on the study of zoology-第1部分

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On　the　Study　of　Zoology


by　Thomas　H。　Huxley





　　

NATURAL　HISTORY　is　the　name　familiarly　applied　to　the　study　of　the
properties　of　such　natural　bodies　as　minerals；　plants；　and　animals；　the
sciences　which　embody　the　knowledge　man　has　acquired　upon　these
subjects　are　commonly　termed　Natural　Sciences；　in　contradistinction　to
other　so…called　〃physical〃　sciences；　and　those　who　devote　themselves
especially　to　the　pursuit　of　such　sciences　have　been　and　are　commonly
termed　〃Naturalists。〃

Linnaeus　was　a　naturalist　in　this　wide　sense；　and　his　'Systema　Naturae'
was　a　work　upon　natural　history；　in　the　broadest　acceptation　of　the
term；　in　it；　that　great　methodising　spirit　embodied　all　that　was　known
in　his　time　of　the　distinctive　characters　of　minerals；　animals；　and
plants。　　But　the　enormous　stimulus　which　Linnaeus　gave　to　the
investigation　of　nature　soon　rendered　it　impossible　that　any　one　man
should　write　another　'Systema　Naturae；'　and　extremely　difficult　for　any
one　to　become　even　a　naturalist　such　as　Linnaeus　was。

Great　as　have　been　the　advances　made　by　all　the　three　branches　of
science；　of　old　included　under　the　title　of　natural　history；　there　can
be　no　doubt　that　zoology　and　botany　have　grown　in　an　enormously　greater
ratio　than　mineralogy；　and　hence；　as　I　suppose；　the　name　of　〃natural
history〃　has　gradually　become　more　and　more　definitely　attached　to　these
prominent　divisions　of　the　subject；　and　by　〃naturalist〃　people　have
meant　more　and　more　distinctly　to　imply　a　student　of　the　structure　and
function　of　living　beings。

However　this　may　be；　it　is　certain　that　the　advance　of　knowledge　has
gradually　widened　the　distance　between　mineralogy　and　its　old
associates；　while　it　has　drawn　zoology　and　botany　closer　together；　so
that　of　late　years　it　has　been　found　convenient　（and　indeed　necessary）
to　associate　the　sciences　which　deal　with　vitality　and　all　its
phenomena　under　the　common　head　of　〃biology〃；　and　the　biologists　have
come　to　repudiate　any　blood…relationship　with　their　foster…brothers；
the　mineralogists。

Certain　broad　laws　have　a　general　application　throughout　both　the　animal
and　the　vegetable　worlds；　but　the　ground　common　to　these　kingdoms　of
nature　is　not　of　very　wide　extent；　and　the　multiplicity　of　details　is
so　great；　that　the　student　of　living　beings　finds　himself　obliged　to
devote　his　attention　exclusively　either　to　the　one　or　the　other。　　If　he
elects　to　study　plants；　under　any　aspect；　we　know　at　once　what　to　call
him。　　He　is　a　botanist；　and　his　science　is　botany。　　But　if　the
investigation　of　animal　life　be　his　choice；　the　name　generally　applied
to　him　will　vary　according　to　the　kind　of　animals　he　studies；　or　the
particular　phenomena　of　animal　life　to　which　he　confines　his
attention。　　If　the　study　of　man　is　his　object；　he　is　called　an
anatomist；　or　a　physiologist；　or　an　ethnologist；　but　if　he　dissects
animals；　or　examines　into　the　mode　in　which　their　functions　are
performed；　he　is　a　comparative　anatomist　or　comparative　physiologist。
If　he　turns　his　attention　to　fossil　animals；　he　is　a　palaeontologist。
If　his　mind　is　more　particularly　directed　to　the　specific　description；
discrimination；　classification；　and　distribution　of　animals；　he　is
termed　a　zoologist。

For　the　purpose　of　the　present　discourse；　however；　I　shall　recognise
none　of　these　titles　save　the　last；　which　I　shall　employ　as　the
equivalent　of　botanist；　and　I　shall　use　the　term　zoology　as　denoting
the　whole　doctrine　of　animal　life；　in　contradistinction　to　botany；　which
signifies　the　whole　doctrine　of　vegetable　life。

