the past condition of organic nature-第3章

按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！


they　could　tell　us　what　was　going　on　at　all　parts　of　the　earth's
surface　during　a　given　epoch；　they　have　talked　of　this　deposit　being
contemporaneous　with　that　deposit；　until；　from　our　little　local
histories　of　the　changes　at　limited　spots　of　the　earth's　surface；　they
have　constructed　a　universal　history　of　the　globe　as　full　of　wonders　and
portents　as　any　other　story　of　antiquity。

But　what　does　this　attempt　to　construct　a　universal　history　of　the　globe
imply？　　It　implies　that　we　shall　not　only　have　a　precise　knowledge　of
the　events　which　have　occurred　at　any　particular　point；　but　that　we
shall　be　able　to　say　what　events；　at　any　one　spot；　took　place　at　the
same　time　with　those　at　other　spots。

Let　us　see　how　far　that　is　in　the　nature　of　things　practicable。　　Suppose
that　here　I　make　a　section　of　the　Lake　of　Killarney；　and　here　the
section　of　another　lakethat　of　Loch　Lomond　in　Scotland　for　instance。
The　rivers　that　flow　into　them　are　constantly　carrying　down　deposits　of
mud；　and　beds；　or　strata；　are　being　as　constantly　formed；　one　above　the
other；　at　the　bottom　of　those　lakes。　　Now；　there　is　not　a　shadow　of
doubt　that　in　these　two　lakes　the　lower　beds　are　all　older　than　the
upperthere　is　no　doubt　about　that；　but　what　does　'this'　tell　us　about
the　age　of　any　given　bed　in　Loch　Lomond；　as　compared　with　that　of　any
given　bed　in　the　Lake　of　Killarney？　　It　is；　indeed；　obvious　that　if　any
two　sets　of　deposits　are　separated　and　discontinuous；　there　is
absolutely　no　means　whatever　given　you　by　the　nature　of　the　deposit　of
saying　whether　one　is　much　younger　or　older　than　the　other；　but　you　may
say；　as　many　have　said　and　think；　that　the　case　is　very　much　altered　if
the　beds　which　we　are　comparing　are　continuous。　　Suppose　two　beds　of
mud　hardened　into　rock；A　and　B…are　seen　in　section。　（Fig。　　5。）

＇Fig。　5。＇

Well；　you　say；　it　is　admitted　that　the　lowermost　bed　is　always　the
older。　　Very　well；　B；　therefore；　is　older　than　A。　　No　doubt；　'as　a
whole'；　it　is　so；　or　if　any　parts　of　the　two　beds　which　are　in　the　same
vertical　line　are　compared；　it　is　so。　　But　suppose　you　take　what　seems
a　very　natural　step　further；　and　say　that　the　part　'a'　of　the　bed　A　is
younger　than　the　part　'b'　of　the　bed　B。　　Is　this　sound　reasoning？　　If
you　find　any　record　of　changes　taking　place　at　'b'；　did　they　occur
before　any　events　which　took　place　while　'a'　was　being　deposited？　　It
looks　all　very　plain　sailing；　indeed；　to　say　that　they　did；　and　yet
there　is　no　proof　of　anything　of　the　kind。　　As　the　former　Director　of
this　Institution；　Sir　H。　De　la　Beche；　long　ago　showed；　this　reasoning
may　involve　an　entire　fallacy。　　It　is　extremely　possible　that　'a'　may
have　been　deposited　ages　before　'b'。　　It　is　very　easy　to　understand　how
that　can　be。　　To　return　to　Fig。　4；　when　A　and　B　were　deposited；　they
were　'substantially'　contemporaneous；　A　being　simply　the　finer　deposit；
and　B　the　coarser　of　the　same　detritus　or　waste　of　land。　　Now　suppose
that　that　sea…bottom　goes　down　（as　shown　in　Fig。　4）；　so　that　the　first
deposit　is　carried　no　farther　than　'a'；　forming　the　bed　Al；　and　the
coarse　no　farther　than　'b'；　forming　the　bed　B1；　the　result　will　be　the
formation　of　two　continuous　beds；　one　of　fine　sediment　（A　A1）
over…lapping　another　of　coarse　sediment　（B　Bl）。　Now　suppose　the　whole
sea…bottom　is　raised　up；　and　a　section　exposed　about　the　point　Al；　no
doubt；　'at　this　spot'；　the　upper　bed　is　younger　than　the　lower。　　But　we
should　obviously　greatly　err　if　we　concluded　that　the　mass　of　the　upper
bed　at　A　was　younger　than　the　lower　bed　at　B；　for　we　have　just　seen
that　they　are　contemporaneous　deposits。　　Still　more　should　we　be　in
error　if　we　supposed　the　upper　bed　at　A　to　be　younger　than　the
continuation　of　the　lower　bed　at　Bl；　for　A　was　deposited　long　before
B1。　　In　fine；　if；　instead　of　comparing　immediately　adjacent　parts　of
two　beds；　one　of　which　lies　upon　another；　we　compare　distant　parts；　it
is　quite　possible　that　the　upper　may　be　any　number　of　years　older　than
the　under；　and　the　under　any　number　of　years　younger　than　the　upper。

