物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 4. NATURAL SELECTION. Page 5
The new species in our diagram descended from the original elevenspecies, will now be fifteen in number. Owing to the divergenttendency of natural selection, the extreme amount of difference incharacter between species a14 and z14 will be much greater than thatbetween the most different of the original eleven species. The newspecies, moreover, will be allied to each other in a widely differentmanner. Of the eight descendants from (A) the three marked a14, q14,p14, will be nearly related from having recently branched off froma10; b14 and f14, from having diverged at an earlier period from a5,will be in some degree distinct from the three first-named species;and lastly, o14, e14, and m14, will be nearly related one to theother, but from having diverged at the first commencement of theprocess of modification, will be widely different from the other fivespecies, and may constitute a sub-genus or even a distinct genus.
The six descendants from (I) will form two sub-genera or even genera.But as the original species (I) differed largely from (A), standingnearly at the extreme points of the original genus, the sixdescendants from (I) will, owing to inheritance, differ considerablyfrom the eight descendants from (A); the two groups, moreover, aresupposed to have gone on diverging in different directions. Theintermediate species, also (and this is a very importantconsideration), which connected the original species (A) and (I), haveall become, excepting (F), extinct, and have left no descendants.Hence the six new species descended from (I), and the eight descendedfrom (A), will have to be ranked as very distinct genera, or even asdistinct sub-families.
Thus it is, as I believe, that two or more genera are produced bydescent, with modification, from two or more species of the samegenus. And the two or more parent-species are supposed to havedescended from some one species of an earlier genus. In our diagram,this is indicated by the broken lines, beneath the capital letters,converging in sub-branches downwards towards a single point; thispoint representing a single species, the supposed single parent of ourseveral new sub-genera and genera.
In the diagram, each horizontal line has hitherto been supposed torepresent a thousand generations, but each may represent a million orhundred million generations, and likewise a section of the successivestrata of the earth's crust including extinct remains. We shall, whenwe come to our chapter on Geology, have to refer again to thissubject, and I think we shall then see that the diagram throws lighton the affinities of extinct beings, which, though generally belongingto the same orders, or families, or genera, with those now living, yetare often, in some degree, intermediate in character between existinggroups; and we can understand this fact, for the extinct species livedat very ancient epochs when the branching lines of descent haddiverged less.
I see no reason to limit the process of modification, as nowexplained, to the formation of genera alone. If, in our diagram, wesuppose the amount of change represented by each successive group ofdiverging dotted lines to be very great, the forms marked a14 to p14,those marked b14 and f14, and those marked o14 to m14, will form threevery distinct genera. We shall also have two very distinct generadescended from (I) and as these latter two genera, both from continueddivergence of character and from inheritance from a different parent,will differ widely from the three genera descended from (A), the twolittle groups of genera will form two distinct families, or evenorders, according to the amount of divergent modification supposed tobe represented in the diagram. And the two new families, or orders,will have descended from two species of the original genus; and thesetwo species are supposed to have descended from one species of a stillmore ancient and unknown genus.
We have seen that in each country it is the species of the largergenera which oftenest present varieties or incipient species. This,indeed, might have been expected; for as natural selection actsthrough one form having some advantage over other forms in thestruggle for existence, it will chiefly act on those which alreadyhave some advantage; and the largeness of any group shows that itsspecies have inherited from a common ancestor some advantage incommon. Hence, the struggle for the production of new and modifieddescendants, will mainly lie between the larger groups, which are alltrying to increase in number. One large group will slowly conqueranother large group, reduce its numbers, and thus lessen its chance offurther variation and improvement. Within the same large group, thelater and more highly perfected sub-groups, from branching out andseizing on many new places in the polity of Nature, will constantlytend to supplant and destroy the earlier and less improved sub-groups.Small and broken groups and sub-groups will finally tend to disappear.Looking to the future, we can predict that the groups of organicbeings which are now large and triumphant, and which are least brokenup, that is, which as yet have suffered least extinction, will for along period continue to increase. But which groups will ultimatelyprevail, no man can predict; for we well know that many groups,formerly most extensively developed, have now become extinct. Lookingstill more remotely to the future, we may predict that, owing to thecontinued and steady increase of the larger groups, a multitude ofsmaller groups will become utterly extinct, and leave no modifieddescendants; and consequently that of the species living at any oneperiod, extremely few will transmit descendants to a remote futurity.I shall have to return to this subject in the chapter onClassification, but I may add that on this view of extremely few ofthe more ancient species having transmitted descendants, and on theview of all the descendants of the same species making a class, we canunderstand how it is that there exist but very few classes in eachmain division of the animal and vegetable kingdoms. Although extremelyfew of the most ancient species may now have living and modifieddescendants, yet at the most remote geological period, the earth mayhave been as well peopled with many species of many genera, families,orders, and classes, as at the present day.
SUMMARY OF CHAPTER.
