the origin of species(物种起源)-第49部分

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ecies　have　been　specially　endowed　with　various　degrees　of　sterility　to　prevent　them　crossing　and　blending　in　nature；　than　to　think　that　trees　have　been　specially　endowed　with　various　and　somewhat　analogous　degrees　of　difficulty　in　being　grafted　together　in　order　to　prevent　them　becoming　inarched　in　our　forests。　
The　sterility　of　first　crosses　between　pure　species；　which　have　their　reproductive　systems　perfect；　seems　to　depend　on　several　circumstances；　in　some　cases　largely　on　the　early　death　of　the　embryo。　The　sterility　of　hybrids；　which　have　their　reproductive　systems　imperfect；　and　which　have　had　this　system　and　their　whole　organisation　disturbed　by　being　compounded　of　two　distinct　species；　seems　closely　allied　to　that　sterility　which　so　frequently　affects　pure　species；　when　their　natural　conditions　of　life　have　been　disturbed。　This　view　is　supported　by　a　parallelism　of　another　kind；　namely；　that　the　crossing　of　forms　only　slightly　different　is　favourable　to　the　vigour　and　fertility　of　their　offspring；　and　that　slight　changes　in　the　conditions　of　life　are　apparently　favourable　to　the　vigour　and　fertility　of　all　organic　beings。　It　is　not　surprising　that　the　degree　of　difficulty　in　uniting　two　species；　and　the　degree　of　sterility　of　their　hybrid…offspring　should　generally　correspond；　though　due　to　distinct　causes；　for　both　depend　on　the　amount　of　difference　of　some　kind　between　the　species　which　are　crossed。　Nor　is　it　surprising　that　the　facility　of　effecting　a　first　cross；　the　fertility　of　the　hybrids　produced；　and　the　capacity　of　being　grafted　together　though　this　latter　capacity　evidently　depends　on　widely　different　circumstances　should　all　run；　to　a　certain　extent；　parallel　with　the　systematic　affinity　of　the　forms　which　are　subjected　to　experiment；　for　systematic　affinity　attempts　to　express　all　kinds　of　resemblance　between　all　species。　
First　crosses　between　forms　known　to　be　varieties；　or　sufficiently　alike　to　be　considered　as　varieties；　and　their　mongrel　offspring；　are　very　generally；　but　not　quite　universally；　fertile。　Nor　is　this　nearly　general　and　perfect　fertility　surprising；　when　we　remember　how　liable　we　are　to　argue　in　a　circle　with　respect　to　varieties　in　a　state　of　nature；　and　when　we　remember　that　the　greater　number　of　varieties　have　been　produced　under　domestication　by　the　selection　of　mere　external　differences；　and　not　of　differences　in　the　reproductive　system。　In　all　other　respects；　excluding　fertility；　there　is　a　close　general　resemblance　between　hybrids　and　mongrels。　Finally；　then；　the　facts　briefly　given　in　this　chapter　do　not　seem　to　me　opposed　to；　but　even　rather　to　support　the　view；　that　there　is　no　fundamental　distinction　between　species　and　varieties。　　
Chapter　9　…　On　the　Imperfection　of　the　Geological　Record

IN　the　sixth　chapter　I　enumerated　the　chief　objections　which　might　be　justly　urged　against　the　views　maintained　in　this　volume。　Most　of　them　have　now　been　discussed。　One；　namely　the　distinctness　of　specific　forms；　and　their　not　being　blended　together　by　innumerable　transitional　links；　is　a　very　obvious　difficulty。　I　assigned　reasons　why　such　links　do　not　commonly　occur　at　the　present　day；　under　the　circumstances　apparently　most　favourable　for　their　presence；　namely　on　an　extensive　and　continuous　area　with　graduated　physical　conditions。　