must　suppose　that　there　is　a　power　always　intently　watching　each　slight　accidental　alteration　in　the　transparent　layers；　and　carefully　selecting　each　alteration　which；　under　varied　circumstances；　may　in　any　way；　or　in　any　degree；　tend　to　produce　a　distincter　image。　We　must　suppose　each　new　state　of　the　instrument　to　be　multiplied　by　the　million；　and　each　to　be　preserved　till　a　better　be　produced；　and　then　the　old　ones　to　be　destroyed。　In　living　bodies；　variation　will　cause　the　slight　alterations；　generation　will　multiply　them　almost　infinitely；　and　natural　selection　will　pick　out　with　unerring　skill　each　improvement。　Let　this　process　go　on　for　millions　on　millions　of　years；　and　during　each　year　on　millions　of　individuals　of　many　kinds；　and　may　we　not　believe　that　a　living　optical　instrument　might　thus　be　formed　as　superior　to　one　of　glass；　as　the　works　of　the　Creator　are　to　those　of　man？
  If　it　could　be　demonstrated　that　any　complex　organ　existed；　which　could　not　possibly　have　been　formed　by　numerous；　successive；　slight　modifications；　my　theory　would　absolutely　break　down。　But　I　can　find　out　no　such　case。　No　doubt　many　organs　exist　of　which　we　do　not　know　the　transitional　grades；　more　especially　if　we　look　to　much…isolated　species；　round　which；　according　to　my　theory；　there　has　been　much　extinction。　Or　again；　if　we　look　to　an　organ　common　to　all　the　members　of　a　large　class；　for　in　this　latter　case　the　organ　must　have　been　first　formed　at　an　extremely　remote　period；　since　which　all　the　many　members　of　the　class　have　been　developed；　and　in　order　to　discover　the　early　transitional　grades　through　which　the　organ　has　passed；　we　should　have　to　look　to　very　ancient　ancestral　forms；　long　since　become　extinct。
  We　should　be　extremely　cautious　in　concluding　that　an　organ　could　not　have　been　formed　by　transitional　gradations　of　some　kind。　Numerous　cases　could　be　given　amongst　the　lower　animals　of　the　same　organ　performing　at　the　same　time　wholly　distinct　functions；　thus　the　alimentary　canal　respires；　digests；　and　excretes　in　the　larva　of　the　dragon…fly　and　in　the　fish　Cobites。　In　the　Hydra；　the　animal　may　be　turned　inside　out；　and　the　exterior　surface　will　then　digest　and　the　stomach　respire。　In　such　cases　natural　selection　might　easily　specialise；　if　any　advantage　were　thus　gained；　a　part　or　organ；　which　had　performed　two　functions；　for　one　function　alone；　and　thus　wholly　change　its　nature　by　insensible　steps。　Two　distinct　organs　sometimes　perform　simultaneously　the　same　function　in　the　same　individual；　to　give　one　instance；　there　are　fish　with　gills　or　branchiae　that　breathe　the　air　dissolved　in　the　water；　at　the　same　time　that　they　breathe　free　air　in　their　swimbladders；　this　latter　organ　having　a　ductus　pneumaticus　for　its　supply；　and　being　divided　by　highly　vascular　partitions。　In　these　cases；　one　of　the　two　organs　might　with　ease　be　modified　and　perfected　so　as　to　perform　all　the　work　by　itself；　being　aided　during　the　process　of　modification　by　the　other　organ；　and　then　this　other　organ　might　be　modified　for　some　other　and　quite　distinct　purpose；　or　be　quite　obliterated。
  The　illustration　of　the　swimbladder　in　fishes　is　a　good　one；　because　it　shows　us　clearly　the　highly　important　fact　that　an　organ　originally　constructed　for　one　purpose；　namely　flotation；　may　be　converted　into　one　for　a　wholly　different　purpose；　namely　respiration。　The　swimbladder　has；　also；　been　worked　in　as　an　accessory　to　the　auditory　organs　of　certain　fish；　or；　for　I　do　not　know　which　view　is　now　generally　held；　a　part　of　the　auditory　apparatus　has　been　worked　in　as　a　complement　to　the　swimbladder。　All　physiologists　admit　that　the　swimbladder　is　homologous；　or　'ideally　similar；'　in　position　and　structure　with　the　lungs　of　the　higher　vertebrate　animals：　hence　there　seems　to　me　to　be　no　great　difficulty　in　believing　that　natural　selection　has　actually　converted　a　swimbladder　into　a　lung；　or　organ　used　exclusively　for　respiration。
  I　can；　indeed；　hardly　doubt　that　all　vertebrate　animals　having　true　lungs　have　descended　by　ordinary　generation　from　an　ancient　prototype；　of　which　we　know　nothing；　furnished　with　a　floating　apparatus　or　swimbladder。　We　can　thus；　as　I　infer　from　Professor　Owen's　interesting　description　of　these　parts；　understand　the　strange　fact　that　every　particle　of　food　and　drink　which　we　swallow　has　to　pass　over　the　orifice　of　the　trachea；　with　some　risk　of　falling　into　the　lungs；　notwithstanding　the　beautiful　contrivance　by　which　the　glottis　is　closed。　In　the　higher　Vertebrata　the　branchiae　have　wholly　disappeared　the　slits　on　the　sides　of　the　neck　and　the　loop…like　course　of　the　arteries　still　marking　in　the　embryo　their　former　position。　But　it　is　conceivable　that　the　now　utterly　lost　branchiae　might　have　been　gradually　worked　in　by　natural　selection　for　some　quite　distinct　purpose：　in　the　same　manner　as；　on　the　view　entertained　by　some　naturalists　that　the　branchiae　and　dorsal　scales　of　Annelids　are　homologous　with　the　wings　and　wing…covers　of　insects；　it　is　probable　that　organs　which　at　a　very　ancient　period　served　for　respiration　have　been　actually　converted　into　organs　of　flight。
