that　are　only　founded　on　mere　assumptions。
  Thus　you　see　that　there　is　a　great　difference　between　〃mongrels；〃　which
  are　crosses　between　distinct　races；　and　〃hybrids；〃　which　are　crosses
  between　distinct　species。　　The　mongrels　are；　so　far　as　we　know；　fertile
  with　one　another。　　But　between　species；　in　many　cases；　you　cannot
  succeed　in　obtaining　even　the　first　cross：　at　any　rate　it　is　quite
  certain　that　the　hybrids　are　often　absolutely　infertile　one　with
  another。
  Here　is　a　feature；　then；　great　or　small　as　it　may　be；　which
  distinguishes　natural　species　of　animals。　　Can　we　find　any
  approximation　to　this　in　the　different　races　known　to　be　produced　by
  selective　breeding　from　a　common　stock？　　Up　to　the　present　time　the
  answer　to　that　question　is　absolutely　a　negative　one。　　As　far　as　we
  know　at　present；　there　is　nothing　approximating　to　this　check。　　In
  crossing　the　breeds　between　the　Fantail　and　the　Pouter；　the　Carrier　and
  the　Tumbler；　or　any　other　variety　or　race　you　may　nameso　far　as　we
  know　at　presentthere　is　no　difficulty　in　breeding　together　the
  mongrels。　　Take　the　Carrier　and　the　Fantail；　for　instance；　and　let　them
  represent　the　Horse　and　the　Ass　in　the　case　of　distinct　species；　then
  you　have；　as　the　result　of　their　breeding；　the　Carrier…Fantail
  mongrel；we　will　say　the　male　and　female　mongrel；and；　as　far　as　we
  know；　these　two　when　crossed　would　not　be　less　fertile　than　the
  original　cross；　or　than　Carrier　with　Carrier。　　Here；　you　see；　is　a
  physiological　contrast　between　the　races　produced　by　selective
  modification　and　natural　species。　　I　shall　inquire　into　the　value　of
  this　fact；　and　of　some　modifying　circumstances　by　and　by；　for　the
  present　I　merely　put　it　broadly　before　you。
  But　while　considering　this　question　of　the　limitations　of　species；　a
  word　must　be　said　about　what　is　called　RECURRENCEthe　tendency　of
  races　which　have　been　developed　by　selective　breeding　from　varieties　to
  return　to　their　primitive　type。　　This　is　supposed　by　many　to　put　an
  absolute　limit　to　the　extent　of　selective　and　all　other　variations。
  People　say；　〃It　is　all　very　well　to　talk　about　producing　these
  different　races；　but　you　know　very　well　that　if　you　turned　all　these
  birds　wild；　these　Pouters；　and　Carriers；　and　so　on；　they　would　all
  return　to　their　primitive　stock。〃　　This　is　very　commonly　assumed　to　be
  a　fact；　and　it　is　an　argument　that　is　commonly　brought　forward　as
  conclusive；　but　if　you　will　take　the　trouble　to　inquire　into　it　rather
  closely；　I　think　you　will　find　that　it　is　not　worth　very　much。　　The
  first　question　of　course　is；　Do　they　thus　return　to　the　primitive
  stock？　　And　commonly　as　the　thing　is　assumed　and　accepted；　it　is
  extremely　difficult　to　get　anything　like　good　evidence　of　it。　　It　is
  constantly　said；　for　example；　that　if　domesticated　Horses　are　turned
  wild；　as　they　have　been　in　some　parts　of　Asia　Minor　and　South　America；
  that　they　return　at　once　to　the　primitive　stock　from　which　they　were
  bred。　But　the　first　answer　that　you　make　to　this　assumption　is；　to　ask
  who　knows　what　the　primitive　stock　was；　and　the　second　answer　is；　that
  in　that　case　the　wild　Horses　of　Asia　Minor　ought　to　be　exactly　like　the
  wild　Horses　of　South　America。　　If　they　are　both　like　the　same　thing；
  they　ought　manifestly　to　be　like　each　other！　　The　best　authorities；
  however；　tell　you　that　it　is　quite　different。　　The　wild　Horse　of　Asia
  is　said　to　be　of　a　dun　colour；　with　a　largish　head；　and　a　great　many
  other　peculiarities；　while　the　best　authorities　on　the　wild　Horses　of
  South　America　tell　you　that　there　is　no　similarity　between　their　wild
  Horses　and　those　of　Asia　Minor；　the　cut　of　their　heads　is　very
  different；　and　they　are　commonly　chestnut　or　bay…coloured。　　It　is　quite
  clear；　therefore；　that　as　by　these　facts　there　ought　to　have　been　two
  primitive　stocks；　they　go　for　nothing　in　support　of　the　assumption　that
  races　recur　to　one　primitive　stock；　and　so　far　as　this　evidence　is
  concerned；　it　falls　to　the　ground。
  Suppose　for　a　moment　that　it　were　so；　and　that　domesticated　races；　when
  turned　wild；　did　return　to　some　common　condition；　I　cannot　see　that
  this　would　prove　much　more　than　that　similar　conditions　are　likely　to
  produce　similar　results；　and　that　when　you　take　back　domesticated
  animals　into　what　we　call　natural　conditions；　you　do　exactly　the　same
  thing　as　if　you　carefully　undid　all　the　work　you　had　gone　through；　for
  the　purpose　of　bringing　the　animal　from　its　wild　to　its　domesticated
  state。　　I　do　not　see　anything　very　wonderful　in　the　fact；　if　it　took
  all　that　trouble　to　get　it　from　a　wild　state；　that　it　should　go　back
