darwin and modern science-第131节

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…　fertilisation　and　hybridisation。　　The　heterostyled　plants　are　normally　dependent　on　a　process　corresponding　to　hybridisation。　　The　view　that　specifically　distinct　species　could　at　best　produce　sterile　hybrids　was　always　opposed　by　Darwin。　　But　if　the　good　results　of　crossing　were　EXCLUSIVELY　dependent　on　the　fact　that　we　are　concerned　with　hybrids；　there　must　then　be　a　demonstration　of　two　distinct　things。　　First；　that　crossing　with　a　fresh　stock　belonging　to　the　same　systematic　entity　or　to　the　same　hybrid；　but　cultivated　for　a　considerable　time　under　different　conditions；　shows　no　superiority　over　self…fertilisation；　and　that　in　pure　species　crossing　gives　no　better　results　than　self…pollination。　　If　this　were　the　case；　we　should　be　better　able　to　understand　why　in　one　plant　crossing　is　advantageous　while　in　others；　such　as　Darwin's　Hero　and　the　forms　of　Mimulus　and　Nicotiana　no　advantage　is　gained；　these　would　then　be　pure　species。　　But　such　a　proof　has　not　been　supplied；　the　inference　drawn　from　cleistogamous　and　cleistopetalous　plants　is　not　supported　by　evidence；　and　the　experiments　on　geitonogamy　and　on　the　advantage　of　cross…fertilisation　in　species　which　are　usually　self…fertilised　are　opposed　to　this　view。　　There　are　still　but　few　researches　on　this　point；　Darwin　found　that　in　Ononis　minutissima；　which　produces　cleistogamous　as　well　as　self…fertile　chasmogamous　flowers；　the　crossed　and　self…fertilised　capsules　produced　seed　in　the　proportion　of　100：65　and　that　the　average　bore　the　proportion　100：86。　　Facts　previously　mentioned　are　also　applicable　to　this　case。　　Further；　it　is　certain　that　the　self…sterility　exhibited　by　many　plants　has　nothing　to　do　with　hybridisation。　　Between　self…sterility　and　reduced　fertility　as　the　result　of　self…fertilisation　there　is　probably　no　fundamental　difference。

It　is　certain　that　so　difficult　a　problem　as　that　of　the　significance　of　sexual　reproduction　requires　much　more　investigation。　　Darwin　was　anything　but　dogmatic　and　always　ready　to　alter　an　opinion　when　it　was　not　based　on　definite　proof：　　he　wrote；　〃But　the　veil　of　secrecy　is　as　yet　far　from　lifted；　nor　will　it　be；　until　we　can　say　why　it　is　beneficial　that　the　sexual　elements　should　be　differentiated　to　a　certain　extent；　and　why；　if　the　differentiation　be　carried　still　further；　injury　follows。〃　　He　has　also　shown　us　the　way　along　which　to　follow　up　this　problem；　it　is　that　of　carefully　planned　and　exact　experimental　research。　　It　may　be　that　eventually　many　things　will　be　viewed　in　a　different　light；　but　Darwin's　investigations　will　always　form　the　foundation　of　Floral　Biology　on　which　the　future　may　continue　to　build。


XXI。　　MENTAL　FACTORS　IN　EVOLUTION。

By　C。　LLOYD　MORGAN；　LL。D。；　F。R。S。

In　developing　his　conception　of　organic　evolution　Charles　Darwin　was　of　necessity　brought　into　contact　with　some　of　the　problems　of　mental　evolution。　　In　〃The　Origin　of　Species〃　he　devoted　a　chapter　to　〃the　diversities　of　instinct　and　of　the　other　mental　faculties　in　animals　of　the　same　class。〃　　（〃Origin　of　Species〃　（6th　edition）；　page　205。）　　When　he　passed　to　the　detailed　consideration　of　〃The　Descent　of　Man〃；　it　was　part　of　his　object　to　show　〃that　there　is　no　fundamental　difference　between　man　and　the　higher　mammals　in　their　mental　faculties。〃　　（〃Descent　of　Man〃　（2nd　edition　1888）；　Vol。　I。　page　99；　Popular　edition　page　99。）　　〃If　no　organic　being　excepting　man；〃　he　said；　〃had　possessed　any　mental　power；　or　if　his　powers　had　been　of　a　wholly　different　nature　from　those　of　the　lower　animals；　then　we　should　never　have　been　able　to　convince　ourselves　that　our　high　faculties　had　been　gradually　developed。〃　　（Ibid。　page　99。）　　In　his　discussion　of　〃The　Expression　of　the　Emotions〃　it　was　important　for　his　purpose　〃fully　to　recognise　that　actions　readily　become　associated　with　other　actions　and　with　various　states　of　the　mind。〃　　（〃The　Expression　of　the　Emotions〃　（2nd　edition）；　page　32。）　　His　hypothesis　of　sexual　selection　is　largely　dependent　upon　the　exercise　of　choice　on　the　part　of　the　female　and　her　preference　for　〃not　only　the　more　attractive　but　at　the　same　time　the　more　vigorous　and　victorious　males。〃　　（〃Descent　of　Man〃；　Vol。　II。　page　435。）　　Mental　processes　and　physiological　processes　were　for　Darwin　closely　correlated；　and　he　accepted　the　conclusion　〃that　the　nervous　system　not　only　regulates　most　of　the　existing　functions　of　the　body；　but　has　indirectly　influenced　the　progressive　development　of　various　bodily　structures　and　of　certain　mental　qualities。〃　　（Ibid。　pages　437；　438。）