I　endeavoured　to　show；　that　the　life　of　each　species　depends　in　a　more　important　manner　on　the　presence　of　other　already　defined　organic　forms；　than　on　climate；　and；　therefore；　that　the　really　governing　conditions　of　life　do　not　graduate　away　quite　insensibly　like　heat　or　moisture。　I　endeavoured；　also；　to　show　that　intermediate　varieties；　from　existing　in　lesser　numbers　than　the　forms　which　they　connect；　will　generally　be　beaten　out　and　exterminated　during　the　course　of　further　modification　and　improvement。　The　main　cause；　however；　of　innumerable　intermediate　links　not　now　occurring　everywhere　throughout　nature　depends　on　the　very　process　of　natural　selection；　through　which　new　varieties　continually　take　the　places　of　and　exterminate　their　parent…forms。　But　just　in　proportion　as　this　process　of　extermination　has　acted　on　an　enormous　scale；　so　must　the　number　of　intermediate　varieties；　which　have　formerly　existed　on　the　earth；　be　truly　enormous。　Why　then　is　not　every　geological　formation　and　every　stratum　full　of　such　intermediate　links？　Geology　assuredly　does　not　reveal　any　such　finely　graduated　organic　chain；　and　this；　perhaps；　is　the　most　obvious　and　gravest　objection　which　can　be　urged　against　my　theory。　The　explanation　lies；　as　I　believe；　in　the　extreme　imperfection　of　the　geological　record。　
In　the　first　place　it　should　always　be　borne　in　mind　what　sort　of　intermediate　forms　must；　on　my　theory；　have　formerly　existed。　I　have　found　it　difficult；　when　looking　at　any　two　species；　to　avoid　picturing　to　myself；　forms　directly　intermediate　between　them。　But　this　is　a　wholly　false　view；　we　should　always　look　for　forms　intermediate　between　each　species　and　a　common　but　unknown　progenitor；　and　the　progenitor　will　generally　have　differed　in　some　respects　from　all　its　modified　descendants。　To　give　a　simple　illustration：　the　fantail　and　pouter　pigeons　have　both　descended　from　the　rock…pigeon；　if　we　possessed　all　the　intermediate　varieties　which　have　ever　existed；　we　should　have　an　extremely　close　series　between　both　and　the　rock…pigeon；　but　we　should　have　no　varieties　directly　intermediate　between　the　fantail　and　pouter；　none；　for　instance；　combining　a　tail　somewhat　expanded　with　a　crop　somewhat　enlarged；　the　characteristic　features　of　these　two　breeds。　These　two　breeds；　moreover；　have　become　so　much　modified；　that　if　we　had　no　historical　or　indirect　evidence　regarding　their　origin；　it　would　not　have　been　possible　to　have　determined　from　a　mere　comparison　of　their　structure　with　that　of　the　rock…pigeon；　whether　they　had　descended　from　this　species　or　from　some　other　allied　species；　such　as　C。　oenas。　
So　with　natural　species；　if　we　look　to　forms　very　distinct；　for　instance　to　the　horse　and　tapir；　we　have　no　reason　to　suppose　that　links　ever　existed　directly　intermediate　between　them；　but　between　each　and　an　unknown　common　parent。　The　common　parent　will　have　had　in　its　whole　organisation　much　general　resemblance　to　the　tapir　and　to　the　horse；　but　in　some　points　of　structure　may　have　differed　considerably　from　both；　even　perhaps　more　than　they　differ　from　each　other。　Hence　in　all　such　cases；　we　should　be　unable　to　recognise　the　parent…form　of　any　two　or　more　species；　even　if　we　closely　compared　the　structure　of　the　parent　with　that　of　its　modified　descendants；　unless　at　the　same　time　we　had　a　nearly　perfect　chain　of　the　intermediate　links。　
