e　that　of　any　other　known　bird。　Finally；　it　may　not　be　a　logical　deduction；　but　to　my　imagination　it　is　far　more　satisfactory　to　look　at　such　instincts　as　the　young　cuckoo　ejecting　its　foster…brothers；　ants　making　slaves；　　the　larvae　of　ichneumonidae　feeding　within　the　live　bodies　of　caterpillars；　not　as　specially　endowed　or　created　instincts；　but　as　small　consequences　of　one　general　law；　leading　to　the　advancement　of　all　organic　beings；　namely；　multiply；　vary；　let　the　strongest　live　and　the　weakest　die。
  Chapter　8　…　Hybridism
  THE　view　generally　entertained　by　naturalists　is　that　species；　when　intercrossed；　have　been　specially　endowed　with　the　quality　of　sterility；　in　order　to　prevent　the　confusion　of　all　organic　forms。　This　view　certainly　seems　at　first　probable；　for　species　within　the　same　country　could　hardly　have　kept　distinct　had　they　been　capable　of　crossing　freely。　The　importance　of　the　fact　that　hybrids　are　very　generally　sterile；　has；　I　think；　been　much　underrated　by　some　late　writers。　On　the　theory　of　natural　selection　the　case　is　especially　important；　inasmuch　as　the　sterility　of　hybrids　could　not　possibly　be　of　any　advantage　to　them；　and　therefore　could　not　have　been　acquired　by　the　continued　preservation　of　successive　profitable　degrees　of　sterility。　I　hope；　however；　to　be　able　to　show　that　sterility　is　not　a　specially　acquired　or　endowed　quality；　but　is　incidental　on　other　acquired　differences。
  In　treating　this　subject；　two　classes　of　facts；　to　a　large　extent　fundamentally　different；　have　generally　been　confounded　together；　namely；　the　sterility　of　two　species　when　first　crossed；　and　the　sterility　of　the　hybrids　produced　from　them。
  Pure　species　have　of　course　their　organs　of　reproduction　in　a　perfect　condition；　yet　when　intercrossed　they　produce　either　few　or　no　offspring。　Hybrids；　on　the　other　hand；　have　their　reproductive　organs　functionally　impotent；　as　may　be　clearly　seen　in　the　state　of　the　male　element　in　both　plants　and　animals；　though　the　organs　themselves　are　perfect　in　structure；　as　far　as　the　microscope　reveals。　In　the　first　case　the　two　sexual　elements　which　go　to　form　the　embryo　are　perfect；　in　the　second　case　they　are　either　not　at　all　developed；　or　are　imperfectly　developed。　This　distinction　is　important；　when　the　cause　of　the　sterility；　which　is　common　to　the　two　cases；　has　to　be　considered。　The　distinction　has　probably　been　slurred　over；　owing　to　the　sterility　in　both　cases　being　looked　on　as　a　special　endowment；　beyond　the　province　of　our　reasoning　powers。
  The　fertility　of　varieties；　that　is　of　the　forms　known　or　believed　to　have　descended　from　common　parents；　when　intercrossed；　and　likewise　the　fertility　of　their　mongrel　offspring；　is；　on　my　theory；　of　equal　importance　with　the　sterility　of　species；　for　it　seems　to　make　a　broad　and　clear　distinction　between　varieties　and　species。
  First；　for　the　sterility　of　species　when　crossed　and　of　their　hybrid　offspring。　It　is　impossible　to　study　the　several　memoirs　and　works　of　those　two　conscientious　and　admirable　observers；　K？lreuter　and　G？rtner；　who　almost　devoted　their　lives　to　this　subject；　without　being　deeply　impressed　with　the　high　generality　of　some　degree　of　sterility。　K？lreuter　makes　the　rule　universal；　but　then　he　cuts　the　knot；　for　in　ten　cases　in　which　he　found　two　forms；　considered　by　most　authors　as　distinct　species；　quite　fertile　together；　he　unhesitatingly　ranks　them　as　varieties。　G？rtner；　also；　makes　the　rule　equally　universal；　and　he　disputes　the　entire　fertility　of　K？lreuter's　ten　cases。　But　in　these　and　in　many　other　cases；　G？rtner　is　obliged　carefully　to　count　the　seeds；　in　order　to　show　that　there　is　any　degree　of　sterility。　He　always　compares　the　maximum　number　of　seeds　produced　by　two　species　when　crossed　and　by　their　hybrid　offspring；　with　the　average　number　produced　by　both　pure　parent…species　in　a　state　of　nature。　But　a　serious　cause　of　error　seems　to　me　to　be　here　introduced：　a　plant　to　be　hybridised　must　be　castrated；　and；　what　is　often　more　important；　must　be　secluded　in　order　to　prevent　pollen　being　brought　to　it　by　insects　from　other　plants。　Nearly　all　the　plants　experimentised　on　by　G？rtner　were　potted；　and　apparently　were　kept　in　a　chamber　in　his　house。　That　these　processes　are　often　injurious　to　the　fertility　of　a　plant　cannot　be　doubted；　for　G？rtner　gives　in　his　table　about　a　score　of　cases　of　plants　which　he　castrated；　and　artificially　fertilised　with　their　own　pollen；　and　（excluding　all　cases　such　as　the　Leguminosae；　in　which　there　is　an　acknowledged　difficulty　in　the　manipulation）　half　of　these　twenty　plants　had　their　fertility　in　some　degree　impaired。　Moreover；　as　G？rtner　during　several　years　repeatedly　crossed　the　primrose　and　cowslip；　which　we　have　such　good　reason　to　believe　to　be　varieties；　and　only　once　or　twice　succeeded　in　getting　fertile　seed；　as　he　found　the　common　red　and　blue　pimpernels　（Anagallis　arvensis　and　coerulea）；　which　the　best　botanists　rank　as　varieties；　absolutely　sterile　together；　and　as　he　came　to　the　same　conclusion　in　several　other　analogous　cases；　it　seems　to　me　that　we　may　well　be　permitted　to　doubt　whether　many　other　species　are　really　so　sterile；　when　intercrossed；　as　G？rtner　believes。
