on the study of zoology-第3部分

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animal　population　of　the　globe；　in　past　ages；　which　is　the　most
striking　fact　revealed　by　geology。

Consider；　now；　where　our　inquiries　have　led　us。　　We　studied　our　type
morphologically；　when　we　determined　its　anatomy　and　its　development；
and　when　comparing　it；　in　these　respects；　with　other　animals；　we　made
out　its　place　in　a　system　of　classification。　　If　we　were　to　examine
every　animal　in　a　similar　manner；　we　should　establish　a　complete　body　of
zoological　morphology。

Again；　we　investigated　the　distribution　of　our　type　in　space　and　in
time；　and；　if　the　like　had　been　done　with　every　animal；　the　sciences　of
geographical　and　geological　distribution　would　have　attained　their
limit。

But　you　will　observe　one　remarkable　circumstance；　that；　up　to　this
point；　the　question　of　the　life　of　these　organisms　has　not　come　under
consideration。　　Morphology　and　distribution　might　be　studied　almost　as
well；　if　animals　and　plants　were　a　peculiar　kind　of　crystals；　and
possessed　none　of　those　functions　which　distinguish　living　beings　so
remarkably。　　But　the　facts　of　morphology　and　distribution　have　to　be
accounted　for；　and　the　science；　whose　aim　it　is　to　account　for　them；　is
Physiology。

Let　us　return　to　our　lobster　once　more。　　If　we　watched　the　creature　in
its　native　element；　we　should　see　it　climbing　actively　the　submerged
rocks；　among　which　it　delights　to　live；　by　means　of　its　strong　legs；　or
swimming　by　powerful　strokes　of　its　great　tail；　the　appendages　of　whose
sixth　joint　are　spread　out　into　a　broad　fan…like　propeller：　seize　it；
and　it　will　show　you　that　its　great　claws　are　no　mean　weapons　of
offence；　suspend　a　piece　of　carrion　among　its　haunts；　and　it　will
greedily　devour　it；　tearing　and　crushing　the　flesh　by　means　of　its
multitudinous　jaws。

Suppose　that　we　had　known　nothing　of　the　lobster　but　as　an　inert　mass；
an　organic　crystal；　if　I　may　use　the　phrase；　and　that　we　could　suddenly
see　it　exerting　all　these　powers；　what　wonderful　new　ideas　and　new
questions　would　arise　in　our　minds！　　The　great　new　question　would　be；
〃How　does　all　this　take　place？〃　　the　chief　new　idea　would　be；　the　idea
of　adaptation　to　purpose；the　notion；　that　the　constituents　of　animal
bodies　are　not　mere　unconnected　parts；　but　organs　working　together　to
an　end。　　Let　us　consider　the　tail　of　the　lobster　again　from　this　point
of　view。　　Morphology　has　taught　us　that　it　is　a　series　of　segments
composed　of　homologous　parts；　which　undergo　various
modificationsbeneath　and　through　which　a　common　plan　of　formation　is
discernible。　　But　if　I　look　at　the　same　part　physiologically；　I　see
that　it　is　a　most　beautifully　constructed　organ　of　locomotion；　by　means
of　which　the　animal　can　swiftly　propel　itself　either　backwards　or
forwards。

But　how　is　this　remarkable　propulsive　machine　made　to　perform　its
functions？　　If　I　were　suddenly　to　kill　one　of　these　animals　and　to　take
out　all　the　soft　parts；　I　should　find　the　shell　to　be　perfectly　inert；
to　have　no　more　power　of　moving　itself　than　is　possessed　by　the
machinery　of　a　mill　when　disconnected　from　its　steam…engine　or
water…wheel。　　But　if　I　were　to　open　it；　and　take　out　the　viscera　only；
leaving　the　white　flesh；　I　should　perceive　that　the　lobster　could　bend
and　extend　its　tail　as　well　as　before。　　If　I　were　to　cut　off　the　tail；　I
should　cease　to　find　any　spontaneous　motion　in　it；　but　on　pinching　any
portion　of　the　flesh；　I　should　observe　that　it　underwent　a　very　curious
changeeach　fibre　becoming　shorter　and　thicker。　　By　this　act　of
contraction；　as　it　is　termed；　the　parts　to　which　the　ends　of　the　fibre
are　attached　are；　of　course；　approximated；　and　according　to　the
relations　of　their　points　of　attachment　to　the　centres　of　motions　of
the　different　rings；　the　bending　or　the　extension　of　the　tail　results。
Close　observation　of　the　newly…opened　lobster　would　soon　show　that　all
its　movements　are　due　to　the　same　causethe　shortening　and　thickening
of　these　fleshy　fibres；　which　are　technically　called　muscles。

