a history of science-4- 第16部分

But　in　point　of　fact；　as　studies　of　a　later　generation　were　to　show；　it　is　the　under　surface　of　the　leaf　that　is　most　abundantly　provided　with　stomata；　or　〃breathing…pores。〃　From　the　stand…point　of　this　later　knowledge；　it　is　of　interest　to　follow　our　author　a　little　farther；　to　illustrate　yet　more　fully　the　possibility　of　combining　correct　observations　with　a　faulty　inference。


〃4。　　As　the　upper　surface　of　leaves　constitutes　the　organ　of　respiration；　on　which　the　sap　is　exposed　in　the　termination　of　arteries　beneath　a　thin　pellicle　to　the　action　of　the　atmosphere；　these　surfaces　in　many　plants　strongly　repel　moisture；　as　cabbage　leaves；　whence　the　particles　of　rain　lying　over　their　surfaces　without　touching　them；　as　observed　by　Mr。　Melville　（Essays　Literary　and　Philosophical：　　Edinburgh）；　have　the　appearance　of　globules　of　quicksilver。　　And　hence　leaves　with　the　upper　surfaces　on　water　wither　as　soon　as　in　the　dry　air；　but　continue　green　for　many　days　if　placed　with　the　under　surface　on　water；　as　appears　in　the　experiments　of　Monsieur　Bonnet　（Usage　des　Feuilles）。　Hence　some　aquatic　plants；　as　the　water…lily　（Nymphoea）；　have　the　lower　sides　floating　on　the　water；　while　the　upper　surfaces　remain　dry　in　the　air。

〃5。　　As　those　insects　which　have　many　spiracula；　or　breathing　apertures；　as　wasps　and　flies；　are　immediately　suffocated　by　pouring　oil　upon　them；　I　carefully　covered　with　oil　the　surfaces　of　several　leaves　of　phlomis；　of　Portugal　laurel；　and　balsams；　and　though　it　would　not　regularly　adhere；　I　found　them　all　die　in　a　day　or　two。

〃It　must　be　added　that　many　leaves　are　furnished　with　muscles　about　their　foot…stalks；　to　turn　their　surfaces　to　the　air　or　light；　as　mimosa　or　Hedysarum　gyrans。　　From　all　these　analogies　I　think　there　can　be　no　doubt　but　that　leaves　of　trees　are　their　lungs；　giving　out　a　phlogistic　material　to　the　atmosphere；　and　absorbing　oxygen；　or　vital　air。

〃6。　　The　great　use　of　light　to　vegetation　would　appear　from　this　theory　to　be　by　disengaging　vital　air　from　the　water　which　they　perspire；　and　thence　to　facilitate　its　union　with　their　blood　exposed　beneath　the　thin　surface　of　their　leaves；　since　when　pure　air　is　thus　applied　it　is　probable　that　it　can　be　more　readily　absorbed。　　Hence；　in　the　curious　experiments　of　Dr。　Priestley　and　Mr。　Ingenhouz；　some　plants　purified　less　air　than　othersthat　is；　they　perspired　less　in　the　sunshine；　and　Mr。　Scheele　found　that　by　putting　peas　into　water　which　about　half　covered　them　they　converted　the　vital　air　into　fixed　air；　or　carbonic…acid　gas；　in　the　same　manner　as　in　animal　respiration。

〃7。　　The　circulation　in　the　lungs　or　leaves　of　plants　is　very　similar　to　that　of　fish。　　In　fish　the　blood；　after　having　passed　through　their　gills；　does　not　return　to　the　heart　as　from　the　lungs　of　air…breathing　animals；　but　the　pulmonary　vein　taking　the　structure　of　an　artery　after　having　received　the　blood　from　the　gills；　which　there　gains　a　more　florid　color；　distributes　it　to　the　other　parts　of　their　bodies。　The　same　structure　occurs　in　the　livers　of　fish；　whence　we　see　in　those　animals　two　circulations　independent　of　the　power　of　the　heartviz。；　that　beginning　at　the　termination　of　the　veins　of　the　gills　and　branching　through　the　muscles；　and　that　which　passes　through　the　liver；　both　which　are　carried　on　by　the　action　of　those　respective　arteries　and　veins。〃＇6＇

