complicated　activities　of　man　are　comprehensible　under　three
  categories。　　Either　they　are　immediately　directed　towards　the
  maintenance　and　development　of　the　body；　or　they　effect　transitory
  changes　in　the　relative　positions　of　parts　of　the　body；　or　they
  tend　towards　the　continuance　of　the　species。　　Even　those
  manifestations　of　intellect；　of　feeling；　and　of　will；　which　we
  rightly　name　the　higher　faculties；　are　not　excluded　from　this
  classification；　inasmuch　as　to　every　one　but　the　subject　of　them；
  they　are　known　only　as　transitory　changes　in　the　relative　positions
  of　parts　of　the　body。　　Speech；　gesture；　and　every　other　form　of
  human　action　are；　in　the　long　run；　resolvable　into　muscular
  contraction；　and　muscular　contraction　is　but　a　transitory　change　in
  the　relative　positions　of　the　parts　of　a　muscle。　　But　the　scheme
  which　is　large　enough　to　embrace　the　activities　of　the　highest　form
  of　life；　covers　all　those　of　the　lower　creatures。　　The　lowest
  plant；　or　animalcule；　feeds；　grows；　and　reproduces　its　kind。　　In
  addition；　all　animals　manifest　those　transitory　changes　of　form
  which　we　class　under　irritability　and　contractility；　and；　it　is
  more　than　probable；　that　when　the　vegetable　world　is　thoroughly
  explored；　we　shall　find　all　plants　in　possession　of　the　same
  powers；　at　one　time　or　other　of　their　existence。
  I　am　not　now　alluding　to　such　phaenomena；　at　once　rare　and
  conspicuous；　as　those　exhibited　by　the　leaflets　of　the　sensitive
  plants；　or　the　stamens　of　the　barberry；　but　to　much　more　widely
  spread；　and　at　the　same　time；　more　subtle　and　hidden；
  manifestations　of　vegetable　contractility。　　You　are　doubtless　aware
  that　the　common　nettle　owes　its　stinging　property　to　the
  innumerable　stiff　and　needle…like；　though　exquisitely　delicate；
  hairs　which　cover　its　surface。　　Each　stinging…needle　tapers　from　a
  broad　base　to　a　slender　summit；　which；　though　rounded　at　the　end；
  is　of　such　microscopic　fineness　that　it　readily　penetrates；　and
  breaks　off　in；　the　skin。　　The　whole　hair　consists　of　a　very
  delicate　outer　case　of　wood；　closely　applied　to　the　inner　surface
  of　which　is　a　layer　of　semi…fluid　matter；　full　of　innumerable
  granules　of　extreme　minuteness。　　This　semi…fluid　lining　is
  protoplasm；　which　thus　constitutes　a　kind　of　bag；　full　of　a　limpid
  liquid；　and　roughly　corresponding　in　form　with　the　interior　of　the
  hair　which　it　fills。　　When　viewed　with　a　sufficiently　high
  magnifying　power；　the　protoplasmic　layer　of　the　nettle　hair　is　seen
  to　be　in　a　condition　of　unceasing　activity。　　Local　contractions　of
  the　whole　thickness　of　its　substance　pass　slowly　and　gradually　from
  point　to　point；　and　give　rise　to　the　appearance　of　progressive
  waves；　just　as　the　bending　of　successive　stalks　of　corn　by　a　breeze
  produces　the　apparent　billows　of　a　cornfield。
  But；　in　addition　to　these　movements；　and　independently　of　them；　the
  granules　are　driven；　in　relatively　rapid　streams；　through　channels
  in　the　protoplasm　which　seem　to　have　a　considerable　amount　of
  persistence。　　Most　commonly；　the　currents　in　adjacent　parts　of　the
  protoplasm　take　similar　directions；　and；　thus；　there　is　a　general
  stream　up　one　side　of　the　hair　and　down　the　other。　　But　this　does
  not　prevent　the　existence　of　partial　currents　which　take　different
  routes；　and　sometimes　trains　of　granules　may　be　seen　coursing
  swiftly　in　opposite　directions　within　a　twenty…thousandth　of　an
  inch　of　one　another；　while；　occasionally；　opposite　streams　come
  into　direct　collision；　and；　after　a　longer　or　shorter　struggle；　one
  predominates。　　The　cause　of　these　currents　seems　to　lie　in
  contractions　of　the　protoplasm　which　bounds　the　channels　in　which
  they　flow；　but　which　are　so　minute　that　the　best　microscopes　show
  only　their　effects；　and　not　themselves。
  The　spectacle　afforded　by　the　wonderful　energies　prisoned　within
  the　compass　of　the　microscopic　hair　of　a　plant；　which　we　commonly
  regard　as　a　merely　passive　organism；　is　not　easily　forgotten　by　one
  who　has　watched　its　display；　continued　hour　after　hour；　without
  pause　or　sign　of　weakening。　　The　possible　complexity　of　many　other
  organic　forms；　seemingly　as　simple　as　the　protoplasm　of　the　nettle；
  dawns　upon　one；　and　the　comparison　of　such　a　protoplasm　to　a　body
  with　an　internal　circulation；　which　has　been　put　forward　by　an
  eminent　physiologist；　loses　much　of　its　startling　character。
  Currents　similar　to　those　of　the　hairs　of　the　nettle　have　been
  observed　in　a　great　multitude　of　very　different　plants；　and　weighty