It　is　just　possible　by　my　theory；　that　one　of　two　living　forms　might　have　descended　from　the　other；　for　instance；　a　horse　from　a　tapir；　and　in　this　case　direct　intermediate　links　will　have　existed　between　them。　But　such　a　case　would　imply　that　one　form　had　remained　for　a　very　long　period　unaltered；　whilst　its　descendants　had　undergone　a　vast　amount　of　change；　and　the　principle　of　competition　between　organism　and　organism；　between　child　and　parent；　will　render　this　a　very　rare　event；　for　in　all　cases　the　new　and　improved　forms　of　life　will　tend　to　supplant　the　old　and　unimproved。　
By　the　theory　of　natural　selection　all　living　species　have　been　connected　with　the　parent…species　of　each　genus；　by　differences　not　greater　than　we　see　between　the　varieties　of　the　same　species　at　the　present　day；　and　these　parent…species；　now　generally　extinct；　have　in　their　turn　been　similarly　connected　with　more　ancient　species；　and　so　on　backwards；　always　converging　to　the　common　ancestor　of　each　great　class。　So　that　the　number　of　intermediate　and　transitional　links；　between　all　living　and　extinct　species；　must　have　been　inconceivably　great。　But　assuredly；　if　this　theory　be　true；　such　have　lived　upon　this　earth。　
On　the　lapse　of　Time。　Independently　of　our　not　finding　fossil　remains　of　such　infinitely　numerous　connecting　links；　it　may　be　objected；　that　time　will　not　have　sufficed　for　so　great　an　amount　of　organic　change；　all　changes　having　been　effected　very　slowly　through　natural　selection。　It　is　hardly　possible　for　me　even　to　recall　to　the　reader；　who　may　not　be　a　practical　geologist；　the　facts　leading　the　mind　feebly　to　comprehend　the　lapse　of　time。　He　who　can　read　Sir　Charles　Lyell's　grand　work　on　the　Principles　of　Geology；　which　the　future　historian　will　recognise　as　having　produced　a　revolution　in　natural　science；　yet　does　not　admit　how　incomprehensibly　vast　have　been　the　past　periods　of　time；　may　at　once　close　this　volume。　Not　that　it　suffices　to　study　the　Principles　of　Geology；　or　to　read　special　treatises　by　different　observers　on　separate　formations；　and　to　mark　how　each　author　attempts　to　give　an　inadequate　idea　of　the　duration　of　each　formation　or　even　each　stratum。　A　man　must　for　years　examine　for　himself　great　piles　of　superimposed　strata；　and　watch　the　sea　at　work　grinding　down　old　rocks　and　making　fresh　sediment；　before　he　can　hope　to　comprehend　anything　of　the　lapse　of　time；　the　monuments　of　which　we　see　around　us。　
It　is　good　to　wander　along　lines　of　sea…coast；　when　formed　of　moderately　hard　rocks；　and　mark　the　process　of　degradation。　The　tides　in　most　cases　reach　the　cliffs　only　for　a　short　time　twice　a　day；　and　the　waves　eat　into　them　only　when　they　are　charged　with　sand　or　pebbles；　for　there　is　reason　to　believe　that　pure　water　can　effect　little　or　nothing　in　wearing　away　rock。　At　last　the　base　of　the　cliff　is　undermined；　huge　fragments　fall　down；　and　these　remaining　fixed；　have　to　be　worn　away；　atom　by　atom；　until　reduced　in　size　they　can　be　rolled　about　by　the　waves；　and　then　are　more　quickly　ground　into　pebbles；　sand；　or　mud。　But　how　often　do　we　see　along　the　bases　of　retreating　cliffs　rounded　boulders；　all　thickly　clothed　by　marine　productions；　showing　how　little　they　are　abraded　and　how　seldom　they　are　rolled　about！　Moreover；　if　we　follow　for　a　few　miles　any　line　of　rocky　cliff；　which　is　undergoing　degradation；　we　find　that　it　is　only　here　and　there；　along　a　short　length　or　round　a　promontory；　that　the　cliffs　are　at　the　present　time　suffering。　The　appearance　of　the　surface　and　the　vegetation　show　that　elsewhere　years　have　elapsed　since