the perpetuation of living beings-第4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！


much　valuable　time　and　attention　on　the　investigation　of　these
variations；　and　getting　together　all　the　facts　that　bear　upon　them。　　I
obtained　from　these　MSS。　the　following　summary　of　the　differences
between　the　domestic　breeds　of　pigeons；　that　is　to　say；　a　notification
of　the　various　points　in　which　their　organization　differs。　　In　the
first　place；　the　back　of　the　skull　may　differ　a　good　deal；　and　the
development　of　the　bones　of　the　face　may　vary　a　great　deal；　the　back
varies　a　good　deal；　the　shape　of　the　lower　jaw　varies；　the　tongue
varies　very　greatly；　not　only　in　correlation　to　the　length　and　size　of
the　beak；　but　it　seems　also　to　have　a　kind　of　independent　variation　of
its　own。　　Then　the　amount　of　naked　skin　round　the　eyes；　and　at　the　base
of　the　beak；　may　vary　enormously；　so　may　the　length　of　the　eyelids；　the
shape　of　the　nostrils；　and　the　length　of　the　neck。　　I　have　already
noticed　the　habit　of　blowing　out　the　gullet；　so　remarkable　in　the
Pouter；　and　comparatively　so　in　the　others。　　There　are　great
differences；　too；　in　the　size　of　the　female　and　the　male；　the　shape　of
the　body；　the　number　and　width　of　the　processes　of　the　ribs；　the
development　of　the　ribs；　and　the　size；　shape；　and　development　of　the
breastbone。　　We　may　notice；　too；and　I　mention　the　fact　because　it　has
been　disputed　by　what　is　assumed　to　be　high　authority；the　variation
in　the　number　of　the　sacral　vertebrae。　　The　number　of　these　varies　from
eleven　to　fourteen；　and　that　without　any　diminution　in　the　number　of
the　vertebrae　of　the　back　or　of　the　tail。　　Then　the　number　and　position
of　the　tail…feathers　may　vary　enormously；　and　so　may　the　number　of　the
primary　and　secondary　feathers　of　the　wings。　　Again；　　the　length　of　the
feet　and　of　the　beak；although　they　have　no　relation　to　each　other；
yet　appear　to　go　together；that　is；　you　have　a　long　beak　wherever　you
have　long　feet。　　There　are　differences　also　in　the　periods　of　the
acquirement　of　the　perfect　plumage；the　size　and　shape　of　the
eggs；the　nature　of　flight；　and　the　powers　of　flight；so…called
〃homing〃　birds　having　enormous　flying　powers；*　while；　on　the　other
hand；　the　little　Tumbler　is　so　called　because　of　its　extraordinary
faculty　of　turning　head　over　heels　in　the　air；　instead　of　pursuing　a
direct　course。　　And；　lastly；　the　dispositions　and　voices　of　the　birds
may　vary。　　Thus　the　case　of　the　pigeons　shows　you　that　there　is　hardly
a　single　particular；whether　of　instinct；　or　habit；　or　bony　structure；
or　of　plumage；of　either　the　internal　economy　or　the　external　shape；
in　which　some　variation　or　change　may　not　take　place；　which；　by
selective　breeding；　may　become　perpetuated；　and　form　the　foundation　of；
and　give　rise　to；　a　new　race。

　　　　＇footnote：　The　〃Carrier；〃　I　learn　from　Mr。　Tegetmeier；　does
　　　　not　'carry'；　a　high…bred　bird　of　this　breed　being　but　a
　　　　poor　flier。　　The　birds　which　fly　long　distances；　and　come
　　　　home；〃homing〃　birds；and　are　consequently　used　as
　　　　carriers；　are　not　〃carriers〃　in　the　fancy　sense。＇

If　you　carry　in　your　mind's　eye　these　four　varieties　of　pigeons；　you
will　bear　with　you　as　good　a　notion　as　you　can　have；　perhaps；　of　the
enormous　extent　to　which　a　deviation　from　a　primitive　type　may　be
carried　by　means　of　this　process　of　selective　breeding。







End