darwin and modern science-第9章

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the　basin…shaped　shell　of　the　limpets　that　live　among　the　breakers　on　the　shore。　　There　can　be　no　doubt　that　the　thickness　of　these　shells；　combined　with　their　flat　form；　protects　the　animals　from　the　force　of　the　waves　breaking　upon　them；but　how　have　they　become　so　thick？　　What　proportion　of　thickness　was　sufficient　to　decide　that　of　two　variants　of　a　limpet　one　should　survive；　the　other　be　eliminated？　　We　can　say　nothing　more　than　that　we　infer　from　the　present　state　of　the　shell；　that　it　must　have　varied　in　regard　to　differences　in　shell…thickness；　and　that　these　differences　must　have　had　selection…value；　no　proof　therefore；　but　an　assumption　which　we　must　show　to　be　convincing。

For　a　long　time　the　marvellously　complex　RADIATE　and　LATTICE…WORK　skeletons　of　Radiolarians　were　regarded　as　a　mere　outflow　of　〃Nature's　infinite　wealth　of　form；〃　as　an　instance　of　a　purely　morphological　character　with　no　biological　significance。　　But　recent　investigations　have　shown　that　these；　too；　have　an　adaptive　significance　（Hacker）。　　The　same　thing　has　been　shown　by　Schutt　in　regard　to　the　lowly　unicellular　plants；　the　Peridineae；　which　abound　alike　on　the　surface　of　the　ocean　and　in　its　depths。　　It　has　been　shown　that　the　long　skeletal　processes　which　grow　out　from　these　organisms　have　significance　not　merely　as　a　supporting　skeleton；　but　also　as　an　extension　of　the　superficial　area；　which　increases　the　contact　with　the　water…particles；　and　prevents　the　floating　organisms　from　sinking。　　It　has　been　established　that　the　processes　are　considerably　shorter　in　the　colder　layers　of　the　ocean；　and　that　they　may　be　twelve　times　as　long　（Chun；　〃Reise　der　Valdivia〃；　Leipzig；　1904。）　in　the　warmer　layers；　thus　corresponding　to　the　greater　or　smaller　amount　of　friction　which　takes　place　in　the　denser　and　less　dense　layers　of　the　water。

The　Peridineae　of　the　warmer　ocean　layers　have　thus　become　long…rayed；　those　of　the　colder　layers　short…rayed；　not　through　the　direct　effect　of　friction　on　the　protoplasm；　but　through　processes　of　selection；　which　favoured　the　longer　rays　in　warm　water；　since　they　kept　the　organism　afloat；　while　those　with　short　rays　sank　and　were　eliminated。　　If　we　put　the　question　as　to　selection…value　in　this　case；　and　ask　how　great　the　variations　in　the　length　of　processes　must　be　in　order　to　possess　selection…value；　what　can　we　answer　except　that　these　variations　must　have　been　minimal；　and　yet　sufficient　to　prevent　too　rapid　sinking　and　consequent　elimination？　　Yet　this　very　case　would　give　the　ideal　opportunity　for　a　mathematical　calculation　of　the　minimal　selection…value；　although　of　course　it　is　not　feasible　from　lack　of　data　to　carry　out　the　actual　calculation。

But　even　in　organisms　of　more　than　microscopic　size　there　must　frequently　be　minute；　even　microscopic　differences　which　set　going　the　process　of　selection；　and　regulate　its　progress　to　the　highest　possible　perfection。

Many　tropical　trees　possess　thick；　leathery　leaves；　as　a　protection　against　the　force　of　the　tropical　rain　drops。　　The　DIRECT　influence　of　the　rain　cannot　be　the　cause　of　this　power　of　resistance；　for　the　leaves；　while　they　were　still　thin；　would　simply　have　been　torn　to　pieces。　　Their　toughness　must　therefore　be　referred　to　selection；　which　would　favour　the　trees　with　slightly　thicker　leaves；　though　we　cannot　calculate　with　any　exactness　how　great　the　first　stages　of　increase　in　thickness　must　have　been。　　Our　hypothesis　receives　further　support　from　the　fact　that；　in　many　such　trees；　the　leaves　are　drawn　out　into　a　beak…like　prolongation　（Stahl　and　Haberlandt）　which　facilitates　the　rapid　falling　off　of　the　rain　water；　and　also　from　the　fact　that　the　leaves；　while　they　are　still　young；　hang　limply　down　in　bunches　which　offer　the　least　possible　resistance　to　the　rain。　　Thus　there　are　here　three　adaptations　which　can　only　be　interpreted　as　due　to　selection。　　The　initial　stages　of　these　adaptations　must　undoubtedly　have　had　selection…value。

