darwin and modern science-第68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！


　the　greater　part　of　its　course　in　North　America。　　So　far　as　it　has　yet　been　traced；　the　line　begins　in　the　lower　Eocene　with　the　genus　Eohippus；　a　little　creature　not　much　larger　than　a　cat；　which　has　a　short　neck；　relatively　short　limbs；　and　in　particular；　short　feet；　with　four　functional　digits　and　a　splint…like　rudiment　in　the　fore…foot；　three　functional　digits　and　a　rudiment　in　the　hind…foot。　　The　forearm　bones　（ulna　and　radius）　are　complete　and　separate；　as　are　also　the　bones　of　the　lower　leg　（fibula　and　tibia）。　　The　skull　has　a　short　face；　with　the　orbit；　or　eye…socket；　incompletely　enclosed　with　bone；　and　the　brain…case　is　slender　and　of　small　capacity。　　The　teeth　are　short…　crowned；　the　incisors　without　〃mark；〃　or　enamel　pit；　on　the　cutting　edge；　the　premolars　are　all　smaller　and　simpler　than　the　molars。　　The　pattern　of　the　upper　molars　is　so　entirely　different　from　that　seen　in　the　modern　horses　that；　without　the　intermediate　connecting　steps；　no　one　would　have　ventured　to　derive　the　later　from　the　earlier　plan。　　This　pattern　is　quadritubercular；　with　four　principal；　conical　cusps　arranged　in　two　transverse　pairs；　forming　a　square；　and　two　minute　cuspules　between　each　transverse　pair；　a　tooth　which　is　much　more　pig…like　than　horse…like。　　In　the　lower　molars　the　cusps　have　already　united　to　form　two　crescents；　one　behind　the　other；　forming　a　pattern　which　is　extremely　common　in　the　early　representatives　of　many　different　families；　both　of　the　Perissodactyla　and　the　Artiodactyla。　　In　spite　of　the　manifold　differences　in　all　parts　of　the　skeleton　between　Eohippus　and　the　recent　horses；　the　former　has　stamped　upon　it　an　equine　character　which　is　unmistakable；　though　it　can　hardly　be　expressed　in　words。

Each　one　of　the　different　Eocene　and　Oligocene　horizons　has　its　characteristic　genus　of　horses；　showing　a　slow；　steady　progress　in　a　definite　direction；　all　parts　of　the　structure　participating　in　the　advance。　　It　is　not　necessary　to　follow　each　of　these　successive　steps　of　change；　but　it　should　be　emphasised　that　the　changes　are　gradual　and　uninterrupted。　　The　genus　Mesohippus；　of　the　middle　Oligocene；　may　be　selected　as　a　kind　of　half…way　stage　in　the　long　progression。　　Comparing　Mesohippus　with　Eohippus；　we　observe　that　the　former　is　much　larger；　some　species　attaining　the　size　of　a　sheep；　and　has　a　relatively　longer　neck；　longer　limbs　and　much　more　elongate　feet；　which　are　tridactyl；　and　the　middle　toe　is　so　enlarged　that　it　bears　most　of　the　weight；　while　the　lateral　digits　are　very　much　more　slender。　　The　fore…arm　bones　have　begun　to　co…ossify　and　the　ulna　is　greatly　reduced；　while　the　fibula；　though　still　complete；　is　hardly　more　than　a　thread　of　bone。　　The　skull　has　a　longer　face　and　a　nearly　enclosed　orbit；　and　the　brain…case　is　fuller　and　more　capacious；　the　internal　cast　of　which　shows　that　the　brain　was　richly　convoluted。　　The　teeth　are　still　very　short…crowned；　but　the　upper　incisors　plainly　show　the　beginning　of　the　〃mark〃；　the　premolars　have　assumed　the　molar　form；　and　the　upper　molars；　though　plainly　derived　from　those　of　Eohippus；　have　made　a　long　stride　toward　the　horse　pattern；　in　that　the　separate　cusps　have　united　to　form　a　continuous　outer　wall　and　two　transverse　crests。

In　the　lower　Miocene　the　interesting　genus　Desmatippus　shows　a　further　advance　in　the　development　of　the　teeth；　which　are　beginning　to　assume　the　long…crowned　shape；　delaying　the　formation　of　roots；　a　thin　layer　of　cement　covers　the　crowns；　and　the　transverse　crests　of　the　upper　grinding　teeth　display　an　incipient　degree　of　their　modern　complexity。　　This　tooth…pattern　is　strictly　intermediate　between　the　recent　type　and　the　ancient　type　seen　in　Mesohippus　and　its　predecessors。　　The　upper　Miocene　genera；　Protohippus　and　Hipparion　are；　to　all　intents　and　purposes；　modern　in　character；　but　their　smaller　size；　tridactyl　feet　and　somewhat　shorter…crowned　teeth　are　reminiscences　of　their　ancestry。