Employed　in　this　sense；　zoology；　like　botany；　is　divisible　into　three
great　but　subordinate　sciences；　morphology；　physiology；　and
distribution；　each　of　which　may；　to　a　very　great　extent；　be　studied
independently　of　the　other。

Zoological　morphology　is　the　doctrine　of　animal　form　or　structure。
Anatomy　is　one　of　its　branches；　development　is　another；　while
classification　is　the　expression　of　the　relations　which　different
animals　bear　to　one　another；　in　respect　of　their　anatomy　and　their
development。

Zoological　distribution　is　the　study　of　animals　in　relation　to　the
terrestrial　conditions　which　obtain　now；　or　have　obtained　at　any
previous　epoch　of　the　earth's　history。

Zoological　physiology；　lastly；　is　the　doctrine　of　the　functions　or
actions　of　animals。　　It　regards　animal　bodies　as　machines　impelled　by
certain　forces；　and　performing　an　amount　of　work　which　can　be　expressed
in　terms　of　the　ordinary　forces　of　nature。　　The　final　object　of
physiology　is　to　deduce　the　facts　of　morphology；　on　the　one　hand；　and
those　of　distribution　on　the　other；　from　the　laws　of　the　molecular
forces　of　matter。

Such　is　the　scope　of　zoology。　　But　if　I　were　to　content　myself　with　the
enunciation　of　these　dry　definitions；　I　should　ill　exemplify　that
method　of　teaching　this　branch　of　physical　science；　which　it　is　my
chief　business　to…night　to　recommend。　　Let　us　turn　away　then　from
abstract　definitions。　　Let　us　take　some　concrete　living　thing；　some
animal；　the　commoner　the　better；　and　let　us　see　how　the　application　of
common　sense　and　common　logic　to　the　obvious　facts　it　presents；
inevitably　leads　us　into　all　these　branches　of　zoological　science。

I　have　before　me　a　lobster。　　When　I　examine　it；　what　appears　to　be　the
most　striking　character　it　presents？　　Why；　I　observe　that　this　part
which　we　call　the　tail　of　the　lobster；　is　made　up　of　six　distinct　hard
rings　and　a　seventh　terminal　piece。　　If　I　separate　one　of　the　middle
rings；　say　the　third；　I　find　it　carries　upon　its　under　surface　a　pair
of　limbs　or　appendages；　each　of　which　consists　of　a　stalk　and　two
terminal　pieces。　　So　that　I　can　represent　a　transverse　section　of　the
ring　and　its　appendages　upon　the　diagram　board　in　this　way。

If　I　now　take　the　fourth　ring；　I　find　it　has　the　same　structure；　and　so
have　the　fifth　and　the　second；　so　that；　in　each　of　these　divisions　of
the　tail；　I　find　parts　which　correspond　with　one　another；　a　ring　and
two　appendages；　and　in　each　appendage　a　stalk　and　two　end　pieces。
These　corresponding　parts　are　called；　in　the　technical　language　of
anatomy；　〃homologous　parts。〃　The　ring　of　the　third　division　is　the
〃homologue〃　of　the　ring　of　the　fifth；　the　appendage　of　the　former　is
the　homologue　of　the　appendage　of　the　latter。　　And；　as　each　division
exhibits　corresponding　parts　in　corresponding　places；　we　say　that　all
the　divisions　are　constructed　upon　the　same　plan。　　But　now　let　us
consider　the　sixth　division。　　It　is　similar　to；　and　yet　different　from；
the　others。　　The　ring　is　essentially　the　same　as　in　the　other　divisions；
but　the　appendages　look　at　first　as　if　they　were　very　different；　and
yet　when　we　regard　them　closely；　what　do　we　find？　　A　stalk　and　two
terminal　divisions；　exactly　as　in　the　others；　but　the　stalk　is　very
short　and　very　thick；　the　terminal　divisions　are　very　broad　and　flat；
and　one　of　them　is　divided　into　two　pieces。

