essays on life, art and science-第29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！


own　toes　owing　to　the　sciatic　nerve　having　been　divided。　　Of　this
latter　fact　thirteen　instances　were　carefully　recorded；　and　a
greater　number　were　seen；　yet　Brown…Sequard　speaks　of　such　cases　as
one　of　the　rarer　forms　of　inheritance。　　It　is　a　still　more
interesting　fact；　'that　the　sciatic　nerve　in　the　congenitally
toeless　animal　has　inherited　the　power　of　passing　through　all　the
different　morbid　states　which　have　occurred　in　one　of　its　parents
from　the　time　of　the　division　till　after　its　reunion　with　the
peripheric　end。　　It　is　not；　therefore；　simply　the　power　of
performing　an　action　which　is　inherited；　but　the　power　of　performing
a　whole　series　of　actions；　in　a　certain　order。'

〃In　most　of　the　cases　of　inheritance　recorded　by　Brown…Sequard　only
one　of　the　two　parents　had　been　operated　upon　and　was　affected。　　He
concludes　by　expressing　his　belief　that　'what　is　transmitted　is　the
morbid　state　of　the　nervous　system；'　due　to　the　operation　performed
on　the　parents。〃

Mr。　Darwin　proceeds　to　give　other　instances　of　inherited　effects　of
mutilations：…

〃With　the　horse　there　seems　hardly　a　doubt　that　exostoses　on　the
legs；　caused　by　too　much　travelling　on　hard　roads；　are　inherited。
Blumenbach　records　the　case　of　a　man　who　had　his　little　finger　on
the　right　hand　almost　cut　off；　and　which　in　consequence　grew
crooked；　and　his　sons　had　the　same　finger　on　the　same　hand　similarly
crooked。　　A　soldier；　fifteen　years　before　his　marriage；　lost　his
left　eye　from　purulent　ophthalmia；　and　his　two　sons　were
microphthalmic　on　the　same　side。〃

The　late　Professor　Rolleston；　whose　competence　as　an　observer　no　one
is　likely　to　dispute；　gave　Mr。　Darwin　two　cases　as　having　fallen
under　his　own　notice；　one　of　a　man　whose　knee　had　been　severely
wounded；　and　whose　child　was　born　with　the　same　spot　marked　or
scarred；　and　the　other　of　one　who　was　severely　cut　upon　the　cheek；
and　whose　child　was　born　scarred　in　the　same　place。　　Mr。　Darwin's
conclusion　was　that　〃the　effects　of　injuries；　especially　when
followed　by　disease；　or　perhaps　exclusively　when　thus　followed；　are
occasionally　inherited。〃

Let　us　now　see　what　Professor　Weismann　has　to　say　against　this。　　He
writes：…

〃The　only　cases　worthy　of　discussion　are　the　well…known　experiments
upon　guinea…pigs　conducted　by　the　French　physiologist；　Brown…
Sequard。　　But　the　explanation　of　his　results　is；　in　my　opinion；　open
to　discussion。　　In　these　cases　we　have　to　do　with　the　apparent
transmission　of　artificially　produced　malformations　。　。　。　All　these
effects　were　said　to　be　transmitted　to　descendants　as　far　as　the
fifth　or　sixth　generation。

〃But　we　must　inquire　whether　these　cases　are　really　due　to　heredity；
and　not　to　simple　infection。　　In　the　case　of　epilepsy；　at　any　rate；
it　is　easy　to　imagine　that　the　passage　of　some　specific　organism
through　the　reproductive　cells　may　take　place；　as　in　the　case　of
syphilis。　　We　are；　however；　entirely　ignorant　of　the　nature　of　the
former　disease。　　This　suggested　explanation　may　not　perhaps　apply　to
the　other　cases；　but　we　must　remember　that　animals　which　have　been
subjected　to　such　severe　operations　upon　the　nervous　system　have
sustained　a　great　shock；　and　if　they　are　capable　of　breeding；　it　is
only　probable　that　they　will　produce　weak　descendants；　and　such　as
are　easily　affected　by　disease。　　Such　a　result　does　not；　however；
explain　why　the　offspring　should　suffer　from　the　same　disease　as
that　which　was　artificially　induced　in　the　parents。　　But　this　does
not　appear　to　have　been　by　any　means　invariably　the　case。　　Brown…
Sequard　himself　says：　　'The　changes　in　the　eye　of　the　offspring　were
of　a　very　variable　nature；　and　were　only　occasionally　exactly
similar　to　those　observed　in　the　parents。'

