the perpetuation of living beings-第1章

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The　Perpetuation　of　Living　Beings


by　Thomas　Henry　Huxley







The　inquiry　which　we　undertook；　at　our　last　meeting；　into　the　state　of
our　knowledge　of　the　causes　of　the　phenomena　of　organic　nature；of　the
past　and　of　the　present；resolved　itself　into　two　subsidiary
inquiries：　the　first　was；　whether　we　know　anything；　either　historically
or　experimentally；　of　the　mode　of　origin　of　living　beings；　the　second
subsidiary　inquiry　was；　whether；　granting　the　origin；　we　know　anything
about　the　perpetuation　and　modifications　of　the　forms　of　organic
beings。　　The　reply　which　I　had　to　give　to　the　first　question　was
altogether　negative；　and　the　chief　result　of　my　last　lecture　was；　that；
neither　historically　nor　experimentally；　do　we　at　present　know　anything
whatsoever　about　the　origin　of　living　forms。　We　saw　that；　historically；
we　are　not　likely　to　know　anything　about　it；　although　we　may　perhaps
learn　something　experimentally；　but　that　at　present　we　are　an　enormous
distance　from　the　goal　I　indicated。

I　now；　then；　take　up　the　next　question；　What　do　we　know　of　the
reproduction；　the　perpetuation；　and　the　modifications　of　the　forms　of
living　beings；　supposing　that　we　have　put　the　question　as　to　their
origination　on　one　side；　and　have　assumed　that　at　present　the　causes　of
their　origination　are　beyond　us；　and　that　we　know　nothing　about　them？
Upon　this　question　the　state　of　our　knowledge　is　extremely　different；
it　is　exceedingly　large；　and；　if　not　complete；　our　experience　is
certainly　most　extensive。　　It　would　be　impossible　to　lay　it　all　before
you；　and　the　most　I　can　do；　or　need　do　to…night；　is　to　take　up　the
principal　points　and　put　them　before　you　with　such　prominence　as　may
subserve　the　purposes　of　our　present　argument。

The　method　of　the　perpetuation　of　organic　beings　is　of　two　kinds；the
asexual　and　the　sexual。　In　the　first　the　perpetuation　takes　place　from
and　by　a　particular　act　of　an　individual　organism；　which　sometimes　may
not　be　classed　as　belonging　to　any　sex　at　all。　　In　the　second　case；　it
is　in　consequence　of　the　mutual　action　and　interaction　of　certain
portions　of　the　organisms　of　usually　two　distinct　individuals；the
male　and　the　female。　　The　cases　of　asexual　perpetuation　are　by　no　means
so　common　as　the　cases　of　sexual　perpetuation；　and　they　are　by　no　means
so　common　in　the　animal　as　in　the　vegetable　world。　　You　are　all
probably　familiar　with　the　fact；　as　a　matter　of　experience；　that　you
can　propagate　plants　by　means　of　what　are　called　〃cuttings；〃　for
example；　that　by　taking　a　cutting　from　a　geranium　plant；　and　rearing　it
properly；　by　supplying　it　with　light　and　warmth　and　nourishment　from
the　earth；　it　grows　up　and　takes　the　form　of　its　parent；　having　all　the
properties　and　peculiarities　of　the　original　plant。

Sometimes　this　process；　which　the　gardener　performs　artificially；　takes
place　naturally；　that　is　to　say；　a　little　bulb；　or　portion　of　the
plant；　detaches　itself；　drops　off；　and　becomes　capable　of　growing　as　a
separate　thing。　　That　is　the　case　with　many　bulbous　plants；　which　throw
off　in　this　way　secondary　bulbs；　which　are　lodged　in　the　ground　and
become　developed　into　plants。　This　is　an　asexual　process；　and　from　it
results　the　repetition　or　reproduction　of　the　form　of　the　original
being　from　which　the　bulb　proceeds。

