物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 13. MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY: EMBRYOLOGY: RUDIMENTARY ORGANS. Page 1
CLASSIFICATION, groups subordinate to groups.Natural system.Rules and difficulties in classification, explained on the theory ofdescent with modification.Classification of varieties.Descent always used in classification.Analogical or adaptive characters.Affinities, general, complex and radiating.Extinction separates and defines groups.MORPHOLOGY, between members of the same class, between parts of thesame individual.EMBRYOLOGY, laws of, explained by variations not supervening at anearly age, and being inherited at a corresponding age.RUDIMENTARY ORGANS; their origin explained.Summary.
From the first dawn of life, all organic beings are found to resembleeach other in descending degrees, so that they can be classed ingroups under groups. This classification is evidently not arbitrarylike the grouping of the stars in constellations. The existence ofgroups would have been of simple signification, if one group had beenexclusively fitted to inhabit the land, and another the water; one tofeed on flesh, another on vegetable matter, and so on; but the case iswidely different in nature; for it is notorious how commonly membersof even the same subgroup have different habits. In our second andfourth chapters, on Variation and on Natural Selection, I haveattempted to show that it is the widely ranging, the much diffused andcommon, that is the dominant species belonging to the larger genera,which vary most. The varieties, or incipient species, thus producedultimately become converted, as I believe, into new and distinctspecies; and these, on the principle of inheritance, tend to produceother new and dominant species. Consequently the groups which are nowlarge, and which generally include many dominant species, tend to goon increasing indefinitely in size. I further attempted to show thatfrom the varying descendants of each species trying to occupy as manyand as different places as possible in the economy of nature, there isa constant tendency in their characters to diverge. This conclusionwas supported by looking at the great diversity of the forms of lifewhich, in any small area, come into the closest competition, and bylooking to certain facts in naturalisation.
I attempted also to show that there is a constant tendency in theforms which are increasing in number and diverging in character, tosupplant and exterminate the less divergent, the less improved, andpreceding forms. I request the reader to turn to the diagramillustrating the action, as formerly explained, of these severalprinciples; and he will see that the inevitable result is that themodified descendants proceeding from one progenitor become broken upinto groups subordinate to groups. In the diagram each letter on theuppermost line may represent a genus including several species; andall the genera on this line form together one class, for all havedescended from one ancient but unseen parent, and, consequently, haveinherited something in common. But the three genera on the left handhave, on this same principle, much in common, and form a sub-family,distinct from that including the next two genera on the right hand,which diverged from a common parent at the fifth stage of descent.These five genera have also much, though less, in common; and theyform a family distinct from that including the three genera stillfurther to the right hand, which diverged at a still earlier period.And all these genera, descended from (A), form an order distinct fromthe genera descended from (I). So that we here have many speciesdescended from a single progenitor grouped into genera; and the generaare included in, or subordinate to, sub-families, families, andorders, all united into one class. Thus, the grand fact in naturalhistory of the subordination of group under group, which, from itsfamiliarity, does not always sufficiently strike us, is in my judgmentfully explained.
Naturalists try to arrange the species, genera, and families in eachclass, on what is called the Natural System. But what is meant by thissystem? Some authors look at it merely as a scheme for arrangingtogether those living objects which are most alike, and for separatingthose which are most unlike; or as an artificial means forenunciating, as briefly as possible, general propositions,--that is,by one sentence to give the characters common, for instance, to allmammals, by another those common to all carnivora, by another thosecommon to the dog-genus, and then by adding a single sentence, a fulldescription is given of each kind of dog. The ingenuity and utility ofthis system are indisputable. But many naturalists think thatsomething more is meant by the Natural System; they believe that itreveals the plan of the Creator; but unless it be specified whetherorder in time or space, or what else is meant by the plan of theCreator, it seems to me that nothing is thus added to our knowledge.Such expressions as that famous one of Linnaeus, and which we oftenmeet with in a more or less concealed form, that the characters do notmake the genus, but that the genus gives the characters, seem to implythat something more is included in our classification, than mereresemblance. I believe that something more is included; and thatpropinquity of descent,--the only known cause of the similarity oforganic beings,--is the bond, hidden as it is by various degrees ofmodification, which is partially revealed to us by ourclassifications.
