The　Conditions　of　Existence
  by　Thomas　H。　Huxley
  IN　the　last　Lecture　I　endeavoured　to　prove　to　you　that；　while；　as　a
  general　rule；　organic　beings　tend　to　reproduce　their　kind；　there　is　in
  them；　also；　a　constantly　recurring　tendency　to　varyto　vary　to　a
  greater　or　to　a　less　extent。　　Such　a　variety；　I　pointed　out　to　you；
  might　arise　from　causes　which　we　do　not　understand；　we　therefore　called
  it　spontaneous；　and　it　might　come　into　existence　as　a　definite　and
  marked　thing；　without　any　gradations　between　itself　and　the　form　which
  preceded　it。　　I　further　pointed　out；　that　such　a　variety　having　once
  arisen；　might　be　perpetuated　to　some　extent；　and　indeed　to　a　very
  marked　extent；　without　any　direct　interference；　or　without　any　exercise
  of　that　process　which　we　called　selection。　　And　then　I　stated　further；
  that　by　such　selection；　when　exercised　artificiallyif　you　took　care　to
  breed　only　from　those　forms　which　presented　the　same　peculiarities　of
  any　variety　which　had　arisen　in　this　mannerthe　variation　might　be
  perpetuated；　as　far　as　we　can　see；　indefinitely。
  The　next　question；　and　it　is　an　important　one　for　us；　is　this：　Is　there
  any　limit　to　the　amount　of　variation　from　the　primitive　stock　which　can
  be　produced　by　this　process　of　selective　breeding？　　In　considering　this
  question；　it　will　be　useful　to　class　the　characteristics；　in　respect　of
  which　organic　beings　vary；　under　two　heads：　we　may　consider　structural
  characteristics；　and　we　may　consider　physiological　characteristics。
  In　the　first　place；　as　regards　structural　characteristics；　I　endeavoured
  to　show　you；　by　the　skeletons　which　I　had　upon　the　table；　and　by
  reference　to　a　great　many　well…ascertained　facts；　that　the　different
  breeds　of　Pigeons；　the　Carriers；　Pouters；　and　Tumblers；　might　vary　in
  any　of　their　internal　and　important　structural　characters　to　a　very
  great　degree；　not　only　might　there　be　changes　in　the　proportions　of　the
  skull；　and　the　characters　of　the　feet　and　beaks；　and　so　on；　but　that
  there　might　be　an　absolute　difference　in　the　number　of　the　vertebrae　of
  the　back；　as　in　the　sacral　vertebrae　of　the　Pouter；　and　so　great　is　the
  extent　of　the　variation　in　these　and　similar　characters　that　I　pointed
  out　to　you；　by　reference　to　the　skeletons　and　the　diagrams；　that　these
  extreme　varieties　may　absolutely　differ　more　from　one　another　in　their
  structural　characters　than　do　what　naturalists　call　distinct　SPECIES　of
  pigeons；　that　is　to　say；　that　they　differ　so　much　in　structure　that
  there　is　a　greater　difference　between　the　Pouter　and　the　Tumbler　than
  there　is　between　such　wild　and　distinct　forms　as　the　Rock　Pigeon　or　the
  Ring　Pigeon；　or　the　Ring　Pigeon　and　the　Stock　Dove；　and　indeed　the
  differences　are　of　greater　value　than　this；　for　the　structural
  differences　between　these　domesticated　pigeons　are　such　as　would　be
  admitted　by　a　naturalist；　supposing　he　knew　nothing　at　all　about　their
  origin；　to　entitle　them　to　constitute　even　distinct　genera。
  As　I　have　used　this　term　SPECIES；　and　shall　probably　use　it　a　good　deal；
  I　had　better　perhaps　devote　a　word　or　two　to　explaining　what　I　mean　by
  it。
  Animals　and　plants　are　divided　into　groups；　which　become　gradually
  smaller；　beginning　with　a　KINGDOM；　which　is　divided　into　SUB…KINGDOMS；
  then　come　the　smaller　divisions　called　PROVINCES；　and　so　on　from　a
  PROVINCE　to　a　CLASS　from　a　CLASS　to　an　ORDER；　from　ORDERS　to　FAMILIES；
  and　from　these　to　GENERA；　until　we　come　at　length　to　the　smallest
  groups　of　animals　which　can　be　defined　one　from　the　other　by　constant
  characters；　which　are　not　sexual；　and　these　are　what　naturalists　call
  SPECIES　in　practice；　whatever　they　may　do　in　theory。
  If；　in　a　state　of　nature；　you　find　any　two　groups　of　living　beings；
  which　are　separated　one　from　the　other　by　some　constantly…recurring
  characteristic；　I　don't　care　how　slight　and　trivial；　so　long　as　it　is
  defined　and　constant；　and　does　not　depend　on　sexual　peculiarities；　then
  all　naturalists　agree　in　calling　them　two　species；　that　is　what　is
  meant　by　the　use　of　the　word　speciesthat　is　to　say；　it　is；　for　the
  practical　naturalist；　a　mere　question　of　structural　differences。*
  ＇footnote＇*　I　lay　stress　here　on　the　'practical'
  signification　of　〃Species。〃　　Whether　a　physiological　test
  between　species　exist　or　not；　it　is　hardly　ever　applicable
  by　the　practical　naturalist。
  We　have　seen　nowto　repeat　this　point　once　more；　and　it　is　very
  essential　that　we　should　rightly　understand　itwe　have　seen　that
  breeds；　known　to　have　been　derived　from　a　common　stock　by　selection；
  may　be　as　different　in　their　structure　from　the　original　stock　as
