the origin of species(物种起源)-第25部分

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is　common　to　most　animals。　On　this　view；　the　capacity　of　enduring　the　most　different　climates　by　man　himself　and　by　his　domestic　animals；　and　such　facts　as　that　former　species　of　the　elephant　and　rhinoceros　were　capable　of　enduring　a　glacial　climate；　whereas　the　living　species　are　now　all　tropical　or　sub…tropical　in　their　habits；　ought　not　to　be　looked　at　as　anomalies；　but　merely　as　examples　of　a　very　common　flexibility　of　constitution；　brought；　under　peculiar　circumstances；　into　play。　
How　much　of　the　acclimatisation　of　species　to　any　peculiar　climate　is　due　to　mere　habit；　and　how　much　to　the　natural　selection　of　varieties　having　different　innate　constitutions；　and　how　much　to　means　combined；　is　a　very　obscure　question。　That　habit　or　custom　has　some　influence　I　must　believe；　both　from　analogy；　and　from　the　incessant　advice　given　in　agricultural　works；　even　in　the　ancient　Encyclopaedias　of　China；　to　be　very　cautious　in　transposing　animals　from　one　district　to　another；　for　it　is　not　likely　that　man　should　have　succeeded　in　selecting　so　many　breeds　and　sub…breeds　with　constitutions　specially　fitted　for　their　own　districts：　the　result　must；　I　think；　be　due　to　habit。　On　the　other　hand；　I　can　see　no　reason　to　doubt　that　natural　selection　will　continually　tend　to　preserve　those　individuals　which　are　born　with　constitutions　best　adapted　to　their　native　countries。　In　treatises　on　many　kinds　of　cultivated　plants；　certain　varieties　are　said　to　withstand　certain　climates　better　than　others：　this　is　very　strikingly　shown　in　works　on　fruit　trees　published　in　the　United　States；　in　which　certain　varieties　are　habitually　recommended　for　the　northern；　and　others　for　the　southern　States；　and　as　most　of　these　varieties　are　of　recent　origin；　they　cannot　owe　their　constitutional　differences　to　habit。　The　case　of　the　Jerusalem　artichoke；　which　is　never　propagated　by　seed；　and　of　which　consequently　new　varieties　have　not　been　produced；　has　even　been　advanced　for　it　is　now　as　tender　as　ever　it　was　　as　proving　that　acclimatisation　cannot　be　effected！　The　case；　also；　of　the　kidney…bean　has　been　often　cited　for　a　similar　purpose；　and　with　much　greater　weight；　but　until　some　one　will　sow；　during　a　score　of　generations；　his　kidney…beans　so　early　that　a　very　large　proportion　are　destroyed　by　frost；　and　then　collect　seed　from　the　few　survivors；　with　care　to　prevent　accidental　crosses；　and　then　again　get　seed　from　these　seedlings；　with　the　same　precautions；　the　experiment　cannot　be　said　to　have　been　even　tried。　Nor　let　it　be　supposed　that　no　differences　in　the　constitution　of　seedling　kidney…beans　ever　appear；　for　an　account　has　been　published　how　much　more　hardy　some　seedlings　appeared　to　be　than　others。　
On　the　whole；　I　think　we　may　conclude　that　habit；　use；　and　disuse；　have；　in　some　cases；　played　a　considerable　part　in　the　modification　of　the　constitution；　and　of　the　structure　of　various　organs；　but　that　the　effects　of　use　and　disuse　have　often　been　largely　combined　with；　and　sometimes　overmastered　by；　the　natural　selection　of　innate　differences。　
Correlation　of　Growth
