essays on life, art and science-第38章

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Contrasting　the　generally　received　view　with　his　own；　Professor
Weismann　says　that　according　to　the　first　of　these　〃the　organism
produces　germ…cells　afresh　again　and　again；　and　that　it　produces
them　entirely　from　its　own　substance。〃　　While　by　the　second　〃the
germ…cells　are　no　longer　looked　upon　as　the　product　of　the　parent's
body；　at　least　as　far　as　their　essential　partthe　specific　germ…
plasmis　concerned；　they　are　rather　considered　as　something　which
is　to　be　placed　in　contrast　with　the　tout　ensemble　of　the　cells
which　make　up　the　parent's　body；　and　the　germ…cells　of　succeeding
generations　stand　in　a　similar　relation　to　one　another　as　a　series
of　generations　of　unicellular　organisms　arising　by　a　continued
process　of　cell…division。〃　｛30｝

On　another　page　he　writes：…

〃I　believe　that　heredity　depends　upon　the　fact　that　a　small　portion
of　the　effective　substance　of　the　germ；　the　germ…plasm；　remains
unchanged　during　the　development　of　the　ovum　into　an　organism；　and
that　this　part　of　the　germ…plasm　serves　as　a　foundation　from　which
the　germ…cells　of　the　new　organism　are　produced。　　There　is；
therefore；　continuity　of　the　germ…plasm　from　one　generation　to
another。　　One　might　represent　the　germ…plasm　by　the　metaphor　of　a
long　creeping　root…stock　from　which　plants　arise　at　intervals；　these
latter　representing　the　individuals　of　successive　generations。〃　｛31｝

Mr。　Wallace；　who　does　not　appear　to　have　read　Professor　Weismann's
essays　themselves；　but　whose　remarks　are；　no　doubt；　ultimately
derived　from　the　sequel　to　the　passage　just　quoted　from　page　266　of
Professor　Weismann's　book；　contends　that　the　impossibility　of　the
transmission　of　acquired　characters　follows　as　a　logical　result　from
Professor　Weismann's　theory；　inasmuch　as　the　molecular　structure　of
the　germ…plasm　that　will　go　to　form　any　succeeding　generation　is
already　predetermined　within　the　still　unformed　embryo　of　its
predecessor；　〃and　Weismann；〃　continues　Mr。　Wallace；　〃holds　that
there　are　no　facts　which　really　prove　that　acquired　characters　can
be　inherited；　although　their　inheritance　has；　by　most　writers；　been
considered　so　probable　as　hardly　to　stand　in　need　of　direct　proof。〃
｛32｝

Professor　Weismann；　in　passages　too　numerous　to　quote；　shows　that　he
recognises　this　necessity；　and　acknowledges　that　the　non…
transmission　of　acquired　characters　〃forms　the　foundation　of　the
views〃　set　forth　in　his　book；　p。　291。

Professor　Ray　Lankester　does　not　commit　himself　absolutely　to　this
view；　but　lends　it　support　by　saying　（Nature；　December　12；　1889）：
〃It　is　hardly　necessary　to　say　that　it　has　never　yet　been　shown
experimentally　that　ANYTHING　acquired　by　one　generation　is
transmitted　to　the　next　（putting　aside　diseases）。〃

Mr。　Romanes；　writing　in　Nature；　March　18；　1890；　and　opposing　certain
details　of　Professor　Weismann's　theory；　so　far　supports　it　as　to　say
that　〃there　is　the　gravest　possible　doubt　lying　against　the
supposition　that　any　really　inherited　decrease　is　due　to　the
inherited　effects　of　disuse。〃　　The　〃gravest　possible　doubt〃　should
mean　that　Mr。　Romanes　regards　it　as　a　moral　certainty　that　disuse
has　no　transmitted　effect　in　reducing　an　organ；　and　it　should　follow
that　he　holds　use　to　have　no　transmitted　effect　in　its　development。
The　sequel；　however；　makes　me　uncertain　how　far　Mr。　Romanes　intends
this；　and　I　would　refer　the　reader　to　the　article　which　Mr。　Romanes
has　just　published　on　Weismann　in　the　Contemporary　Review　for　this
current　month。

The　burden　of　Mr。　Thiselton　Dyer's　controversy　with　the　Duke　of
Argyll　（see　Nature；　January　16；　1890；　et　seq。）　was　that　there　was　no
evidence　in　support　of　the　transmission　of　any　acquired
modification。　　The　orthodoxy　of　science；　therefore；　must　be　held　as
giving　at　any　rate　a　provisional　support　to　Professor　Weismann；　but
all　of　them；　including　even　Professor　Weismann　himself；　shrink　from
committing　themselves　to　the　opinion　that　the　germ…cells　of　any
organisms　remain　in　all　cases　unaffected　by　the　events　that　occur　to
the　other　cells　of　the　same　organism；　and　until　they　do　this　they
have　knocked　the　bottom　out　of　their　case。