Now　you　must　not　suppose　that　I　put　this　before　you　for　the　purpose　of
raising　a　paradoxical　difficulty；　the　fact　is；　that　the　great　mass　of
deposits　have　taken　place　in　sea…bottoms　which　are　gradually　sinking；
and　have　been　formed　under　the　very　conditions　I　am　here　supposing。

Do　not　run　away　with　the　notion　that　this　subverts　the　principle　I　laid
down　at　first。　　The　error　lies　in　extending　a　principle　which　is
perfectly　applicable　to　deposits　in　the　same　vertical　line　to　deposits
which　are　not　in　that　relation　to　one　another。

It　is　in　consequence　of　circumstances　of　this　kind；　and　of　others　that　I
might　mention　to　you；　that　our　conclusions　on　and　interpretations　of
the　record　are　really　and　strictly　only　valid　so　long　as　we　confine
ourselves　to　one　vertical　section。　　I　do　not　mean　to　tell　you　that　there
are　no　qualifying　circumstances；　so　that；　even　in　very　considerable
areas；　we　may　safely　speak　of　conformably　superimposed　beds　being　older
or　younger　than　others　at　many　different　points。　But　we　can　never　be
quite　sure　in　coming　to　that　conclusion；　and　especially　we　cannot　he
sure　if　there　is　any　break　in　their　continuity；　or　any　very　great
distance　between　the　points　to　be　compared。

Well　now；　so　much　for　the　record　itself；so　much　for　its
imperfections；so　much　for　the　conditions　to　be　observed　in
interpreting　it；　and　its　chronological　indications；　the　moment　we　pass
beyond　the　limits　of　a　vertical　linear　section。

Now　let　us　pass　from　the　record　to　that　which　it　contains；from　the
book　itself　to　the　writing　and　the　figures　on　its　pages。　　This　writing
and　these　figures　consist　of　remains　of　animals　and　plants　which；　in
the　great　majority　of　cases；　have　lived　and　died　in　the　very　spot　in
which　we　now　find　them；　or　at　least　in　the　immediate　vicinity。　　You
must　all　of　you　be　awareand　I　referred　to　the　fact　in　my　last
lecturethat　there　are　vast　numbers　of　creatures　living　at　the　bottom
of　the　sea。　　These　creatures；　like　all　others；　sooner　or　later　die；　and
their　shells　and　hard　parts　lie　at　the　bottom；　and　then　the　fine　mud
which　is　being　constantly　brought　down　by　rivers　and　the　action　of　the
wear　and　tear　of　the　sea；　covers　them　over　and　protects　them　from　any
further　change　or　alteration；　and；　of　course；　as　in　process　of　time　the
mud　becomes　hardened　and　solidified；　the　shells　of　these　animals　are
preserved　and　firmly　imbedded　in　the　limestone　or　sandstone　which　is
being　thus　formed。　　You　may　see　in　the　galleries　of　the　Museum　up
stairs　specimens　of　limestones　in　which　such　fossil　remains　of　existing
animals　are　imbedded。　　There　are　some　specimens　in　which　turtles'　eggs
have　been　imbedded　in　calcareous　sand；　and　before　the　sun　had　hatched
the　young　turtles；　they　became　covered　over　with　calcareous　mud；　and
thus　have　been　preserved　and　fossilized。

Not　only　does　this　process　of　imbedding　and　fossilization　occur　with
marine　and　other　aquatic　animals　and　plants；　but　it　affects　those　land
animals　and　plants　which　are　drifted　away　to　sea；　or　become　buried　in
bogs　or　morasses；　and　the　animals　which　have　been　trodden　down　by　their
fellows　and　crushed　in　the　mud　at　the　river's　bank；　as　the　herd　have
come　to　drink。　　In　any　of　these　cases；　the　organisms　may　be　crushed　or
be　mutilated；　before　or　after　putrefaction；　in　such　a　manner　that
perhaps　only　a　part　will　be　left　in　the　form　in　which　it　reaches　us。　　It
is；　indeed；　a　most　remarkable　fact；　that　it　is　quite　an　exceptional
case　to　find　a　skeleton　of　any　one　of　all　the　thousands　of　wild　land
animals　that　we　know　are　constantly　being　killed；　or　dying　in　the
course　of　nature：　they　are　preyed　on　and　devoured　by　other　animals　or
die　in　places　where　their　bodies　are　not　afterwards　protected　by　mud。
There　are　other　animals　existing　in　the　sea；　the　shells　of　which　form
exceedingly　large　deposits。　　You　are　probably　aware　that　before　the
attempt　was　made　to　lay　the　Atlantic　telegraphic　cable；　the　Government
employed　vessels　in　making　a　series　of　very　careful　observations　and
soundings　of　the　bottom　of　the　Atlantic；　and　although；　as　we　must　all
regret；　up　to　the　present　time　that　project　has　not　succeeded；　w