great　thickness；　and　a　thick　bed　of　mould　checks　in　many　ways　the
  disintegration　of　the　underlying　rocks　and　fragments　of　rock。
  The　removal　of　worm…castings　by　the　above　means　leads　to　results
  which　are　far　from　insignificant。　　It　has　been　shown　that　a　layer
  of　earth；　0。2　of　an　inch　in　thickness；　is　in　many　places　annually
  brought　to　the　surface；　and　if　a　small　part　of　this　amount　flows；
  or　rolls；　or　is　washed；　even　for　a　short　distance；　down　every
  inclined　surface；　or　is　repeatedly　blown　in　one　direction；　a　great
  effect　will　be　produced　in　the　course　of　ages。　　It　was　found　by
  measurements　and　calculations　that　on　a　surface　with　a　mean
  inclination　of　9　degrees　26　seconds；　2。4　cubic　inches　of　earth
  which　had　been　ejected　by　worms　crossed；　in　the　course　of　a　year；　a
  horizontal　line　one　yard　in　length；　so　that　240　cubic　inches　would
  cross　a　line　100　yards　in　length。　　This　latter　amount　in　a　damp
  state　would　weigh　11。5　pounds。　　Thus　a　considerable　weight　of　earth
  is　continually　moving　down　each　side　of　every　valley；　and　will　in
  time　reach　its　bed。　　Finally　this　earth　will　be　transported　by　the
  streams　flowing　in　the　valleys　into　the　ocean；　the　great　receptacle
  for　all　matter　denuded　from　the　land。　　It　is　known　from　the　amount
  of　sediment　annually　delivered　into　the　sea　by　the　Mississippi；
  that　its　enormous　drainage…area　must　on　an　average　be　lowered
  。00263　of　an　inch　each　year；　and　this　would　suffice　in　four　and
  half　million　years　to　lower　the　whole　drainage…area　to　the　level　of
  the　sea…shore。　　So　that；　if　a　small　fraction　of　the　layer　of　fine
  earth；　0。2　of　an　inch　in　thickness；　which　is　annually　brought　to
  the　surface　by　worms；　is　carried　away；　a　great　result　cannot　fail
  to　be　produced　within　a　period　which　no　geologist　considers
  extremely　long。
  Archaeologists　ought　to　be　grateful　to　worms；　as　they　protect　and
  preserve　for　an　indefinitely　long　period　every　object；　not　liable
  to　decay；　which　is　dropped　on　the　surface　of　the　land；　by　burying
  it　beneath　their　castings。　　Thus；　also；　many　elegant　and　curious
  tesselated　pavements　and　other　ancient　remains　have　been　preserved；
  though　no　doubt　the　worms　have　in　these　cases　been　largely　aided　by
  earth　washed　and　blown　from　the　adjoining　land；　especially　when
  cultivated。　　The　old　tesselated　pavements　have；　however；　often
  suffered　by　having　subsided　unequally　from　being　unequally
  undermined　by　the　worms。　　Even　old　massive　walls　may　be　undermined
  and　subside；　and　no　building　is　in　this　respect　safe；　unless　the
  foundations　lie　6　or　7　feet　beneath　the　surface；　at　a　depth　at
  which　worms　cannot　work。　　It　is　probable　that　many　monoliths　and
  some　old　walls　have　fallen　down　from　having　been　undermined　by
  worms。
  Worms　prepare　the　ground　｛82｝　in　an　excellent　manner　for　the　growth
  of　fibrous…rooted　plants　and　for　seedlings　of　all　kinds。　　They
  periodically　expose　the　mould　to　the　air；　and　sift　it　so　that　no
  stones　larger　than　the　particles　which　they　can　swallow　are　left　in
  it。　　They　mingle　the　whole　intimately　together；　like　a　gardener　who