  It　is　certain；　on　the　one　hand；　that　the　sterility　of　various　species　when　crossed　is　so　different　in　degree　and　graduates　away　so　insensibly；　and；　on　the　other　hand；　that　the　fertility　of　pure　species　is　so　easily　affected　by　various　circumstances；　that　for　all　practical　purposes　it　is　most　difficult　to　say　where　perfect　fertility　ends　and　sterility　begins。　I　think　no　better　evidence　of　this　can　be　required　than　that　the　two　most　experienced　observers　who　have　ever　lived；　namely；　K？lreuter　and　G？rtner；　should　have　arrived　at　diametrically　opposite　conclusions　in　regard　to　the　very　same　species。　It　is　also　most　instructive　to　compare　but　I　have　not　space　here　to　enter　on　details　the　evidence　advanced　by　our　best　botanists　on　the　question　whether　certain　doubtful　forms　should　be　ranked　as　species　or　varieties；　with　the　evidence　from　fertility　adduced　by　different　hybridisers；　or　by　the　same　author；　from　experiments　made　during　different　years。　It　can　thus　be　shown　that　neither　sterility　nor　fertility　affords　any　clear　distinction　between　species　and　varieties；　but　that　the　evidence　from　this　source　graduates　away；　and　is　doubtful　in　the　same　degree　as　is　the　evidence　derived　from　other　constitutional　and　structural　differences。
  In　regard　to　the　sterility　of　hybrids　in　successive　generations；　though　G？rtner　was　enabled　to　rear　some　hybrids；　carefully　guarding　them　from　a　cross　with　either　pure　parent；　for　six　or　seven；　and　in　one　case　for　ten　generations；　yet　he　asserts　positively　that　their　fertility　never　increased；　but　generally　greatly　decreased。　I　do　not　doubt　that　this　is　usually　the　case；　and　that　the　fertility　often　suddenly　decreases　in　the　first　few　generations。　Nevertheless　I　believe　that　in　all　these　experiments　the　fertility　has　been　diminished　by　an　independent　cause；　namely；　from　close　interbreeding。　I　have　collected　so　large　a　body　of　facts；　showing　that　close　interbreeding　lessens　fertility；　and；　on　the　other　hand；　that　an　occasional　cross　with　a　distinct　individual　or　variety　increases　fertility；　that　I　cannot　doubt　the　correctness　of　this　almost　universal　belief　amongst　breeders。　Hybrids　are　seldom　raised　by　experimentalists　in　great　numbers；　and　as　the　parent…species；　or　other　allied　hybrids；　generally　grow　in　the　same　garden；　the　visits　of　insects　must　be　carefully　prevented　during　the　flowering　season：　hence　hybrids　will　generally　be　fertilised　during　each　generation　by　their　own　individual　pollen；　and　I　am　convinced　that　this　would　be　injurious　to　their　fertility；　already　lessened　by　their　hybrid　origin。　I　am　strengthened　in　this　conviction　by　a　remarkable　statement　repeatedly　made　by　G？rtner；　namely；　that　if　even　the　less　fertile　hybrids　be　artificially　fertilised　with　hybrid　pollen　of　the　same　kind；　their　fertility；　notwithstanding　the　frequent　ill　effects　of　manipulation；　sometimes　decidedly　increases；　and　goes　on　increasing。　Now；　in　artificial　fertilisation　pollen　is　as　often　taken　by　chance　（as　I　know　from　my　own　experience）　from　the　anthers　of　another　flower；　as　from　the　anthers　of　the　flower　itself　which　is　to　be　fertilised；　so　that　a　cross　between　two　flowers；　though　probably　on　the　same　plant；　would　be　thus　effected。　Moreover；　whenever　complicated　experiments　are　in　progress；　so　careful　an　observer　as　G？rtner　would　have　castrated　his　hybrids；　and　this　would　have　insured　in　each　generation　a　cross　with　the　pollen　from　a　distinct　flower；　either　from　the　same　plant　or　from　another　plant　of　the　same　hybrid　nature。　And　thus；　the　strange　fact　of　the　increase　of　fertility　in　the　successive　generations　of　artificially　fertilised　hybrids　may；　I　believe；　be　accounted　for　by　close　interbreeding　having　been　avoided。
  Now　let　us　turn　to　the　results　arrived　at　by　the　third　most　experienced　hybridiser；　namely；　the　Hon。　and　Rev。　W。　Herbert。　He　is　as　emphatic　in　his　conclusion　that　some　hybrids　are　perfectly　fertile　as　fertile　as　the　pure　parent…species　as　are　K？lreuter　and　G？rtner　that　some　degree　of　sterility　between　distinct　species　is　a　universal　law　of　nature。　He　experimentised