Darwin　is　here　a　trifle　fanciful　in　forcing　the　analogy　between　plants　and　animals。　　The　circulatory　system　of　plants　is　really　not　quite　so　elaborately　comparable　to　that　of　fishes　as　he　supposed。　But　the　all…important　idea　of　the　uniformity　underlying　the　seeming　diversity　of　Nature　is　here　exemplified；　as　elsewhere　in　the　writings　of　Erasmus　Darwin；　and；　more　specifically；　a　clear　grasp　of　the　essentials　of　the　function　of　respiration　is　fully　demonstrated。


ZOOLOGY　AT　THE　CLOSE　OF　THE　EIGHTEENTH　CENTURY


Several　causes　conspired　to　make　exploration　all　the　fashion　during　the　closing　epoch　of　the　eighteenth　century。　New　aid　to　the　navigator　had　been　furnished　by　the　perfected　compass　and　quadrant；　and　by　the　invention　of　the　chronometer；　medical　science　had　banished　scurvy；　which　hitherto　had　been　a　perpetual　menace　to　the　voyager；　and；　above　all；　the　restless　spirit　of　the　age　impelled　the　venturesome　to　seek　novelty　in　fields　altogether　new。　　Some　started　for　the　pole；　others　tried　for　a　northeast　or　northwest　passage　to　India；　yet　others　sought　the　great　fictitious　antarctic　continent　told　of　by　tradition。　All　these　of　course　failed　of　their　immediate　purpose；　but　they　added　much　to　the　world's　store　of　knowledge　and　its　fund　of　travellers'　tales。

Among　all　these　tales　none　was　more　remarkable　than　those　which　told　of　strange　living　creatures　found　in　antipodal　lands。　And　here；　as　did　not　happen　in　every　field；　the　narratives　were　often　substantiated　by　the　exhibition　of　specimens　that　admitted　no　question。　Many　a　company　of　explorers　returned　more　or　less　laden　with　such　trophies　from　the　animal　and　vegetable　kingdoms；　to　the　mingled　astonishment；　delight；　and　bewilderment　of　the　closet　naturalists。　The　followers　of　Linnaeus　in　the　〃golden　age　of　natural　history；〃　a　few　decades　before；　had　increased　the　number　of　known　species　of　fishes　to　about　four　hundred；　of　birds　to　one　thousand；　of　insects　to　three　thousand；　and　of　plants　to　ten　thousand。　But　now　these　sudden　accessions　from　new　territories　doubled　the　figure　for　plants；　tripled　it　for　fish　and　birds；　and　brought　the　number　of　described　insects　above　twenty　thousand。　　Naturally　enough；　this　wealth　of　new　material　was　sorely　puzzling　to　the　classifiers。　The　more　discerning　began　to　see　that　the　artificial　system　of　Linnaeus；　wonderful　and　useful　as　it　had　been；　must　be　advanced　upon　before　the　new　material　could　be　satisfactorily　disposed　of。　The　way　to　a　more　natural　system；　based　on　less　arbitrary　signs；　had　been　pointed　out　by　Jussieu　in　botany；　but　the　zoologists　were　not　prepared　to　make　headway　towards　such　a　system　until　they　should　gain　a　wider　understanding　of　the　organisms　with　which　they　had　to　deal　through　comprehensive　studies　of　anatomy。　Such　studies　of　individual　forms　in　their　relations　to　the　entire　scale　of　organic　beings　were　pursued　in　these　last　decades　of　the　century；　but　though　two　or　three　most　important　generalizations　were　achieved　（notably　Kaspar　Wolff's　conception　of　the　cell　as　the　basis　of　organic　life；　and　Goethe's　all…important　doctrine　of　metamorphosis　of　parts）；　yet；　as　a　whole；　the　work　of　the　anatomists　of　the　period　was　germinative　rather　than　fruit…bearing。　Bichat's　volumes；　telling　of　the　recognition　of　the　fundamental　tissues　of　the　body；　did　not　begin　to　appear　till　the　last　year　of　the　century。　The　announcement　by　Cuvier　of　the　doctrine　of　correlation　of　parts　bears　the　same　date；　but　in　general　the　studies　of　this　great　naturalist；　which　in　due　time　were　to　stamp　him　as　the　successor　of　Linnaeus；　were　as　yet　only　fairly　begun。



