pecies；　for　according　as　the　one　species　or　the　other　is　used　as　the　father　or　the　mother；　there　is　generally　some　difference；　and　occasionally　the　widest　possible　difference；　in　the　facility　of　effecting　an　union。　The　hybrids；　moreover；　produced　from　reciprocal　crosses　often　differ　in　fertility。
  Now　do　these　complex　and　singular　rules　indicate　that　species　have　been　endowed　with　sterility　simply　to　prevent　their　becoming　confounded　in　nature？　I　think　not。　For　why　should　the　sterility　be　so　extremely　different　in　degree；　when　various　species　are　crossed；　all　of　which　we　must　suppose　it　would　be　equally　important　to　keep　from　blending　together？　Why　should　the　degree　of　sterility　be　innately　variable　in　the　individuals　of　the　same　species？　Why　should　some　species　cross　with　facility；　and　yet　produce　very　sterile　hybrids；　and　other　species　cross　with　extreme　difficulty；　and　yet　produce　fairly　fertile　hybrids？　Why　should　there　often　be　so　great　a　difference　in　the　result　of　a　reciprocal　cross　between　the　same　two　species？　Why；　it　may　even　be　asked；　has　the　production　of　hybrids　been　permitted？　To　grant　to　species　the　special　power　of　producing　hybrids；　and　then　to　stop　their　further　propagation　by　different　degrees　of　sterility；　not　strictly　related　to　the　facility　of　the　first　union　between　their　parents；　seems　to　be　a　strange　arrangement。
  The　foregoing　rules　and　facts；　on　the　other　hand；　appear　to　me　clearly　to　indicate　that　the　sterility　both　of　first　crosses　and　of　hybrids　is　simply　incidental　or　dependent　on　unknown　differences；　chiefly　in　the　reproductive　systems；　of　the　species　which　are　crossed。　The　differences　being　of　so　peculiar　and　limited　a　nature；　that；　in　reciprocal　crosses　between　two　species　the　male　sexual　element　of　the　one　will　often　freely　act　on　the　female　sexual　element　of　the　other；　but　not　in　a　reversed　direction。　It　will　be　advisable　to　explain　a　little　more　fully　by　an　example　what　I　mean　by　sterility　being　incidental　on　other　differences；　and　not　a　specially　endowed　quality。　As　the　capacity　of　one　plant　to　be　grafted　or　budded　on　another　is　so　entirely　unimportant　for　its　welfare　in　a　state　of　nature；　I　presume　that　no　one　will　suppose　that　this　capacity　is　a　specially　endowed　quality；　but　will　admit　that　it　is　incidental　on　differences　in　the　laws　of　growth　of　the　two　plants。　We　can　sometimes　see　the　reason　why　one　tree　will　not　take　on　another；　from　differences　in　their　rate　of　growth；　in　the　hardness　of　their　wood；　in　the　period　of　the　flow　or　nature　of　their　sap；　&c。；　but　in　a　multitude　of　cases　we　can　assign　no　reason　whatever。　Great　diversity　in　the　size　of　two　plants；　one　being　woody　and　the　other　herbaceous；　one　being　evergreen　and　the　other　deciduous；　and　adaptation　to　widely　different　climates；　does　not　always　prevent　the　two　grafting　together。　As　in　hybridisation；　so　with　grafting；　the　capacity　is　limited　by　systematic　affinity；　for　no　one　has　been　able　to　graft　trees　together　belonging　to　quite　distinct　families；　and；　on　the　other　hand；　closely　allied　species；　and　varieties　of　the　same　species；　can　usually；　but　not　invariably；　be　grafted　with　ease。　But　this　capacity；　as　in　hybridisation；　is　by　no　means　absolutely　governed　by　systematic　affinity。　Although　many　distinct　genera　within　the　same　family　have　been　grafted　together；　in　other　cases　species　of　the　same　genus　will　not　take　on　each　other。　The　pear　can　be　grafted　far　more　readily　on　the　quince；　which　is　ranked　as　a　distinct　genus；　than　on　the　apple；　which　is　a　member　of　the　same　genus。　Even　different　varieties　of　the　pear　take　with　different　degrees　of　facility　on　the　quince；　so　do　different　varieties　of　the　apricot　and　peach　on　certain　varieties　of　the　plum。
  As　G？rtner　found　that　there　was　sometimes　an　innate　difference　in　different　individuals　of　the　same　two　species　in　crossing；　so　Sagaret　believes　this　to　be　the　case　with　different　individuals　of　the　same　two　species　in　being　grafted　together。　As　in　reciprocal　crosses；　the　facility　of　effecting　an　union　is　often　very　far　from　equal；　so　it　sometimes　is　in　grafting；　the　common　gooseberry；　for　instance；　cannot　be　grafted　on　the　currant；　whereas　the　currant　will　take；　though　with　difficulty；　on　the　gooseberry。
