on the study of zoology-第2部分

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a　unity　of　plan；　of　the　same　kind　as　that　discoverable　in　the　tail　or
abdomen　of　the　lobster；　pervades　the　whole　organization　of　its
skeleton；　so　that　I　can　return　to　the　diagram　representing　any　one　of
the　rings　of　the　tail；　which　I　drew　upon　the　board；　and　by　adding　a
third　division　to　each　appendage；　I　can　use　it　as　a　sort　of　scheme　or
plan　of　any　ring　of　the　body。　　I　can　give　names　to　all　the　parts　of
that　figure；　and　then　if　I　take　any　segment　of　the　body　of　the　lobster；
I　can　point　out　to　you　exactly；　what　modification　the　general　plan　has
undergone　in　that　particular　segment；　what　part　has　remained　movable；
and　what　has　become　fixed　to　another；　what　has　been　excessively
developed　and　metamorphosed　and　what　has　been　suppressed。

But　I　imagine　I　hear　the　question；　How　is　all　this　to　be　tested？　　No
doubt　it　is　a　pretty　and　ingenious　way　of　looking　at　the　structure　of
any　animal；　but　is　it　anything　more？　　Does　Nature　acknowledge；　in　any
deeper　way；　this　unity　of　plan　we　seem　to　trace？

The　objection　suggested　by　these　questions　is　a　very　valid　and　important
one；　and　morphology　was　in　an　unsound　state　so　long　as　it　rested　upon
the　mere　perception　of　the　analogies　which　obtain　between　fully　formed
parts。　　The　unchecked　ingenuity　of　speculative　anatomists　proved　itself
fully　competent　to　spin　any　number　of　contradictory　hypotheses　out　of
the　same　facts；　and　endless　morphological　dreams　threatened　to　supplant
scientific　theory。

Happily；　however；　there　is　a　criterion　of　morphological　truth；　and　a
sure　test　of　all　homologies。　Our　lobster　has　not　always　been　what　we
see　it；　it　was　once　an　egg；　a　semifluid　mass　of　yolk；　not　so　big　as　a
pin's　head；　contained　in　a　transparent　membrane；　and　exhibiting　not　the
least　trace　of　any　one　of　those　organs；　whose　multiplicity　and
complexity；　in　the　adult；　are　so　surprising。　　After　a　time　a　delicate
patch　of　cellular　membrane　appeared　upon　one　face　of　this　yolk；　and
that　patch　was　the　foundation　of　the　whole　creature；　the　clay　out　of
which　it　would　be　moulded。　　Gradually　investing　the　yolk；　it　became
subdivided　by　transverse　constrictions　into　segments；　the　forerunners
of　the　rings　of　the　body。　　Upon　the　ventral　surface　of　each　of　the
rings　thus　sketched　out；　a　pair　of　bud…like　prominences　made　their
appearancethe　rudiments　of　the　appendages　of　the　ring。　　At　first；　all
the　appendages　were　alike；　but；　as　they　grew；　most　of　them　became
distinguished　into　a　stem　and　two　terminal　divisions；　to　which　in　the
middle　part　of　the　body；　was　added　a　third　outer　division；　and　it　was
only　at　a　later　period；　that　by　the　modification；　or　absorption；　of
certain　of　these　primitive　constituents；　the　limbs　acquired　their
perfect　form。

Thus　the　study　of　development　proves　that　the　doctrine　of　unity　of　plan
is　not　merely　a　fancy；　that　it　is　not　merely　one　way　of　looking　at　the
matter；　but　that　it　is　the　expression　of　deep…seated　natural　facts。　The
legs　and　jaws　of　the　lobster　may　not　merely　be　regarded　as
modifications　of　a　common　type；in　fact　and　in　nature　they　are　so；the
leg　and　the　jaw　of　the　young　animal　being；　at　first；　indistinguishable。

