ally　been　endowed　with　this　quality；　in　order　to　prevent　their　crossing　and　blending　together　in　utter　confusion。　The　following　rules　and　conclusions　are　chiefly　drawn　up　from　G？rtner's　admirable　work　on　the　hybridisation　of　plants。　I　have　taken　much　pains　to　ascertain　how　far　the　rules　apply　to　animals；　and　considering　how　scanty　our　knowledge　is　in　regard　to　hybrid　animals；　I　have　been　surprised　to　find　how　generally　the　same　rules　apply　to　both　kingdoms。
  It　has　been　already　remarked；　that　the　degree　of　fertility；　both　of　first　crosses　and　of　hybrids；　graduates　from　zero　to　perfect　fertility。　It　is　surprising　in　how　many　curious　ways　this　gradation　can　be　shown　to　exist；　but　only　the　barest　outline　of　the　facts　can　here　be　given。　When　pollen　from　a　plant　of　one　family　is　placed　on　the　stigma　of　a　plant　of　a　distinct　family；　it　exerts　no　more　influence　than　so　much　inorganic　dust。　From　this　absolute　zero　of　fertility；　the　pollen　of　different　species　of　the　same　genus　applied　to　the　stigma　of　some　one　species；　yields　a　perfect　gradation　in　the　number　of　seeds　produced；　up　to　nearly　complete　or　even　quite　complete　fertility；　and；　as　we　have　seen；　in　certain　abnormal　cases；　even　to　an　excess　of　fertility；　beyond　that　which　the　plant's　own　pollen　will　produce。　So　in　hybrids　themselves；　there　are　some　which　never　have　produced；　and　probably　never　would　produce；　even　with　the　pollen　of　either　pure　parent；　a　single　fertile　seed：　but　in　some　of　these　cases　a　first　trace　of　fertility　may　be　detected；　by　the　pollen　of　one　of　the　pure　parent…species　causing　the　flower　of　the　hybrid　to　wither　earlier　than　it　otherwise　would　have　done；　and　the　early　withering　of　the　flower　is　well　known　to　be　a　sign　of　incipient　fertilisation。　From　this　extreme　degree　of　sterility　we　have　self…fertilised　hybrids　producing　a　greater　and　greater　number　of　seeds　up　to　perfect　fertility。
  Hybrids　from　two　species　which　are　very　difficult　to　cross；　and　which　rarely　produce　any　offspring；　are　generally　very　sterile；　but　the　parallelism　between　the　difficulty　of　making　a　first　cross；　and　the　sterility　of　the　hybrids　thus　produced　two　classes　of　facts　which　are　generally　confounded　together　is　by　no　means　strict。　There　are　many　cases；　in　which　two　pure　species　can　be　united　with　unusual　facility；　and　produce　numerous　hybrid…offspring；　yet　these　hybrids　are　remarkably　sterile。　On　the　other　hand；　there　are　species　which　can　be　crossed　very　rarely；　or　with　extreme　difficulty；　but　the　hybrids；　when　at　last　produced；　are　very　fertile。　Even　within　the　limits　of　the　same　genus；　for　instance　in　Dianthus；　these　two　opposite　cases　occur。
  The　fertility；　both　of　first　crosses　and　of　hybrids；　is　more　easily　affected　by　unfavourable　conditions；　than　is　the　fertility　of　pure　species。　But　the　degree　of　fertility　is　likewise　innately　variable；　for　it　is　not　always　the　same　when　the　same　two　species　are　crossed　under　the　same　circumstances；　but　depends　in　part　upon　the　constitution　of　the　individuals　which　happen　to　have　been　chosen　for　the　experiment。　So　it　is　with　hybrids；　for　their　degree　of　fertility　is　often　found　to　differ　greatly　in　the　several　individuals　raised　from　seed　out　of　the　same　capsule　and　exposed　to　exactly　the　same　conditions。
  By　the　term　systematic　affinity　is　meant；　the　resemblance　between　species　in　structure　and　in　constitution；　more　especially　in　the　structure　of　parts　which　are　of　high　physiological　importance　and　which　differ　little　in　the　allied　species。　Now　the　fertility　of　first　crosses　between　species；　and　of　the　hybrids　produced　from　them；　is　largely　governed　by　their　systematic　affinity。　This　is　clearly　shown　by　hybrids　never　having　been　raised　between　species　ranked　by　systematists　in　distinct　families；　and　on　the　other　hand；　by　very　closely　allied　species　generally　uniting　with　facility。　But　the　correspondence　between　systematic　affinity　and　the　facility　of　crossing　is　by　no　means　strict。　A　multitude　of　cases　could　be　given　of　very　closely　allied　species　which　will　not　unite；　or　only　with　extreme　difficulty；　and　on　the　other　hand　of　very　distinct　species　which　unite　with　the　utmost　facility。　In　the　same　family　there　may　be　a　genus；　as　Dianthus；　in　which　very　many　species　can　most　readily　be　crossed；　and　another　genus；　as　Silene；　in　which　the　most　persevering　efforts　have　failed　to　produce　between　extremely　close　species　a　single　hybrid。　Even　within　the　limits　of　the　same　genus；　we　meet　with　this　same　difference；　for　instance；　the　many　species　of　Nicotiana　have　been　more　largely　crossed　than　the　species　of　almost　any　other　genus；　but　G？rtner　found　that　N。　acuminata；　which　is　not　a　particularly　distinct　species；　obstinately　failed　to　fertilise；　or　to　be　fertilised　by；　no　less　than　eight　other　species　of　Nicotiana。　Very　many　analogous　facts　could　be　given。
