the origin of species(物种起源)-第19章

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qual　periods。　Only　those　variations　which　are　in　some　way　profitable　will　be　preserved　or　naturally　selected。　And　here　the　importance　of　the　principle　of　benefit　being　derived　from　divergence　of　character　comes　in；　for　this　will　generally　lead　to　the　most　different　or　divergent　variations　（represented　by　the　outer　dotted　lines）　being　preserved　and　accumulated　by　natural　selection。　When　a　dotted　line　reaches　one　of　the　horizontal　lines；　and　is　there　marked　by　a　small　numbered　letter；　a　sufficient　amount　of　variation　is　supposed　to　have　been　accumulated　to　have　formed　a　fairly　well…marked　variety；　such　as　would　be　thought　worthy　of　record　in　a　systematic　work。　
The　intervals　between　the　horizontal　lines　in　the　diagram；　may　represent　each　a　thousand　generations；　but　it　would　have　been　better　if　each　had　represented　ten　thousand　generations。　After　a　thousand　generations；　species　（A）　is　supposed　to　have　produced　two　fairly　well…marked　varieties；　namely　a1　and　m1。　These　two　varieties　will　generally　continue　to　be　exposed　to　the　same　conditions　which　made　their　parents　variable；　and　the　tendency　to　variability　is　in　itself　hereditary；　consequently　they　will　tend　to　vary；　and　generally　to　vary　in　nearly　the　same　manner　as　their　parents　varied。　Moreover；　these　two　varieties；　being　only　slightly　modified　forms；　will　tend　to　inherit　those　advantages　which　made　their　common　parent　（A）　more　numerous　than　most　of　the　other　inhabitants　of　the　same　country；　they　will　likewise　partake　of　those　more　general　advantages　which　made　the　genus　to　which　the　parent…species　belonged；　a　large　genus　in　its　own　country。　And　these　circumstances　we　know　to　be　favourable　to　the　production　of　new　varieties。　
If；　then；　these　two　varieties　be　variable；　the　most　divergent　of　their　variations　will　generally　be　preserved　during　the　next　thousand　generations。　And　after　this　interval；　variety　a1　is　supposed　in　the　diagram　to　have　produced　variety　a2；　which　will；　owing　to　the　principle　of　divergence；　differ　more　from　（A）　than　did　variety　a1。　Variety　m1　is　supposed　to　have　produced　two　varieties；　namely　m　2　and　s2；　differing　from　each　other；　and　more　considerably　from　their　common　parent　（A）。　We　may　continue　the　process　by　similar　steps　for　any　length　of　time；　some　of　the　varieties；　after　each　thousand　generations；　producing　only　a　single　variety；　but　in　a　more　and　more　modified　condition；　some　producing　two　or　three　varieties；　and　some　failing　to　produce　any。　Thus　the　varieties　or　modified　descendants；　proceeding　from　the　common　parent　（A）；　will　generally　go　on　increasing　in　number　and　diverging　in　character。　In　the　diagram　the　process　is　represented　up　to　the　ten…thousandth　generation；　and　under　a　condensed　and　simplified　form　up　to　the　fourteen…thousandth　generation。　
But　I　must　here　remark　that　I　do　not　suppose　that　the　process　ever　goes　on　so　regularly　as　is　represented　in　the　diagram；　though　in　itself　made　somewhat　irregular。　I　am　far　from　thinking　that　the　most　divergent　varieties　will　invariably　prevail　and　multiply：　a　medium　form　may　often　long　endure；　and　may　or　may　not　produce　more　than　one　modified　descendant；　for　natural　selection　will　always　act　according　to　the　nature　of　the　places　which　are　either　unoccupied　or　not　perfectly　occupied　by　other　beings；　and　this　will　depend　on　infinitely　complex　relations。　But　as　a　general　rule；　the　more　diversified　in　structure　the　descendants　from　any　one　species　can　be　rendered；　the　more　places　they　will　be　enabled　to　seize　on；　and　the　more　their　modified　progeny　will　be　increased。　In　our　diagram　the　line　of　succession　is　broken　at　regular　intervals　by　small　numbered　letters　marking　the　successive　forms　which　have　become　sufficiently　distinct　to　be　recorded　as　varieties。　But　these　breaks　are　imaginary；　and　might　have　been　inserted　anywhere；　after　intervals　long　enough　to　have　allowed　the　accumulation　of　a　considerable　amount　of　divergent　variation。　
