按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
stwood in regard to insects； that in large genera the amount of difference between the species is often exceedingly small。 I have endeavoured to test this numerically by averages； and； as far as my imperfect results go； they always confirm the view。 I have also consulted some sagacious and most experienced observers； and； after deliberation； they concur in this view。 In this respect； therefore； the species of the larger genera resemble varieties； more than do the species 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 of varieties or incipient species greater than the average are now manufacturing； many of the species already manufactured still to a certain extent resemble varieties； for they differ from each other by a 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 to each other。 No naturalist pretends that all the species of a genus are equally distinct from each other； they may generally be divided into sub…genera； or sections； or lesser groups。 As Fries has well remarked； little groups of species are generally clustered like satellites around certain other species。 And what are varieties but groups of forms； unequally related to each other； and clustered round certain forms that is； round their parent…species？ Undoubtedly there is one most important point of difference between varieties and species； namely； that the amount of difference between varieties； when compared with each other or with their parent…species； is much less than that between the species of the same genus。 But when we come to discuss the principle； as I call it； of Divergence of Character； we shall see how this may be explained； and how the lesser differences between varieties will tend to increase into the greater differences between species。
There is one other point which seems to me worth notice。 Varieties generally have much restricted ranges： this statement is indeed scarcely more than a truism； for if a variety were found to have a wider range than that of its supposed parent…species； their denominations ought to be reversed。 But there is also reason to believe； that those species which are very closely allied to other species； and in so far resemble varieties； often have much restricted ranges。 For instance； Mr H。 C。 Watson has marked for me in the well…sifted London Catalogue of plants （4th edition） 63 plants which are therein ranked as species； but which he considers as so closely allied to other species as to be of doubtful value： these 63 reputed species range on an average over 6。9 of the provinces into which Mr Watson has divided Great Britain。 Now； in this same catalogue； 53 acknowledged varieties are recorded； and these range over 7。7 provinces； whereas； the species to which these varieties belong range over 14。3 provinces。 So that the acknowledged varieties have very nearly the same restricted average range； as have those very closely allied forms； marked for me by Mr Watson as doubtful species； but which are almost universally ranked by British botanists as good and true species。
Finally； then； varieties have the same general characters as species； for they cannot be distinguished from species； except； firstly； by the discovery of intermediate linking forms； and the occurrence of such links cannot affect the actual characters of the forms which they connect； and except； secondly； by a certain amount of difference； for two forms； if differing very little； are generally ranked as varieties； notwithstanding that intermediate linking forms have not been discovered； but the amount of difference considered necessary to give to two forms the rank of species is quite indefinite。 In genera having more than the average number of species in any country； the species of these genera have more than the average number of varieties。 In large genera the species are apt to be closely； but unequally； allied together； forming little clusters round certain species。 Species very closely allied to other species apparently have restricted ranges。 In all these several respects the species of large genera present a strong analogy with varieties。 And we can clearly understand these analogies； if species have once existed as varieties； and have thus originated： whereas； these analogies are utterly inexplicable if each species has been independently created。
We have； also； seen that it is the most flourishing and dominant species of the larger genera which on an average vary most； and varieties； as we shall hereafter see； tend to become converted into new and distinct species。 The larger genera thus tend to become larger； and throughout nature the forms of life which are now dominant tend to become still more dominant by leaving many modified and dominant descendants。 But by steps hereafter to be explained； the larger genera also tend to break up into smaller genera。 And thus； the forms of life throughout the universe become divided into groups subordinate to groups。
The Origin of Species
by Charles Darwin
Previous Chapter
Next Chapter
Chapter 3 … Struggle for Existence
BEF0RE entering on the subject of this chapter； I must make a few preliminary remarks； to show how the struggle for existence bears on Natural Selection。 It has been seen in the last chapter that amongst organic beings in a state of nature there is some individual variability； indeed I am not aware that this has ever been disputed。 It is immaterial for us whether a multitude of doubtful forms be called species or sub…species or varieties； what rank； for instance； the two or three hundred doubtful forms of British plants are entitled to hold； if the existence of any well…marked varieties be admitted。 But the mere existence of individual variability and of some few well…marked varieties； though necessary as the foundation for the work； helps us but little in understanding how species arise in nature。 How have all those exquisite adaptations of one part of the organisation to another part； and to the conditions of life； and of one distinct organic being to another being； been perfected？ We see these beautiful co…adaptations most plainly in the woodpecker and missletoe； and only a little less plainly in the humblest parasite which clings to the hairs of a quadruped or feathers of a bird； in the structure of the beetle which dives through the water； in the plumed seed which is wafted by the gentlest breeze； in short； we see beautiful adaptations everywhere and in every part of the organic world。
Again； it may be asked； how is it that varieties； which I have called incipient species； become ultimately converted into good and distinct species； which in most cases obviously differ from each other far more than do the varieties of the same species？ How do those groups of species； which constitute what are called distinct genera； and which differ from each other more than do the species of the same genus； arise？ All these results； as we shall more fully see in the next chapter； follow inevitably from the struggle for life。 Owing to this struggle for life； any variation； however slight