物种起源 英文版 On the Origin of Species
达尔文 Charles Darwin
CHAPTER 2. VARIATION UNDER NATURE. Page 2
From looking at species as only strongly-marked and well-definedvarieties, I was led to anticipate that the species of the largergenera in each country would oftener present varieties, than thespecies of the smaller genera; for wherever many closely relatedspecies (i.e. species of the same genus) have been formed, manyvarieties or incipient species ought, as a general rule, to be nowforming. Where many large trees grow, we expect to find saplings.Where many species of a genus have been formed through variation,circumstances have been favourable for variation; and hence we mightexpect that the circumstances would generally be still favourable tovariation. On the other hand, if we look at each species as a specialact of creation, there is no apparent reason why more varieties shouldoccur in a group having many species, than in one having few.
To test the truth of this anticipation I have arranged the plants oftwelve countries, and the coleopterous insects of two districts, intotwo nearly equal masses, the species of the larger genera on one side,and those of the smaller genera on the other side, and it hasinvariably proved to be the case that a larger proportion of thespecies on the side of the larger genera present varieties, than onthe side of the smaller genera. Moreover, the species of the largegenera which present any varieties, invariably present a largeraverage number of varieties than do the species of the small genera.Both these results follow when another division is made, and when allthe smallest genera, with from only one to four species, areabsolutely excluded from the tables. These facts are of plainsignification on the view that species are only strongly marked andpermanent varieties; for wherever many species of the same genus havebeen formed, or where, if we may use the expression, the manufactoryof species has been active, we ought generally to find the manufactorystill in action, more especially as we have every reason to believethe process of manufacturing new species to be a slow one. And thiscertainly is the case, if varieties be looked at as incipient species;for my tables clearly show as a general rule that, wherever manyspecies of a genus have been formed, the species of that genus presenta number of varieties, that is of incipient species, beyond theaverage. It is not that all large genera are now varying much, and arethus increasing in the number of their species, or that no smallgenera are now varying and increasing; for if this had been so, itwould have been fatal to my theory; inasmuch as geology plainly tellsus that small genera have in the lapse of time often increased greatlyin size; and that large genera have often come to their maxima,declined, and disappeared. All that we want to show is, that wheremany species of a genus have been formed, on an average many are stillforming; and this holds good.
There are other relations between the species of large genera andtheir recorded varieties which deserve notice. We have seen that thereis no infallible criterion by which to distinguish species andwell-marked varieties; and in those cases in which intermediate linkshave not been found between doubtful forms, naturalists are compelledto come to a determination by the amount of difference between them,judging by analogy whether or not the amount suffices to raise one orboth to the rank of species. Hence the amount of difference is onevery important criterion in settling whether two forms should beranked as species or varieties. Now Fries has remarked in regard toplants, and Westwood in regard to insects, that in large genera theamount of difference between the species is often exceedingly small. Ihave endeavoured to test this numerically by averages, and, as far asmy imperfect results go, they always confirm the view. I have alsoconsulted some sagacious and most experienced observers, and, afterdeliberation, they concur in this view. In this respect, therefore,the species of the larger genera resemble varieties, more than do thespecies of the smaller genera. Or the case may be put in another way,and it may be said, that in the larger genera, in which a number ofvarieties or incipient species greater than the average are nowmanufacturing, many of the species already manufactured still to acertain extent resemble varieties, for they differ from each other bya less than usual amount of difference.
Moreover, the species of the large genera are related to each other,in the same manner as the varieties of any one species are related toeach other. No naturalist pretends that all the species of a genus areequally distinct from each other; they may generally be divided intosub-genera, or sections, or lesser groups. As Fries has well remarked,little groups of species are generally clustered like satellitesaround certain other species. And what are varieties but groups offorms, unequally related to each other, and clustered round certainforms--that is, round their parent-species? Undoubtedly there is onemost important point of difference between varieties and species;namely, that the amount of difference between varieties, when comparedwith each other or with their parent-species, is much less than thatbetween the species of the same genus. But when we come to discuss theprinciple, as I call it, of Divergence of Character, we shall see howthis may be explained, and how the lesser differences betweenvarieties will tend to increase into the greater differences betweenspecies.
There is one other point which seems to me worth notice. Varietiesgenerally have much restricted ranges: this statement is indeedscarcely more than a truism, for if a variety were found to have awider range than that of its supposed parent-species, theirdenominations ought to be reversed. But there is also reason tobelieve, that those species which are very closely allied to otherspecies, and in so far resemble varieties, often have much restrictedranges. For instance, Mr. H. C. Watson has marked for me in thewell-sifted London Catalogue of plants (4th edition) 63 plants whichare therein ranked as species, but which he considers as so closelyallied to other species as to be of doubtful value: these 63 reputedspecies range on an average over 6.9 of the provinces into which Mr.Watson has divided Great Britain. Now, in this same catalogue, 53acknowledged varieties are recorded, and these range over 7.7provinces; whereas, the species to which these varieties belong rangeover 14.3 provinces. So that the acknowledged varieties have verynearly the same restricted average range, as have those very closelyallied forms, marked for me by Mr. Watson as doubtful species, butwhich are almost universally ranked by British botanists as good andtrue species.
Finally, then, varieties have the same general characters as species,for they cannot be distinguished from species,--except, firstly, bythe discovery of intermediate linking forms, and the occurrence ofsuch links cannot affect the actual characters of the forms which theyconnect; and except, secondly, by a certain amount of difference, fortwo forms, if differing very little, are generally ranked asvarieties, notwithstanding that intermediate linking forms have notbeen discovered; but the amount of difference considered necessary togive to two forms the rank of species is quite indefinite. In generahaving more than the average number of species in any country, thespecies of these genera have more than the average number ofvarieties. In large genera the species are apt to be closely, butunequally, allied together, forming little clusters round certainspecies. Species very closely allied to other species apparently haverestricted ranges. In all these several respects the species of largegenera present a strong analogy with varieties. And we can clearlyunderstand these analogies, if species have once existed as varieties,and have thus originated: whereas, these analogies are utterlyinexplicable if each species has been independently created.
We have, also, seen that it is the most flourishing and dominantspecies of the larger genera which on an average vary most; andvarieties, as we shall hereafter see, tend to become converted intonew and distinct species. The larger genera thus tend to becomelarger; and throughout nature the forms of life which are now dominanttend to become still more dominant by leaving many modified anddominant descendants. But by steps hereafter to be explained, thelarger genera also tend to break up into smaller genera. And thus, theforms of life throughout the universe become divided into groupssubordinate to groups.