As　all　the　modified　descendants　from　a　common　and　widely…diffused　species；　belonging　to　a　large　genus；　will　tend　to　partake　of　the　same　advantages　which　made　their　parent　successful　in　life；　they　will　generally　go　on　multiplying　in　number　as　well　as　diverging　in　character：　this　is　represented　in　the　diagram　by　the　several　divergent　branches　proceeding　from　（A）。　The　modified　offspring　from　the　later　and　more　highly　improved　branches　in　the　lines　of　descent；　will；　it　is　probable；　often　take　the　place　of；　and　so　destroy；　the　earlier　and　less　improved　branches：　this　is　represented　in　the　diagram　by　some　of　the　lower　branches　not　reaching　to　the　upper　horizontal　lines。　In　some　cases　I　do　not　doubt　that　the　process　of　modification　will　be　confined　to　a　single　line　of　descent；　and　the　number　of　the　descendants　will　not　be　increased；　although　the　amount　of　divergent　modification　may　have　been　increased　in　the　successive　generations。　This　case　would　be　represented　in　the　diagram；　if　all　the　lines　proceeding　from　（A）　were　removed；　excepting　that　from　a1　to　a10　In　the　same　way；　for　instance；　the　English　race…horse　and　English　pointer　have　apparently　both　gone　on　slowly　diverging　in　character　from　their　original　stocks；　without　either　having　given　off　any　fresh　branches　or　races。　
After　ten　thousand　generations；　species　（A）　is　supposed　to　have　produced　three　forms；　a10；　f10；　and　m10；　which；　from　having　diverged　in　character　during　the　successive　generations；　will　have　come　to　differ　largely；　but　perhaps　unequally；　from　each　other　and　from　their　common　parent。　If　we　suppose　the　amount　of　change　between　each　horizontal　line　in　our　diagram　to　be　excessively　small；　these　three　forms　may　still　be　only　well…marked　varieties；　or　they　may　have　arrived　at　the　doubtful　category　of　sub…species；　but　we　have　only　to　suppose　the　steps　in　the　process　of　modification　to　be　more　numerous　or　greater　in　amount；　to　convert　these　three　forms　into　well…defined　species：　thus　the　diagram　illustrates　the　steps　by　which　the　small　differences　distinguishing　varieties　are　increased　into　the　larger　differences　distinguishing　species。　By　continuing　the　same　process　for　a　greater　number　of　generations　（as　shown　in　the　diagram　in　a　condensed　and　simplified　manner）；　we　get　eight　species；　marked　by　the　letters　between　a14　and　m14；　all　descended　from　（A）。　Thus；　as　I　believe；　species　are　multiplied　and　genera　are　formed。　
In　a　large　genus　it　is　probable　that　more　than　one　species　would　vary。　In　the　diagram　I　have　assumed　that　a　second　species　（I）　has　produced；　by　analogous　steps；　after　ten　thousand　generations；　either　two　well…marked　varieties　（w10　and　z10）　or　two　species；　according　to　the　amount　of　change　supposed　to　be　represented　between　the　horizontal　lines。　After　fourteen　thousand　generations；　six　new　species；　marked　by　the　letters　n14　to　z14；　are　supposed　to　have　been　produced。　In　each　genus；　the　species；　which　are　already　extremely　different　in　character；　will　generally　tend　to　produce　the　greatest　number　of　modified　descendants；　for　these　will　have　the　best　chance　of　filling　new　and　widely　different　places　in　the　polity　of　nature：　hence　in　the　diagram　I　have　chosen　the　extreme　species　（A）；　and　the　nearly　extreme　species　（I）；　as　those　which　have　largely　varied；　and　have　given　rise　to　new　varieties　and　species。　The　other　nine　species　（marked　by　capital　letters）　of　our　original　genus；　may　for　a　long　period　continue　transmitting　unaltered　descendants；　and　this　is　shown　in　the　diagram　by　the　dotted　lines　not　prolonged　far　upwards　from　want　of　space。　
