the origin of species(物种起源)-第10节

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rms　which　in　the　opinion　of　some　naturalists　come　very　near　to；　but　do　not　quite　arrive　at　the　rank　of　species；　or；　again；　between　sub…species　and　well…marked　varieties；　or　between　lesser　varieties　and　individual　differences。　These　differences　blend　into　each　other　in　an　insensible　series；　and　a　series　impresses　the　mind　with　the　idea　of　an　actual　passage。　
Hence　I　look　at　individual　differences；　though　of　small　interest　to　the　systematist；　as　of　high　importance　for　us；　as　being　the　first　step　towards　such　slight　varieties　as　are　barely　thought　worth　recording　in　works　on　natural　history。　And　I　look　at　varieties　which　are　in　any　degree　more　distinct　and　permanent；　as　steps　leading　to　more　strongly　marked　and　more　permanent　varieties；　and　at　these　latter；　as　leading　to　sub…species；　and　to　species。　The　passage　from　one　stage　of　difference　to　another　and　higher　stage　may　be；　in　some　cases；　due　merely　to　the　long…continued　action　of　different　physical　conditions　in　two　different　regions；　but　I　have　not　much　faith　in　this　view；　and　I　attribute　the　passage　of　a　variety；　from　a　state　in　which　it　differs　very　slightly　from　its　parent　to　one　in　which　it　differs　more；　to　the　action　of　natural　selection　in　accumulating　（as　will　hereafter　be　more　fully　explained）　differences　of　structure　in　certain　definite　directions。　Hence　I　believe　a　well…marked　variety　may　be　justly　called　an　incipient　species；　but　whether　this　belief　be　justifiable　must　be　judged　of　by　the　general　weight　of　the　several　facts　and　views　given　throughout　this　work。　
It　need　not　be　supposed　that　all　varieties　or　incipient　species　necessarily　attain　the　rank　of　species。　They　may　whilst　in　this　incipient　state　become　extinct；　or　they　may　endure　as　varieties　for　very　long　periods；　as　has　been　shown　to　be　the　case　by　Mr　Wollaston　with　the　varieties　of　certain　fossil　land…shells　in　Madeira。　If　a　variety　were　to　flourish　so　as　to　exceed　in　numbers　the　parent　species；　it　would　then　rank　as　the　species；　and　the　species　as　the　variety；　or　it　might　come　to　supplant　and　exterminate　the　parent　species；　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；　as　one　arbitrarily　given　for　the　sake　of　convenience　to　a　set　of　individuals　closely　resembling　each　other；　and　that　it　does　not　essentially　differ　from　the　term　variety；　which　is　given　to　less　distinct　and　more　fluctuating　forms。　The　term　variety；　again；　in　comparison　with　mere　individual　differences；　is　also　applied　arbitrarily；　and　for　mere　convenience　sake。　
Guided　by　theoretical　considerations；　I　thought　that　some　interesting　results　might　be　obtained　in　regard　to　the　nature　and　relations　of　the　species　which　vary　most；　by　tabulating　all　the　varieties　in　several　well…worked　floras。　At　first　this　seemed　a　simple　task；　but　Mr　H。　C。　Watson；　to　whom　I　am　much　indebted　for　valuable　advice　and　assistance　on　this　subject；　soon　convinced　me　that　there　were　many　difficulties；　as　did　subsequently　Dr　Hooker；　even　in　stronger　terms。　I　shall　reserve　for　my　future　work　the　discussion　of　these　difficulties；　and　the　tables　themselves　of　the　proportional　numbers　of　the　varying　species。　Dr　Hooker　permits　me　to　add；　that　after　having　carefully　read　my　manuscript；　and　examined　the　tables；　he　thinks　that　the　following　statements　are　fairly　well　established。　The　whole　subject；　however；　treated　as　it　necessarily　here　is　with　much　brevity；　is　rather　perplexing；　and　allusions　cannot　be　avoided　to　the　'struggle　for　existence；'　'divergence　of　character；'　and　other　questions；　hereafter　to　be　discussed。　
Alph。　De　Candolle　and　others　have　shown　that　plants　which　have　very　wide　ranges　generally　present　varieties；　and　this　might　have　been　expected；　as　they　become　exposed　to　diverse　physical　conditions；　and　as　they　come　into　competition　（which；　as　we　shall　hereafter　see；　is　a　far　more　important　circumstance）　with　different　sets　of　organic　beings。　But　my　tables　further　show　that；　in　any　limited　country；　the　species　which　are　most　common；　that　is　abound　most　in　individuals；　and　the　species　which　are　most　widely　diffused　within　their　own　country　（and　this　is　a　different　consideration　from　wide　range；　and　to　a　certain　extent　from　commonness）；　often　give　rise　to　varieties　sufficiently　well…marked　to　have　been　recorded　in　botanical　works。　Hence　it　is　the　most　flourishing；　or；　as　they　may　be　called；　the　dominant　species；　those　which　range　widely　over　the　world；　are　the　most　diffused　in　their　own　country；　and　are　the　most　numerous　in　individuals；　which　oftenest　produce　well…marked　varieties；　or；　as　I　consider　them；　incipient　species。　And　this；　perhaps；　might　have　been　anticipated；　for；　as　varieties；　in　order　to　become　in　any　degree　permanent；　necessarily　have　to　struggle　with　the　other　inhabitants　of　the　country；　the　species　which　are　already　dominant　will　be　the　most　likely　to　yield　offspring　which；　though　in　some　slight　degree　modified；　will　still　inherit　those　advantages　that　enabled　their　parents　to　become　dominant　over　their　compatriots。　
