the perpetuation of living beings-第2部分

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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　another。　　This　is　what　is　called
the　'sporting'　of　plants。

In　animals　the　phenomena　of　asexual　propagation　are　so　obscure；　that　at
present　we　cannot　be　said　to　know　much　about　them；　but　if　we　turn　to
that　mode　of　perpetuation　which　results　from　the　sexual　process；　then
we　find　variation　a　perfectly　constant　occurrence；　to　a　certain　extent；
and；　indeed；　I　think　that　a　certain　amount　of　variation　from　the
primitive　stock　is　the　necessary　result　of　the　method　of　sexual
propagation　itself；　for；　inasmuch　as　the　thing　propagated　proceeds　from
two　organisms　of　different　sexes　and　different　makes　and　temperaments；
and　as　the　offspring　is　to　be　either　of　one　sex　or　the　other；　it　is
quite　clear　that　it　cannot　be　an　exact　diagonal　of　the　two；　or　it　would
be　of　no　sex　at　all；　it　cannot　be　an　exact　intermediate　form　between
that　of　each　of　its　parentsit　must　deviate　to　one　side　or　the　other。
You　do　not　find　that　the　male　follows　the　precise　type　of　the　male
parent；　nor　does　the　female　always　inherit　the　precise　characteristics
of　the　mother；there　is　always　a　proportion　of　the　female　character　in
the　male　offspring；　and　of　the　male　character　in　the　female　offspring。
That　must　be　quite　plain　to　all　of　you　who　have　looked　at　all
attentively　on　your　own　children　or　those　of　your　neighbours；　you　will
have　noticed　how　very　often　it　may　happen　that　the　son　shall　exhibit
the　maternal　type　of　character；　or　the　daughter　possess　the
characteristics　of　the　father's　family。　There　are　all　sorts　of
intermixtures　and　intermediate　conditions　between　the　two；　where
complexion；　or　beauty；　or　fifty　other　different　peculiarities　belonging
to　either　side　of　the　house；　are　reproduced　in　other　members　of　the
same　family。　　Indeed；　it　is　sometimes　to　be　remarked　in　this　kind　of
variation；　that　the　variety　belongs；　strictly　speaking；　to　neither　of
the　immediate　parents；　you　will　see　a　child　in　a　family　who　is　not　like
either　its　father　or　its　mother；　but　some　old　person　who　knew　its
grandfather　or　grandmother；　or；　it　may　be；　an　uncle；　or；　perhaps；　even
a　more　distant　relative；　will　see　a　great　similarity　between　the　child
and　one　of　these。　　In　this　way　it　constantly　happens　that　the
characteristic　of　some　previous　member　of　the　family　comes　out　and　is
reproduced　and　recognised　in　the　most　unexpected　manner。

But　apart　from　that　matter　of　general　experience；　there　are　some　cases
which　put　that　curious　mixture　in　a　very　clear　light。　　You　are　aware
that　the　offspring　of　the　Ass　and　the　Horse；　or　rather　of　the　he…Ass
and　the　Mare；　is　what　is　called　a　Mule；　and；　on　the　other　hand；　the
offspring　of　the　Stallion　and　the　she…Ass　is　what　is　called　a　'Hinny'。
I　never　saw　one　myself；　but　they　have　been　very　carefully　studied。　Now；
the　curious　thing　is　this；　that　although　you　have　the　same　elements　in
the　experiment　in　each　case；　the　offspring　is　entirely　different　in
character；　according　as　the　male　influence　comes　from　the　Ass　or　the
Horse。　　Where　the　Ass　is　the　male；　as　in　the　case　of　the　Mule；　you　find
that　the　head　is　like　that　of　the　Ass；　that　the　ears　are　long；　the　tail
is　tufted　at　the　end；　the　feet　are　small；　and　the　voice　is　an
unmistakable　bray；　these　are　all　points　of　similarity　to　the　Ass；　but；
on　the　other　hand；　the　barrel　of　the　body　and　the　cut　of　the　neck　are
much　more　like　those　of　the　Mare。　　Then；　if　you　look　at　the　Hinny；the
result　of　the　union　of　the　Stallion　and　the　she…Ass；　then　you　find　it　is
the　Horse　that　has　the　predominance；　that　the　head　is　more　like　that　of
the　Horse；　the　ears　are　shorter；　the　legs　coarser；　and　the　type　is
altogether　altered；　while　the　voice；　instead　of　being　a　bray；　is　the
ordinary　neigh　of　the　Horse。　　Here；　you　see；　is　a　most　curious　thing：
you　take　exactly　the　same　elements；　Ass　and　Horse；　but　you　combine　the
sexes　in　a　different　manner；　and　the　result　is　modified　accordingly。
You　have　in　this　case；　however；　a　result　which　is　not　general　and
universalthere　is　usually　an　important　preponderance；　but　not　always
on　the　same　side。

Here；　then；　is　one　intelligible；　and；　perhaps；　necessary　cause　of
variation：　the　fact；　that　there　are　two　sexes　sharing　in　the　production
of　the　offspring；　and　that　the　share　taken　by　each　is　different　and
variable；　not　only　for　each　combination；　but　also　for　different　members
of　the　same　family。