But　during　the　process　of　modification；　represented　in　the　diagram；　another　of　our　principles；　namely　that　of　extinction；　will　have　played　an　important　part。　As　in　each　fully　stocked　country　natural　selection　necessarily　acts　by　the　selected　form　having　some　advantage　in　the　struggle　for　life　over　other　forms；　there　will　be　a　constant　tendency　in　the　improved　descendants　of　any　one　species　to　supplant　and　exterminate　in　each　stage　of　descent　their　predecessors　and　their　original　parent。　For　it　should　be　remembered　that　the　competition　will　generally　be　most　severe　between　those　forms　which　are　most　nearly　related　to　each　other　in　habits；　constitution；　and　structure。　Hence　all　the　intermediate　forms　between　the　earlier　and　later　states；　that　is　between　the　less　and　more　improved　state　of　a　species；　as　well　as　the　original　parent…species　itself；　will　generally　tend　to　become　extinct。　So　it　probably　will　be　with　many　whole　collateral　lines　of　descent；　which　will　be　conquered　by　later　and　improved　lines　of　descent。　If；　however；　the　modified　offspring　of　a　species　get　into　some　distinct　country；　or　become　quickly　adapted　to　some　quite　new　station；　in　which　child　and　parent　do　not　come　into　competition；　both　may　continue　to　exist。　
If　then　our　diagram　be　assumed　to　represent　a　considerable　amount　of　modification；　species　（A）　and　all　the　earlier　varieties　will　have　become　extinct；　having　been　replaced　by　eight　new　species　（a14　to　m14）；　and　（I）　will　have　been　replaced　by　six　（n14　to　z14）　new　species。　
But　we　may　go　further　than　this。　The　original　species　of　our　genus　were　supposed　to　resemble　each　other　in　unequal　degrees；　as　is　so　generally　the　case　in　nature；　species　（A）　being　more　nearly　related　to　B；　C；　and　D；　than　to　the　other　species；　and　species　（I）　more　to　G；　H；　K；　L；　than　to　the　others。　These　two　species　（A）　and　（I）；　were　also　supposed　to　be　very　common　and　widely　diffused　species；　so　that　they　must　originally　have　had　some　advantage　over　most　of　the　other　species　of　the　genus。　Their　modified　descendants；　fourteen　in　number　at　the　fourteen…thousandth　generation；　will　probably　have　inherited　some　of　the　same　advantages：　they　have　also　been　modified　and　improved　in　a　diversified　manner　at　each　stage　of　descent；　so　as　to　have　become　adapted　to　many　related　places　in　the　natural　economy　of　their　country。　It　seems；　therefore；　to　me　extremely　probable　that　they　will　have　taken　the　places　of；　and　thus　exterminated；　not　only　their　parents　（A）　and　（I）；　but　likewise　some　of　the　original　species　which　were　most　nearly　related　to　their　parents。　Hence　very　few　of　the　original　species　will　have　transmitted　offspring　to　the　fourteen…thousandth　generation。　We　may　suppose　that　only　one　（F）；　of　the　two　species　which　were　least　closely　related　to　the　other　nine　original　species；　has　transmitted　descendants　to　this　late　stage　of　descent。　
The　new　species　in　our　diagram　descended　from　the　original　eleven　species；　will　now　be　fifteen　in　number。　Owing　to　the　divergent　tendency　of　natural　selection；　the　extreme　amount　of　difference　in　character　between　species　a14　and　z14　will　be　much　greater　than　that　between　the　most　different　of　the　original　eleven　species。　The　new　species；　moreover；　will　be　allied　to　each　other　in　a　widely　different　manner。　Of　the　eight　descendants　from　（A）　the　three　marked　a14；　q14；　p14；　will　be　nearly　related　from　having　recently　branched　off　from　a14；　b14　and　f14；　from　having　diverged　at　an　earlier　period　from　a5；　will　be　in　some　degree　distinct　from　the　three　first…named　species；　and　lastly；　o14；　e14；　and　m14；　will　be　nearly　related　one　to　the　other；　but　from　having　diverged　at　the　first　commencement　of　the　process　of　modification；　will　be　widely　different　from　the　other　five　species；　and　may　constitute　a　sub…genus　or　even　a　distinct　genus。　The　six　descendants　from　（I）　will　form