  We　have　seen　that　the　sterility　of　hybrids；　which　have　their　reproductive　organs　in　an　imperfect　condition；　is　a　very　different　case　from　the　difficulty　of　uniting　two　pure　species；　which　have　their　reproductive　organs　perfect；　yet　these　two　distinct　cases　run　to　a　certain　extent　parallel。　Something　analogous　occurs　in　grafting；　for　Thouin　found　that　three　species　of　Robinia；　which　seeded　freely　on　their　own　roots；　and　which　could　be　grafted　with　no　great　difficulty　on　another　species；　when　thus　grafted　were　rendered　barren。　On　the　other　hand；　certain　species　of　Sorbus；　when　grafted　on　other　species；　yielded　twice　as　much　fruit　as　when　on　their　own　roots。　We　are　reminded　by　this　latter　fact　of　the　extraordinary　case　of　Hippeastrum；　Lobelia；　&c。；　which　seeded　much　more　freely　when　fertilised　with　the　pollen　of　distinct　species；　than　when　self…fertilised　with　their　own　pollen。
  We　thus　see；　that　although　there　is　a　clear　and　fundamental　difference　between　the　mere　adhesion　of　grafted　stocks；　and　the　union　of　the　male　and　female　elements　in　the　act　of　reproduction；　yet　that　there　is　a　rude　degree　of　parallelism　in　the　results　of　grafting　and　of　crossing　distinct　species。　And　as　we　must　look　at　the　curious　and　complex　laws　governing　the　facility　with　which　trees　can　be　grafted　on　each　other　as　incidental　on　unknown　differences　in　their　vegetative　systems；　so　I　believe　that　the　still　more　complex　laws　governing　the　facility　of　first　crosses；　are　incidental　on　unknown　differences；　chiefly　in　their　reproductive　systems。　These　differences；　in　both　cases；　follow　to　a　certain　extent；　as　might　have　been　expected；　systematic　affinity；　by　which　every　kind　of　resemblance　and　dissimilarity　between　organic　beings　is　attempted　to　be　expressed。　The　facts　by　no　means　seem　to　me　to　indicate　that　the　greater　or　lesser　difficulty　of　either　grafting　or　crossing　together　various　species　has　been　a　special　endowment；　although　in　the　case　of　crossing；　the　difficulty　is　as　important　for　the　endurance　and　stability　of　specific　forms；　as　in　the　case　of　grafting　it　is　unimportant　for　their　welfare。
  Causes　of　the　Sterility　of　first　Crosses　and　of　Hybrids。　We　may　now　look　a　little　closer　at　the　probable　causes　of　the　sterility　of　first　crosses　and　of　hybrids。　These　two　cases　are　fundamentally　different；　for；　as　just　remarked；　in　the　union　of　two　pure　species　the　male　and　female　sexual　elements　are　perfect；　whereas　in　hybrids　they　are　imperfect。　Even　in　first　crosses；　the　greater　or　lesser　difficulty　in　effecting　a　union　apparently　depends　on　several　distinct　causes。　There　must　sometimes　be　a　physical　impossibility　in　the　male　element　reaching　the　ovule；　as　would　be　the　case　with　a　plant　having　a　pistil　too　long　for　the　pollen…tubes　to　reach　the　ovarium。　It　has　also　been　observed　that　when　pollen　of　one　species　is　placed　on　the　stigma　of　a　distantly　allied　species；　though　the　pollen…tubes　protrude；　they　do　not　penetrate　the　stigmatic　surface。　Again；　the　male　element　may　reach　the　female　element；　but　be　incapable　of　causing　an　embryo　to　be　developed；　as　seems　to　have　been　the　case　with　some　of　Thuret's　experiments　on　Fuci。　No　explanation　can　be　given　of　these　facts；　any　more　than　why　certain　trees　cannot　be　grafted　on　others。　Lastly；　an　embryo　may　be　developed；　and　then　perish　at　an　early　period。　This　latter　alternative　has　not　been　sufficiently　attended　to；　but　I　believe；　from　observations　communicated　to　me　by　Mr。　Hewitt；　who　has　had　great　experience　in　hybridising　gallinaceous　birds；　that　the　early　death　of　the　embryo　is　a　very　frequent　cause　of　sterility　in　first　crosses。　I　was　at　first　very　unwilling　to　believe　in　this　view；　as　hybrids；　when　once　born；　are　generally　healthy　and　long…lived；　as　we　see　in　the　case　of　the　common　mule。　Hybrids；　however；　are　differently　circumstanced　before　and　after　birth：　when　born　and　living　in　a　country　where　their　two　parents　can　live；　they　are　generally　placed　under　suitable　conditions　of　life。　But　a　hybrid　partakes　of　only　half　of　the　nature　and　constitution　of　its　mother；　and　therefore　before　birth；　as　long　as　it　is　nourished　within　its　mother's　womb　or　within　the　egg　or　seed　produced　by　the　mother；　it　may　be　exposed　to　conditions　in　some　degree　unsuitable；　and　consequently　be　liable　to　perish　at　an　early　period；　more　especially　as　all　very　young　beings　seem　eminently　sensitive　to　injurious　or　unnatural　conditions　of　life。
  In　regard　to　the　sterility　of　hybrids；　in　which　the　sexual　elements　are　imperfectly　developed；　the　case　is　very　different。　I　have　more　than　once　alluded　to　a　large　body　of　facts；　which　I　have　collected；　showing　that　when　animals　and　plants　are　removed　from　their　natural　conditions；　they　are　extremely　liable　to　have　their　reproductive　systems　seriously　affected。　This；　in　fact；　is　the　great　bar　to　the　domestication　of　animals。　Between　the　sterility　thus　superinduced　and　that　of　hybrids；　there　are　many　points　of　similarity。　In　both　cases　the　sterility　is　independent　of　general　health；　and　is　often　accompanied　by　excess　of　size　or　great　luxuriance。　In　both　ca