the origin of species(物种起源)-第34部分

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；　likewise；　very　slightly　aided　the　act　of　respiration；　have　been　gradually　converted　by　natural　selection　into　branchiae；　simply　through　an　increase　in　their　size　and　the　obliteration　of　their　adhesive　glands。　If　all　pedunculated　cirripedes　had　become　extinct；　and　they　have　already　suffered　far　more　extinction　than　have　sessile　cirripedes；　who　would　ever　have　imagined　that　the　branchiae　in　this　latter　family　had　originally　existed　as　organs　for　preventing　the　ova　from　being　washed　out　of　the　sack？　
Although　we　must　be　extremely　cautious　in　concluding　that　any　organ　could　not　possibly　have　been　produced　by　successive　transitional　gradations；　yet；　undoubtedly；　grave　cases　of　difficulty　occur；　some　of　which　will　be　discussed　in　my　future　work。　
One　of　the　gravest　is　that　of　neuter　insects；　which　are　often　very　differently　constructed　from　either　the　males　or　fertile　females；　but　this　case　will　be　treated　of　in　the　next　chapter。　The　electric　organs　of　fishes　offer　another　case　of　special　difficulty；　it　is　impossible　to　conceive　by　what　steps　these　wondrous　organs　have　been　produced；　but；　as　Owen　and　others　have　remarked；　their　intimate　structure　closely　resembles　that　of　common　muscle；　and　as　it　has　lately　been　shown　that　Rays　have　an　organ　closely　analogous　to　the　electric　apparatus；　and　yet　do　not；　as　Matteuchi　asserts；　discharge　any　electricity；　we　must　own　that　we　are　far　too　ignorant　to　argue　that　no　transition　of　any　kind　is　possible。　
The　electric　organs　offer　another　and　even　more　serious　difficulty；　for　they　occur　in　only　about　a　dozen　fishes；　of　which　several　are　widely　remote　in　their　affinities。　Generally　when　the　same　organ　appears　in　several　members　of　the　same　class；　especially　if　in　members　having　very　different　habits　of　life；　we　may　attribute　its　presence　to　inheritance　from　a　common　ancestor；　and　its　absence　in　some　of　the　members　to　its　loss　through　disuse　or　natural　selection。　But　if　the　electric　organs　had　been　inherited　from　one　ancient　progenitor　thus　provided；　we　might　have　expected　that　all　electric　fishes　would　have　been　specially　related　to　each　other。　Nor　does　geology　at　all　lead　to　the　belief　that　formerly　most　fishes　had　electric　organs；　which　most　of　their　modified　descendants　have　lost。　The　presence　of　luminous　organs　in　a　few　insects；　belonging　to　different　families　and　orders；　offers　a　parallel　case　of　difficulty。　Other　cases　could　be　given；　for　instance　in　plants；　the　very　curious　contrivance　of　a　mass　of　pollen…grains；　borne　on　a　foot…stalk　with　a　sticky　gland　at　the　end；　is　the　same　in　Orchis　and　Asclepias；　genera　almost　as　remote　as　possible　amongst　flowering　plants。　In　all　these　cases　of　two　very　distinct　species　furnished　with　apparently　the　same　anomalous　organ；　it　should　be　observed　that；　although　the　general　appearance　and　function　of　the　organ　may　be　the　same；　yet　some　fundamental　difference　can　generally　be　detected。　I　am　inclined　to　believe　that　in　nearly　the　same　way　as　two　men　have　sometimes　independently　hit　on　the　very　same　invention；　so　natural　selection；　working　for　the　good　of　each　being　and　taking　advantage　of　analogous　variations；　has　sometimes　modified　in　very　nearly　the　same　manner　two　parts　in　two　organic　beings；　which　owe　but　little　of　their　structure　in　common　to　inheritance　from　the　same　ancestor。　