Among　animals　the　same　thing　takes　place。　　Among　the　lower　forms　of
animal　life；　the　infusorial　animalculae　we　have　already　spoken　of　throw
off　certain　portions；　or　break　themselves　up　in　various　directions；
sometimes　transversely　or　sometimes　longitudinally；　or　they　may　give
off　buds；　which　detach　themselves　and　develop　into　their　proper　forms。
There　is　the　common　fresh…water　Polype；　for　instance；　which　multiplies
itself　in　this　way。　　Just　in　the　same　way　as　the　gardener　is　able　to
multiply　and　reproduce　the　peculiarities　and　characters　of　particular
plants　by　means　of　cuttings；　so　can　the　physiological
experimentalistas　was　shown　by　the　Abbe　Trembley　many　years　agoso
can　he　do　the　same　thing　with　many　of　the　lower　forms　of　animal　life。
M。　de　Trembley　showed　that　you　could　take　a　polype　and　cut　it　into　two；
or　four；　or　many　pieces；　mutilating　it　in　all　directions；　and　the　pieces
would　still　grow　up　and　reproduce　completely　the　original　form　of　the
animal。　　These　are　all　cases　of　asexual　multiplication；　and　there　are
other　instances；　and　still　more　extraordinary　ones；　in　which　this
process　takes　place　naturally；　in　a　more　hidden；　a　more　recondite　kind
of　way。　　You　are　all　of　you　familiar　with　those　little　green　insects；
the　'Aphis'　or　blight；　as　it　is　called。　　These　little　animals；　during　a
very　considerable　part　of　their　existence；　multiply　themselves　by　means
of　a　kind　of　internal　budding；　the　buds　being　developed　into
essentially　asexual　animals；　which　are　neither　male　nor　female；　they
become　converted　into　young　'Aphides'；　which　repeat　the　process；　and
their　offspring　after　them；　and　so　on　again；　you　may　go　on　for　nine　or
ten；　or　even　twenty　or　more　successions；　and　there　is　no　very　good
reason　to　say　how　soon　it　might　terminate；　or　how　long　it　might　not　go
on　if　the　proper　conditions　of　warmth　and　nourishment　were　kept　up。

Sexual　reproduction　is　quite　a　distinct　matter。　　Here；　in　all　these
cases；　what　is　required　is　the　detachment　of　two　portions　of　the
parental　organisms；　which　portions　we　know　as　the　egg　and　the
spermatozoon。　　In　plants　it　is　the　ovule　and　the　pollen…grain；　as　in　the
flowering　plants；　or　the　ovule　and　the　antherozooid；　as　in　the
flowerless。　　Among　all　forms　of　animal　life；　the　spermatozoa　proceed
from　the　male　sex；　and　the　egg　is　the　product　of　the　female。　　Now；　what
is　remarkable　about　this　mode　of　reproduction　is　this；　that　the　egg　by
itself；　or　the　spermatozoa　by　themselves；　are　unable　to　assume　the
parental　form；　but　if　they　be　brought　into　contact　with　one　another；
the　effect　of　the　mixture　of　organic　substances　proceeding　from　two
sources　appears　to　confer　an　altogether　new　vigour　to　the　mixed　product。
This　process　is　brought　about；　as　we　all　know；　by　the　sexual
intercourse　of　the　two　sexes；　and　is　called　the　act　of　impregnation。
The　result　of　this　act　on　the　part　of　the　male　and　female　is；　that　the
formation　of　a　new　being　is　set　up　in　the　ovule　or　egg；　this　ovule　or
egg　soon　begins　to　be　divided　and　subdivided；　and　to　be　fashioned　into
various　complex　organisms；　and　eventually　to　develop　into　the　form　of
one　of　its　parents；　as　I　explained　in　the　first　lecture。　　These　are　the
processes　by　which　the　perpetuation　of　organic　beings　is　secured。　　Why
there　should　be　the　two　modeswhy　this　re…invigoration　should　be
required　on　the　part　of　the　female　element　we　do　not　know；　but　it　is
most　assuredly　the　fact；　and　it　is　presumable；　that；　however　long　the
process　of　asexual　multiplication　could　be　continued；　I　say　there　is
good　reason　to　believe　that　it　would　come　to　an　end　if　a　new
commencement　were　not　obtained　by　a　conjunction　of　the　two　sexual
elements。

That　character　which　is　common　to　these　two　distinct　processes　is　this；
that；　whether　we　consider　the　reproduction；　or　perpetuation；　or
modification　of　organic　beings　as　they　take　place　asexually；　or　as　they
may　take　place　sexually；in　either　case；　I　say；　the　offspring　has　a
constant　tendency　to　assume；　speaking　generally；　the　character　of　the
parent。　　As　I　said　just　now；　if　you　take　a　slip　of　a　plant；　and　tend　it
with　care；　it　will　eventually　grow　up　and　develop　into　a　plant　like
that　from　which　it　had　sprung；　and　this　tendency　is　so　strong　that；　as
gardeners　know；　this　mode　of　multiplying　by　means　of　cuttings　is　the
only　secure　mode　of　propagating　very　many　varieties　of　plants；　the
peculiarity　of　the　primitive　stock　seems　to　be　better　preserved　if　you
propagate　it　by　means　of　a　slip　than　if　you　resort　to　the　sexual　mode。

