a history of science-4-第30节

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species　gradually　become　modified。〃　From　then　on；　as　he　afterwards　asserted；　the　subject　haunted　him；　hence　the　journal　of　1837。

It　will　thus　be　seen　that　the　idea　of　the　variability　of　species　came　to　Charles　Darwin　as　an　inference　from　personal　observations　in　the　field；　not　as　a　thought　borrowed　from　books。　　He　had；　of　course；　read　the　works　of　his　grandfather　much　earlier　in　life；　but　the　arguments　of　Zoonomia　and　The　Temple　of　Nature　had　not　served　in　the　least　to　weaken　his　acceptance　of　the　current　belief　in　fixity　of　species。　Nor　had　he　been　more　impressed　with　the　doctrine　of　Lamarck；　so　closely　similar　to　that　of　his　grandfather。　　Indeed；　even　after　his　South…American　experience　had　aroused　him　to　a　new　point　of　view　he　was　still　unable　to　see　anything　of　value　in　these　earlier　attempts　at　an　explanation　of　the　variation　of　species。　In　opening　his　journal；　therefore；　he　had　no　preconceived　notion　of　upholding　the　views　of　these　or　any　other　makers　of　hypotheses；　nor　at　the　time　had　he　formulated　any　hypothesis　of　his　own。　His　mind　was　open　and　receptive；　he　was　eager　only　for　facts　which　might　lead　him　to　an　understanding　of　a　problem　which　seemed　utterly　obscure。　It　was　something　to　feel　sure　that　species　have　varied；　but　how　have　such　variations　been　brought　about？

It　was　not　long　before　Darwin　found　a　clew　which　he　thought　might　lead　to　the　answer　he　sought。　　In　casting　about　for　facts　he　had　soon　discovered　that　the　most　available　field　for　observation　lay　among　domesticated　animals；　whose　numerous　variations　within　specific　lines　are　familiar　to　every　one。　　Thus　under　domestication　creatures　so　tangibly　different　as　a　mastiff　and　a　terrier　have　sprung　from　a　common　stock。　So　have　the　Shetland　pony；　the　thoroughbred；　and　the　draught…horse。　In　short；　there　is　no　domesticated　animal　that　has　not　developed　varieties　deviating　more　or　less　widely　from　the　parent　stock。　Now；　how　has　this　been　accomplished？　　Why；　clearly；　by　the　preservation；　through　selective　breeding；　of　seemingly　accidental　variations。　Thus　one　horseman；　by　constantly　selecting　animals　that　〃chance〃　to　have　the　right　build　and　stamina；　finally　develops　a　race　of　running…horses；　while　another　horseman；　by　selecting　a　different　series　of　progenitors；　has　developed　a　race　of　slow；　heavy　draught　animals。

So　far；　so　good；　the　preservation　of　〃accidental〃　variations　through　selective　breeding　is　plainly　a　means　by　which　races　may　be　developed　that　are　very　different　from　their　original　parent　form。　But　this　is　under　man's　supervision　and　direction。　　By　what　process　could　such　selection　be　brought　about　among　creatures　in　a　state　of　nature？　Here　surely　was　a　puzzle；　and　one　that　must　be　solved　before　another　step　could　be　taken　in　this　direction。

The　key　to　the　solution　of　this　puzzle　came　into　Darwin's　mind　through　a　chance　reading　of　the　famous　essay　on　〃Population〃　which　Thomas　Robert　Malthus　had　published　almost　half　a　century　before。　This　essay；　expositing　ideas　by　no　means　exclusively　original　with　Malthus；　emphasizes　the　fact　that　organisms　tend　to　increase　at　a　geometrical　ratio　through　successive　generations；　and　hence　would　overpopulate　the　earth　if　not　somehow　kept　in　check。　Cogitating　this　thought；　Darwin　gained　a　new　insight　into　the　processes　of　nature。　　He　saw　that　in　virtue　of　this　tendency　of　each　race　of　beings　to　overpopulate　the　earth；　the　entire　organic　world；　animal　and　vegetable；　must　be　in　a　state　of　perpetual　carnage　and　strife；　individual　against　individual；　fighting　for　sustenance　and　life。

That　idea　fully　imagined；　it　becomes　plain　that　a　selective　influence　is　all　the　time　at　work　in　nature；　since　only　a　few　individuals；　relatively；　of　each　generation　can　come　to　maturity；　and　these　few　must；　naturally；　be　those　best　fitted　to　battle　with　the　particular　circumstances　in　the　midst　of　which　they　are　placed。　In　other　words；　the　individuals　best　adapted　to　their　surroundings　will；　on　the　average；　be　those　that　grow　to　maturity　and　produce　offspring。　To　these　offspring　will　be　transmitted　the　favorable　peculiarities。　Thus　these　peculiarities　will　become　permanent；　and　nature　will　have　accomplished　precisely　what　the　human　breeder　is　seen　to　accomplish。　Grant　that　organisms　in　a　state　of　nature　vary；　however　slightly；　one　from　another　（which　is　indubitable）；　and　that　such　variations　will　be　transmitted　by　a　parent　to　its　offspring　（which　no　one　then　doubted）；　grant；　further；　that　there　is　incessant　strife　among　the　various　organisms；　so　that　only　a　small　proportion　can　come　to　maturitygrant　these　things；　said　Darwin；　and　we　have　an　explanation　of　the　preservation　of　variations　which　leads　on　to　the　transmutation　of　species　themselves。