Although　in　many　cases　it　is　most　difficult　to　conjecture　by　what　transitions　an　organ　could　have　arrived　at　its　present　state；　yet；　considering　that　the　proportion　of　living　and　known　forms　to　the　extinct　and　unknown　is　very　small；　I　have　been　astonished　how　rarely　an　organ　can　be　named；　towards　which　no　transitional　grade　is　known　to　lead。　The　truth　of　this　remark　is　indeed　shown　by　that　old　canon　in　natural　history　of　'Natura　non　facit　saltum。'　We　meet　with　this　admission　in　the　writings　of　almost　every　experienced　naturalist；　or；　as　Milne　Edwards　has　well　expressed　it；　nature　is　prodigal　in　variety；　but　niggard　in　innovation。　Why；　on　the　theory　of　Creation；　should　this　be　so？　Why　should　all　the　parts　and　organs　of　many　independent　beings；　each　supposed　to　have　been　separately　created　for　its　proper　place　in　nature；　be　so　invariably　linked　together　by　graduated　steps？　Why　should　not　Nature　have　taken　a　leap　from　structure　to　structure？　On　the　theory　of　natural　selection；　we　can　clearly　understand　why　she　should　not；　for　natural　selection　can　act　only　by　taking　advantage　of　slight　successive　variations；　she　can　never　take　a　leap；　but　must　advance　by　the　shortest　and　slowest　steps。　
Organs　of　little　apparent　importance。　As　natural　selection　acts　by　life　and　death；　by　the　preservation　of　individuals　with　any　favourable　variation；　and　by　the　destruction　of　those　with　any　unfavourable　deviation　of　structure；　I　have　sometimes　felt　much　difficulty　in　understanding　the　origin　of　simple　parts；　of　which　the　importance　does　not　seem　sufficient　to　cause　the　preservation　of　successively　varying　individuals。　I　have　sometimes　felt　as　much　difficulty；　though　of　a　very　different　kind；　on　this　head；　as　in　the　case　of　an　organ　as　perfect　and　complex　as　the　eye。　
In　the　first　place；　we　are　much　too　ignorant　in　regard　to　the　whole　economy　of　any　one　organic　being；　to　say　what　slight　modifications　would　be　of　importance　or　not。　In　a　former　chapter　I　have　given　instances　of　most　trifling　characters；　such　as　the　down　on　fruit　and　the　colour　of　the　flesh；　which；　from　determining　the　attacks　of　insects　or　from　being　correlated　with　constitutional　differences；　might　assuredly　be　acted　on　by　natural　selection。　The　tail　of　the　giraffe　looks　like　an　artificially　constructed　fly…flapper；　and　it　seems　at　first　incredible　that　this　could　have　been　adapted　for　its　present　purpose　by　successive　slight　modifications；　each　better　and　better；　for　so　trifling　an　object　as　driving　away　flies；　yet　we　should　pause　before　being　too　positive　even　in　this　case；　for　we　know　that　the　distribution　and　existence　of　cattle　and　other　animals　in　South　America　absolutely　depends　on　their　power　of　resisting　the　attacks　of　insects：　so　that　individuals　which　could　by　any　means　defend　themselves　from　these　small　enemies；　would　be　able　to　range　into　new　pastures　and　thus　gain　a　great　advantage。　It　is　not　that　the　larger　quadrupeds　are　actually　destroyed　（except　in　some　rare　cases）　by　the　flies；　but　they　are　incessantly　harassed　and　their　strength　reduced；　so　that　they　are　more　subject　to　disease；　or　not　so　well　enabled　in　a　coming　dearth　to　search　for　food；　or　to　escape　from　beasts　of　prey。　
