endent　of　general　health；　and　is　often　accompanied　by　excess　of　size　or　great　luxuriance。　In　both　cases；　the　sterility　occurs　in　various　degrees；　in　both；　the　male　element　is　the　most　liable　to　be　affected；　but　sometimes　the　female　more　than　the　male。　In　both；　the　tendency　goes　to　a　certain　extent　with　systematic　affinity；　or　whole　groups　of　animals　and　plants　are　rendered　impotent　by　the　same　unnatural　conditions；　and　whole　groups　of　species　tend　to　produce　sterile　hybrids。　On　the　other　hand；　one　species　in　a　group　will　sometimes　resist　great　changes　of　conditions　with　unimpaired　fertility；　and　certain　species　in　a　group　will　produce　unusually　fertile　hybrids。　No　one　can　tell；　till　he　tries；　whether　any　particular　animal　will　breed　under　confinement　or　any　plant　seed　freely　under　culture；　nor　can　he　tell；　till　he　tries；　whether　any　two　species　of　a　genus　will　produce　more　or　less　sterile　hybrids。　Lastly；　when　organic　beings　are　placed　during　several　generations　under　conditions　not　natural　to　them；　they　are　extremely　liable　to　vary；　which　is　due；　as　I　believe；　to　their　reproductive　systems　having　been　specially　affected；　though　in　a　lesser　degree　than　when　sterility　ensues。　So　it　is　with　hybrids；　for　hybrids　in　successive　generations　are　eminently　liable　to　vary；　as　every　experimentalist　has　observed。
  Thus　we　see　that　when　organic　beings　are　placed　under　new　and　unnatural　conditions；　and　when　hybrids　are　produced　by　the　unnatural　crossing　of　two　species；　the　reproductive　system；　independently　of　the　general　state　of　health；　is　affected　by　sterility　in　a　very　similar　manner。　In　the　one　case；　the　conditions　of　life　have　been　disturbed；　though　often　in　so　slight　a　degree　as　to　be　inappreciable　by　us；　in　the　other　case；　or　that　of　hybrids；the　external　conditions　have　remained　the　same；　but　the　organisation　has　been　disturbed　by　two　different　structures　and　constitutions　having　been　blended　into　one。　For　it　is　scarcely　possible　that　two　organisations　should　be　compounded　into　one；　without　some　disturbance　occurring　in　the　development；　or　periodical　action；　or　mutual　relation　of　the　different　parts　and　organs　one　to　another；　or　to　the　conditions　of　life。　When　hybrids　are　able　to　breed　inter　se；　they　transmit　to　their　offspring　from　generation　to　generation　the　same　compounded　organisation；　and　hence　we　need　not　be　surprised　that　their　sterility；　though　in　some　degree　variable；　rarely　diminishes。
  It　must；　however；　be　confessed　that　we　cannot　understand；　excepting　on　vague　hypotheses；　several　facts　with　respect　to　the　sterility　of　hybrids；　for　instance；　the　unequal　fertility　of　hybrids　produced　from　reciprocal　crosses；　or　the　increased　sterility　in　those　hybrids　which　occasionally　and　exceptionally　resemble　closely　either　pure　parent。　Nor　do　I　pretend　that　the　foregoing　remarks　go　to　the　root　of　the　matter：　no　explanation　is　offered　why　an　organism；　when　placed　under　unnatural　conditions；　is　rendered　sterile。　All　that　I　have　attempted　to　show；　is　that　in　two　cases；　in　some　respects　allied；　sterility　is　the　common　result；　in　the　one　case　from　the　conditions　of　life　having　been　disturbed；　in　the　other　case　from　the　organisation　having　been　disturbed　by　two　organisations　having　been　compounded　into　one。
  It　may　seem　fanciful；　but　I　suspect　that　a　similar　parallelism　extends　to　an　allied　yet　very　different　class　of　facts。　It　is　an　old　and　almost　universal　belief；　founded；　I　think；　on　a　considerable　body　of　evidence；　that　slight　changes　in　the　conditions　of　life　are　beneficial　to　all　living　things。　We　see　this　acted　on　by　farmers　and　gardeners　in　their　frequent　exchanges　of　seed；　tubers；　&c。；　from　one　soil　or　climate　to　another；　and　back　again。　During　the　convalescence　of　animals；　we　plainly　see　that　great　benefit　is　derived　from　almost　any　change　in　the　habits　of　life。　Again；　both　with　plants　and　animals；　there　is　abundant　evidence；　that　a　cross　between　very　distinct　individuals　of　the　same　species；　that　is　between　members　of　different　strains　or　sub…breeds；　gives　vigour　and　fertility　to　the　offspring。　I　believe；　indeed；　from　the　facts　alluded　to　in　our　fourth　chapter；　that　a　certain　amount　of　crossing　is　indispensable　even　with　hermaphrodites；　and　that　close　interbreeding　continued　during　several　generations　between　the　nearest　relations；　especially　if　these　be　kept　under　the　same　conditions　of　life；　always　induces　weakness　and　sterility　in　the　progeny。