  into　its　original　state　as　soon　as　you　removed　the　conditions　which
  produced　the　variation　to　the　domesticated　form。　There　is　an　important
  fact；　however；　forcibly　brought　forward　by　Mr。　Darwin；　which　has　been
  noticed　in　connection　with　the　breeding　of　domesticated　pigeons；　and　it
  is；　that　however　different　these　breeds　of　pigeons　may　be　from　each
  other；　and　we　have　already　noticed　the　great　differences　in　these
  breeds；　that　if；　among　any　of　those　variations；　you　chance　to　have　a
  blue　pigeon　turn　up；　it　will　be　sure　to　have　the　black　bars　across　the
  wings；　which　are　characteristic　of　the　original　wild　stock；　the　Rock
  Pigeon。
  Now；　this　is　certainly　a　very　remarkable　circumstance；　but　I　do　not　see
  myself　how　it　tells　very　strongly　either　one　way　or　the　other。　　I
  think；　in　fact；　that　this　argument　in　favour　of　recurrence　to　the
  primitive　type　might　prove　a　great　deal　too　much　for　those　who　so
  constantly　bring　it　forward。　　For　example；　Mr。　Darwin　has　very　forcibly
  urged；　that　nothing　is　commoner　than　if　you　examine　a　dun　horseand　I
  had　an　opportunity　of　verifying　this　illustration　lately；　while　in　the
  islands　of　the　West　Highlands；　where　there　are　a　great　many　dun
  horsesto　find　that　horse　exhibit　a　long　black　stripe　down　his　back；
  very　often　stripes　on　his　shoulder；　and　very　often　stripes　on　his
  legs。　　I；　myself；　saw　a　pony　of　this　description　a　short　time　ago；　in　a
  baker's　cart；　near　Rothesay；　in　Bute：　it　had　the　long　stripe　down　the
  back；　and　stripes　on　the　shoulders　and　legs；　just　like　those　of　the
  Ass；　the　Quagga；　and　the　Zebra。　　Now；　if　we　interpret　the　theory　of
  recurrence　as　applied　to　this　case；　might　it　not　be　said　that　here　was
  a　case　of　a　variation　exhibiting　the　characters　and　conditions　of　an
  animal　occupying　something　like　an　intermediate　position　between　the
  Horse；　the　Ass；　the　Quagga；　and　the　Zebra；　and　from　which　these　had
  been　developed？　　In　the　same　way　with　regard　even　to　Man。　Every
  anatomist　will　tell　you　that　there　is　nothing　commoner；　in　dissecting
  the　human　body；　than　to　meet　with　what　are　called　muscular
  variationsthat　is；　if　you　dissect　two　bodies　very　carefully；　you　will
  probably　find　that　the　modes　of　attachment　and　insertion　of　the　muscles
  are　not　exactly　the　same　in　both；　there　being　great　peculiarities　in
  the　mode　in　which　the　muscles　are　arranged；　and　it　is　very　singular；
  that　in　some　dissections　of　the　human　body　you　will　come　upon
  arrangements　of　the　muscles　very　similar　indeed　to　the　same　parts　in　the
  Apes。　　Is　the　conclusion　in　that　case　to　be；　that　this　is　like　the
  black　bars　in　the　case　of　the　Pigeon；　and　that　it　indicates　a
  recurrence　to　the　primitive　type　from　which　the　animals　have　been
  probably　developed？　　Truly；　I　think　that　the　opponents　of　modification
  and　variation　had　better　leave　the　argument　of　recurrence　alone；　or　it
  may　prove　altogether　too　strong　for　them。
  To　sum　up；the　evidence　as　far　as　we　have　gone　is　against　the　argument
  as　to　any　limit　to　divergences；　so　far　as　structure　is　concerned；　and
  in　favour　of　a　physiological　limitation。　　By　selective　breeding　we　can
  produce　structural　divergences　as　great　as　those　of　species；　but　we
  cannot　produce　equal　physiological　divergences。　　For　the　present　I　leave
  the　question　there。
  Now；　the　next　problem　that　lies　before　usand　it　is　an　extremely
  important　oneis　this：　Does　this　selective　breeding　occur　in　nature？
  Because；　if　there　is　no　proof　of　it；　all　that　I　have　been　telling　you
  goes　for　nothing　in　accounting　for　the　origin　of　species。　　Are　natural
  causes　competent　to　play　the　part　of　selection　in　perpetuating
  varieties？　　Here　we　labour　under　very　great　difficulties。　　In　the　last
  lecture　I　had　occasion　to　point　out　to　you　the　extreme　difficulty　of
  obtaining　evidence　even　of　the　first　origin　of　those　varieties　which　we
  know　to　have　occurred　in　domesticated　animals。　　I　told　you；　that　almost
  always　the　origin　of　these　varieties　is　overlooked；　so　that　I　could
  only　produce　two　of　three　cases；　as　that　of　Gratio　Kelleia　and　of　the
  Ancon　sheep。　　People　forget；　or　do　not　take　notice　of　them　until　they
  come　to　have　a　prominence；　and　if　that　is　true　of　artificial　cases；
  under　our　own　eyes；　and　in　animals　in　our　own　care；　how　much　more
  difficult　it　must　be　to　have　at　first　hand　good　evidence　of　the　origin
  of　varieties　in　nature！　　Indeed；　I　do　not　know　that　it　is　possible　by
  direct　evidence　to　prove　the　origin　of　a　variety　in　nature；　or　to　prove
  selective　breeding；　but　I　will　tell　you　what　we　can　proveand　this
  comes　to　the　same　thingthat　varieties　exist　in　nature　within　the
  limits　of　species；　and；　what　is　more；　that　when　a　variety　has　come　into
  e