物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 2. VARIATION UNDER NATURE. Page 1
Variability.Individual differences.Doubtful species.Wide ranging, much diffused, and common species vary most.Species of the larger genera in any country vary more than the speciesof the smaller genera.Many of the species of the larger genera resemble varieties in beingvery closely, but unequally, related to each other, and in havingrestricted ranges.
Before applying the principles arrived at in the last chapter toorganic beings in a state of nature, we must briefly discuss whetherthese latter are subject to any variation. To treat this subject atall properly, a long catalogue of dry facts should be given; but theseI shall reserve for my future work. Nor shall I here discuss thevarious definitions which have been given of the term species. No onedefinition has as yet satisfied all naturalists; yet every naturalistknows vaguely what he means when he speaks of a species. Generally theterm includes the unknown element of a distinct act of creation. Theterm "variety" is almost equally difficult to define; but herecommunity of descent is almost universally implied, though it canrarely be proved. We have also what are called monstrosities; but theygraduate into varieties. By a monstrosity I presume is meant someconsiderable deviation of structure in one part, either injurious toor not useful to the species, and not generally propagated. Someauthors use the term "variation" in a technical sense, as implying amodification directly due to the physical conditions of life; and"variations" in this sense are supposed not to be inherited: but whocan say that the dwarfed condition of shells in the brackish waters ofthe Baltic, or dwarfed plants on Alpine summits, or the thicker fur ofan animal from far northwards, would not in some cases be inheritedfor at least some few generations? and in this case I presume that theform would be called a variety.
Again, we have many slight differences which may be called individualdifferences, such as are known frequently to appear in the offspringfrom the same parents, or which may be presumed to have thus arisen,from being frequently observed in the individuals of the same speciesinhabiting the same confined locality. No one supposes that all theindividuals of the same species are cast in the very same mould. Theseindividual differences are highly important for us, as they affordmaterials for natural selection to accumulate, in the same manner asman can accumulate in any given direction individual differences inhis domesticated productions. These individual differences generallyaffect what naturalists consider unimportant parts; but I could showby a long catalogue of facts, that parts which must be calledimportant, whether viewed under a physiological or classificatorypoint of view, sometimes vary in the individuals of the same species.I am convinced that the most experienced naturalist would be surprisedat the number of the cases of variability, even in important parts ofstructure, which he could collect on good authority, as I havecollected, during a course of years. It should be remembered thatsystematists are far from pleased at finding variability in importantcharacters, and that there are not many men who will laboriouslyexamine internal and important organs, and compare them in manyspecimens of the same species. I should never have expected that thebranching of the main nerves close to the great central ganglion of aninsect would have been variable in the same species; I should haveexpected that changes of this nature could have been effected only byslow degrees: yet quite recently Mr. Lubbock has shown a degree ofvariability in these main nerves in Coccus, which may almost becompared to the irregular branching of the stem of a tree. Thisphilosophical naturalist, I may add, has also quite recently shownthat the muscles in the larvae of certain insects are very far fromuniform. Authors sometimes argue in a circle when they state thatimportant organs never vary; for these same authors practically rankthat character as important (as some few naturalists have honestlyconfessed) which does not vary; and, under this point of view, noinstance of an important part varying will ever be found: but underany other point of view many instances assuredly can be given.
There is one point connected with individual differences, which seemsto me extremely perplexing: I refer to those genera which havesometimes been called "protean" or "polymorphic," in which the speciespresent an inordinate amount of variation; and hardly two naturalistscan agree which forms to rank as species and which as varieties. Wemay instance Rubus, Rosa, and Hieracium amongst plants, several generaof insects, and several genera of Brachiopod shells. In mostpolymorphic genera some of the species have fixed and definitecharacters. Genera which are polymorphic in one country seem to be,with some few exceptions, polymorphic in other countries, andlikewise, judging from Brachiopod shells, at former periods of time.These facts seem to be very perplexing, for they seem to show thatthis kind of variability is independent of the conditions of life. Iam inclined to suspect that we see in these polymorphic generavariations in points of structure which are of no service ordisservice to the species, and which consequently have not been seizedon and rendered definite by natural selection, as hereafter will beexplained.