  prepares　fine　soil　for　his　choicest　plants。　　In　this　state　it　is
  well　fitted　to　retain　moisture　and　to　absorb　all　soluble
  substances；　as　well　as　for　the　process　of　nitrification。　　The　bones
  of　dead　animals；　the　harder　parts　of　insects；　the　shells　of　land…
  molluscs；　leaves；　twigs；　&c。；　are　before　long　all　buried　beneath
  the　accumulated　castings　of　worms；　and　are　thus　brought　in　a　more
  or　less　decayed　state　within　reach　of　the　roots　of　plants。　　Worms
  likewise　drag　an　infinite　number　of　dead　leaves　and　other　parts　of
  plants　into　their　burrows；　partly　for　the　sake　of　plugging　them　up
  and　partly　as　food。
  The　leaves　which　are　dragged　into　the　burrows　as　food；　after　being
  torn　into　the　finest　shreds；　partially　digested；　and　saturated　with
  the　intestinal　and　urinary　secretions；　are　commingled　with　much
  earth。　　This　earth　forms　the　dark　coloured；　rich　humus　which　almost
  everywhere　covers　the　surface　of　the　land　with　a　fairly　well…
  defined　layer　or　mantle。　　Hensen　｛83｝　placed　two　worms　in　a　vessel
  18　inches　in　diameter；　which　was　filled　with　sand；　on　which　fallen
  leaves　were　strewed；　and　these　were　soon　dragged　into　their　burrows
  to　a　depth　of　3　inches。　　After　about　6　weeks　an　almost　uniform
  layer　of　sand；　a　centimeter　（0。4　inch）　in　thickness；　was　converted
  into　humus　by　having　passed　through　the　alimentary　canals　of　these
  two　worms。　　It　is　believed　by　some　persons　that　worm…burrows；　which
  often　penetrate　the　ground　almost　perpendicularly　to　a　depth　of　5
  or　6　feet；　materially　aid　in　its　drainage；　notwithstanding　that　the
  viscid　castings　piled　over　the　mouths　of　the　burrows　prevent　or
  check　the　rain…water　directly　entering　them。　　They　allow　the　air　to
  penetrate　deeply　into　the　ground。　　They　also　greatly　facilitate　the
  downward　passage　of　roots　of　moderate　size；　and　these　will　be
  nourished　by　the　humus　with　which　the　burrows　are　lined。　　Many
  seeds　owe　their　germination　to　having　been　covered　by　castings；　and
  others　buried　to　a　considerable　depth　beneath　accumulated　castings
  lie　dormant；　until　at　some　future　time　they　are　accidentally
  uncovered　and　germinate。
  Worms　are　poorly　provided　with　sense…organs；　for　they　cannot　be
  said　to　see；　although　they　can　just　distinguish　between　light　and
  darkness；　they　are　completely　deaf；　and　have　only　a　feeble　power　of
  smell；　the　sense　of　touch　alone　is　well　developed。　　They　can
  therefore　learn　but　little　about　the　outside　world；　and　it　is
  surprising　that　they　should　exhibit　some　skill　in　lining　their
  burrows　with　their　castings　and　with　leaves；　and　in　the　case　of
  some　species　in　piling　up　their　castings　into　tower…like
  constructions。　　But　it　is　far　more　surprising　that　they　should
  apparently　exhibit　some　degrees　of　intelligence　instead　of　a　mere
  blind　instinctive　impulse；　in　their　manner　of　plugging　up　the
  mouths　of　their　burrows。　　They　act　in　nearly　the　same　manner　as
  would　a　man；　who　had　to　close　a　cylindrical　tube　with　different
  kinds　of　leaves；　petioles；　triangles　of　paper；　&c。；　for　they