I　mean　by　this　expression　that　the　whole　organisation　is　so　tied　together　during　its　growth　and　development；　that　when　slight　variations　in　any　one　part　occur；　and　are　accumulated　through　natural　selection；　other　parts　become　modified。　This　is　a　very　important　subject；　most　imperfectly　understood。　The　most　obvious　case　is；　that　modifications　accumulated　solely　for　the　good　of　the　young　or　larva；　will；　it　may　safely　be　concluded；　affect　the　structure　of　the　adult；　in　the　same　manner　as　any　malconformation　affecting　the　early　embryo；　seriously　affects　the　whole　organisation　of　the　adult。　The　several　parts　of　the　body　which　are　homologous；　and　which；　at　an　early　embryonic　period；　are　alike；　seem　liable　to　vary　in　an　allied　manner：　we　see　this　in　the　right　and　left　sides　of　the　body　varying　in　the　same　manner；　in　the　front　and　hind　legs；　and　even　in　the　jaws　and　limbs；　varying　together；　for　the　lower　jaw　is　believed　to　be　homologous　with　the　limbs。　These　tendencies；　I　do　not　doubt；　may　be　mastered　more　or　less　completely　by　natural　selection：　thus　a　family　of　stags　once　existed　with　an　antler　only　on　one　side；　and　if　this　had　been　of　any　great　use　to　the　breed　it　might　probably　have　been　rendered　permanent　by　natural　selection。　
Homologous　parts；　as　has　been　remarked　by　some　authors；　tend　to　cohere；　this　is　often　seen　in　monstrous　plants；　and　nothing　is　more　common　than　the　union　of　homologous　parts　in　normal　structures；　as　the　union　of　the　petals　of　the　corolla　into　a　tube。　Hard　parts　seem　to　affect　the　form　of　adjoining　soft　parts；　it　is　believed　by　some　authors　that　the　diversity　in　the　shape　of　the　pelvis　in　birds　causes　the　remarkable　diversity　in　the　shape　of　their　kidneys。　Others　believe　that　the　shape　of　the　pelvis　in　the　human　mother　influences　by　pressure　the　shape　of　the　head　of　the　child。　In　snakes；　according　to　Schlegel；　the　shape　of　the　body　and　the　manner　of　swallowing　determine　the　position　of　several　of　the　most　important　viscera。　
The　nature　of　the　bond　of　correlation　is　very　frequently　quite　obscure。　M。　Is。　Geoffroy　St　Hilaire　has　forcibly　remarked；　that　certain　malconformations　very　frequently；　and　that　others　rarely　coexist；　without　our　being　able　to　assign　any　reason。　What　can　be　more　singular　than　the　relation　between　blue　eyes　and　deafness　in　cats；　and　the　tortoise…shell　colour　with　the　female　sex；　the　feathered　feet　and　skin　between　the　outer　toes　in　pigeons；　and　the　presence　of　more　or　less　down　on　the　young　birds　when　first　hatched；　with　the　future　colour　of　their　plumage；　or；　again；　the　relation　between　the　hair　and　teeth　in　the　naked　Turkish　dog；　though　here　probably　homology　comes　into　play？　With　respect　to　this　latter　case　of　correlation；　I　think　it　can　hardly　be　accidental；　that　if　we　pick　out　the　two　orders　of　mammalia　which　are　most　abnormal　in　their　dermal　coverings；　viz。　Cetacea　（whales）　and　Edentata　（armadilloes；　scaly　ant…eaters；　&c。）；　that　these　are　likewise　the　most　abnormal　in　their　teeth。　
I　know　of　no　case　better　adapted　to　show　the　importance　of　the　laws　of　correlation　in　modifying　important　structures；　independently　of　utility　and；　therefore；　of　natural　selection；　than　that　of　the　difference　between　the　outer　and　inner　flowers　in　some　Compositous　and　Umbelliferous　plants。　Every　one　knows　the　difference　in　the　ray　and　central　florets　of；　for　instance；　the　daisy；　and　this　difference　is　often　accompanied　with　the　abortion　of　parts　of　the　flower。　But；　in　some　Compositous　plants；　the　seeds　also　differ　in　shape　and　sculpture；　and　even　the　ovary　itself；　with　its　accessory　parts；　differs；　as　has　been　described　by　Cassini。　