These　are　wonderful　truths；　the　more　so　because　the　zoologist　finds　them
to　be　of　universal　application。　　The　investigation　of　a　polype；　of　a
snail；　of　a　fish；　of　a　horse；　or　of　a　man；　would　have　led　us；　though　by
a　less　easy　path；　perhaps；　to　exactly　the　same　point。　　Unity　of　plan
everywhere　lies　hidden　under　the　mask　of　diversity　of　structurethe
complex　is　everywhere　evolved　out　of　the　simple。　　Every　animal　has　at
first　the　form　of　an　egg；　and　every　animal　and　every　organic　part；　in
reaching　its　adult　state；　passes　through　conditions　common　to　other
animals　and　other　adult　parts；　and　this　leads　me　to　another　point。　　I
have　hitherto　spoken　as　if　the　lobster　were　alone　in　the　world；　but；　as
I　need　hardly　remind　you；　there　are　myriads　of　other　animal　organisms。
Of　these；　some；　such　as　men；　horses；　birds；　fishes；　snails；　slugs；
oysters；　corals；　and　sponges；　are　not　in　the　least　like　the　lobster。
But　other　animals；　though　they　may　differ　a　good　deal　from　the　lobster；
are　yet　either　very　like　it；　or　are　like　something　that　is　like　it。　The
cray　fish；　the　rock　lobster；　and　the　prawn；　and　the　shrimp；　for
example；　however　different；　are　yet　so　like　lobsters；　that　a　child
would　group　them　as　of　the　lobster　kind；　in　contradistinction　to　snails
and　slugs；　and　these　last　again　would　form　a　kind　by　themselves；　in
contradistinction　to　cows；　horses；　and　sheep；　the　cattle　kind。

But　this　spontaneous　grouping　into　〃kinds〃　is　the　first　essay　of　the
human　mind　at　classification；　or　the　calling　by　a　common　name　of　those
things　that　are　alike；　and　the　arranging　them　in　such　a　manner　as　best
to　suggest　the　sum　of　their　likenesses　and　unlikenesses　to　other
things。

Those　kinds　which　include　no　other　subdivisions　than　the　sexes；　or
various　breeds；　are　called；　in　technical　language；　species。　　The
English　lobster　is　a　species；　our　cray　fish　is　another；　our　prawn　is
another。　　In　other　countries；　however；　there　are　lobsters；　cray　fish；
and　prawns；　very　like　ours；　and　yet　presenting　sufficient　differences
to　deserve　distinction。　　Naturalists；　therefore；　express　this
resemblance　and　this　diversity　by　grouping　them　as　distinct　species　of
the　same　〃genus。〃　　But　the　lobster　and　the　cray　fish；　though　belonging
to　distinct　genera；　have　many　features　in　common；　and　hence　are　grouped
together　in　an　assemblage　which　is　called　a　family。　　More　distant
resemblances　connect　the　lobster　with　the　prawn　and　the　crab；　which　are
expressed　by　putting　all　these　into　the　same　order。　　Again；　more　remote；
but　still　very　definite；　resemblances　unite　the　lobster　with　the
woodlouse；　the　king　crab；　the　water　flea；　and　the　barnacle；　and
separate　them　from　all　other　animals；　whence　they　collectively
constitute　the　larger　group；　or　class；　'Crustacea'。　　But　the
'Crustacea'　exhibit　many　peculiar　features　in　common　with　insects；
spiders；　and　centipedes；　so　that　these　are　grouped　into　the　still
larger　assemblage　or　〃province〃　'Articulata'；　and；　finally；　the
relations　which　these　have　to　worms　and　other　lower　animals；　are
expressed　by　combining　the　whole　vast　aggregate　into　the　sub…kingdom　of
'Annulosa'。

If　I　had　worked　my　way　from　a　sponge　instead　of　a　lobster；　I　should　have
found　it　associated；　by　like　ties；　with　a　great　number　of　other　animals
into　the　sub…kingdom　'Protozoa'；　if　I　had　selected　a　fresh…water　polype
or　a　coral；　the　members　of　what　naturalists　term　the　sub…kingdom
'Coelenterata'；　would　have　grouped　themselves　around　my　type；　had　a
snail　been　chosen；　the　inhabitants　of　all　univalve　and　bivalve；　land
and　water；　shells；　the　lamp　shells；　the　squids；　and　the　sea…mat　would
have　gradually　linked　themselves　on　to　it　as　members　of　the　same
sub…kingdom　of　'Mollusca'；　and　finally；　starting　from　man；　I　should　have
been　compelled　to　admit　first；　the　ape；　the　rat；　the　horse；　the　dog；
into　the　same　class；　and　then　the　bird；　the　crocodile；　the　turtle；　the
frog；　and　the　fish；　into　the　same　sub…kingdom　of　'Vertebrata'。