Organs　now　of　trifling　importance　have　probably　in　some　cases　been　of　high　importance　to　an　early　progenitor；　and；　after　having　been　slowly　perfected　at　a　former　period；　have　been　transmitted　in　nearly　the　same　state；　although　now　become　of　very　slight　use；　and　any　actually　injurious　deviations　in　their　structure　will　always　have　been　checked　by　natural　selection。　Seeing　how　important　an　organ　of　locomotion　the　tail　is　in　most　aquatic　animals；　its　general　presence　and　use　for　many　purposes　in　so　many　land　animals；　which　in　their　lungs　or　modified　swim…bladders　betray　their　aquatic　origin；　may　perhaps　be　thus　accounted　for。　A　well…developed　tail　having　been　formed　in　an　aquatic　animal；　it　might　subsequently　come　to　be　worked　in　for　all　sorts　of　purposes；　as　a　fly…flapper；　an　organ　of　prehension；　or　as　an　aid　in　turning；　as　with　the　dog；　though　the　aid　must　be　slight；　for　the　hare；　with　hardly　any　tail；　can　double　quickly　enough。　
In　the　second　place；　we　may　sometimes　attribute　importance　to　characters　which　are　really　of　very　little　importance；　and　which　have　originated　from　quite　secondary　causes；　independently　of　natural　selection。　We　should　remember　that　climate；　food；　&c。；　probably　have　some　little　direct　influence　on　the　organisation；　that　characters　reappear　from　the　law　of　reversion；；　that　correlation　of　growth　will　have　had　a　most　important　influence　in　modifying　various　structures；　and　finally；　that　sexual　selection　will　often　have　largely　modified　the　external　characters　of　animals　having　a　will；　to　give　one　male　an　advantage　in　fighting　with　another　or　in　charming　the　females。　Moreover　when　a　modification　of　structure　has　primarily　arisen　from　the　above　or　other　unknown　causes；　it　may　at　first　have　been　of　no　advantage　to　the　species；　but　may　subsequently　have　been　taken　advantage　of　by　the　descendants　of　the　species　under　new　conditions　of　life　and　with　newly　acquired　habits。　
To　give　a　few　instances　to　illustrate　these　latter　remarks。　If　green　woodpeckers　alone　had　existed；　and　we　did　not　know　that　there　were　many　black　and　pied　kinds；　I　dare　say　that　we　should　have　thought　that　the　green　colour　was　a　beautiful　adaptation　to　hide　this　tree…frequenting　bird　from　its　enemies；　and　consequently　that　it　was　a　character　of　importance　and　might　have　been　acquired　through　natural　selection；　as　it　is；　I　have　no　doubt　that　the　colour　is　due　to　some　quite　distinct　cause；　probably　to　sexual　selection。　A　trailing　bamboo　in　the　Malay　Archipelago　climbs　the　loftiest　trees　by　the　aid　of　exquisitely　constructed　hooks　clustered　around　the　ends　of　the　branches；　and　this　contrivance；　no　doubt；　is　of　the　highest　service　to　the　plant；　but　as　we　see　nearly　similar　hooks　on　many　trees　which　are　not　climbers　the　hooks　on　the　bamboo　may　have　arisen　from　unknown　laws　of　growth；　and　have　been　subsequently　taken　advantage　of　by　the　plant　undergoing　further　modification　and　becoming　a　climber。　The　naked　skin　on　the　head　of　a　vulture　is　generally　looked　at　as　a　direct　adaptation　for　wallowing　in　putridity；　and　so　it　may　be；　or　it　may　possibly　be　due　to　the　direct　action　of　putrid　matter；　but　we　should　be　very　cautious　in　drawing　any　such　inference；　when　we　see　that　the　skin　on　the　head　of　the　clean…feeding　male　turkey　is　likewise　naked。　The　sutures　in　the　skulls　of　young　mammals　have　been　advanced　as　a　beautiful　adaptation　for　aiding　parturition；　and　no　doubt　they　facilitate；　or　may　be　indispensable　for　this　act；　but　as　sutures　occur　in　the　skulls　of　young　birds　and　reptiles；　which　have　only　to　escape　from　a　broken　egg；　we　may　infer　that　this　structure　has　arisen　from　the　laws　of　growth；　and　has　been　taken　advantage　of　in　the　parturition　of　the　higher　animals。　
We　are　profoundly　ignorant　of　the　causes　producing　slight　and　unimportant　variations；　and　we　are　immediate