From　time　to　time；　when　a　land…connection　between　North　America　and　Eurasia　was　established；　some　of　the　successive　equine　genera　migrated　to　the　Old　World；　but　they　do　not　seem　to　have　gained　a　permanent　footing　there　until　the　end　of　the　Miocene　or　beginning　of　the　Pliocene；　eventually　diversifying　into　the　horses；　asses；　and　zebras　of　Africa；　Asia　and　Europe。　　At　about　the　same　period；　the　family　extended　its　range　to　South　America　and　there　gave　rise　to　a　number　of　species　and　genera；　some　of　them　extremely　peculiar。　　For　some　unknown　reason；　all　the　horse　tribe　had　become　extinct　in　the　western　hemisphere　before　the　European　discovery；　but　not　until　after　the　native　race　of　man　had　peopled　the　continents。

In　addition　to　the　main　stem　of　equine　descent；　briefly　considered　in　the　foregoing　paragraphs；　several　side…branches　were　given　off　at　successive　levels　of　the　stem。　　Most　of　these　branches　were　short…lived；　but　some　of　them　flourished　for　a　considerable　period　and　ramified　into　many　species。

Apparently　related　to　the　horses　and　derived　from　the　same　root…stock　is　the　family　of　the　Palaeotheres；　confined　to　the　Eocene　and　Oligocene　of　Europe；　dying　out　without　descendants。　　In　the　earlier　attempts　to　work　out　the　history　of　the　horses；　as　in　the　famous　essay　of　Kowalevsky　（〃Sur　l'Anchitherium　aurelianense　Cuv。　et　sur　l'histoire　paleontologique　des　Chevaux〃；　〃Mem。　de　l'Acad。　Imp。　des　Sc。　de　St　Petersbourg〃；　XX。　no。　5；　1873。）；　the　Palaeotheres　were　placed　in　the　direct　line；　because　the　number　of　adequately　known　Eocene　mammals　was　then　so　small；　that　Cuvier's　types　were　forced　into　various　incongruous　positions；　to　serve　as　ancestors　for　unrelated　series。

The　American　family　of　the　Titanotheres　may　also　be　distantly　related　to　the　horses；　but　passed　through　an　entirely　different　course　of　development。　　From　the　lower　Eocene　to　the　lower　sub…stage　of　the　middle　Oligocene　the　series　is　complete；　beginning　with　small　and　rather　lightly　built　animals。　Gradually　the　stature　and　massiveness　increase；　a　transverse　pair　of　nasal　horns　make　their　appearance　and；　as　these　increase　in　size；　the　canine　tusks　and　incisors　diminish　correspondingly。　　Already　in　the　oldest　known　genus　the　number　of　digits　had　been　reduced　to　four　in　the　fore…foot　and　three　in　the　hind；　but　there　the　reduction　stops；　for　the　increasing　body…　weight　made　necessary　the　development　of　broad　and　heavy　feet。　　The　final　members　of　the　series　comprise　only　large；　almost　elephantine　animals；　with　immensely　developed　and　very　various　nasal　horns；　huge　and　massive　heads；　and　altogether　a　grotesque　appearance。　　The　growth　of　the　brain　did　not　at　all　keep　pace　with　the　increase　of　the　head　and　body；　and　the　ludicrously　small　brain　may　will　have　been　one　of　the　factors　which　determined　the　startlingly　sudden　disappearance　and　extinction　of　the　group。

Less　completely　known；　but　of　unusual　interest；　is　the　genealogy　of　the　rhinoceros　family；　which　probably；　though　not　certainly；　was　likewise　of　American　origin。　　The　group　in　North　America　at　least；　comprised　three　divisions；　or　sub…families；　of　very　different　proportions；　appearance　and　habits；　representing　three　divergent　lines　from　the　same　stem。　　Though　the　relationship　between　the　three　lines　seems　hardly　open　to　question；　yet　the　form　ancestral　to　all　of　them　has　not　yet　been　identified。　　This　is　because　of　our　still　very　incomplete　knowledge　of　several　perissodactyl　genera　of　the　Eocene；　any　one　of　which　may　eventually　prove　to　be　the　ancestor　sought　for。

The　first　sub…family　is　the　entirely　extinct　group　of　Hyracodonts；　which　may　be　traced　in　successive　modifications　through　the　upper　Eocene；　lower　and　middle　Oligocene；　then　disappearing　altogether。　　As　yet；　the　hyracodonts　have　been　found　only　in　North　America；　and　the　last　genus　of　the　series；　Hyracodon；　was　a　cursorial　animal。　　Very　briefly　stated；　the　modifications　consist　in　a　gradual　increase　in　size；　with　greater　slenderness　of　proportions；　accompanied　by　elongation　of　the　neck；　limbs；　and　feet；　which　become　tridactyl　and　very　narrow。　　The　grinding　teeth　have　assumed　the　rhinoceros…like　pattern　and　the　premolars　resemble　the　molars　in　form；　on　the　other　hand；　the　front　teeth；　incisors　and　canines；　have　become　very　small　and　are　useless　as　weapons。　　As　the　anim