  In　considering　transitions　of　organs；　it　is　so　important　to　bear　in　mind　the　probability　of　conversion　from　one　function　to　another；　that　I　will　give　one　more　instance。　Pedunculated　cirripedes　have　two　minute　folds　of　skin；　called　by　me　the　ovigerous　frena；　which　serve；　through　the　means　of　a　sticky　secretion；　to　retain　the　eggs　until　they　are　hatched　within　the　sack。　These　cirripedes　have　no　branchiae；　the　whole　surface　of　the　body　and　sack；　including　the　small　frena；　serving　for　respiration。　The　Balanidae　or　sessile　cirripedes；　on　the　other　hand；　have　no　ovigerous　frena；　the　eggs　lying　loose　at　the　bottom　of　the　sack；　in　the　well…enclosed　shell；　but　they　have　large　folded　branchiae。　Now　I　think　no　one　will　dispute　that　the　ovigerous　frena　in　the　one　family　are　strictly　homologous　with　the　branchiae　of　the　other　family；　indeed；　they　graduate　into　each　other。　Therefore　I　do　not　doubt　that　little　folds　of　skin；　which　originally　served　as　ovigerous　frena；　but　which；　likewise；　very　slightly　aided　the　act　of　respiration；　have　been　gradually　converted　by　natural　selection　into　branchiae；　simply　through　an　increase　in　their　size　and　the　obliteration　of　their　adhesive　glands。　If　all　pedunculated　cirripedes　had　become　extinct；　and　they　have　already　suffered　far　more　extinction　than　have　sessile　cirripedes；　who　would　ever　have　imagined　that　the　branchiae　in　this　latter　family　had　originally　existed　as　organs　for　preventing　the　ova　from　being　washed　out　of　the　sack？
  Although　we　must　be　extremely　cautious　in　concluding　that　any　organ　could　not　possibly　have　been　produced　by　successive　transitional　gradations；　yet；　undoubtedly；　grave　cases　of　difficulty　occur；　some　of　which　will　be　discussed　in　my　future　work。
  One　of　the　gravest　is　that　of　neuter　insects；　which　are　often　very　differently　constructed　from　either　the　males　or　fertile　females；　but　this　case　will　be　treated　of　in　the　next　chapter。　The　electric　organs　of　fishes　offer　another　case　of　special　difficulty；　it　is　impossible　to　conceive　by　what　steps　these　wondrous　organs　have　been　produced；　but；　as　Owen　and　others　have　remarked；　their　intimate　structure　closely　resembles　that　of　common　muscle；　and　as　it　has　lately　been　shown　that　Rays　have　an　organ　closely　analogous　to　the　electric　apparatus；　and　yet　do　not；　as　Matteuchi　asserts；　discharge　any　electricity；　we　must　own　that　we　are　far　too　ignorant　to　argue　that　no　transition　of　any　kind　is　possible。
  The　electric　organs　offer　another　and　even　more　serious　difficulty；　for　they　occur　in　only　about　a　dozen　fishes；　of　which　several　are　widely　remote　in　their　affinities。　Generally　when　the　same　organ　appears　in　several　members　of　the　same　class；　especially　if　in　members　having　very　different　habits　of　life；　we　may　attribute　its　presence　to　inheritance　from　a　common　ancestor；　and　its　absence　in　some　of　the　members　to　its　loss　through　disuse　or　natural　selection。　But　if　the　electric　organs　had　been　inherited　from　one　ancient　progenitor　thus　provided；　we　might　have　expected　that　all　electric　fishes　would　have　been　specially　related　to　each　other。　Nor　does　geology　at　all　lead　to　the　belief　that　formerly　most　fishes　had　electric　organs；　which　most　of　their　modified　descendants　have　lost。　The　presence　of　luminous　organs　in　a　few　insects；　belonging　to　different　families　and　orders；　offers　a　parallel　case　of　difficulty。　Other　cases　could　be　given；　for　instance　in　plants；　the　very　curious　contrivance　of　a　mass　of　pollen…grains；　borne　on　a　foot…stalk　with　a　sticky　gland　at　the　end；　is　the　same　in　Orchis　and　Asclepias；　genera　almost　as　remote　as　possible　amongst　flowering　plants。　In　all　these　cases　of　two　very　distinct　species　furnished　with　apparently　the　same　anomalous　organ；　it　should　be　observed　that；　although　the　general　appearance　and　function　of　the　organ　may　be　the　same；　yet　some　fundamental　difference　can　generally　be　detected。　I　am　inclined　to　believe　that　in　nearly　the　same　way　as　two　men　have　sometimes　independently　hit　on　the　very　same　invention；　so　natural　selection；　working　for　the　good　of　each　being　and　taking　advantage　of　analogous　variations；　has　sometimes　modified　in　ver