I　may　say；　therefore；　that　the　sixth　segment　is　like　the　others　in　plan；
but　that　it　is　modified　in　its　details。

The　first　segment　is　like　the　others；　so　far　as　its　ring　is　concerned；
and　though　its　appendages　differ　from　any　of　those　yet　examined　in　the
simplicity　of　their　structure；　parts　corresponding　with　the　stem　and
one　of　the　divisions　of　the　appendages　of　the　other　segments　can　be
readily　discerned　in　them。

Thus　it　appears　that　the　lobster's　tail　is　composed　of　a　series　of
segments　which　are　fundamentally　similar；　though　each　presents　peculiar
modifications　of　the　plan　common　to　all。　But　when　I　turn　to　the
forepart　of　the　body　I　see；　at　first；　nothing　but　a　great　shield…like
shell；　called　technically　the　〃carapace；〃　ending　in　front　in　a　sharp
spine；　on　either　side　of　which　are　the　curious　compound　eyes；　set　upon
the　ends　of　stout　movable　stalks。　　Behind　these；　on　the　under　side　of
the　body；　are　two　pairs　of　long　feelers；　or　antennae；　followed　by　six
pairs　of　jaws　folded　against　one　another　over　the　mouth；　and　five　pairs
of　legs；　the　foremost　of　these　being　the　great　pinchers；　or　claws；　of
the　lobster。

It　looks；　at　first；　a　little　hopeless　to　attempt　to　find　in　this　complex
mass　a　series　of　rings；　each　with　its　pair　of　appendages；　such　as　I
have　shown　you　in　the　abdomen；　and　yet　it　is　not　difficult　to
demonstrate　their　existence。　　Strip　off　the　legs；　and　you　will　find　that
each　pair　is　attached　to　a　very　definite　segment　of　the　under　wall　of
the　body；　but　these　segments；　instead　of　being　the　lower　parts　of　free
rings；　as　in　the　tail；　are　such　parts　of　rings　which　are　all　solidly
united　and　bound　together；　and　the　like　is　true　of　the　jaws；　the
feelers；　and　the　eye…stalks；　every　pair　of　which　is　borne　upon　its　own
special　segment。　　Thus　the　conclusion　is　gradually　forced　upon　us；　that
the　body　of　the　lobster　is　composed　of　as　many　rings　as　there　are　pairs
of　appendages；　namely；　twenty　in　all；　but　that　the　six　hindmost　rings
remain　free　and　movable；　while　the　fourteen　front　rings　become　firmly
soldered　together；　their　backs　forming　one　continuous　shieldthe
carapace。

Unity　of　plan；　diversity　in　execution；　is　the　lesson　taught　by　the　study
of　the　rings　of　the　body；　and　the　same　instruction　is　given　still　more
emphatically　by　the　appendages。　　If　I　examine　the　outermost　jaw　I　find
it　consists　of　three　distinct　portions；　an　inner；　a　middle；　and　an
outer；　mounted　upon　a　common　stem；　and　if　I　compare　this　jaw　with　the
legs　behind　it；　or　the　jaws　in　front　of　it；　I　find　it　quite　easy　to
see；　that；　in　the　legs；　it　is　the　part　of　the　appendage　which
corresponds　with　the　inner　division；　which　becomes　modified　into　what　we
know　familiarly　as　the　〃leg；〃　while　the　middle　division　disappears；　and
the　outer　division　is　hidden　under　the　carapace。　　Nor　is　it　more
difficult　to　discern　that；　in　the　appendages　of　the　tail；　the　middle
division　appears　again　and　the　outer　vanishes；　while；　on　the　other　hand；
in　the　foremost　jaw；　the　so…called　mandible；　the　inner　division　only　is
left；　and；　in　the　same　way；　the　parts　of　the　feelers　and　of　the
eye…stalks　can　be　identified　with　those　of　the　legs　and　jaws。

But　whither　does　all　this　tend？　　To　the　very　remarkable　conclusion　that
a　unity　of　plan；　of　the　same　kind　as　that　dis