  No　one　has　been　able　to　point　out　what　kind；　or　what　amount；　of　difference　in　any　recognisable　character　is　sufficient　to　prevent　two　species　crossing。　It　can　be　shown　that　plants　most　widely　different　in　habit　and　general　appearance；　and　having　strongly　marked　differences　in　every　part　of　the　flower；　even　in　the　pollen；　in　the　fruit；　and　in　the　cotyledons；　can　be　crossed。　Annual　and　perennial　plants；　deciduous　and　evergreen　trees；　plants　inhabiting　different　stations　and　fitted　for　extremely　different　climates；　can　often　be　crossed　with　ease。
  By　a　reciprocal　cross　between　two　species；　I　mean　the　case；　for　instance；　of　a　stallion…horse　being　first　crossed　with　a　female…ass；　and　then　a　male…ass　with　a　mare：　these　two　species　may　then　be　said　to　have　been　reciprocally　crossed。　There　is　often　the　widest　possible　difference　in　the　facility　of　making　reciprocal　crosses。　Such　cases　are　highly　important；　for　they　prove　that　the　capacity　in　any　two　species　to　cross　is　often　completely　independent　of　their　systematic　affinity；　or　of　any　recognisable　difference　in　their　whole　organisation。　On　the　other　hand；　these　cases　clearly　show　that　the　capacity　for　crossing　is　connected　with　constitutional　differences　imperceptible　by　us；　and　confined　to　the　reproductive　system。　This　difference　in　the　result　of　reciprocal　crosses　between　the　same　two　species　was　long　ago　observed　by　K？lreuter。　To　give　an　instance：　Mirabilis　jalappa　can　easily　be　fertilised　by　the　pollen　of　M。　longiflora；　and　the　hybrids　thus　produced　are　sufficiently　fertile；　but　K？lreuter　tried　more　than　two　hundred　times；　during　eight　following　years；　to　fertilise　reciprocally　M。　longiflora　with　the　pollen　of　M。　jalappa；　and　utterly　failed。　Several　other　equally　striking　cases　could　be　given。　Thuret　has　observed　the　same　fact　with　certain　sea…weeds　or　Fuci。　G？rtner；　moreover；　found　that　this　difference　of　facility　in　making　reciprocal　crosses　is　extremely　common　in　a　lesser　degree。　He　has　observed　it　even　between　forms　so　closely　related　（as　Matthiola　annua　and　glabra）　that　many　botanists　rank　them　only　as　varieties。　It　is　also　a　remarkable　fact；　that　hybrids　raised　from　reciprocal　crosses；　though　of　course　compounded　of　the　very　same　two　species；　the　one　species　having　first　been　used　as　the　father　and　then　as　the　mother；　generally　differ　in　fertility　in　a　small；　and　occasionally　in　a　high　degree。
  Several　other　singular　rules　could　be　given　from　G？rtner：　for　instance；　some　species　have　a　remarkable　power　of　crossing　with　other　species；　other　species　of　the　same　genus　have　a　remarkable　power　of　impressing　their　likeness　on　their　hybrid　offspring；　but　these　two　powers　do　not　at　all　necessarily　go　together。　There　are　certain　hybrids　which　instead　of　having；　as　is　usual；　an　intermediate　character　between　their　two　parents；　always　closely　resemble　one　of　them；　and　such　hybrids；　though　externally　so　like　one　of　their　pure　parent…species；　are　with　rare　exceptions　extremely　sterile。　So　again　amongst　hybrids　which　are　usually　intermediate　in　structure　between　their　parents；　exceptional　and　abnormal　individuals　sometimes　are　born；　which　closely　resemble　one　of　their　pure　parents；　and　these　hybrids　are　almost　always　utterly　sterile；　even　when　the　other　hybrids　raised　from　seed　from　the　same　capsule　have　a　considerable　degree　of　fertility。　These　facts　show　how　completely　fertility　in　the　hybrid　is　independent　of　its　external　resemblance　to　either　pure　parent。
  Considering　the　several　rules　now　given；　which　govern　the　fertility　of　first　crosses　and　of　hybrids；　we　see　that　when　forms；　which　must　be　considered　as　good　and　distinct　species；　are　united；　their　fertility　graduates　from　zero　to　perfect　fertility；　or　even　to　fertility　under　certain　conditions　in　excess。　That　their　fertility；　besides　being　eminently　susceptible　to　favourable　and　unfavourable　conditions；　is　innately　variable。　That　it　is　by　no　means　always　the　same　in　degree　in　the　first　cross　and　in　the　hybrids　produced　from　this　cross。　That　the　fertility　of　hybrids　is　not　related　to　the　degree　in　which　they　resemble　in　external　appearance　either　parent。　And　lastly；　that　the　facility　of　making　a　first　cross　between　any　two　species　is　not　always　governed　by　their　systematic　affinity　or　degree　of　resemblance　to　each　other。　This　latter　statement　is　clearly　proved　by　reciprocal　crosses　between　the　same　two　species；　for　according　as　the　one　species　or　the　other　is　used　as　the　father　or　the　mother；　there　is