Let us now consider the rules followed in classification, and thedifficulties which are encountered on the view that classificationeither gives some unknown plan of creation, or is simply a scheme forenunciating general propositions and of placing together the formsmost like each other. It might have been thought (and was in ancienttimes thought) that those parts of the structure which determined thehabits of life, and the general place of each being in the economy ofnature, would be of very high importance in classification. Nothingcan be more false. No one regards the external similarity of a mouseto a shrew, of a dugong to a whale, of a whale to a fish, as of anyimportance. These resemblances, though so intimately connected withthe whole life of the being, are ranked as merely "adaptive oranalogical characters;" but to the consideration of these resemblanceswe shall have to recur. It may even be given as a general rule, thatthe less any part of the organisation is concerned with specialhabits, the more important it becomes for classification. As aninstance: Owen, in speaking of the dugong, says, "The generativeorgans being those which are most remotely related to the habits andfood of an animal, I have always regarded as affording very clearindications of its true affinities. We are least likely in themodifications of these organs to mistake a merely adaptive for anessential character." So with plants, how remarkable it is that theorgans of vegetation, on which their whole life depends, are of littlesignification, excepting in the first main divisions; whereas theorgans of reproduction, with their product the seed, are of paramountimportance!
We must not, therefore, in classifying, trust to resemblances in partsof the organisation, however important they may be for the welfare ofthe being in relation to the outer world. Perhaps from this cause ithas partly arisen, that almost all naturalists lay the greatest stresson resemblances in organs of high vital or physiological importance.No doubt this view of the classificatory importance of organs whichare important is generally, but by no means always, true. But theirimportance for classification, I believe, depends on their greaterconstancy throughout large groups of species; and this constancydepends on such organs having generally been subjected to less changein the adaptation of the species to their conditions of life. That themere physiological importance of an organ does not determine itsclassificatory value, is almost shown by the one fact, that in alliedgroups, in which the same organ, as we have every reason to suppose,has nearly the same physiological value, its classificatory value iswidely different. No naturalist can have worked at any group withoutbeing struck with this fact; and it has been most fully acknowledgedin the writings of almost every author. It will suffice to quote thehighest authority, Robert Brown, who in speaking of certain organs inthe Proteaceae, says their generic importance, "like that of all theirparts, not only in this but, as I apprehend, in every natural family,is very unequal, and in some cases seems to be entirely lost." Againin another work he says, the genera of the Connaraceae "differ inhaving one or more ovaria, in the existence or absence of albumen, inthe imbricate or valvular aestivation. Any one of these characterssingly is frequently of more than generic importance, though here evenwhen all taken together they appear insufficient to separate Cnestisfrom Connarus." To give an example amongst insects, in one greatdivision of the Hymenoptera, the antennae, as Westwood has remarked,are most constant in structure; in another division they differ much,and the differences are of quite subordinate value in classification;yet no one probably will say that the antennae in these two divisionsof the same order are of unequal physiological importance. Any numberof instances could be given of the varying importance forclassification of the same important organ within the same group ofbeings.
Again, no one will say that rudimentary or atrophied organs are ofhigh physiological or vital importance; yet, undoubtedly, organs inthis condition are often of high value in classification. No one willdispute that the rudimentary teeth in the upper jaws of youngruminants, and certain rudimentary bones of the leg, are highlyserviceable in exhibiting the close affinity between Ruminants andPachyderms. Robert Brown has strongly insisted on the fact that therudimentary florets are of the highest importance in theclassification of the Grasses.