Those forms which possess in some considerable degree the character ofspecies, but which are so closely similar to some other forms, or areso closely linked to them by intermediate gradations, that naturalistsdo not like to rank them as distinct species, are in several respectsthe most important for us. We have every reason to believe that manyof these doubtful and closely-allied forms have permanently retainedtheir characters in their own country for a long time; for as long, asfar as we know, as have good and true species. Practically, when anaturalist can unite two forms together by others having intermediatecharacters, he treats the one as a variety of the other, ranking themost common, but sometimes the one first described, as the species,and the other as the variety. But cases of great difficulty, which Iwill not here enumerate, sometimes occur in deciding whether or not torank one form as a variety of another, even when they are closelyconnected by intermediate links; nor will the commonly-assumed hybridnature of the intermediate links always remove the difficulty. In verymany cases, however, one form is ranked as a variety of another, notbecause the intermediate links have actually been found, but becauseanalogy leads the observer to suppose either that they do nowsomewhere exist, or may formerly have existed; and here a wide doorfor the entry of doubt and conjecture is opened.
Hence, in determining whether a form should be ranked as a species ora variety, the opinion of naturalists having sound judgment and wideexperience seems the only guide to follow. We must, however, in manycases, decide by a majority of naturalists, for few well-marked andwell-known varieties can be named which have not been ranked asspecies by at least some competent judges.
That varieties of this doubtful nature are far from uncommon cannot bedisputed. Compare the several floras of Great Britain, of France or ofthe United States, drawn up by different botanists, and see what asurprising number of forms have been ranked by one botanist as goodspecies, and by another as mere varieties. Mr. H. C. Watson, to whom Ilie under deep obligation for assistance of all kinds, has marked forme 182 British plants, which are generally considered as varieties,but which have all been ranked by botanists as species; and in makingthis list he has omitted many trifling varieties, but whichnevertheless have been ranked by some botanists as species, and he hasentirely omitted several highly polymorphic genera. Under genera,including the most polymorphic forms, Mr. Babington gives 251 species,whereas Mr. Bentham gives only 112,--a difference of 139 doubtfulforms! Amongst animals which unite for each birth, and which arehighly locomotive, doubtful forms, ranked by one zoologist as aspecies and by another as a variety, can rarely be found within thesame country, but are common in separated areas. How many of thosebirds and insects in North America and Europe, which differ veryslightly from each other, have been ranked by one eminent naturalistas undoubted species, and by another as varieties, or, as they areoften called, as geographical races! Many years ago, when comparing,and seeing others compare, the birds from the separate islands of theGalapagos Archipelago, both one with another, and with those from theAmerican mainland, I was much struck how entirely vague and arbitraryis the distinction between species and varieties. On the islets of thelittle Madeira group there are many insects which are characterized asvarieties in Mr. Wollaston's admirable work, but which it cannot bedoubted would be ranked as distinct species by many entomologists.Even Ireland has a few animals, now generally regarded as varieties,but which have been ranked as species by some zoologists. Several mostexperienced ornithologists consider our British red grouse as only astrongly-marked race of a Norwegian species, whereas the greaternumber rank it as an undoubted species peculiar to Great Britain. Awide distance between the homes of two doubtful forms leads manynaturalists to rank both as distinct species; but what distance, ithas been well asked, will suffice? if that between America and Europeis ample, will that between the Continent and the Azores, or Madeira,or the Canaries, or Ireland, be sufficient? It must be admitted thatmany forms, considered by highly-competent judges as varieties, haveso perfectly the character of species that they are ranked by otherhighly-competent judges as good and true species. But to discusswhether they are rightly called species or varieties, before anydefinition of these terms has been generally accepted, is vainly tobeat the air.
