essays on life, art and science-第39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！


severity　does　not　warrant　Professor　Weismann　in　ascribing　to　Mr。
Darwin　an　opinion　which　he　emphatically　condemned。　　His　tone；
however；　is　so　offhand；　that　those　who　have　little　acquaintance　with
the　literature　of　evolution　would　hardly　guess　that　he　is　not　much
better　informed　on　this　subject　than　themselves。

Returning　to　the　inheritance　of　acquired　characters；　Professor
Weismann　says　that　this　has　never　been　proved　either　by　means　of
direct　observation　or　by　experiment。　　〃It　must　be　admitted；〃　he
writes；　〃that　there　are　in　existence　numerous　descriptions　of　cases
which　tend　to　prove　that　such　mutilations　as　the　loss　of　fingers；
the　scars　of　wounds；　&c。；　are　inherited　by　the　offspring；　but　in
these　descriptions　the　previous　history　is　invariably　obscure；　and
hence　the　evidence　loses　all　scientific　value。〃

The　experiments　of　M。　　Brown…Sequard　throw　so　much　light　upon　the
question　at　issue　that　I　will　quote　at　some　length　from　the　summary
given　by　Mr。　Darwin　in　his　〃Variation　of　Animals　and　Plants　under
Domestication。〃　｛34｝　　Mr。　Darwin　writes：…

〃With　respect　to　the　inheritance　of　structures　mutilated　by　injuries
or　altered　by　disease；　it　was　until　lately　difficult　to　come　to　any
definite　conclusion。〃　　＇Then　follow　several　cases　in　which
mutilations　practised　for　many　generations　are　not　found　to　be
transmitted。＇　　〃Notwithstanding；〃　continues　Mr。　Darwin；　〃the　above
several　negative　cases；　we　now　possess　conclusive　evidence　that　the
effects　of　operations　are　sometimes　inherited。　　Dr。　Brown…Sequard
gives　the　following　summary　of　his　observations　on　guinea…pigs；　and
this　summary　is　so　important　that　I　will　quote　the　whole：…

〃'1st。　　Appearance　of　epilepsy　in　animals　born　of　parents　having
been　rendered　epileptic　by　an　injury　to　the　spinal　cord。

〃'2nd。　　Appearance　of　epilepsy　also　in　animals　born　of　parents
having　been　rendered　epileptic　by　the　section　of　the　sciatic　nerve。

〃'3rd。　　A　change　in　the　shape　of　the　ear　in　animals　born　of　parents
in　which　such　a　change　was　the　effect　of　a　division　of　the　cervical
sympathetic　nerve。

〃'4th。　　Partial　closure　of　the　eyelids　in　animals　born　of　parents　in
which　that　state　of　the　eyelids　had　been　caused　either　by　the
section　of　the　cervical　sympathetic　nerve　or　the　removal　of　the
superior　cervical　ganglion。

〃'5th。　　Exophthalmia　in　animals　born　of　parents　in　which　an　injury
to　the　restiform　body　had　produced　that　protrusion　of　the　eyeball。
This　interesting　fact　I　have　witnessed　a　good　many　times；　and　I　have
seen　the　transmission　of　the　morbid　state　of　the　eye　continue
through　four　generations。　　In　these　animals　modified　by　heredity；
the　two　eyes　generally　protruded；　although　in　the　parents　usually
only　one　showed　exophthalmia；　the　lesion　having　been　made　in　most
cases　only　on　one　of　the　corpora　restiformia。

〃'6th。　　Haematoma　and　dry　gangrene　of　the　ears　in　animals　born　of
parents　in　which　these　ear…alterations　had　been　caused　by　an　injury
to　the　restiform　body　near　the　nib　of　the　calamus。

〃'7th。　　Absence　of　two　toes　out　of　the　three　of　the　hind　leg；　and
sometimes　of　the　three；　in　animals　whose　parents　had　eaten　up　their
hind…leg　toes　which　had　become　anaesthetic　from　a　section　of　the
sciatic　nerve　alone；　or　of　that　nerve　and　also　of　the　crural。
Sometimes；　instead　of　complete　absence　of　the　toes；　only　a　part　of
one　or　two　or　three　was　missing　in　the　young；　although　in　the　parent
not　only　the　toes　but　the　whole　foot　was　absent　（partly　eaten　off；
partly　destroyed　by　inflammation；　ulceration；　or　gangrene）。

〃'8th。　　Appearance　of　various　morbid　states　of　the　skin　and　hair　of
the　neck　and　face　in　animals　born　of　parents　having　had　similar
alterations　in　the　same　parts；　as　effects　of　an　injury　to　the
sciatic　nerve。'

〃It　should　be　especially　observed　that　Brown…Sequard　has　bred　during
thirty　years　many　thousand　guinea…pigs　from　animals　which　had　not
been　operated　upon；　and　not　one　of　these　manifested　the　epileptic
tendency。　　Nor　has　he　ever　seen　a　guinea…pig　born　without　toes；
which　was　not　the　offspring　of　parents　which　had　gnawed　off　their
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　on