Again；　in　experiments　upon　the　lower　animals；　such　as　the　polype；　to
which　I　have　referred；　it　is　most　extraordinary　that；　although　cut　up
into　various　pieces；　each　particular　piece　will　grow　up　into　the　form
of　the　primitive　stock；　the　head；　if　separated；　will　reproduce　the　body
and　the　tail；　and　if　you　cut　off　the　tail；　you　will　find　that　that　will
reproduce　the　body　and　all　the　rest　of　the　members；　without　in　any　way
deviating　from　the　plan　of　the　organism　from　which　these　portions　have
been　detached。　　And　so　far　does　this　go；　that　some　experimentalists
have　carefully　examined　the　lower　orders　of　animals；among　them　the
Abbe　Spallanzani；　who　made　a　number　of　experiments　upon　snails　and
salamanders；and　have　found　that　they　might　mutilate　them　to　an
incredible　extent；　that　you　might　cut　off　the　jaw　or　the　greater　part　of
the　head；　or　the　leg　or　the　tail；　and　repeat　the　experiment　several
times；　perhaps；　cutting　off　the　same　member　again　and　again；　and　yet
each　of　those　types　would　be　reproduced　according　to　the　primitive
type：　nature　making　no　mistake；　never　putting　on　a　fresh　kind　of　leg；　or
head；　or　tail；　but　always　tending　to　repeat　and　to　return　to　the
primitive　type。

It　is　the　same　in　sexual　reproduction：　it　is　a　matter　of　perfectly
common　experience；　that　the　tendency　on　the　part　of　the　offspring
always　is；　speaking　broadly；　to　reproduce　the　form　of　the　parents。　　The
proverb　has　it　that　the　thistle　does　not　bring　forth　grapes；　so；　among
ourselves；　there　is　always　a　likeness；　more　or　less　marked　and
distinct；　between　children　and　their　parents。　　That　is　a　matter　of
familiar　and　ordinary　observation。　　We　notice　the　same　thing　occurring
in　the　cases　of　the　domestic　animalsdogs；　for　instance；　and　their
offspring。　　In　all　these　cases　of　propagation　and　perpetuation；　there
seems　to　be　a　tendency　in　the　offspring　to　take　the　characters　of　the
parental　organisms。　　To　that　tendency　a　special　name　is　given　it　is
called　'Atavism'；　it　expresses　this　tendency　to　revert　to　the　ancestral
type；　and　comes　from　the　Latin　word　'atavus'；　ancestor。

Well；　this　'Atavism'　which　I　shall　speak　of；　is；　as　I　said　before；　one
of　the　most　marked　and　striking　tendencies　of　organic　beings；　but；　side
by　side　with　this　hereditary　tendency　there　is　an　equally　distinct　and
remarkable　tendency　to　variation。　　The　tendency　to　reproduce　the
original　stock　has；　as　it　were；　its　limits；　and　side　by　side　with　it
there　is　a　tendency　to　vary　in　certain　directions；　as　if　there　were　two
opposing　powers　working　upon　the　organic　being；　one　tending　to　take　it
in　a　straight　line；　and　the　other　tending　to　make　it　diverge　from　that
straight　line；　first　to　one　side　and　then　to　the　other。

So　that　you　see　these　two　tendencies　need　not　precisely　contradict　one
another；　as　the　ultimate　result　may　not　always　be　very　remote　from　what
would　have　been　the　case　if　the　line　had　been　quite　straight。

This　tendency　to　variation　is　less　marked　in　that　mode　of　propagation
which　takes　place　asexually；　it　is　in　that　mode　that　the　minor
characters　of　animal　and　vegetable　structures　are　most　completely
preserved。　　Still；　it　will　happen　sometimes；　that　the　gardener；　when　he
has　planted　a　cutting　of　some　favourite　plant；　will　find；　contrary　to
his　expectation；　that　the　slip　grows　up　a　little　different　from　the
primitive　stockthat　it　produces　flowers　of　a　different　colour　or
make；　or　some　deviation　in　one　way　or　an