Many of the cases of strongly-marked varieties or doubtful specieswell deserve consideration; for several interesting lines of argument,from geographical distribution, analogical variation, hybridism, etc.,have been brought to bear on the attempt to determine their rank. Iwill here give only a single instance,--the well-known one of theprimrose and cowslip, or Primula veris and elatior. These plantsdiffer considerably in appearance; they have a different flavour andemit a different odour; they flower at slightly different periods;they grow in somewhat different stations; they ascend mountains todifferent heights; they have different geographical ranges; andlastly, according to very numerous experiments made during severalyears by that most careful observer Gartner, they can be crossed onlywith much difficulty. We could hardly wish for better evidence of thetwo forms being specifically distinct. On the other hand, they areunited by many intermediate links, and it is very doubtful whetherthese links are hybrids; and there is, as it seems to me, anoverwhelming amount of experimental evidence, showing that theydescend from common parents, and consequently must be ranked asvarieties.
Close investigation, in most cases, will bring naturalists to anagreement how to rank doubtful forms. Yet it must be confessed, thatit is in the best-known countries that we find the greatest number offorms of doubtful value. I have been struck with the fact, that if anyanimal or plant in a state of nature be highly useful to man, or fromany cause closely attract his attention, varieties of it will almostuniversally be found recorded. These varieties, moreover, will beoften ranked by some authors as species. Look at the common oak, howclosely it has been studied; yet a German author makes more than adozen species out of forms, which are very generally considered asvarieties; and in this country the highest botanical authorities andpractical men can be quoted to show that the sessile and pedunculatedoaks are either good and distinct species or mere varieties.
When a young naturalist commences the study of a group of organismsquite unknown to him, he is at first much perplexed to determine whatdifferences to consider as specific, and what as varieties; for heknows nothing of the amount and kind of variation to which the groupis subject; and this shows, at least, how very generally there is somevariation. But if he confine his attention to one class within onecountry, he will soon make up his mind how to rank most of thedoubtful forms. His general tendency will be to make many species, forhe will become impressed, just like the pigeon or poultry-fancierbefore alluded to, with the amount of difference in the forms which heis continually studying; and he has little general knowledge ofanalogical variation in other groups and in other countries, by whichto correct his first impressions. As he extends the range of hisobservations, he will meet with more cases of difficulty; for he willencounter a greater number of closely-allied forms. But if hisobservations be widely extended, he will in the end generally beenabled to make up his own mind which to call varieties and whichspecies; but he will succeed in this at the expense of admitting muchvariation,--and the truth of this admission will often be disputed byother naturalists. When, moreover, he comes to study allied formsbrought from countries not now continuous, in which case he can hardlyhope to find the intermediate links between his doubtful forms, hewill have to trust almost entirely to analogy, and his difficultieswill rise to a climax.
Certainly no clear line of demarcation has as yet been drawn betweenspecies and sub-species--that is, the forms which in the opinion ofsome naturalists come very near to, but do not quite arrive at therank of species; or, again, between sub-species and well-markedvarieties, or between lesser varieties and individual differences.These differences blend into each other in an insensible series; and aseries impresses the mind with the idea of an actual passage.
Hence I look at individual differences, though of small interest tothe systematist, as of high importance for us, as being the first steptowards such slight varieties as are barely thought worth recording inworks on natural history. And I look at varieties which are in anydegree more distinct and permanent, as steps leading to more stronglymarked and more permanent varieties; and at these latter, as leadingto sub-species, and to species. The passage from one stage ofdifference to another and higher stage may be, in some cases, duemerely to the long-continued action of different physical conditionsin two different regions; but I have not much faith in this view; andI attribute the passage of a variety, from a state in which it differsvery slightly from its parent to one in which it differs more, to theaction of natural selection in accumulating (as will hereafter be morefully explained) differences of structure in certain definitedirections. Hence I believe a well-marked variety may be justly calledan incipient species; but whether this belief be justifiable must bejudged of by the general weight of the several facts and views giventhroughout this work.