  species　may　be　distinct　from　each　other。
  But　is　the　like　true　of　the　physiological　characteristics　of　animals？
  Do　the　physiological　differences　of　varieties　amount　in　degree　to　those
  observed　between　forms　which　naturalists　call　distinct　species？　　This
  is　a　most　important　point　for　us　to　consider。
  As　regards　the　great　majority　of　physiological　characteristics；　there　is
  no　doubt　that　they　are　capable　of　being　developed；　increased；　and
  modified　by　selection。
  There　is　no　doubt　that　breeds　may　be　made　as　different　as　species　in
  many　physiological　characters。　　I　have　already　pointed　out　to　you　very
  briefly　the　different　habits　of　the　breeds　of　Pigeons；　all　of　which
  depend　upon　their　physiological　peculiarities；as　the　peculiar　habit
  of　tumbling；　in　the　Tumblerthe　peculiarities　of　flight；　in　the
  〃homing〃　birds；the　strange　habit　of　spreading　out　the　tail；　and
  walking　in　a　peculiar　fashion；　in　the　Fantail；and；　lastly；　the　habit
  of　blowing　out　the　gullet；　so　characteristic　of　the　Pouter。　These　are
  all　due　to　physiological　modifications；　and　in　all　these　respects　these
  birds　differ　as　much　from　each　other　as　any　two　ordinary　species　do。
  So　with　Dogs　in　their　habits　and　instincts。　　It　is　a　physiological
  peculiarity　which　leads　the　Greyhound　to　chase　its　prey　by　sight；that
  enables　the　Beagle　to　track　it　by　the　scent；that　impels　the　Terrier
  to　its　rat…hunting　propensity；and　that　leads　the　Retriever　to　its
  habit　of　retrieving。　　These　habits　and　instincts　are　all　the　results　of
  physiological　differences　and　peculiarities；　which　have　been　developed
  from　a　common　stock；　at　least　there　is　every　reason　to　believe　so。　　But
  it　is　a　most　singular　circumstance；　that　while　you　may　run　through
  almost　the　whole　series　of　physiological　processes；　without　finding　a
  check　to　your　argument；　you　come　at　last　to　a　point　where　you　do　find　a
  check；　and　that　is　in　the　reproductive　processes。　For　there　is　a　most
  singular　circumstance　in　respect　to　natural　speciesat　least　about　some
  of　themand　it　would　be　sufficient　for　the　purposes　of　this　argument
  if　it　were　true　of　only　one　of　them；　but　there　is；　in　fact；　a　great
  number　of　such　casesand　that　is；　that；　similar　as　they　may　appear　to
  be　to　mere　races　or　breeds；　they　present　a　marked　peculiarity　in　the
  reproductive　process。　If　you　breed　from　the　male　and　female　of　the　same
  race；　you　of　course　have　offspring　of　the　like　kind；　and　if　you　make
  the　offspring　breed　together；　you　obtain　the　same　result；　and　if　you
  breed　from　these　again；　you　will　still　have　the　same　kind　of　offspring；
  there　is　no　check。　But　if　you　take　members　of　two　distinct　species；
  however　similar　they　may　be　to　each　other　and　make　them　breed　together；
  you　will　find　a　check；　with　some　modifications　and　exceptions；　however；
  which　I　shall　speak　of　presently。　　If　you　cross　two　such　species　with
  each　other；　then；although　you　may　get　offspring　in　the　case　of　the
  first　cross；　yet；　if　you　attempt　to　breed　from　the　products　of　that
  crossing；　which　are　what　are　called　HYBRIDSthat　is；　if　you　couple　a
  male　and　a　female　hybridthen　the　result　is　that　in　ninety…nine　cases
  out　of　a　hundred　you　will　get　no　offspring　at　all；　there　will　be　no
  result　whatsoever。
  The　reason　of　this　is　quite　obvious　in　some　cases；　the　male　hybrids；
  although　possessing　all　the　external　appearances　and　characteristics　of
  perfect　animals；　are　physiologically　imperfect　and　deficient　in　the
  structural　parts　of　the　reproductive　elements　necessary　to　generation。
  It　is　said　to　be　invariably　the　case　with　the　male　mule；　the　cross
  between　the　Ass　and　the　Mare；　and　hence　it　is；　that；　although　crossing
  the　Horse　with　the　Ass　is　easy　enough；　and　is　constantly　done；　as　far
  as　I　am　aware；　if　you　take　two　mules；　a　male　and　a　female；　and　endeavour
  to　breed　from　them；　you　get　no　offspring　whatever；　no　generation　will
  take　place。　　This　is　what　is　called　the　sterility　of　the　hybrids
  between　two　distinct　species。
  You　see　that　this　is　a　very　extraordinary　circumstance；　one　does　not　see
  why　it　should　be。　　The　common　teleological　explanation　is；　that　it　is
  to　prevent　the　impurity　of　the　blood　resulting　from　the　crossing　of　one
  species　with　another；　but　you　see　it　does　not　in　reality　do　anything　of
  the　kind。　　There　is　nothing　in　this　fact　that　hybrids　cannot　breed　with
  each　other；　to　establish　such　a　theory；　there　is　nothing　to　prevent　the
  Horse　breeding　with　the　Ass；　or　the　Ass　with　the　Horse。　　So　that　this
  explanation　breaks　down；　as　a　great　many　explanations　of　this　kind　do；
  that　are　only　founded　on　mere　assumptions。
  Thus　you　see　that　there　is　a　g