Numerous instances could be given of characters derived from partswhich must be considered of very trifling physiological importance,but which are universally admitted as highly serviceable in thedefinition of whole groups. For instance, whether or not there is anopen passage from the nostrils to the mouth, the only character,according to Owen, which absolutely distinguishes fishes andreptiles--the inflection of the angle of the jaws in Marsupials--themanner in which the wings of insects are folded--mere colour incertain Algae--mere pubescence on parts of the flower in grasses--thenature of the dermal covering, as hair or feathers, in the Vertebrata.If the Ornithorhynchus had been covered with feathers instead of hair,this external and trifling character would, I think, have beenconsidered by naturalists as important an aid in determining thedegree of affinity of this strange creature to birds and reptiles, asan approach in structure in any one internal and important organ.
The importance, for classification, of trifling characters, mainlydepends on their being correlated with several other characters ofmore or less importance. The value indeed of an aggregate ofcharacters is very evident in natural history. Hence, as has oftenbeen remarked, a species may depart from its allies in severalcharacters, both of high physiological importance and of almostuniversal prevalence, and yet leave us in no doubt where it should beranked. Hence, also, it has been found, that a classification foundedon any single character, however important that may be, has alwaysfailed; for no part of the organisation is universally constant. Theimportance of an aggregate of characters, even when none areimportant, alone explains, I think, that saying of Linnaeus, that thecharacters do not give the genus, but the genus gives the characters;for this saying seems founded on an appreciation of many triflingpoints of resemblance, too slight to be defined. Certain plants,belonging to the Malpighiaceae, bear perfect and degraded flowers; inthe latter, as A. de Jussieu has remarked, "the greater number of thecharacters proper to the species, to the genus, to the family, to theclass, disappear, and thus laugh at our classification." But whenAspicarpa produced in France, during several years, only degradedflowers, departing so wonderfully in a number of the most importantpoints of structure from the proper type of the order, yet M. Richardsagaciously saw, as Jussieu observes, that this genus should still beretained amongst the Malpighiaceae. This case seems to me well toillustrate the spirit with which our classifications are sometimesnecessarily founded.
Practically when naturalists are at work, they do not troublethemselves about the physiological value of the characters which theyuse in defining a group, or in allocating any particular species. Ifthey find a character nearly uniform, and common to a great number offorms, and not common to others, they use it as one of high value; ifcommon to some lesser number, they use it as of subordinate value.This principle has been broadly confessed by some naturalists to bethe true one; and by none more clearly than by that excellentbotanist, Aug. St. Hilaire. If certain characters are always foundcorrelated with others, though no apparent bond of connexion can bediscovered between them, especial value is set on them. As in mostgroups of animals, important organs, such as those for propelling theblood, or for aerating it, or those for propagating the race, arefound nearly uniform, they are considered as highly serviceable inclassification; but in some groups of animals all these, the mostimportant vital organs, are found to offer characters of quitesubordinate value.
We can see why characters derived from the embryo should be of equalimportance with those derived from the adult, for our classificationsof course include all ages of each species. But it is by no meansobvious, on the ordinary view, why the structure of the embryo shouldbe more important for this purpose than that of the adult, which aloneplays its full part in the economy of nature. Yet it has been stronglyurged by those great naturalists, Milne Edwards and Agassiz, thatembryonic characters are the most important of any in theclassification of animals; and this doctrine has very generally beenadmitted as true. The same fact holds good with flowering plants, ofwhich the two main divisions have been founded on characters derivedfrom the embryo,--on the number and position of the embryonic leavesor cotyledons, and on the mode of development of the plumule andradicle. In our discussion on embryology, we shall see why suchcharacters are so valuable, on the view of classification tacitlyincluding the idea of descent.
Our classifications are often plainly influenced by chains ofaffinities. Nothing can be easier than to define a number ofcharacters common to all birds; but in the case of crustaceans, suchdefinition has hitherto been found impossible. There are crustaceansat the opposite ends of the series, which have hardly a character incommon; yet the species at both ends, from being plainly allied toothers, and these to others, and so onwards, can be recognised asunequivocally belonging to this, and to no other class of theArticulata.
Geographical distribution has often been used, though perhaps notquite logically, in classification, more especially in very largegroups of closely allied forms. Temminck insists on the utility oreven necessity of this practice in certain groups of birds; and it hasbeen followed by several entomologists and botanists.