  commonly　seize　such　objects　by　their　pointed　ends。　　But　with　thin
  objects　a　certain　number　are　drawn　in　by　their　broader　ends。　　They
  do　not　act　in　the　same　unvarying　manner　in　all　cases；　as　do　most　of
  the　lower　animals；　for　instance；　they　do　not　drag　in　leaves　by
  their　foot…stalks；　unless　the　basal　part　of　the　blade　is　as　narrow
  as　the　apex；　or　narrower　than　it。
  When　we　behold　a　wide；　turf…covered　expanse；　we　should　remember
  that　its　smoothness；　on　which　so　much　of　its　beauty　depends；　is
  mainly　due　to　all　the　inequalities　having　been　slowly　levelled　by
  worms。　　It　is　a　marvellous　reflection　that　the　whole　of　the
  superficial　mould　over　any　such　expanse　has　passed；　and　will　again
  pass；　every　few　years　through　the　bodies　of　worms。　　The　plough　is
  one　of　the　most　ancient　and　most　valuable　of　man's　inventions；　but
  long　before　he　existed　the　land　was　in　fact　regularly　ploughed；　and
  still　continues　to　be　thus　ploughed　by　earth…worms。　　It　may　be
  doubted　whether　there　are　many　other　animals　which　have　played　so
  important　a　part　in　the　history　of　the　world；　as　have　these　lowly
  organized　creatures。　　Some　other　animals；　however；　still　more　lowly
  organized；　namely　corals；　have　done　far　more　conspicuous　work　in
  having　constructed　innumerable　reefs　and　islands　in　the　great
  oceans；　but　these　are　almost　confined　to　the　tropical　zones。
  Footnotes：
  ｛1｝　　'Lecons　de　Geologie　Pratique；'　tom。　i。　1845；　p。　140。
  ｛2｝　　'Transactions　Geolog。　Soc。'　vol。　v。　p。　505。　　Read　November　1；
  1837。
  ｛3｝　　'Histoire　des　progres　de　la　Geologie；'　tom。　i。　1847；　p。　224。
  ｛4｝　　'Zeitschrift　fur　wissenschaft。　　Zoologie；'　B。　xxviii。　1877；　p。
  361。
  ｛5｝　　'Gardeners'　Chronicle；'　April　17；　1869；　p。　418。
  ｛6｝　　Mr。　Darwin's　attention　was　called　by　Professor　Hensen　to　P。　E。
  Muller's　work　on　Humus　in　'Tidsskrift　for　Skovbrug；'　Band　iii。　Heft
  1　and　2；　Copenhagen；　1878。　　He　had；　however；　no　opportunity　of
  consulting　Muller's　work。　　Dr。　Muller　published　a　second　paper　in
  1884　in　the　same　periodicala　Danish　journal　of　forestry。　　His
  results　have　also　been　published　in　German；　in　a　volume　entitled
  'Studien　uber　die　naturlichen　Humusformen；　unter　deren　Einwirkung
  auf　Vegetation　und　Boden；'　8vo。；　Berlin；　1887。
  ｛7｝　　'Bidrag　till　Skandinaviens　Oligochaetfauna；'　1871。
  ｛8｝　　'Die　bis　jetzt　bekannten　Arten　aus　der　Familie　der
  Regenwurmer；'　1845。
  ｛9｝　　There　is　even　some　reason　to　believe　that　pressure　is　actually
  favourable　to　the　growth　of　grasses；　for　Professor　Buckman；　who
  made　many　observations　on　their　growth　in　the　experimental　gardens
  of　the　Royal　Agricultural　College；　remarks　（'Gardeners'　Chronicle；'
  1854；　p。　619）：　　〃Another　circumstance　in　the　cultivation　of　grasses
  in　the　separate　form　or　small　patches；　is　the　impossibility　of
  rolling　or　treading　them　firmly；　without　which　no　pasture　can
  continue　good。〃
  ｛10｝　　I　shall　have　occasion　often　to　refer　to　M。　Perrier's
  admirable　memoir；　'Organisation　des　Lombriciens　terrestres'　in
  'Archives　de　Zoolog。　exper。'　tom。　iii。　1874；　p。　372。　　C。　F。　Morren
  （'De　Lumbrici　terrestris　Hist。　Nat。'