But　even　in　regard　to　this　case　we　are　reasoning　in　a　circle；　not　giving　〃proofs；〃　and　no　one　who　does　not　wish　to　believe　in　the　selection…value　of　the　initial　stages　can　be　forced　to　do　so。　　Among　the　many　pieces　of　presumptive　evidence　a　particularly　weighty　one　seems　to　me　to　be　THE　SMALLNESS　OF　THE　STEPS　OF　PROGRESS　which　we　can　observe　in　certain　cases；　as　for　instance　in　leaf…imitation　among　butterflies；　and　in　mimicry　generally。　　The　resemblance　to　a　leaf；　for　instance　of　a　particular　Kallima；　seems　to　us　so　close　as　to　be　deceptive；　and　yet　we　find　in　another　individual；　or　it　may　be　in　many　others；　a　spot　added　which　increases　the　resemblance；　and　which　could　not　have　become　fixed　unless　the　increased　deceptiveness　so　produced　had　frequently　led　to　the　overlooking　of　its　much　persecuted　possessor。　　But　if　we　take　the　selection…value　of　the　initial　stages　for　granted；　we　are　confronted　with　the　further　question　which　I　myself　formulated　many　years　ago：　　How　does　it　happen　THAT　THE　NECESSARY　BEGINNINGS　OF　A　USEFUL　VARIATION　ARE　ALWAYS　PRESENT？　　How　could　insects　which　live　upon　or　among　green　leaves　become　all　green；　while　those　that　live　on　bark　become　brown？　　How　have　the　desert　animals　become　yellow　and　the　Arctic　animals　white？　　Why　were　the　necessary　variations　always　present？　　How　could　the　green　locust　lay　brown　eggs；　or　the　privet　caterpillar　develop　white　and　lilac…coloured　lines　on　its　green　skin？

It　is　of　no　use　answering　to　this　that　the　question　is　wrongly　formulated　（Plate；　〃Selektionsprinzip　u。　Probleme　der　Artbildung〃　（3rd　edition）；　Leipzig；　1908。）　and　that　it　is　the　converse　that　is　true；　that　the　process　of　selection　takes　place　in　accordance　with　the　variations　that　present　themselves。　　This　proposition　is　undeniably　true；　but　so　also　is　another；　which　apparently　negatives　it：　　the　variation　required　has　in　the　majority　of　cases　actually　presented　itself。　　Selection　cannot　solve　this　contradiction；　it　does　not　call　forth　the　useful　variation；　but　simply　works　upon　it。　　The　ultimate　reason　why　one　and　the　same　insect　should　occur　in　green　and　in　brown；　as　often　happens　in　caterpillars　and　locusts；　lies　in　the　fact　that　variations　towards　brown　presented　themselves；　and　so　also　did　variations　towards　green：　　THE　KERNEL　OF　THE　RIDDLE　LIES　IN　THE　VARYING；　and　for　the　present　we　can　only　say；　that　small　variations　in　different　directions　present　themselves　in　every　species。　　Otherwise　so　many　different　kinds　of　variations　could　not　have　arisen。　　I　have　endeavoured　to　explain　this　remarkable　fact　by　means　of　the　intimate　processes　that　must　take　place　within　the　germ…plasm；　and　I　shall　return　to　the　problem　when　dealing　with　〃germinal　selection。〃

We　have；　however；　to　make　still　greater　demands　on　variation；　for　it　is　not　enough　that　the　necessary　variation　should　occur　in　isolated　individuals；　because　in　that　case　there　would　be　small　prospect　of　its　being　preserved；　notwithstanding　its　utility。　　Darwin　at　first　believed；　that　even　single　variations　might　lead　to　transformation　of　the　species；　but　later　he　became　convinced　that　this　was　impossible；　at　least　without　the　cooperation　of　other　factors；　such　as　isolation　and　sexual　selection。