Finally, with respect to the comparative value of the various groupsof species, such as orders, sub-orders, families, sub-families, andgenera, they seem to be, at least at present, almost arbitrary.Several of the best botanists, such as Mr. Bentham and others, havestrongly insisted on their arbitrary value. Instances could be givenamongst plants and insects, of a group of forms, first ranked bypractised naturalists as only a genus, and then raised to the rank ofa sub-family or family; and this has been done, not because furtherresearch has detected important structural differences, at firstoverlooked, but because numerous allied species, with slightlydifferent grades of difference, have been subsequently discovered.
All the foregoing rules and aids and difficulties in classificationare explained, if I do not greatly deceive myself, on the view thatthe natural system is founded on descent with modification; that thecharacters which naturalists consider as showing true affinity betweenany two or more species, are those which have been inherited from acommon parent, and, in so far, all true classification isgenealogical; that community of descent is the hidden bond whichnaturalists have been unconsciously seeking, and not some unknown planof creation, or the enunciation of general propositions, and the mereputting together and separating objects more or less alike.
But I must explain my meaning more fully. I believe that theARRANGEMENT of the groups within each class, in due subordination andrelation to the other groups, must be strictly genealogical in orderto be natural; but that the AMOUNT of difference in the severalbranches or groups, though allied in the same degree in blood to theircommon progenitor, may differ greatly, being due to the differentdegrees of modification which they have undergone; and this isexpressed by the forms being ranked under different genera, families,sections, or orders. The reader will best understand what is meant, ifhe will take the trouble of referring to the diagram in the fourthchapter. We will suppose the letters A to L to represent alliedgenera, which lived during the Silurian epoch, and these havedescended from a species which existed at an unknown anterior period.Species of three of these genera (A, F, and I) have transmittedmodified descendants to the present day, represented by the fifteengenera (a14 to z14) on the uppermost horizontal line. Now all thesemodified descendants from a single species, are represented as relatedin blood or descent to the same degree; they may metaphorically becalled cousins to the same millionth degree; yet they differ widelyand in different degrees from each other. The forms descended from A,now broken up into two or three families, constitute a distinct orderfrom those descended from I, also broken up into two families. Nor canthe existing species, descended from A, be ranked in the same genuswith the parent A; or those from I, with the parent I. But theexisting genus F14 may be supposed to have been but slightly modified;and it will then rank with the parent-genus F; just as some few stillliving organic beings belong to Silurian genera. So that the amount orvalue of the differences between organic beings all related to eachother in the same degree in blood, has come to be widely different.Nevertheless their genealogical ARRANGEMENT remains strictly true, notonly at the present time, but at each successive period of descent.All the modified descendants from A will have inherited something incommon from their common parent, as will all the descendants from I;so will it be with each subordinate branch of descendants, at eachsuccessive period. If, however, we choose to suppose that any of thedescendants of A or of I have been so much modified as to have more orless completely lost traces of their parentage, in this case, theirplaces in a natural classification will have been more or lesscompletely lost,--as sometimes seems to have occurred with existingorganisms. All the descendants of the genus F, along its whole line ofdescent, are supposed to have been but little modified, and they yetform a single genus. But this genus, though much isolated, will stilloccupy its proper intermediate position; for F originally wasintermediate in character between A and I, and the several generadescended from these two genera will have inherited to a certainextent their characters. This natural arrangement is shown, as far asis possible on paper, in the diagram, but in much too simple a manner.If a branching diagram had not been used, and only the names of thegroups had been written in a linear series, it would have been stillless possible to have given a natural arrangement; and it isnotoriously not possible to represent in a series, on a flat surface,the affinities which we discover in nature amongst the beings of thesame group. Thus, on the view which I hold, the natural system isgenealogical in its arrangement, like a pedigree; but the degrees ofmodification which the different groups have undergone, have to beexpressed by ranking them under different so-called genera,sub-families, families, sections, orders, and classes.