Here；　then；　is　a　capital　fact。　　The　movements　of　the　lobster　are　due　to
muscular　contractility。　But　why　does　a　muscle　contract　at　one　time　and
not　at　another？　　Why　does　one　whole　group　of　muscles　contract　when　the
lobster　wishes　to　extend　his　tail；　and　another　group　when　he　desires　to
bend　it？　　What　is　it　originates；　directs；　and　controls　the　motive
power？

Experiment；　the　great　instrument　for　the　ascertainment　of　truth　in
physical　science；　answers　this　question　for　us。　　In　the　head　of　the
lobster　there　lies　a　small　mass　of　that　peculiar　tissue　which　is　known
as　nervous　substance。　　Cords　of　similar　matter　connect　this　brain　of
the　lobster；　directly　or　indirectly；　with　the　muscles。　　Now；　if　these
communicating　cords　are　cut；　the　brain　remaining　entire；　the　power　of
exerting　what　we　call　voluntary　motion　in　the　parts　below　the　section
is　destroyed；　and　on　the　other　hand；　if；　the　cords　remaining　entire；　the
brain　mass　be　destroyed；　the　same　voluntary　mobility　is　equally　lost。
Whence　the　inevitable　conclusion　is；　that　the　power　of　originating
these　motions　resides　in　the　brain；　and　is　propagated　along　the　nervous
cords。

In　the　higher　animals　the　phenomena　which　attend　this　transmission　have
been　investigated；　and　the　exertion　of　the　peculiar　energy　which
resides　in　the　nerves　has　been　found　to　be　accompanied　by　a　disturbance
of　the　electrical　state　of　their　molecules。

If　we　could　exactly　estimate　the　signification　of　this　disturbance；　if
we　could　obtain　the　value　of　a　given　exertion　of　nerve　force　by
determining　the　quantity　of　electricity；　or　of　heat；　of　which　it　is　the
equivalent；　if　we　could　ascertain　upon　what　arrangement；　or　other
condition　of　the　molecules　of　matter；　the　manifestation　of　the　nervous
and　muscular　energies　depends　（and　doubtless　science　will　some　day　or
other　ascertain　these　points）；　physiologists　would　have　attained　their
ultimate　goal　in　this　direction；　they　would　have　determined　the　relation
of　the　motive　force　of　animals　to　the　other　forms　of　force　found　in
nature；　and　if　the　same　process　had　been　successfully　performed　for　all
the　operations　which　are　carried　on　in；　and　by；　the　animal　frame；
physiology　would　be　perfect；　and　the　facts　of　morphology　and
distribution　would　be　deducible　from　the　laws　which　physiologists　had
established；　combined　with　those　determining　the　condition　of　the
surrounding　universe。