In　the　case　of　the　GREEN　CATERPILLARS　WITH　BRIGHT　LONGITUDINAL　STRIPES；　numerous　individuals　exhibiting　this　useful　variation　must　have　been　produced　to　start　with。　　In　all　higher；　that　is；　multicellular　organisms；　the　germ…substance　is　the　source　of　all　transmissible　variations；　and　this　germ…plasm　is　not　a　simple　substance　but　is　made　up　of　many　primary　constituents。　　The　question　can　therefore　be　more　precisely　stated　thus：　　How　does　it　come　about　that　in　so　many　cases　the　useful　variations　present　themselves　in　numbers　just　where　they　are　required；　the　white　oblique　lines　in　the　leaf…caterpillar　on　the　under　surface　of　the　body；　the　accompanying　coloured　stripes　just　above　them？　　And；　further；　how　has　it　come　about　that　in　grass　caterpillars；　not　oblique　but　longitudinal　stripes；　which　are　more　effective　for　concealment　among　grass　and　plants；　have　been　evolved？　　And　finally；　how　is　it　that　the　same　Hawk…moth　caterpillars；　which　to…day　show　oblique　stripes；　possessed　longitudinal　stripes　in　Tertiary　times？　　We　can　read　this　fact　from　the　history　of　their　development；　and　I　have　before　attempted　to　show　the　biological　significance　of　this　change　of　colour。　　（〃Studien　zur　Descendenz…Theorie〃　II。；　〃Die　Enstehung　der　Zeichnung　bei　den　Schmetterlings…raupen；〃　Leipzig；　1876。）

For　the　present　I　need　only　draw　the　conclusion　that　one　and　the　same　caterpillar　may　exhibit　the　initial　stages　of　both；　and　that　it　depends　on　the　manner　in　which　these　marking　elements　are　INTENSIFIED　and　COMBINED　by　natural　selection　whether　whitish　longitudinal　or　oblique　stripes　should　result。　　In　this　case　then　the　〃useful　variations〃　were　actually　〃always　there；〃　and　we　see　that　in　the　same　group　of　Lepidoptera；　e。g。　species　of　Sphingidae；　evolution　has　occurred　in　both　directions　according　to　whether　the　form　lived　among　grass　or　on　broad　leaves　with　oblique　lateral　veins；　and　we　can　observe　even　now　that　the　species　with　oblique　stripes　have　longitudinal　stripes　when　young；　that　is　to　say；　while　the　stripes　have　no　biological　significance。　　The　white　places　in　the　skin　which　gave　rise；　probably　first　as　small　spots；　to　this　protective　marking　could　be　combined　in　one　way　or　another　according　to　the　requirements　of　the　species。　　They　must　therefore　either　have　possessed　selection…value　from　the　first；　or；　if　this　was　not　the　case　at　their　earliest　occurrence；　there　must　have　been　SOME　OTHER　FACTORS　which　raised　them　to　the　point　of　selection…value。　　I　shall　return　to　this　in　discussing　germinal　selection。　　But　the　case　may　be　followed　still　farther；　and　leads　us　to　the　same　alternative　on　a　still　more　secure　basis。

Many　years　ago　I　observed　in　caterpillars　of　Smerinthus　populi　（the　poplar　hawk…moth）；　which　also　possess　white　oblique　stripes；　that　certain　individuals　showed　RED　SPOTS　above　these　stripes；　these　spots　occurred　only　on　certain　segments；　and　never　flowed　together　to　form　continuous　stripes。　In　another　species　（Smerinthus　tiliae）　similar　blood…red　spots　unite　to　form　a　line…like　coloured　seam　in　the　last　stage　of　larval　life；　while　in　S。　ocellata　rust…red　spots　appear　in　individual　caterpillars；　but　more　rarely　than　in　S。　Populi；　and　they　show　no　tendency　to　flow　together。

Thus　we　have　here　the　origin　of　a　new　character；　arising　from　small　beginnings；　at　least　in　S。　tiliae；　in　which　species　the　coloured　stripes　are　a　normal　specific　character。　　In　the　other　species；　S。　populi　and　S。　ocellata；　we　find　the　beginnings　of　the　same　variation；　in　one　more　rarely　than　in　the　other；　and　we　can　imagine　that；　in　the　course　of　time；　in　these　two　species；　coloured　lines　over　the　oblique　stripes　will　arise。　　In　any　case　these　spots　are　the　elements　of　variation；　out　of　which　coloured　lines　MAY　be　evolved；　if　they　are　combined　in　this　direction　through　the　agency　of　natural　selection。　　In　S。　populi　the　spots　are　often　small；　but　sometimes　it　seems　as　though　several　had　united　to　form　large　spots。　　Whether　a　process　of　selection　in　this　direction　will　arise　in　S。　populi　and　S。　ocellata；　or　whether　it　is　now　going　on　cannot　be　determined；　since　we　cannot　tell　in　advance　what　biologica