If　the　plants　inhabiting　a　country　and　described　in　any　Flora　be　divided　into　two　equal　masses；　all　those　in　the　larger　genera　being　placed　on　one　side；　and　all　those　in　the　smaller　genera　on　the　other　side；　a　somewhat　larger　number　of　the　very　common　and　much　diffused　or　dominant　species　will　be　found　on　the　side　of　the　larger　genera。　This；　again；　might　have　been　anticipated；　for　the　mere　fact　of　many　species　of　the　same　genus　inhabiting　any　country；　shows　that　there　is　something　in　the　organic　or　inorganic　conditions　of　that　country　favourable　to　the　genus；　and；　consequently；　we　might　have　expected　to　have　found　in　the　larger　genera；　or　those　including　many　species；　a　large　proportional　number　of　dominant　species。　But　so　many　causes　tend　to　obscure　this　result；　that　I　am　surprised　that　my　tables　show　even　a　small　majority　on　the　side　of　the　larger　genera。　I　will　here　allude　to　only　two　causes　of　obscurity。　Fresh…water　and　salt…loving　plants　have　generally　very　wide　ranges　and　are　much　diffused；　but　this　seems　to　be　connected　with　the　nature　of　the　stations　inhabited　by　them；　and　has　little　or　no　relation　to　the　size　of　the　genera　to　which　the　species　belong。　Again；　plants　low　in　the　scale　of　organisation　are　generally　much　more　widely　diffused　than　plants　higher　in　the　scale；　and　here　again　there　is　no　close　relation　to　the　size　of　the　genera。　The　cause　of　lowly…organised　plants　ranging　widely　will　be　discussed　in　our　chapter　on　geographical　distribution。　
From　looking　at　species　as　only　strongly…marked　and　well…defined　varieties；　I　was　led　to　anticipate　that　the　species　of　the　larger　genera　in　each　country　would　oftener　present　varieties；　than　the　species　of　the　smaller　genera；　for　wherever　many　closely　related　species　（i。e。　species　of　the　same　genus）　have　been　formed；　many　varieties　or　incipient　species　ought；　as　a　general　rule；　to　be　now　forming。　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　might　expect　that　the　circumstances　would　generally　be　still　favourable　to　variation。　On　the　other　hand；　if　we　look　at　each　species　as　a　special　act　of　creation；　there　is　no　apparent　reason　why　more　varieties　should　occur　in　a　group　having　many　species；　than　in　one　having　few。　
To　test　the　truth　of　this　anticipation　I　have　arranged　the　plants　of　twelve　countries；　and　the　coleopterous　insects　of　two　districts；　into　two　nearly　equal　masses；　the　species　of　the　larger　genera　on　one　side；　and　those　of　the　smaller　genera　on　the　other　side；　and　it　has　invariably　proved　to　be　the　case　that　a　larger　proportion　of　the　species　on　the　side　of　the　larger　genera　present　varieties；　than　on　the　side　of　the　smaller　genera。　Moreover；　the　species　of　the　large　genera　which　present　any　varieties；　invariably　present　a　larger　average　number　of　varieties　than　do　the　species　of　the　small　genera。　Both　these　results　follow　when　another　division　is　made；　and　when　all　the　smallest　genera；　with　from　only　one　to　four　species；　are　absolutely　excluded　from　the　tables。　These　facts　are　of　plain　signification　on　the　view　that　species　are　only　strongly　marked　and　permanent　varieties；　for　whenever　many　species　of　the　same　genus　have　been　formed；　or　where；　if　we　may　use　the　expression；　the　manufactory　of　species　has　been　active；　we　ought　generally　to　find　the　manufactory　still　in　action；　more　especially　as　we　have　every　reason　to　believe　the　process　of　manufacturing　new　species　to　be　a　slow　one。　And　this　certainly　is　the　case；　if　varieties　be　looked　at　as　incipient　species；　for　my　tables　clearly　show　as　a　general　rule　that；　wherever　many　species　of　a　genus　have　been　formed；　the　species　of　that　genus　present　a　number　of　varieties；　that　is　of　incipient　species；　beyond　the　average。　It　is　not　that　all　large　genera　are　now　varying　much；　and　are　thus　increasing　in　the　number　of　their　species；　or　that　no　small　genera　are　now　varying　and　increasing；　for　if　this　had　been　so；　it　would　have　been　fatal　to　my　theory；　inasmuch　as　geology　plainly　tells　us　that　small　genera　have　in　the　lapse　of　time　often　increased　greatly　in