Secondly；　there　is　a　variation；　to　a　certain　extentthough；　in　all
probability；　the　influence　of　this　cause　has　been　very　much
exaggeratedbut　there　is　no　doubt　that　variation　is　produced；　to　a
certain　extent；　by　what　are　commonly　known　as　external　conditions；such
as　temperature；　food；　warmth；　and　moisture。　　In　the　long　run；　every
variation　depends；　in　some　sense；　upon　external　conditions；　seeing　that
everything　has　a　cause　of　its　own。　　I　use　the　term　〃external
conditions〃　now　in　the　sense　in　which　it　is　ordinarily　employed：　certain
it　is；　that　external　conditions　have　a　definite　effect。　　You　may　take　a
plant　which　has　single　flowers；　and　by　dealing　with　the　soil；　and
nourishment；　and　so　on；　you　may　by…and…by　convert　single　flowers　into
double　flowers；　and　make　thorns　shoot　out　into　branches。　　You　may
thicken　or　make　various　modifications　in　the　shape　of　the　fruit。　　In
animals；　too；　you　may　produce　analogous　changes　in　this　way；　as　in　the
case　of　that　deep　bronze　colour　which　persons　rarely　lose　after　having
passed　any　length　of　time　in　tropical　countries。　　You　may　also　alter
the　development　of　the　muscles　very　much；　by　dint　of　training；　all　the
world　knows　that　exercise　has　a　great　effect　in　this　way；　we　always
expect　to　find　the　arm　of　a　blacksmith　hard　and　wiry；　and　possessing　a
large　development　of　the　brachial　muscles。　　No　doubt　training；　which　is
one　of　the　forms　of　external　conditions；　converts　what　are　originally
only　instructions；　teachings；　into　habits；　or；　in　other　words；　into
organizations；　to　a　great　extent；　but　this　second　cause　of　variation
cannot　be　considered　to　be　by　any　means　a　large　one。　　The　third　cause
that　I　have　to　mention；　however；　is　a　very　extensive　one。　　It　is　one
that；　for　want　of　a　better　name；　has　been　called　〃spontaneous
variation；〃　which　means　that　when　we　do　not　know　anything　about　the
cause　of　phenomena；　we　call　it　spontaneous。　　In　the　orderly　chain　of
causes　and　effects　in　this　world；　there　are　very　few　things　of　which　it
can　be　said　with　truth　that　they　are　spontaneous。　Certainly　not　in
these　physical　matters；in　these　there　is　nothing　of　the
kind；everything　depends　on　previous　conditions。　　But　when　we　cannot
trace　the　cause　of　phenomena；　we　call　them　spontaneous。

Of　these　variations；　multitudinous　as　they　are；　but　little　is　known　with
perfect　accuracy。　　I　will　mention　to　you　some　two　or　three　cases；
because　they　are　very　remarkable　in　themselves；　and　also　because　I
shall　want　to　use　them　afterwards。　　Reaumur；　a　famous　French　naturalist；
a　great　many　years　ago；　in　an　essay　which　he　wrote　upon　the　art　of
hatching　chickens；which　was　indeed　a　very　curious　essay；had
occasion　to　speak　of　variations　and　monstrosities。　　One　very　remarkable
case　had　come　under　his　notice　of　a　variation　in　the　form　of　a　human
member；　in　the　person　of　a　Maltese；　of　the　name　of　Gratio　Kelleia；　who
was　born　with　six　fingers　upon　each　hand；　and　the　like　number　of　toes
to　each　of　his　feet。　　That　was　a　case　of　spontaneous　variation。　　Nobody
knows　why　he　was　born　with　that　number　of　fingers　and　toes；　and　as　we
don't　know；　we　call　it　a　case　of　〃spontaneous〃　variation。　　There　is
another　remarkable　case　also。　　I　select　these；　because　they　happen　to
have　been　observed　and　noted　very　carefully　at　the　time。　　It　frequently
happens　that　a　variation　occurs；　but　the　persons　who　notice　it　do　not
take　any　care　in　noting　down　the　particulars；　until　at　length；　when
inquiries　come　to　be　made；　the　exact　circumstances　are　forgotten；　and
hence；　multitudinous　as　may　be　such　〃spontaneous〃　variations；　it　is
exceedingly　difficult　to　get　at　the　origin　of　them。

The　second　case　is　one　of　which　you　may　find　the　whole　details　in　the
〃Philosophical　Transactions〃　for　the　year　1813；　in　a　paper　communicated
by　Colonel　Humphrey　to　the　President　of　the　Royal　Society；〃On　a　new
Variety　in　the　Breed　of　Sheep；〃　giving　an　account　of　a　very　remarkable
breed　of　sheep；　which　at　one　time　was　well　known　in　the　northern　states
of　America；　and　which　went　by　the　name　of　the　Ancon　or　the　Otter　breed
of　sheep。　　In　the　year　1791；　there　was　a　farmer　of　the　name　of　Seth
Wright　in　Massachusetts；　who　had　a　flock　of　sheep；　consisting　of　a　ram
and；　I　think；　of　some　twelve　or　thirteen　ewes。　　Of　this　flock　of　ewes；
one　at　the　breeding…time　bore　a　lamb　which　was　very　singularly　formed；
it　had　a　very　long　body；　very　short　legs；　and　those　legs　were　bowed！　　I
will　tell　you　by…and…by　how　this　singular　variation　in　the　breed　of
sheep　came　to　be　noted；　and　to　have　the　prominence　that　it　now　has。　For
the　present；　I　mention　only　these　two　cases；　but　the　extent　of　variation
in　the　breed　of　animals　is　perfectly　obvious　to　any　one　who　has　studied
natural　history　with　ordinary　attention；　or　to　any　person　who　compares
animals　with　others　of　the　same　kind。　　It　is　strictly　true　that　there
are　never　any