These　differences　have　been　attributed　by　some　authors　to　pressure；　and　the　shape　of　the　seeds　in　the　ray…florets　in　some　Compositae　countenances　this　idea；　but；　in　the　case　of　the　corolla　of　the　Umbelliferae；　it　is　by　no　means；　as　Dr　Hooker　informs　me；　in　species　with　the　densest　heads　that　the　inner　and　outer　flowers　most　frequently　differ。　It　might　have　been　thought　that　the　development　of　the　ray…petals　by　drawing　nourishment　from　certain　other　parts　of　the　flower　had　caused　their　abortion；　but　in　some　Compositae　there　is　a　difference　in　the　seeds　of　the　outer　and　inner　florets　without　any　difference　in　the　corolla。　Possibly；　these　several　differences　may　be　connected　with　some　difference　in　the　flow　of　nutriment　towards　the　central　and　external　flowers：　we　know；　at　least；　that　in　irregular　flowers；　those　nearest　to　the　axis　are　oftenest　subject　to　peloria；　and　become　regular。　I　may　add；　as　an　instance　of　this；　and　of　a　striking　case　of　correlation；　that　I　have　recently　observed　in　some　garden　pelargoniums；　that　the　central　flower　of　the　truss　often　loses　the　patches　of　darker　colour　in　the　two　upper　petals；　and　that　when　this　occurs；　the　adherent　nectary　is　quite　aborted；　when　the　colour　is　absent　from　only　one　of　the　two　upper　petals；　the　nectary　is　only　much　shortened。　
With　respect　to　the　difference　in　the　corolla　of　the　central　and　exterior　flowers　of　a　head　or　umbel；　I　do　not　feel　at　all　sure　that　C。　C。　Sprengel's　idea　that　the　ray…florets　serve　to　attract　insects；　whose　agency　is　highly　advantageous　in　the　fertilisation　of　plants　of　these　two　orders；　is　so　far…fetched；　as　it　may　at　first　appear：　and　if　it　be　advantageous；　natural　selection　may　have　come　into　play。　But　in　regard　to　the　differences　both　in　the　internal　and　external　structure　of　the　seeds；　which　are　not　always　correlated　with　any　differences　in　the　flowers；　it　seems　impossible　that　they　can　be　in　any　way　advantageous　to　the　plant：　yet　in　the　Umbelliferae　these　differences　are　of　such　apparent　importance　the　seeds　being　in　some　cases；　according　to　Tausch；　orthospermous　in　the　exterior　flowers　and　coelospermous　in　the　central　flowers；　that　the　elder　De　Candolle　founded　his　main　divisions　of　the　order　on　analogous　differences。　Hence　we　see　that　modifications　of　structure；　viewed　by　systematists　as　of　high　value；　may　be　wholly　due　to　unknown　laws　of　correlated　growth；　and　without　being；　as　far　as　we　can　see；　of　the　slightest　service　to　the　species。　
We　may　often　falsely　attribute　to　correlation　of　growth；　structures　which　are　common　to　whole　groups　of　species；　and　which　in　truth　are　simply　due　to　inheritance；　for　an　ancient　progenitor　may　have　acquired　through　natural　selection　some　one　modification　in　structure；　and；　after　thousands　of　generations；　some　other　and　independent　modification；　and　these　two　modifications；　having　been　transmitted　to　a　whole　group　of　descendants　with　diverse　habits；　would　naturally　be　thought　to　be　correlated　in　some　necessary　manner。　So；　again；　I　do　not　doubt　that　some　apparent　correlations；　occurring　throughout　whole　orders；　are　entirely　due　to　the　manner　alone　in　which　natural　selection　can　act。　For　instance；　Alph。　De　Candolle　has　remarked　that　winged　seeds　are　never　found　in　fruits　which　do　not　open：　I　should　explain　the　rule　by　the　fact　that　seeds　could　not　gradually　become　winged　through　natural　selection；　except　in　fruits　which　opened；　so　that　the　individual　plants　producing　seeds　which　were　a　little　better　fitted　to　be　wafted　further；　might　get　an　advantage　over　those　producing　seed　less　fitted　for　dispersal；　and　this　process　could　not　possi