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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£º¡¡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£º¡¡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£º¡¡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£º¡¡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¡¡

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