  Hence　it　seems　that；　on　the　one　hand；　slight　changes　in　the　conditions　of　life　benefit　all　organic　beings；　and　on　the　other　hand；　that　slight　crosses；　that　is　crosses　between　the　males　and　females　of　the　same　species　which　have　varied　and　become　slightly　different；　give　vigour　and　fertility　to　the　offspring。　But　we　have　seen　that　greater　changes；　or　changes　of　a　particular　nature；　often　render　organic　beings　in　some　degree　sterile；　and　that　greater　crosses；　that　is　crosses　between　males　and　females　which　have　become　widely　or　specifically　different；　produce　hybrids　which　are　generally　sterile　in　some　degree。　I　cannot　persuade　myself　that　this　parallelism　is　an　accident　or　an　illusion。　Both　series　of　facts　seem　to　be　connected　together　by　some　common　but　unknown　bond；　which　is　essentially　related　to　the　principle　of　life。
  Fertility　of　Varieties　when　crossed；　and　of　their　Mongrel　off…spring。　It　may　be　urged；　as　a　most　forcible　argument；　that　there　must　be　some　essential　distinction　between　species　and　varieties；　and　that　there　must　be　some　error　in　all　the　foregoing　remarks；　inasmuch　as　varieties；　however　much　they　may　differ　from　each　other　in　external　appearance；　cross　with　perfect　facility；　and　yield　perfectly　fertile　offspring。　I　fully　admit　that　this　is　almost　invariably　the　case。　But　if　we　look　to　varieties　produced　under　nature；　we　are　immediately　involved　in　hopeless　difficulties；　for　if　two　hitherto　reputed　varieties　be　found　in　any　degree　sterile　together；　they　are　at　once　ranked　by　most　naturalists　as　species。　For　instance；　the　blue　and　red　pimpernel；　the　primrose　and　cowslip；　which　are　considered　by　many　of　our　best　botanists　as　varieties；　are　said　by　G？rtner　not　to　be　quite　fertile　when　crossed；　and　he　consequently　ranks　them　as　undoubted　species。　If　we　thus　argue　in　a　circle；　the　fertility　of　all　varieties　produced　under　nature　will　assuredly　have　to　be　granted。
  If　we　turn　to　varieties；　produced；　or　supposed　to　have　been　produced；　under　domestication；　we　are　still　involved　in　doubt。　For　when　it　is　stated；　for　instance；　that　the　German　Spitz　dog　unites　more　easily　than　other　dogs　with　foxes；　or　that　certain　South　American　indigenous　domestic　dogs　do　not　readily　cross　with　European　dogs；　the　explanation　which　will　occur　to　everyone；　and　probably　the　true　one；　is　that　these　dogs　have　descended　from　several　aboriginally　distinct　species。　Nevertheless　the　perfect　fertility　of　so　many　domestic　varieties；　differing　widely　from　each　other　in　appearance；　for　instance　of　the　pigeon　or　of　the　cabbage；　is　a　remarkable　fact；　more　especially　when　we　reflect　how　many　species　there　are；　which；　though　resembling　each　other　most　closely；　are　utterly　sterile　when　intercrossed。　Several　considerations；　however；　render　the　fertility　of　domestic　varieties　less　remarkable　than　at　first　appears。　It　can；　in　the　first　place；　be　clearly　shown　that　mere　external　dissimilarity　between　two　species　does　not　determine　their　greater　or　lesser　degree　of　sterility　when　crossed；　and　we　may　apply　the　same　rule　to　domestic　varieties。　In　the　second　place；　some　eminent　naturalists　believe　that　a　long　course　of　domestication　tends　to　eliminate　sterility　in　the　successive　generations　of　hybrids；　which　were　at　first　only　slightly　sterile；　and　if　this　be　so；　we　surely　ought　not　to　expect　to　find　sterility　both　appearing　and　disappearing　under　nearly　the　same　conditions　of　life。　Lastly；　and　this　seems　to　me　by　far　the　most　important　consideration；　new　races　of　animals　and　plants　are　produced　under　domestication　by　man's　methodical　and　unconscious　power　of　selection；　for　his　own　use　and　pleasure：　he　neither　wishes　to　select；　nor　could　select；　slight　differences　in　the　reproductive　system；　or　other　constitutional　difference　correlated　with　the　reproductive　system。　He　supplies　his　several　varieties　with　the　same　food；　treats　them　in　nearly　the　same　manner；　and　does　not　wish　to　alter　their　general　habits　of　life。　Nature　acts　uniformly　and　slowly　during　vast　periods　of　time　on　the　whole　organization；　in　any　way　which　may　be　for　each　creature's　own　good；　and　thus　she　may；　either　directly；　or　more　probably　indirectly；　through　correlation；　modify　the　reproductive　system　in　the　several　descendants　from　any　one　species。　Seeing　this　difference　in　the　process　of　selection；　as　carried　on　by　man　and　nature；　we　need　not　be　surprised　at　some　difference　in　the　result。
  I　have　as　yet　spoken　as　if　the　varieties　of　the　same　species　were　invariably　fertile　when　intercrossed。　But　it　seems　to　me　impossible　to　resist　the　evidence　of　the　existence　of　a　certain　amount　of　sterility　in　the　few　following　cases；　which　I　will　briefly　abstract。　The　evidence　is　at　least　as　good　as　that　from　which　we　believe　in　the　sterility　of　a　multitude　of　species。　The　evidence　is；　also；　derived　from　hostile　witnesses；　who　in　all　other　cases　consider　fertility　and　ste