There　is　not　a　fragment　of　the　organism　of　this　humble　animal　whose
study　would　not　lead　us　into　regions　of　thought　as　large　as　those　which
I　have　briefly　opened　up　to　you；　but　what　I　have　been　saying；　I　trust；
has　not　only　enabled　you　to　form　a　conception　of　the　scope　and　purport
of　zoology；　but　has　given　you　an　imperfect　example　of　the　manner　in
which；　in　my　opinion；　that　science；　or　indeed　any　physical　science；　may
be　best　taught。　　The　great　matter　is；　to　make　teaching　real　and
practical；　by　fixing　the　attention　of　the　student　on　particular　facts；
but　at　the　same　time　it　should　be　rendered　broad　and　comprehensive；　by
constant　reference　to　the　generalizations　of　which　all　particular　facts
are　illustrations。　　The　lobster　has　served　as　a　type　of　the　whole
animal　kingdom；　and　its　anatomy　and　physiology　have　illustrated　for　us
some　of　the　greatest　truths　of　biology。　　The　student　who　has　once　seen
for　himself　the　facts　which　I　have　described；　has　had　their　relations
explained　to　him；　and　has　clearly　comprehended　them；　has；　so　far；　a
knowledge　of　zoology；　which　is　real　and　genuine；　however　limited　it　may
be；　and　which　is　worth　more　than　all　the　mere　reading　knowledge　of　the
science　he　could　ever　acquire。　　His　zoological　information　is；　so　far；
knowledge　and　not　mere　hear…say。

And　if　it　were　my　business　to　fit　you　for　the　certificate　in　zoological
science　granted　by　this　department；　I　should　pursue　a　course　precisely
similar　in　principle　to　that　which　I　have　taken　to…night。　　I　should
select　a　fresh…water　sponge；　a　fresh…water　polype　or　a　'Cyanaea'；　a
fresh…water　mussel；　a　lobster；　a　fowl；　as　types　of　the　five　primary
divisions　of　the　animal　kingdom。　　I　should　explain　their　structure　very
fully；　and　show　how　each　illustrated　the　great　principles　of　zoology。
Having　gone　very　carefully　and　fully　over　this　ground；　I　should　feel
that　you　had　a　safe　foundation；　and　I　should　then　take　you　in　the　same
way；　but　less　minutely；　over　similarly　selected　illustrative　types　of
the　classes；　and　then　I　should　direct　your　attention　to　the　special
forms　enumerated　under　the　head　of　types；　in　this　syllabus；　and　to　the
other　facts　there　mentioned。

That　would；　speaking　generally；　be　my　plan。　　But　I　have　undertaken　to
explain　to　you　the　best　mode　of　acquiring　and　communicating　a　knowledge
of　zoology；　and　you　may　therefore　fairly　ask　me　for　a　more　detailed　and
precise　account　of　the　manner　in　which　I　should　propose　to　furnish　you
with　the　information　I　refer　to。

My　own　impression　is；　that　the　best　model　for　all　kinds　of　training　in
physical　science　is　that　afforded　by　the　method　of　teaching　anatomy；　in
use　in　the　medical　schools。　　This　method　consists　of　three
elementslectures；　demonstrations；　and　examinations。

The　object　of　lectures　is；　in　the　first　place；　to　awaken　the　attention
and　excite　the　enthusiasm　of　the　student；　and　this；　I　am　sure；　may　be
effected　to　a　far　greater　extent　by　the　oral　discourse　and　by　the
personal　influence　of　a　respected　teacher　than　in　any　other　way。
Secondly；　lectures　have　the　double　use　of　guiding　the　student　to　the
salient　points　of　a　subject；　and　at　the　same　time　forcing　him　to　attend
to　the　whole　of　it；　and　not　merely　to　that　part　which　takes　his　fancy。
And　lastly；　lectures　afford　the　student　the　opportunity　of　seeking
explanations　of　those　difficulties　which　will；　and　indeed　ought　to；
arise　in　the　course　of　his　studies。

But　for　a　student　to　derive　the　utmost　possible　value　from　lectures；
several　precautions　are　needful。

I　have　a　strong　impression　that　the　better　a　discourse　is；　as　an
oration；　the　worse　it　is　as　a　lecture。　　The　flow　of　the　discourse
carries　you　on　without　proper　attention　to　its　sense；　you　drop　a　word
or　a　phrase；　you　lose　the　exact　meaning　for　a　moment；　and　while　you
strive　to　recover　yourself；　the　speaker　has　passed　on　to　something
els