　V。　ANATOMY　AND　PHYSIOLOGY　IN　THE　NINETEENTH　CENTURY

CUVIER　AND　THE　CORRELATION　OF　PARTS

We　have　seen　that　the　focal　points　of　the　physiological　world　towards　the　close　of　the　eighteenth　century　were　Italy　and　England；　but　when　Spallanzani　and　Hunter　passed　away　the　scene　shifted　to　France。　　The　time　was　peculiarly　propitious；　as　the　recent　advances　in　many　lines　of　science　had　brought　fresh　data　for　the　student　of　animal　life　which　were　in　need　of　classification；　and；　as　several　minds　capable　of　such　a　task　were　in　the　field；　it　was　natural　that　great　generalizations　should　have　come　to　be　quite　the　fashion。　Thus　it　was　that　Cuvier　came　forward　with　a　brand…new　classification　of　the　animal　kingdom；　establishing　four　great　types　of　being；　which　he　called　vertebrates；　mollusks；　articulates；　and　radiates。　Lamarck　had　shortly　before　established　the　broad　distinction　between　animals　with　and　those　without　a　backbone；　Cuvier's　Classification　divided　the　latterthe　invertebratesinto　three　minor　groups。　And　this　division；　familiar　ever　since　to　all　students　of　zoology；　has　only　in　very　recent　years　been　supplanted；　and　then　not　by　revolution；　but　by　a　further　division；　which　the　elaborate　recent　studies　of　lower　forms　of　life　seemed　to　make　desirable。

In　the　course　of　those　studies　of　comparative　anatomy　which　led　to　his　new　classification；　Cuvier's　attention　was　called　constantly　to　the　peculiar　co…ordination　of　parts　in　each　individual　organism。　Thus　an　animal　with　sharp　talons　for　catching　living　preyas　a　member　of　the　cat　tribehas　also　sharp　teeth；　adapted　for　tearing　up　the　flesh　of　its　victim；　and　a　particular　type　of　stomach；　quite　different　from　that　of　herbivorous　creatures。　This　adaptation　of　all　the　parts　of　the　animal　to　one　another　extends　to　the　most　diverse　parts　of　the　organism；　and　enables　the　skilled　anatomist；　from　the　observation　of　a　single　typical　part；　to　draw　inferences　as　to　the　structure　of　the　entire　animala　fact　which　was　of　vast　aid　to　Cuvier　in　his　studies　of　paleontology。　It　did　not　enable　Cuvier；　nor　does　it　enable　any　one　else；　to　reconstruct　fully　the　extinct　animal　from　observation　of　a　single　bone；　as　has　sometimes　been　asserted；　but　what　it　really　does　establish；　in　the　hands　of　an　expert；　is　sufficiently　astonishing。

〃While　the　study　of　the　fossil　remains　of　the　greater　quadrupeds　is　more　satisfactory；〃　he　writes；　〃by　the　clear　results　which　it　affords；　than　that　of　the　remains　of　other　animals　found　in　a　fossil　state；　it　is　also　complicated　with　greater　and　more　numerous　difficulties。　Fossil　shells　are　usually　found　quite　entire；　and　retaining　all　the　characters　requisite　for　comparing　them　with　the　specimens　contained　in　collections　of　natural　history；　or　represented　in　the　works　of　naturalists。　Even　the　skeletons　of　fishes　are　found　more　or　less　entire；　so　that　the　general　forms　of　their　bodies　can；　for　the　most　part；　be　ascertained；　and　usually；　at　least；　their　generic　and　specific　characters　are　determinable；　as　these　are　mostly　drawn　from　their　solid　parts。　In　quadrupeds；　on　the　contrary；　even　when　their　entire　skeletons　are　found；　there　is　great　difficulty　in　discovering　their　distinguishing　characters；　as　these　are　chiefly　founded　upon　their　hairs　and　colors　and　other　marks　which　have　disappeared　previous　to　their　incrustat