Homologous　parts　tend　to　vary　in　the　same　way；　and　homologous　parts　tend　to　cohere。　Modifications　in　hard　parts　and　in　external　parts　sometimes　affect　softer　and　internal　parts。　When　one　part　is　largely　developed；　perhaps　it　tends　to　draw　nourishment　from　the　adjoining　parts；　and　every　part　of　the　structure　which　can　be　saved　without　detriment　to　the　individual；　will　be　saved。　Changes　of　structure　at　an　early　age　will　generally　affect　parts　subsequently　developed；　and　there　are　very　many　other　correlations　of　growth；　the　nature　of　which　we　are　utterly　unable　to　understand。　Multiple　parts　are　variable　in　number　and　in　structure；　perhaps　arising　from　such　parts　not　having　been　closely　specialized　to　any　particular　function；　so　that　their　modifications　have　not　been　closely　checked　by　natural　selection。　It　is　probably　from　this　same　cause　that　organic　beings　low　in　the　scale　of　nature　are　more　variable　than　those　which　have　their　whole　organisation　more　specialized；　and　are　higher　in　the　scale。　Rudimentary　organs；　from　being　useless；　will　be　disregarded　by　natural　selection；　and　hence　probably　are　variable。　Specific　characters　that　is；　the　characters　which　have　come　to　differ　since　the　several　species　of　the　same　genus　branched　off　from　a　common　parent　are　more　variable　than　generic　characters；　or　those　which　have　long　been　inherited；　and　have　not　differed　within　this　same　period。　In　these　remarks　we　have　referred　to　special　parts　or　organs　being　still　variable；　because　they　have　recently　varied　and　thus　come　to　differ；　but　we　have　also　seen　in　the　second　Chapter　that　the　same　principle　applies　to　the　whole　individual；　for　in　a　district　where　many　species　of　any　genus　are　found　that　is；　where　there　has　been　much　former　variation　and　differentiation；　or　where　the　manufactory　of　new　specific　forms　has　been　actively　at　work　there；　on　an　average；　we　now　find　most　varieties　or　incipient　species。　Secondary　sexual　characters　are　highly　variable；　and　such　characters　differ　much　in　the　species　of　the　same　group。　Variability　in　the　same　parts　of　the　organisation　has　generally　been　taken　advantage　of　in　giving　secondary　sexual　differences　to　the　sexes　of　the　same　species；　and　specific　differences　to　the　several　species　of　the　same　genus。　Any　part　or　organ　developed　to　an　extraordinary　size　or　in　an　extraordinary　manner；　in　comparison　with　the　same　part　or　organ　in　the　allied　species；　must　have　gone　through　an　extraordinary　amount　of　modification　since　the　genus　arose；　and　thus　we　can　understand　why　it　should　often　still　be　variable　in　a　much　higher　degree　than　other　parts；　for　variation　is　a　long…continued　and　slow　process；　and　natural　selection　will　in　such　cases　not　as　yet　have　had　time　to　overcome　the　tendency　to　further　variability　and　to　reversion　to　a　less　modified　state。　But　when　a　species　with　any　extraordinarily…developed　organ　has　become　the　parent　of　many　modified　descendants　which　on　my　view　must　be　a　very　slow　process；　requiring　a　long　lapse　of　time　in　this　case；　natural　selection　may　readily　have　succeeded　in　giving　a　fixed　character　to　the　organ；　in　however　extraordinary　a　manner　it　may　be　developed。　Species　inheriting　nearly　the　same　constitution　from　a　common　parent　and　exposed　to　similar　influences　will　naturally　tend　to　present　analogous　variations；　and　these　same　species　may　occasionally　revert　to　some　of　the　characters　of　their　ancient　progenitors。　Although　new　and　important　modifications　may　not　arise　from　reversion　and　analogous　variation；　such　modifications　will　add　to　the　beautiful　and　harmonious　diversity　of　nature。
  Whatever　the　cause　may　be　of　each　slight　difference　in　the　offspring　from　their　parents　and　a　cause　for　each　must　exist　it　is　the　steady　accumulation；　through　natural　selection；　of　such　differences；　when　beneficial　to　the　individual；　that　gives　rise　to　all　the　more　important　modifications　of　structure；　by　which　the　innumerable　beings　on　the　face　of　this　earth　are　enabled　to　struggle　with　each　other；　and　the　best　adapted　to　survive。
  Chapter　6　…　Difficulties　on　Theory
  LONG　before　having　arrived　at　this　part　of　my　work；　a　crowd　of　difficulties　will　have　occurred　to　the　reader。　Some　of　them　are　so　grave　that　to　this　day　I　can　never　reflect　on　them　without　being　staggered；　but；　to　the　best　of　my　judgment；　the　greater　number　are　only　apparent；　and　those　that　are　real　are　not；　I　think；　fatal　to　my　theory。
  These　difficulties　and　objections　may　be　classed　under　the　following　heads：…Firstly；　why；　if　species　have　descended　from　other　species　by　insensibly　fine　gradations；　do　we　not　everywhere　see　innumerable　transitional　forms？　Why　is　not　all　nature　in　confusion　instead　of　the　species　being；　as　we　see　them；　well　defined？