〃There　is　no　doubt；　however；　that　these　experiments　demand　careful
consideration；　but　before　they　can　claim　scientific　recognition；
they　must　be　subjected　to　rigid　criticism　as　to　the　precautions
taken；　the　nature　and　number　of　the　control　experiments；　&c。

〃Up　to　the　present　time　such　necessary　conditions　have　not　been
sufficiently　observed。　　The　recent　experiments　themselves　are　only
described　in　short　preliminary　notices；　which；　as　regards　their
accuracy；　the　possibility　of　mistake；　the　precautions　taken；　and　the
exact　succession　of　individuals　affected；　afford　no　data　on　which　a
scientific　opinion　can　be　founded〃　（pp。　81；　82）。

The　line　Professor　Weismann　takes；　therefore；　is　to　discredit　the
facts；　yet　on　a　later　page　we　find　that　the　experiments　have　since
been　repeated　by　Obersteiner；　〃who　has　described　them　in　a　very
exact　and　unprejudiced　manner；〃　and　that　〃the　fact〃（I　imagine　that
Professor　Weismann　intends　〃the　facts〃）〃cannot　be　doubted。〃

On　a　still　later　page；　however；　we　read：…

〃If；　for　instance；　it　could　be　shown　that　artificial　mutilation
spontaneously　reappears　in　the　offspring　with　sufficient　frequency
to　exclude　all　possibilities　of　chance；　then　such　proof　＇i。e。；　that
acquired　characters　can　be　transmitted＇　would　be　forthcoming。　　The
transmission　of　mutilations　has　been　frequently　asserted；　and　has
been　even　recently　again　brought　forward；　but　all　the　supposed
instances　have　broken　down　when　carefully　examined〃　（p。　390）。

Here；　then；　we　are　told　that　proof　of　the　occasional　transmission　of
mutilations　would　be　sufficient　to　establish　the　fact；　but　on　p。　267
we　find　that　no　single　fact　is　known　which　really　proves　that
acquired　characters　can　be　transmitted；　〃FOR　THE　ASCERTAINED　FACTS
WHICH　SEEM　TO　POINT　TO　THE　TRANSMISSION　OF　ARTIFICIALLY　PRODUCED
DISEASES　CANNOT　BE　CONSIDERED　AS　PROOF〃　＇Italics　mine。＇　　Perhaps；
but　it　was　mutilation　in　many　cases　that　Professor　Weismann
practically　admitted　to　have　been　transmitted　when　he　declared　that
Obersteiner　had　verified　Brown…Sequard's　experiments。

That　Professor　Weismann　recognises　the　vital　importance　to　his　own
theory　of　the　question　whether　or　no　mutilations　can　be　transmitted
under　any　circumstances；　is　evident　from　a　passage　on　p。　425　of　his
work；　on　which　he　says：　　〃It　can　hardly　be　doubted　that　mutilations
are　acquired　characters；　they　do　not　arise　from　any　tendency
contained　in　the　germ；　but　are　merely　the　reaction　of　the　body　under
certain　external　influences。　　They　are；　as　I　have　recently　expressed
it；　purely　somatogenic　charactersviz。；　characters　which　emanate
from　the　body　（soma）　only；　as　opposed　to　the　germ…cells；　they　are；
therefore；　characters　that　do　not　arise　from　the　germ　itself。

〃If　mutilations　must　necessarily　be　transmitted〃　＇which　no　one　that
I　know　of　has　maintained＇；　〃or　even　if　they　might　occasionally　be
transmitted〃　＇which　cannot；　I　imagine；　be　reasonably　questioned＇；　〃a
powerful　support　would　be　given　to　the　Lamarckian　principle；　and　the
transmission　of　functional　hypertrophy　or　atrophy　would　thus　become
highly　probable。〃

I　have　not　found　any　further　attempt　in　Professor　Weismann's　book　to
deal　with　the　evidence　adduced　by　Mr。　Darwin　to　show　that
mutilations；　if　followed　by　diseases；　are　sometimes　inherited；　and　I
must　leave　it　to　the　reader　to　determine　how　far　Professor　Weismann
has　shown　reason　for　rejecting　Mr。　Darwin's　conclusion。　　I　do　not；
however；　dwell　upon　these　facts　now　as　evidence　of　a　transmitted
change　of　bodily　form；　or　of　instinct　due　to　use　and　disuse　or
habit；　what　they　prove　is　that　the　germ…cells　within　the　parent's
body　do　not　stand　apart　from　the　other　cells　of　the　body　so
completely　as　Professor　Weismann　would　have　us　believe；　but　that；　as
Professor　Hering；　of　Prague；　has　aptly　said；　they　echo　with　more　or
less　frequency　and　force　to　the　profounder　impressions　made　upon
other　cells。