And　if　I　had　followed　out　all　these　various　lines　of　classification
fully；　I　should　discover　in　the　end　that　there　was　no　animal；　either
recent　or　fossil；　which　did　not　at　once　fall　into　one　or　other　of　these
sub…kingdoms。　　In　other　words；　every　animal　is　organized　upon　one　or
other　of　the　five；　or　more；　plans；　whose　existence　renders　our
classification　possible。　　And　so　definitely　and　precisely　marked　is　the
structure　of　each　animal；　that；　in　the　present　state　of　our　knowledge；
there　is　not　the　least　evidence　to　prove　that　a　form；　in　the　slightest
degree　transitional　between　any　of　the　two　groups　'Vertebrata'；
'Annulosa'；　'Mollusca'；　and　'Coelenterata'；　either　exists；　or　has
existed；　during　that　period　of　the　earth's　history　which　is　recorded　by
the　geologist。　　Nevertheless；　you　must　not　for　a　moment　suppose；　because
no　such　transitional　forms　are　known；　that　the　members　of　the
sub…kingdoms　are　disconnected　from；　or　independent　of；　one　another。　　On
the　contrary；　in　their　earliest　condition　they　are　all　alike；　and　the
primordial　germs　of　a　man；　a　dog；　a　bird；　a　fish；　a　beetle；　a　snail；　and
a　polype　are；　in　no　essential　structural　respects；　distinguishable。

In　this　broad　sense；　it　may　with　truth　be　said；　that　all　living　animals；
and　all　those　dead　creations　which　geology　reveals；　are　bound　together
by　an　all…pervading　unity　of　organization；　of　the　same　character；
though　not　equal　in　degree；　to　that　which　enables　us　to　discern　one　and
the　same　plan　amidst　the　twenty　different　segments　of　a　lobster's　body。
Truly　it　has　been　said；　that　to　a　clear　eye　the　smallest　fact　is　a
window　through　which　the　Infinite　may　be　seen。

Turning　from　these　purely　morphological　considerations；　let　us　now
examine　into　the　manner　in　which　the　attentive　study　of　the　lobster
impels　us　into　other　lines　of　research。

Lobsters　are　found　in　all　the　European　seas；　but　on　the　opposite　shores
of　the　Atlantic　and　in　the　seas　of　the　southern　hemisphere　they　do　not
exist。　　They　are；　however；　represented　in　these　regions　by　very　closely
allied；　but　distinct　formsthe　'Homarus　Americanus'　and　the　'Homarus
Capensis'：　so　that　we　may　say　that　the　European　has　one　species　of
'Homarus'；　the　American；　another；　the　African；　another；　and　thus　the
remarkable　facts　of　geographical　distribution　begin　to　dawn　upon　us。

Again；　if　we　examine　the　contents　of　the　earth's　crust；　we　shall　find　in
the　latter　of　those　deposits；　which　have　served　as　the　great　burying
grounds　of　past　ages；　numberless　lobster…like　animals；　but　none　so
similar　to　our　living　lobster　as　to　make　zoologists　sure　that　they
belonged　even　to　the　same　genus。　　If　we　go　still　further　back　in　time；
we　discover；　in　the　oldest　rocks　of　all；　the　remains　of　animals；
constructed　on　the　same　general　plan　as　the　lobster；　and　belonging　to
the　same　great　group　of　'Crustacea'；　but　for　the　most　part　totally
different　from　the　lobster；　and　indeed　from　any　other　living　form　of
crustacean；　and　thus　we　gain　a　notion　of　that　successive　change　of　the
animal　population　of　the　globe；　in　past　ages；　which　is　the　most
striking　fact　revealed　by　geology。

Consider；　now；　where　our　inqui