在最完善的器官如人类的眼睛里，也不是完全的。没有人怀疑过赫姆霍尔兹（Helmholtz）的判断，他强调地描述了人类的眼睛具有奇异的能力之后，又说了以下值得注意的话：“我们发现在这种光学器具里和视网膜上的影像里有不正确和不完善的情形，这种情形不能与我们刚刚遇到的感觉领域内的各种不调和相比较。人们可以说，自然界为了要否定外界和内界之间预存有协调的理论的所有基础，是喜欢积累矛盾的。”如果我们的理性引导我们热烈地赞美自然界里有无数不能模仿的装置，那么这一理性又告诉我们说（纵然我们在两方面都容易犯错误），某些其他装置是比较不完善的。我们能够认为蜜蜂的刺针是完善的吗？当它用刺针刺多种敌害的时候，不能把它拔出来，因为它有倒生的小锯齿，这样，自己的内脏就被拉出，不可避免地要引起死亡。 如果我们把蜜蜂的刺针看作在遥远的祖先里已经存在，原是穿孔用的锯齿状的器具，就像这个大目里的许多成员的情形那样，后来为了现在的目的它被改变了，但没有改变得完全，它的毒素原本是适于别种用处的，例如产生树瘿，后来才变得强烈，这样，我们大概能够理解为什么蜜蜂一用它的刺针就会如此经常地引起自己的死亡：因为，如果从整体来看，刺针的能力对于社会生活有用处，虽然可以引起少数成员的死亡，却可以满足自然选择的一切要求的。如果我们赞叹许多昆虫中的雄虫依靠嗅觉的真正奇异能力去寻找它们的雌虫，那么，只为了生殖目的而产生的成千的雄蜂，对于群没有一点其他用处，终于被那些劳动而不育的姊妹弄死，我们对此也赞叹吗？也许是难以赞叹的，但是我们应当赞叹后蜂的野蛮的本能的恨，这种恨鼓动它在幼小的后蜂——它的女儿刚产生出来的时候，就把它们弄死，或者自己在这场战斗中死亡；因为没有疑问，这对于群是有好处的；母爱或母恨（幸而后者很少），对于自然选择的坚定原则都是一样的。如果我们赞叹兰科植物和许多其他植物的几种巧妙装置，它们据此通过昆虫的助力来受精，那么枞树产生出来的密云一般的花粉，其中只有少数几粒能够碰巧吹到胚珠上去，我们能够认为它们是同等完善的吗？ 提要：自然选择学说所包括的模式统一法则和生存条件法则 我们在这一章里，已经把可以用来反对这一学说的一些难点和异议讨论过了，其中有许多是严重的；但是，我想在这个讨论里，对于一些事实已经提出了若干说明，如果依照特创论的信条，这些事实是完全弄不清的。我们已经看到，物种在任何一个时期的变异都不是无限的，也没有由无数的中间诸级联系起来，一部分原因是自然选择的过程永远是极其缓慢的，在任何一个时期只对少数类型发生作用；一部分原因是自然选择这一过程本身就包含着先驱的中间诸级不断地受到排斥和绝灭。现今生存于连续地域上的亲缘密切的物种，一定往往在这个地域还没有连续起来并且生活条件还没有从这一处不知不觉地逐渐变化到另一处的时候，就已经形成了。当两个变种在连续地域的两处形成的时候，常有适于中间地带的一个中间变种形成；但依照上述的理由，中间变种的个体数量通常要比它所连接的两个变种为少；结果，这两个变种，在进一步变异的过程中，由于个体数量较多，便比个体数量较少的中间变种占有强大的优势，因此，一般就会成功地把中间变种排斥掉和消灭掉。 我们在本章里已经看到，要断言极其不同的生活习性不能逐渐彼此转化；譬如断言蝙蝠不能通过自然选择从一种最初只在空中滑翔的动物而形成，我们应该怎样地慎重。 我们已经看到，一个物种在新的生活条件下可以改变它的习性；或者它可以有多样的习性，其中有些和它的最近同类的习性很不相同。因此，只要记住各生物都在试图生活于任何可以生活的地方，我们就能理解脚上有蹼的高地鹅、栖居地上的啄木鸟、潜水的鸫和具有海鸟习性的海燕是怎样发生的了。 