the origin of species(物种起源)-第9节

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phic　genera　some　of　the　species　have　fixed　and　definite　characters。　Genera　which　are　polymorphic　in　one　country　seem　to　be；　with　some　few　exceptions；　polymorphic　in　other　countries；　and　likewise；　judging　from　Brachiopod　shells；　at　former　periods　of　time。　These　facts　seem　to　be　very　perplexing；　for　they　seem　to　show　that　this　kind　of　variability　is　independent　of　the　conditions　of　life。　I　am　inclined　to　suspect　that　we　see　in　these　polymorphic　genera　variations　in　points　of　structure　which　are　of　no　service　or　disservice　to　the　species；　and　which　consequently　have　not　been　seized　on　and　rendered　definite　by　natural　selection；　as　hereafter　will　be　explained。　
Those　forms　which　possess　in　some　considerable　degree　the　character　of　species；　but　which　are　so　closely　similar　to　some　other　forms；　or　are　so　closely　linked　to　them　by　intermediate　gradations；　that　naturalists　do　not　like　to　rank　them　as　distinct　species；　are　in　several　respects　the　most　important　for　us。　We　have　every　reason　to　believe　that　many　of　these　doubtful　and　closely…allied　forms　have　permanently　retained　their　characters　in　their　own　country　for　a　long　time；　for　as　long；　as　far　as　we　know；　as　have　good　and　true　species。　practically；　when　a　naturalist　can　unite　two　forms　together　by　others　having　intermediate　characters；　he　treats　the　one　as　a　variety　of　the　other；　ranking　the　most　common；　but　sometimes　the　one　first　described；　as　the　species；　and　the　other　as　the　variety。　But　cases　of　great　difficulty；　which　I　will　not　here　enumerate；　sometimes　occur　in　deciding　whether　or　not　to　rank　one　form　as　a　variety　of　another；　even　when　they　are　closely　connected　by　intermediate　links；　nor　will　the　commonly…assumed　hybrid　nature　of　the　intermediate　links　always　remove　the　difficulty。　In　very　many　cases；　however；　one　form　is　ranked　as　a　variety　of　another；　not　because　the　intermediate　links　have　actually　been　found；　but　because　analogy　leads　the　observer　to　suppose　either　that　they　do　now　somewhere　exist；　or　may　formerly　have　existed；　and　here　a　wide　door　for　the　entry　of　doubt　and　conjecture　is　opened。　
Hence；　in　determining　whether　a　form　should　be　ranked　as　a　species　or　a　variety；　the　opinion　of　naturalists　having　sound　judgement　and　wide　experience　seems　the　only　guide　to　follow。　We　must；　however；　in　many　cases；　decide　by　a　majority　of　naturalists；　for　few　well…marked　and　well…known　varieties　can　be　named　which　have　not　been　ranked　as　species　by　at　least　some　competent　judges。　
That　varieties　of　this　doubtful　nature　are　far　from　uncommon　cannot　be　disputed。　Compare　the　several　floras　of　Great　Britain；　of　France　or　of　the　United　States；　drawn　up　by　different　botanists；　and　see　what　a　surprising　number　of　forms　have　been　ranked　by　one　botanist　as　good　species；　and　by　another　as　mere　varieties。　Mr　H。　C。　Watson；　to　whom　I　lie　under　deep　obligation　for　assistance　of　all　kinds；　has　marked　for　me　182　British　plants；　which　are　generally　considered　as　varieties；　but　which　have　all　been　ranked　by　botanists　as　species；　and　in　making　this　list　he　has　omitted　many　trifling　varieties；　but　which　nevertheless　have　been　ranked　by　some　botanists　as　species；　and　he　has　entirely　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　doubtful　forms！　Amongst　animals　which　unite　for　each　birth；　and　which　are　highly　locomotive；　doubtful　forms；　ranked　by　one　zoologist　as　a　species　and　by　another　as　a　variety；　can　rarely　be　found　within　the　same　country；　but　are　common　in　separated　areas。　How　many　of　those　birds　and　insects　in　North　America　and　Europe；　which　differ　very　slightly　from　each　other；　have　been　ranked　by　one　eminent　naturalist　as　undoubted　species；　and　by　another　as　varieties；　or；　as　they　are　often　called；　as　geographical　races！　Many　years　ago；　when　comparing；　and　seeing　others　compare；　the　birds　from　the　separate　islands　of　the　Galapagos　Archipelago；　both　one　with　another；　and　with　those　from　the　American　mainland；　I　was　much　struck　how　entirely　vague　and　arbitrary　is　the　distinction　between　species　and　varieties。　On　the　islets　of　the　little　Madeira　group　there　are　many　insects　which　are　characterized　as　varieties　in　Mr　Wollaston's　admirable　work；　but　which　it　cannot　be　doubted　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　most　experienced　ornithologists　consider　our　British　red　grouse　as　only　a　strongly…marked　race　of　a　Norwegian　species；　whereas　the　greater　number　rank　it　as　an　undoubted　species　peculiar　to　Great　Britain。　A　wide　distance　between　the　homes　of　two　doubtful　forms　leads　many　naturalists　to　rank　both　as　distinct　species；　but　what　distance；　it　has　been　well　asked；　will　suffice？　if　that　between　America　and　Europe　is　ample；　will　that　between　the　Continent　and　the　Azores；　or　Madeira；　or　the　Canaries；　or　Ireland；　be　sufficient？　It　must　be　admitted　that　many　forms；　considered　by　highly…competent　judges　as　varieties；　have　so　perfectly　the　character　of　species　that　they　are　ranked　by　other　highly…competent　judges　as　good　and　true　species。　But　to　discuss　whether　they　are　rightly　called　species　or　varieties；　before　any　definition　of　these　terms　has　been　generally　accepted；　is　vainly　to　beat　the　air。　