  Secondly；　is　it　possible　that　an　animal　having；　for　instance；　the　structure　and　habits　of　a　bat；　could　have　been　formed　by　the　modification　of　some　animal　with　wholly　different　habits？　Can　we　believe　that　natural　selection　could　produce；　on　the　one　hand；　organs　of　trifling　importance；　such　as　the　tail　of　a　giraffe；　which　serves　as　a　fly…flapper；　and；　on　the　other　hand；　organs　of　such　wonderful　structure；　as　the　eye；　of　which　we　hardly　as　yet　fully　understand　the　inimitable　perfection？
  Thirdly；　can　instincts　be　acquired　and　modified　through　natural　selection？　What　shall　we　say　to　so　marvellous　an　instinct　as　that　which　leads　the　bee　to　make　cells；　which　have　practically　anticipated　the　discoveries　of　profound　mathematicians？
  Fourthly；　how　can　we　account　for　species；　when　crossed；　being　sterile　and　producing　sterile　offspring；　whereas；　when　varieties　are　crossed；　their　fertility　is　unimpaired？
  The　two　first　heads　shall　be　here　discussed　Instinct　and　Hybridism　in　separate　chapters。
  On　the　absence　or　rarity　of　transitional　varieties。　As　natural　selection　acts　solely　by　the　preservation　of　profitable　modifications；　each　new　form　will　tend　in　a　fully…stocked　country　to　take　the　place　of；　and　finally　to　exterminate；　its　own　less　improved　parent　or　other　less…favoured　forms　with　which　it　comes　into　competition。　Thus　extinction　and　natural　selection　will；　as　we　have　seen；　go　hand　in　hand。　Hence；　if　we　look　at　each　species　as　descended　from　some　other　unknown　form；　both　the　parent　and　all　the　transitional　varieties　will　generally　have　been　exterminated　by　the　very　process　of　formation　and　perfection　of　the　new　form。
  But；　as　by　this　theory　innumerable　transitional　forms　must　have　existed；　why　do　we　not　find　them　embedded　in　countless　numbers　in　the　crust　of　the　earth？　It　will　be　much　more　convenient　to　discuss　this　question　in　the　chapter　on　the　Imperfection　of　the　geological　record；　and　I　will　here　only　state　that　I　believe　the　answer　mainly　lies　in　the　record　being　incomparably　less　perfect　than　is　generally　supposed；　the　imperfection　of　the　record　being　chiefly　due　to　organic　beings　not　inhabiting　profound　depths　of　the　sea；　and　to　their　remains　being　embedded　and　preserved　to　a　future　age　only　in　masses　of　sediment　sufficiently　thick　and　extensive　to　withstand　an　enormous　amount　of　future　degradation；　and　such　fossiliferous　masses　can　be　accumulated　only　where　much　sediment　is　deposited　on　the　shallow　bed　of　the　sea；　whilst　it　slowly　subsides。　These　contingencies　will　concur　only　rarely；　and　after　enormously　long　intervals。　Whilst　the　bed　of　the　sea　is　stationary　or　is　rising；　or　when　very　little　sediment　is　being　deposited；　there　will　be　blanks　in　our　geological　history。　The　crust　of　the　earth　is　a　vast　museum；　but　the　natural　collections　have　been　made　only　at　intervals　of　time　immensely　remote。
  But　it　may　be　urged　that　when　several　closely…allied　species　inhabit　the　same　territory　we　surely　ought　to　find　at　the　present　time　many　transitional　forms。　Let　us　take　a　simple　case：　in　travelling　from　north　to　south　over　a　continent；　we　generally　meet　at　successive　intervals　with　closely　allied　or　representative　species；　evidently　filling　nearly　the　same　place　in　the　natural　economy　of　the　land。　These　representative　species　often　meet　and　interlock；　and　as　the　one　becomes　rarer　and　rarer；　the　other　becomes　more　and　more　frequent；　till　the　one　replaces　the　other。　But　if　we　compare　these　species　where　they　intermingle；　they　are　generally　as　absolutely　distinct　from　each　other　in　every　detail　of　structure　as　are　specimens　taken　from　the　metropolis　inhabited　by　each。　By　my　theory　these　allied　species　have　descended　from　a　common　parent；　and　during　the　process　of　modification；　each　has　become　adapted　to　the　conditions　of　life　of　its　own　region；　and　has　supplanted　and　exterminated　its　original　parent　and　all　the　transitional　varieties　between　its　past　and　present　states。　Hence　we　ought　not　to　expect　at　the　present　time　to　meet　with　numerous　transitional　varieties　in　each　region；　though　they　must　have　existed　there；　and　may　be　embedded　there　in　a　fossil　condition。　But　in　the　intermediate　region；　having　intermediate　conditions　of　life；　why　do　we　not　now　find　closely…linking　intermediate　varieties？　This　difficulty　for　a　long　time　quite　confounded　me。　But　I　think　it　can　be　in　large　part　explained。
  In　the　first　place　we　should　be　extremely　cautious　in　inferring；　because　an　area　is　now　continuous；　that　it　has　been　continuous　during　a　long　period。　Geology　would　lead　us　to　believe　that　almost　every　continen