像眼睛那样完善的器官，要说能够由自然选择而形成，这足以使任何人踌躇；但是不论何种器官，只要我们知道其一系列逐渐的、复杂的过渡诸级，各各对于所有者都有益处，那末，在改变着的生活条件下，通过自然选择而达到任何可以想像的完善程度，在逻辑上并不是不可能的。在我们还不知道有中间状态或过渡状态的情形里，要断言不能有这些状态曾经存在过，必须极端慎重。因为许多器官的变态阐明了，机能上的奇异变化至少是可能的。例如，鳔显然已经转变成呼吸空气的肺了。同时进行多种不同机能的、然后一部分或全部变为专营一种机能的同一器官；同时进行同种机能的、一种器官受到另一种器官的帮助而完善化的两种不同器官，一定常常会大大地促进它们的过渡。 我们已经看到，在自然系统中彼此相距很远的两种生物里，供同样用途的并且外表很相像的器官，可以各自独立形成；但是对这等器官仔细加以检查，差不多常常可以发现它们的构造在本质上有所不同；依照自然选择的原理，结果当然是这样。另一方面，为了达到同一目的的构造的无限多样性，是整个自然界的普遍规律；这也是依照同一伟大原理的当然结果。 在许多情形里，我们实在太无知无识了，以致主张：因为一个部分或器官对于物种的利益极其不重要，所以它的构造上的变异，不能由自然选择而徐徐累积起来。在许多别的情形里，变异大概是变异法则或生长法则的直接结果，与由此获得的任何利益无关。但是，甚至这等构造，后来在新的生活条件下为了物种的利益，也常常被利用，并且还要进一步地变异下去，我们觉得这是可以确信的。我们还可以相信，从前曾经是高度重要的部分，虽然它已变得这样不重要，以致在它的目前状态下，它已不能由自然选择而获得，但往往还会保留着（如水栖动物的尾巴仍然保留在它的陆栖后代里）。 自然选择不能在一个物种里产生出完全为着另一个物种的利益或为着损害另一物种的任何东西；虽然它能够有效地产生出对于另一物种极其有用的或者甚至不可缺少的，或者对于另一物种极其有害的部分、器官和分泌物，但是在一切情形里，同时也是对于它们的所有者有用的。在生物繁生的各个地方，自然选择通过生物的竞争而发生作用，结果，只是依照这个地方的标准，在生活战斗中产生出成功者。因此，一个地方——通常是较小地方——的生物，常常屈服于另一个地方——通常是较大地方——的生物。因为在大的地方里，有比较多的个体和比较多样为类型存在，所以竞争比较剧烈，这样，完善化的标准也就比较高。自然选择不一定能导致绝对的完善化；依照我们的有限才能来判断，绝对的完善化，也不是随处可以断定的。 依据自然选择的学说，我们就能明白地理解博物学里“自然界里没有飞跃”这个古代格言的充分意义。如果我们只看到世界上的现存生物，这句格言并不是严格正确的；但如果我们把过去的一切生物都包括在内，无论已知或未知的生物，这句格言按照这个学说一定是严格正确的了。 一般承认一切生物都是依照两大法则——“模式统一”和“生存条件”——形成的。模式统一是指同纲生物的、与生活习性十分无关的构造上的基本一致而言。依照我的学说，模式的统一可以用祖先的统一来解释。曾被著名的居维尔所经常坚持的生存条件的说法，完全可以包括在自然选择的原理之内。因为自然选择的作用在于使各生物的变异部分现今适用于有机的和无机的生存条件，或者在于使它们在过去的时代里如此去适应；在许多情形里，适应受到器官的增多使用或不使用的帮助，受到外界生活条件的直接作用的影响，并且在一切场合里受到生长和变异的若干法则所支配。因此，事实上“生存条件法则”乃是比较高级的法则；因为通过以前的变异和适应的遗传，它把“模式统一法则”包括在内了。