If during the long course of ages and under varying conditions oflife, organic beings vary at all in the several parts of theirorganisation, and I think this cannot be disputed; if there be, owingto the high geometrical powers of increase of each species, at someage, season, or year, a severe struggle for life, and this certainlycannot be disputed; then, considering the infinite complexity of therelations of all organic beings to each other and to their conditionsof existence, causing an infinite diversity in structure,constitution, and habits, to be advantageous to them, I think it wouldbe a most extraordinary fact if no variation ever had occurred usefulto each being's own welfare, in the same way as so many variationshave occurred useful to man. But if variations useful to any organicbeing do occur, assuredly individuals thus characterised will have thebest chance of being preserved in the struggle for life; and from thestrong principle of inheritance they will tend to produce offspringsimilarly characterised. This principle of preservation, I havecalled, for the sake of brevity, Natural Selection. Natural selection,on the principle of qualities being inherited at corresponding ages,can modify the egg, seed, or young, as easily as the adult. Amongstmany animals, sexual selection will give its aid to ordinaryselection, by assuring to the most vigorous and best adapted males thegreatest number of offspring. Sexual selection will also givecharacters useful to the males alone, in their struggles with othermales.
Whether natural selection has really thus acted in nature, inmodifying and adapting the various forms of life to their severalconditions and stations, must be judged of by the general tenour andbalance of evidence given in the following chapters. But we alreadysee how it entails extinction; and how largely extinction has acted inthe world's history, geology plainly declares. Natural selection,also, leads to divergence of character; for more living beings can besupported on the same area the more they diverge in structure, habits,and constitution, of which we see proof by looking at the inhabitantsof any small spot or at naturalised productions. Therefore during themodification of the descendants of any one species, and during theincessant struggle of all species to increase in numbers, the morediversified these descendants become, the better will be their chanceof succeeding in the battle of life. Thus the small differencesdistinguishing varieties of the same species, will steadily tend toincrease till they come to equal the greater differences betweenspecies of the same genus, or even of distinct genera.
We have seen that it is the common, the widely-diffused, andwidely-ranging species, belonging to the larger genera, which varymost; and these will tend to transmit to their modified offspring thatsuperiority which now makes them dominant in their own countries.Natural selection, as has just been remarked, leads to divergence ofcharacter and to much extinction of the less improved and intermediateforms of life. On these principles, I believe, the nature of theaffinities of all organic beings may be explained. It is a trulywonderful fact--the wonder of which we are apt to overlook fromfamiliarity--that all animals and all plants throughout all time andspace should be related to each other in group subordinate to group,in the manner which we everywhere behold--namely, varieties of thesame species most closely related together, species of the same genusless closely and unequally related together, forming sections andsub-genera, species of distinct genera much less closely related, andgenera related in different degrees, forming sub-families, families,orders, sub-classes, and classes. The several subordinate groups inany class cannot be ranked in a single file, but seem rather to beclustered round points, and these round other points, and so on inalmost endless cycles. On the view that each species has beenindependently created, I can see no explanation of this great fact inthe classification of all organic beings; but, to the best of myjudgment, it is explained through inheritance and the complex actionof natural selection, entailing extinction and divergence ofcharacter, as we have seen illustrated in the diagram.
The affinities of all the beings of the same class have sometimes beenrepresented by a great tree. I believe this simile largely speaks thetruth. The green and budding twigs may represent existing species; andthose produced during each former year may represent the longsuccession of extinct species. At each period of growth all thegrowing twigs have tried to branch out on all sides, and to overtopand kill the surrounding twigs and branches, in the same manner asspecies and groups of species have tried to overmaster other speciesin the great battle for life. The limbs divided into great branches,and these into lesser and lesser branches, were themselves once, whenthe tree was small, budding twigs; and this connexion of the formerand present buds by ramifying branches may well represent theclassification of all extinct and living species in groups subordinateto groups. Of the many twigs which flourished when the tree was a merebush, only two or three, now grown into great branches, yet surviveand bear all the other branches; so with the species which livedduring long-past geological periods, very few now have living andmodified descendants. From the first growth of the tree, many a limband branch has decayed and dropped off; and these lost branches ofvarious sizes may represent those whole orders, families, and generawhich have now no living representatives, and which are known to usonly from having been found in a fossil state. As we here and theresee a thin straggling branch springing from a fork low down in a tree,and which by some chance has been favoured and is still alive on itssummit, so we occasionally see an animal like the Ornithorhynchus orLepidosiren, which in some small degree connects by its affinities twolarge branches of life, and which has apparently been saved from fatalcompetition by having inhabited a protected station. As buds give riseby growth to fresh buds, and these, if vigorous, branch out andovertop on all sides many a feebler branch, so by generation I believeit has been with the great Tree of Life, which fills with its dead andbroken branches the crust of the earth, and covers the surface withits ever branching and beautiful ramifications.