Many　of　the　cases　of　strongly…marked　varieties　or　doubtful　species　well　deserve　consideration；　for　several　interesting　lines　of　argument；　from　geographical　distribution；　analogical　variation；　hybridism；　&c。；　have　been　brought　to　bear　on　the　attempt　to　determine　their　rank。　I　will　here　give　only　a　single　instance；　the　well…known　one　of　the　primrose　and　cowslip；　or　Primula　veris　and　elatior。　These　plants　differ　considerably　in　appearance；　they　have　a　different　flavour　and　emit　a　different　odour；　they　flower　at　slightly　different　periods；　they　grow　in　somewhat　different　stations；　they　ascend　mountains　to　different　heights；　they　have　different　geographical　ranges；　and　lastly；　according　to　very　numerous　experiments　made　during　several　years　by　that　most　careful　observer　G？rtner；　they　can　be　crossed　only　with　much　difficulty。　We　could　hardly　wish　for　better　evidence　of　the　two　forms　being　specifically　distinct。　On　the　other　hand；　they　are　united　by　many　intermediate　links；　and　it　is　very　doubtful　whether　these　links　are　hybrids；　and　there　is；　as　it　seems　to　me；　an　overwhelming　amount　of　experimental　evidence；　showing　that　they　descend　from　common　parents；　and　consequently　must　be　ranked　as　varieties。　
Close　investigation；　in　most　cases；　will　bring　naturalists　to　an　agreement　how　to　rank　doubtful　forms。　Yet　it　must　be　confessed；　that　it　is　in　the　best…known　countries　that　we　find　the　greatest　number　of　forms　of　doubtful　value。　I　have　been　struck　with　the　fact；　that　if　any　animal　or　plant　in　a　state　of　nature　be　highly　useful　to　man；　or　from　any　cause　closely　attract　his　attention；　varieties　of　it　will　almost　universally　be　found　recorded。　These　varieties；　moreover；　will　be　often　ranked　by　some　authors　as　species。　Look　at　the　common　oak；　how　closely　it　has　been　studied；　yet　a　German　author　makes　more　than　a　dozen　species　out　of　forms；　which　are　very　generally　considered　as　varieties；　and　in　this　country　the　highest　botanical　authorities　and　practical　men　can　be　quoted　to　show　that　the　sessile　and　pedunculated　oaks　are　either　good　and　distinct　species　or　mere　varieties。　
When　a　young　naturalist　commences　the　study　of　a　group　of　organisms　quite　unknown　to　him；　he　is　at　first　much　perplexed　to　determine　what　differences　to　consider　as　specific；　and　what　as　varieties；　for　he　knows　nothing　of　the　amount　and　kind　of　variation　to　which　the　group　is　subject；　and　this　shows；　at　least；　how　very　generally　there　is　some　variation。　But　if　he　confine　his　attention　to　one　class　within　one　country；　he　will　soon　make　up　his　mind　how　to　rank　most　of　the　doubtful　forms。　His　general　tendency　will　be　to　make　many　species；　for　he　will　become　impressed；　just　like　the　pigeon　or　poultry…fancier　before　alluded　to；　with　the　amount　of　difference　in　the　forms　which　he　is　continually　studying；　and　he　has　little　general　knowledge　of　analogical　variation　in　other　groups　and　in　other　countries；　by　which　to　correct　his　first　impressions。　As　he　extends　the　range　of　his　observations；　he　will　meet　with　more　cases　of　difficulty；　for　he　will　encounter　a　greater　number　of　closely…allied　forms。　But　if　his　observations　be　widely　extended；　he　will　in　the　end　generally　be　enabled　to　make　up　his　own　mind　which　to　call　varieties　and　which　species；　but　he　will　succeed　in　this　at　the　expense　of　admitting　much　variation；　and　the　truth　of　this　admission　will　often　be　disputed　by　other　naturalists。　When；　moreover；　he　comes　to　study　allied　forms　brought　from　countries　not　now　continuous；　in　which　case　he　can　hardly　hope　to　find　the　intermediate　links　between　his　doubtful　forms；　he　will　have　to　trust　almost　entirely　to　analogy；　and　his　difficulties　will　rise　to　a　climax。　
Certainly　no　clear　line　of　demarcation　has　as　yet　been　drawn　between　species　and　sub…species　that　is；　the　forms　which　in　the　opinion　of　some　naturalists　come　very　near　to；　but　do　not　quite　arrive　at　the　rank