I　may　say　that　Professor　Weismann　does　not　more　cavalierly　wave
aside　the　mass　of　evidence　collected　by　Mr。　Darwin　and　a　host　of
other　writers；　to　the　effect　that　mutilations　are　sometimes
inherited；　than　does　Mr。　Wallace；　who　says　that；　〃as　regards
mutilations；　it　is　generally　admitted　that　they　are　not　inherited；
and　there　is　ample　evidence　on　this　point。〃　　It　is　indeed　generally
admitted　that　mutilations；　when　not　followed　by　disease；　are　very
rarely；　if　ever；　inherited；　and　Mr。　Wallace's　appeal　to　the　〃ample
evidence〃　which　he　alleges　to　exist　on　this　head；　is　much　as　though
he　should　say　that　there　is　ample　evidence　to　show　that　the　days　are
longer　in　summer　than　in　winter。　　〃Nevertheless；〃　he　continues；　〃a
few　cases　of　apparent　inheritance　of　mutilations　have　been　recorded；
and　these；　if　trustworthy；　are　difficulties　in　the　way　of　the
theory。〃　。　。　。　〃The　often…quoted　case　of　a　disease　induced　by
mutilation　being　inherited　（Brown…Sequard's　epileptic　guinea…pigs）
has　been　discussed　by　Professor　Weismann　and　shown　to　be　not
conclusive。　　The　mutilation　itselfa　section　of　certain　nerveswas
never　inherited；　but　the　resulting　epilepsy；　or　a　general　state　of
weakness；　deformity；　or　sores；　was　sometimes　inherited。　　It　is；
however；　possible　that　the　mere　injury　introduced　and　encouraged　the
growth　of　certain　microbes；　which；　spreading　through　the　organism；
sometimes　reached　the　germ…cells；　and　thus　transmitted　a　diseased
condition　to　the　offspring。〃　｛35｝

I　suppose　a　microbe　which　made　guinea…pigs　eat　their　toes　off　was
communicated　to　the　germ…cells　of　an　unfortunate　guinea…pig　which
had　been　already　microbed　by　it；　and　made　the　offspring　bite　its
toes　off　too。　　The　microbe　has　a　good　deal　to　answer　for。

On　the　case　of　the　deterioration　of　horses　in　the　Falkland　Islands
after　a　few　generations；　Professor　Weismann　says：…

〃In　such　a　case　we　have　only　to　assume　that　the　climate　which　is
unfavourable；　and　the　nutriment　which　is　insufficient　for　horses；
affect　not　only　the　animal　as　a　whole　but　also　its　germ…cells。　　This
would　result　in　the　diminution　in　size　of　the　germ…cells；　the
effects　upon　the　offspring　being　still　further　intensified　by　the
insufficient　nourishment　supplied　during　growth。　　But　such　results
would　not　depend　upon　the　transmission　by　the　germ…cells　of　certain
peculiarities　due　to　the　unfavourable　climate；　which　only　appear　in
the　full…grown　horse。〃

But　Professor　Weismann　does　not　like　such　cases；　and　admits　that　he
cannot　explain　the　facts　in　connection　with　the　climatic　varieties
of　certain　butterflies；　except　〃by　supposing　the　passive　acquisition
of　characters　produced　by　the　direct　influence　of　climate。〃

Nevertheless　in　his　next　paragraph　but　one　he　calls　such　cases
〃doubtful；〃　and　proposes　that　for　the　moment　they　should　be　left
aside。　　He　accordingly　leaves　them；　but　I　have　not　yet　found　what
other　moment　he　considered　auspicious　for　returning　to　them。　　He
tells　us　that　〃new　experiments　will　be　necessary；　and　that　he　has
himself　already　begun　to　undertake　them。〃　　Perhaps　he　will　give　us
the　results　of　these　experiments　in　some　future　bookfor　that　they
will　prove　satisfactory　to　him　can　hardly；　I　think；　be　doubted。　　He
writes：…

〃Leaving　on　one　side；　for　the　moment；　these　doubtful　and
insufficiently　investigated　cases；　we　may　still　maintain　that　the
assumption　that　changes　induced　by　external　conditions　in　the
organism　as　a　whole　are　communicated　to　the　germ…cells　after　the
manner　indicated　in　Darwin's　hypothesis　of　pangenesis；　is　wholly
unnecessary　for　the　explanation　of　these　phenomena。　　Still　we　can