It need not be supposed that all varieties or incipient speciesnecessarily attain the rank of species. They may whilst in thisincipient state become extinct, or they may endure as varieties forvery long periods, as has been shown to be the case by Mr. Wollastonwith the varieties of certain fossil land-shells in Madeira. If avariety were to flourish so as to exceed in numbers the parentspecies, it would then rank as the species, and the species as thevariety; or it might come to supplant and exterminate the parentspecies; or both might co-exist, and both rank as independent species.But we shall hereafter have to return to this subject.
From these remarks it will be seen that I look at the term species, asone arbitrarily given for the sake of convenience to a set ofindividuals closely resembling each other, and that it does notessentially differ from the term variety, which is given to lessdistinct and more fluctuating forms. The term variety, again, incomparison with mere individual differences, is also appliedarbitrarily, and for mere convenience sake.
Guided by theoretical considerations, I thought that some interestingresults might be obtained in regard to the nature and relations of thespecies which vary most, by tabulating all the varieties in severalwell-worked floras. At first this seemed a simple task; but Mr. H. C.Watson, to whom I am much indebted for valuable advice and assistanceon this subject, soon convinced me that there were many difficulties,as did subsequently Dr. Hooker, even in stronger terms. I shallreserve for my future work the discussion of these difficulties, andthe tables themselves of the proportional numbers of the varyingspecies. Dr. Hooker permits me to add, that after having carefullyread my manuscript, and examined the tables, he thinks that thefollowing statements are fairly well established. The whole subject,however, treated as it necessarily here is with much brevity, israther perplexing, and allusions cannot be avoided to the "strugglefor existence," "divergence of character," and other questions,hereafter to be discussed.
Alph. De Candolle and others have shown that plants which have verywide ranges generally present varieties; and this might have beenexpected, as they become exposed to diverse physical conditions, andas they come into competition (which, as we shall hereafter see, is afar more important circumstance) with different sets of organicbeings. But my tables further show that, in any limited country, thespecies which are most common, that is abound most in individuals, andthe species which are most widely diffused within their own country(and this is a different consideration from wide range, and to acertain extent from commonness), often give rise to varietiessufficiently well-marked to have been recorded in botanical works.Hence it is the most flourishing, or, as they may be called, thedominant species,--those which range widely over the world, are themost diffused in their own country, and are the most numerous inindividuals,--which oftenest produce well-marked varieties, or, as Iconsider them, incipient species. And this, perhaps, might have beenanticipated; for, as varieties, in order to become in any degreepermanent, necessarily have to struggle with the other inhabitants ofthe country, the species which are already dominant will be the mostlikely to yield offspring which, though in some slight degreemodified, will still inherit those advantages that enabled theirparents to become dominant over their compatriots.
If the plants inhabiting a country and described in any Flora bedivided into two equal masses, all those in the larger genera beingplaced on one side, and all those in the smaller genera on the otherside, a somewhat larger number of the very common and much diffused ordominant species will be found on the side of the larger genera. This,again, might have been anticipated; for the mere fact of many speciesof the same genus inhabiting any country, shows that there issomething in the organic or inorganic conditions of that countryfavourable to the genus; and, consequently, we might have expected tohave found in the larger genera, or those including many species, alarge proportional number of dominant species. But so many causes tendto obscure this result, that I am surprised that my tables show even asmall majority on the side of the larger genera. I will here allude toonly two causes of obscurity. Fresh-water and salt-loving plants havegenerally very wide ranges and are much diffused, but this seems to beconnected with the nature of the stations inhabited by them, and haslittle or no relation to the size of the genera to which the speciesbelong. Again, plants low in the scale of organisation are generallymuch more widely diffused than plants higher in the scale; and hereagain there is no close relation to the size of the genera. The causeof lowly-organised plants ranging widely will be discussed in ourchapter on geographical distribution.