  authorities　have　suggested　that　they　probably　occur；　in　more　or
  less　perfection；　in　all　young　vegetable　cells。　　If　such　be　the
  case；　the　wonderful　noonday　silence　of　a　tropical　forest　is；　after
  all；　due　only　to　the　dulness　of　our　hearing；　and　could　our　ears
  catch　the　murmur　of　these　tiny　Maelstroms；　＇96＇　as　they　whirl　in　the
  innumerable　myriads　of　living　cells　which　constitute　each　tree；　we
  should　be　stunned；　as　with　the　roar　of　a　great　city。
  Among　the　lower　plants；　it　is　the　rule　rather　than　the　exception；
  that　contractility　should　be　still　more　openly　manifested　at　some
  periods　of　their　existence。　　The　protoplasm　of　Algae　and　Fungi
  becomes；　under　many　circumstances；　partially；　or　completely；　freed
  from　its　woody　case；　and　exhibits　movements　of　its　whole　mass；　or
  is　propelled　by　the　contractility　of　one；　or　more；　hair…like
  prolongations　of　its　body；　which　are　called　vibratile　cilia。　　And；
  so　far　as　the　conditions　of　the　manifestation　of　the　phaenomena　of
  contractility　have　yet　been　studied；　they　are　the　same　for　the
  plant　as　for　the　animal。　　Heat　and　electric　shocks　influence　both；
  and　in　the　same　way；　though　it　may　be　in　different　degrees。　　It　is
  by　no　means　my　intention　to　suggest　that　there　is　no　difference　in
  faculty　between　the　lowest　plant　and　the　highest；　or　between　plants
  and　animals。　　But　the　difference　between　the　powers　of　the　lowest
  plant；　or　animal；　and　those　of　the　highest；　is　one　of　degree；　not
  of　kind；　and　depends；　as　Milne…Edwards　＇97＇　long　ago　so　well　pointed
  out；　upon　the　extent　to　which　the　principle　of　the　division　of
  labour　is　carried　out　in　the　living　economy。　　In　the　lowest
  organism　all　parts　are　competent　to　perform　all　functions；　and　one
  and　the　same　portion　of　protoplasm　may　successfully　take　on　the
  function　of　feeding；　moving；　or　reproducing　apparatus。　　In　the
  highest；　on　the　contrary；　a　great　number　of　parts　combine　to
  perform　each　function；　each　part　doing　its　allotted　share　of　the
  work　with　great　accuracy　and　efficiency；　but　being　useless　for　any
  other　purpose。
  On　the　other　hand；　notwithstanding　all　the　fundamental　resemblances
  which　exist　between　the　powers　of　the　protoplasm　in　plants　and　in
  animals；　they　present　a　striking　difference　（to　which　I　shall
  advert　more　at　length　presently）；　in　the　fact　that　plants　can
  manufacture　fresh　protoplasm　out　of　mineral　compounds；　whereas
  animals　are　obliged　to　procure　it　ready　made；　and　hence；　in　the
  long　run；　depend　upon　plants。　　Upon　what　condition　this　difference
  in　the　powers　of　the　two　great　divisions　of　the　world　of　life
  depends；　nothing　is　at　present　known。
  With　such　qualifications　as　arises　＇98＇　out　of　the　last…mentioned
  fact；　it　may　be　truly　said　that　the　acts　of　all　living　things　are
  fundamentally　one。　　Is　any　such　unity　predicable　of　their　forms？
  Let　us　seek　in　easily　verified　facts　for　a　reply　to　this　question。
  If　a　drop　of　blood　be　drawn　by　pricking　one's　finger；　and　viewed
  with　proper　precautions；　and　under　a　sufficiently　high　microscopic
  power；　there　will　be　seen；　among　the　innumerable　multitude　of
  little；　circular；　discoidal　bodies；　or　corpuscles；　which　float　in
  it　and　give　it　its　colour；　a　comparatively　small　number　of
  colourless　corpuscles；　of　somewhat　larger　size　and　very　irregular
  shape。　　If　the　drop　of　blood　be　kept　at　the　temperature　of　the
  body；　these　colourless　corpuscles　will　be　seen　to　exhibit　a
  marvellous　activity；　changing　their　forms　with　great　rapidity；
  drawing　in　and　thrusting　out　prolongations　of　their　substance；　and
  creeping　about　as　if　they　were　independent　organisms。
  The　substance　which　is　thus　active　is　a　mass　of　protoplasm；　and　its
  activity　differs　in　detail；　rather　than　in　principle；　from　that　of
  the　protoplasm　of　the　nettle。　　Under　sundry　circumstances　the
  corpuscle　dies　and　becomes　distended　into　a　round　mass；　in　the
  midst　of　which　is　seen　a　smaller　spherical　body；　which　existed；　but
  was　more　or　less　hidden；　in　the　living　corpuscle；　and　is　called　its
  nucleus。　　Corpuscles　of　essentially　similar　structure　are　to　be
  found　in　the　skin；　in　the　lining　of　the　mouth；　and　scattered
  through　the　whole　framework　of　the　body。　　Nay；　more；　in　the
  earliest　condition　of　the　human　organism；　in　that　state　in　which　it
  has　but　just　become　distinguishable　from　the　egg　in　which　it
  arises；　it　is　nothing　but　an　aggregation　of　such　corpuscles；　and
  every　organ　of　the　body　was；　once；　no　more　than　such　an
  aggregation。
  Thus　a　nucleated　mass　of　protoplasm　tur