From　among　the　passages　in　which　Professor　Weismann　himself　shows　a
desire　to　hedge　I　may　take　the　following　from　page　170　of　his　book：…

〃I　am　also　far　from　asserting　that　the　germ…plasm　which；　as　I　hold；
is　transmitted　as　the　basis　of　heredity　from　one　generation　to
another；　is　absolutely　unchangeable　or　totally　uninfluenced　by
forces　residing　in　the　organism　within　which　it　is　transformed　into
germ…cells。　　I　am　also　compelled　to　admit　it　as　conceivable　that
organisms　may　exert　a　modifying　influence　upon　their　germ…cells；　and
even　that　such　a　process　is　to　a　certain　extent　inevitable。　　The
nutrition　and　growth　of　the　individual　must　exercise　some　influence
upon　its　germ…cells　。　。　。　〃

Professor　Weismann　does　indeed　go　on　to　say　that　this　influence　must
be　extremely　slight；　but　we　do　not　care　how　slight　the　changes
produced　may　be　provided　they　exist　and　can　be　transmitted。　　On　an
earlier　page　（p。　101）　he　said　in　regard　to　variations　generally　that
we　should　not　expect　to　find　them　conspicuous；　their　frequency　would
be　enough；　if　they　could　be　accumulated。　　The　same　applies　here；　if
stirring　events　that　occur　to　the　somatic　cells　can　produce　any
effect　at　all　on　offspring。　　A　very　small　effect；　provided　it　can　be
repeated　and　accumulated　in　successive　generations；　is　all　that　even
the　most　exacting　Lamarckian　will　ask　for。

Having　now　made　the　reader　acquainted　with　the　position　taken　by　the
leading　Charles…Darwinian　authorities；　I　will　return　to　Professor
Weismann　himself；　who　declares　that　the　transmission　of　acquired
characters　〃at　first　sight　certainly　seems　necessary；〃　and　that　〃it
appears　rash　to　attempt　to　dispense　with　its　aid。〃　　He　continues：…

〃Many　phenomena　only　appear　to　be　intelligible　if　we　assume　the
hereditary　transmission　of　such　acquired　characters　as　the　changes
which　we　ascribe　to　the　use　or　disuse　of　particular　organs；　or　to
the　direct　influence　of　climate。　　Furthermore；　how　can　we　explain
instinct　as　hereditary　habit；　unless　it　has　gradually　arisen　by　the
accumulation；　through　heredity；　of　habits　which　were　practised　in
succeeding　generations？〃　｛33｝

I　may　say　in　passing　that　Professor　Weismann　appears　to　suppose　that
the　view　of　instinct　just　given　is　part　of　the　Charles…Darwinian
system；　for　on　page　889　of　his　book　he　says　〃that　many　observers　had
followed　Darwin　in　explaining　them　＇instincts＇　as　inherited　habits。〃
This　was　not　Mr。　Darwin's　own　view　of　the　matter。　　He　wrote：…

〃If　we　suppose　any　habitual　action　to　become　inheritedand　I　think
it　can　be　shown　that　this　does　sometimes　happenthen　the
resemblance　between　what　originally　was　a　habit　and　an　instinct
becomes　so　close　as　not　to　be　distinguished。　。　。　But　it　would　be　the
most　serious　error　to　suppose　that　the　greater　number　of　instincts
have　been　acquired　by　habit　in　one　generation；　and　then　transmitted
by　inheritance　to　succeeding　generations。　　It　can　be　clearly　shown
that　the　most　wonderful　instincts　with　which　we　are　acquainted；
namely；　those　of　the　hive…bee　and　of　many　ants；　could　not　possibly
have　been　thus　acquired。〃＇〃Origin　of　Species；〃　ed。；　1859；　p。　209。＇

Again　we　read：　　〃Domestic　instincts　are　sometimes　spoken　of　as
actions　which　have　become　inherited　solely　from　long…continued　and
compulsory　habit；　but　this；　I　think；　is　not　true。〃Ibid。；　p。　214。

Again：　　〃I　am　surprised　that　no　one　has　advanced　this　demonstrative
case　of　neuter　insects；　against　the　well…known　doctrine　of　inherited
habit；　as　advanced　by　Lamarck。〃＇〃Origin　of　Species；〃　ed。　1872；　p。
283。＇

I　am　not　aware　that　Lamarck　advanced　the　doctrine　that　instinct　is
inherited　habit；　but　he　may　have　done　so　in　some　work　that　I　have
not　seen。

It　is　true；　as　I　have　more　than　once　pointed　out；　that　in　the　later
editions　of　the　〃Origin　of　Species〃　it　is　no　longer　〃the　MOST
serious〃　error　to　refer　instincts　generally　to　inherited　habit；　but
it　still　remains　〃a　serious　error；〃　and　this　slight　relaxation　of
severity　does　not　warrant　Professor　Weismann　in　ascribing　to　Mr。
Darwin　an　opinion　which　he　emphatically　condemned。　　His　tone；
how