Throughout　his　treatment；　mental　evolution　was　for　Darwin　incidental　to　and　contributory　to　organic　evolution。　　For　specialised　research　in　comparative　and　genetic　psychology；　as　an　independent　field　of　investigation；　he　had　neither　the　time　nor　the　requisite　training。　　None　the　less　his　writings　and　the　spirit　of　his　work　have　exercised　a　profound　influence　on　this　department　of　evolutionary　thought。　　And；　for　those　who　follow　Darwin's　lead；　mental　evolution　is　still　in　a　measure　subservient　to　organic　evolution。　　Mental　processes　are　the　accompaniments　or　concomitants　of　the　functional　activity　of　specially　differentiated　parts　of　the　organism。　　They　are　in　some　way　dependent　on　physiological　and　physical　conditions。　　But　though　they　are　not　physical　in　their　nature；　and　though　it　is　difficult　or　impossible　to　conceive　that　they　are　physical　in　their　origin；　they　are；　for　Darwin　and　his　followers；　factors　in　the　evolutionary　process　in　its　physical　or　organic　aspect。　　By　the　physiologist　within　his　special　and　well…defined　universe　of　discourse　they　may　be　properly　regarded　as　epiphenomena；　but　by　the　naturalist　in　his　more　catholic　survey　of　nature　they　cannot　be　so　regarded；　and　were　not　so　regarded　by　Darwin。　　Intelligence　has　contributed　to　evolution　of　which　it　is　in　a　sense　a　product。

The　facts　of　observation　or　of　inference　which　Darwin　accepted　are　these：　　Conscious　experience　accompanies　some　of　the　modes　of　animal　behaviour；　it　is　concomitant　with　certain　physiological　processes；　these　processes　are　the　outcome　of　development　in　the　individual　and　evolution　in　the　race；　the　accompanying　mental　processes　undergo　a　like　development。　　Into　the　subtle　philosophical　questions　which　arise　out　of　the　naive　acceptance　of　such　a　creed　it　was　not　Darwin's　province　to　enter；　〃I　have　nothing　to　do；〃　he　said　（〃Origin　of　Species〃　（6th　edition）；　page　205。）；　〃with　the　origin　of　the　mental　powers；　any　more　than　I　have　with　that　of　life　itself。〃　　He　dealt　with　the　natural　history　of　organisms；　including　not　only　their　structure　but　their　modes　of　behaviour；　with　the　natural　history　of　the　states　of　consciousness　which　accompany　some　of　their　actions；　and　with　the　relation　of　behaviour　to　experience。　　We　will　endeavour　to　follow　Darwin　in　his　modesty　and　candour　in　making　no　pretence　to　give　ultimate　explanations。　　But　we　must　note　one　of　the　implications　of　this　self…　denying　ordinance　of　science。　　Development　and　evolution　imply　continuity。　　For　Darwin　and　his　followers　the　continuity　is　organic　through　physical　heredity。　　Apart　from　speculative　hypothesis；　legitimate　enough　in　its　proper　place　but　here　out　of　court；　we　know　nothing　of　continuity　of　mental　evolution　as　such：　　consciousness　appears　afresh　in　each　succeeding　generation。　　Hence　it　is　that　for　those　who　follow　Darwin's　lead；　mental　evolution　is　and　must　ever　be；　within　his　universe　of　discourse；　subservient　to　organic　evolution。　　Only　in　so　far　as　conscious　experience；　or　its　neural　correlate；　effects　some　changes　in　organic　structure　can　it　influence　the　course　of　heredity；　and　conversely　only　in　so　far　as　changes　in　organic　structure　are　transmitted　through　heredity；　is　mental　evolution　rendered　possible。　　Such　is　the　logical　outcome　of　Darwin's　teaching。

Those　who　abide　by　the　cardinal　results　of　this　teaching　are　bound　to　regard　all　behaviour　as　the　expression　of　the　functional　activities　of　the　living　tissues　of　the　organism；　and　all　conscious　experience　as　correlated　with　such　activities。　　For　the　purposes　of　scientific　treatment；　mental　processes　are　one　mode　of　expression　of　the　same　changes　of　which　the　physiological　processes　accompanying　behaviour　are　another　mode　of　expression。　　This　is　simply　accepted　as　a　fact　which　others　may　seek　to　explain。　　The　behaviour　itself　is　the　adaptive　application　of　the　energies　of　the　organism；　it　is　called　forth　by　some　form　of　presentation　or　stimulation　brought　to　bear　on　the　organism　by　the　environment。　　This　presentation　is　always　an　individual　or　personal　matter。　　But　in　order　that　the　organism　may　be　fitted　to　respond　to　the　presentation　of　the　environment　it　must　have　undergone　in　some　way　a　suitable　preparation。　　According　to　the　theory　of　evolution　this　preparation　is　primarily　racial　and　is　transmitted　through　heredity。　　Darwin's　main　thesis　was　that　the　method　of　preparation　is　predominantly　by　natural　selection。　　Subordinate　to　racial　preparation；　and　always　dependent　thereon；　is　individual　or　personal　preparation　through　some　kind　of　acquisition；　of　which　the　guidance　of　behaviour　through　individually　won　experience　is　a　typical　example。　　We　here　introduce　the　mental　factor　because　the　facts　seem　to　justify　the　inference。　　Thus　there　are　s