This　wonderful　coign　of　vantage　Darwin　had　reached　by　1839。　Here　was　the　full　outline　of　his　theory；　here　were　the　ideas　which　afterwards　came　to　be　embalmed　in　familiar　speech　in　the　phrases　〃spontaneous　variation；〃　and　the　〃survival　of　the　fittest；〃　through　〃natural　selection。〃　　After　such　a　discovery　any　ordinary　man　would　at　once　have　run　through　the　streets　of　science；　so　to　speak；　screaming　〃Eureka！〃　　Not　so　Darwin。　　He　placed　the　manuscript　outline　of　his　theory　in　his　portfolio；　and　went　on　gathering　facts　bearing　on　his　discovery。　　In　1844　he　made　an　abstract　in　a　manuscript　book　of　the　mass　of　facts　by　that　time　accumulated。　He　showed　it　to　his　friend　Hooker；　made　careful　provision　for　its　publication　in　the　event　of　his　sudden　death；　then　stored　it　away　in　his　desk　and　went　ahead　with　the　gathering　of　more　data。　This　was　the　unexploded　powder…mine　to　which　I　have　just　referred。

Twelve　years　more　elapsedyears　during　which　the　silent　worker　gathered　a　prodigious　mass　of　facts；　answered　a　multitude　of　objections　that　arose　in　his　own　mind；　vastly　fortified　his　theory。　All　this　time　the　toiler　was　an　invalid；　never　knowing　a　day　free　from　illness　and　discomfort；　obliged　to　husband　his　strength；　never　able　to　work　more　than　an　hour　and　a　half　at　a　stretch；　yet　he　accomplished　what　would　have　been　vast　achievements　for　half　a　dozen　men　of　robust　health。　　Two　friends　among　the　eminent　scientists　of　the　day　knew　of　his　laborsSir　Joseph　Hooker；　the　botanist；　and　Sir　Charles　Lyell；　the　geologist。　　Gradually　Hooker　had　come　to　be　more　than　half　a　convert　to　Darwin's　views。　Lyell　was　still　sceptical；　yet　he　urged　Darwin　to　publish　his　theory　without　further　delay　lest　he　be　forestalled。　At　last　the　patient　worker　decided　to　comply　with　this　advice；　and　in　1856　he　set　to　work　to　make　another　and　fuller　abstract　of　the　mass　of　data　he　had　gathered。

And　then　a　strange　thing　happened。　　After　Darwin　had　been　at　work　on　his　〃abstract〃　about　two　years；　but　before　he　had　published　a　line　of　it；　there　came　to　him　one　day　a　paper　in　manuscript；　sent　for　his　approval　by　a　naturalist　friend　named　Alfred　Russel　Wallace；　who　had　been　for　some　time　at　work　in　the　East　India　Archipelago。　　He　read　the　paper；　and；　to　his　amazement；　found　that　it　contained　an　outline　of　the　same　theory　of　〃natural　selection〃　which　he　himself　had　originated　and　for　twenty　years　had　worked　upon。　Working　independently；　on　opposite　sides　of　the　globe；　Darwin　and　Wallace　had　hit　upon　the　same　explanation　of　the　cause　of　transmutation　of　species。　〃Were　Wallace's　paper　an　abstract　of　my　unpublished　manuscript　of　1844；〃　said　Darwin；　〃it　could　not　better　express　my　ideas。〃

Here　was　a　dilemma。　　To　publish　this　paper　with　no　word　from　Darwin　would　give　Wallace　priority；　and　wrest　from　Darwin　the　credit　of　a　discovery　which　he　had　made　years　before　his　codiscoverer　entered　the　field。　　Yet；　on　the　other　hand；　could　Darwin　honorably　do　otherwise　than　publish　his　friend's　paper　and　himself　remain　silent？　It　was　a　complication　well　calculated　to　try　a　man's　soul。　Darwin's　was　equal　to　the　test。　　Keenly　alive　to　the　delicacy　of　the　position；　he　placed　the　whole　matter　before　his　friends　Hooker　and　Lyell；　and　left　the　decision　as　to　a　course　of　action　absolutely　to　them。　　Needless　to　say；　these　great　men　did　the　one　thing　which　insured　full　justice　to　all　concerned。　They　counselled　a　joint　publication；　to　include　on　the　one　hand　Wallace's　paper；　and　on　the　other　an　abstract　of　Darwin's　ideas；　in　the　exact　form　in　which　it　had　been　outlined　by　the　author　in　a　letter　to　Asa　Gray　in　the　previous　yearan　abstract　which　was　in　Gray's　hands　before　Wallace's　paper　was　in　existence。　This　joint　production；　together　with　a　full　statement　of　the　facts　of　the　case；　was　presented　to　the　Linnaean　Society　of　London　by　Hooker　and　Lyell　on　the　evening　of　July　1；　1858；　this　being；　by　an　odd　coincidence；　the　twenty…first　anniversary　of　the　day　on　which　Darwin　had　opened　his　journal　to　collect　facts　bearing　on　the　〃species　question。〃　　Not　often　before　in　the　history　of　science　has　it　happened　that　a　great　theory　has　been　nurtured　in　its　author's　brain　through　infancy　and　adolescence　to　its　full　legal　majority　before　being　sent　out　into　the　world。

Thus　the　fuse　that　led　to　the　great　powder…mine　had　been　lighted。　The　explosion　itself　came　more　than　a　year　later；　in　November；　1859；　when　Darwin；　after　thirteen　months　of　further　effort；　completed　the　outline　of　his　theory；　which　was　at　first　begun　as　an　abstract　for　the　Linnaean　Society；　but　which　grew　to　the　size