darwin and modern science-第61节

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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　animal　had　no　horns；　it　was　quite　defenceless　and　must　have　found　its　safety　in　its　swift　running；　for　Hyracodon　displays　many　superficial　resemblances　to　the　contemporary　Oligocene　horses；　and　was　evidently　adapted　for　speed。　　It　may　well　have　been　the　competition　of　the　horses　which　led　to　the　extinction　of　these　cursorial　rhinoceroses。

The　second　sub…family；　that　of　the　Amynodonts；　followed　a　totally　different　course　of　development；　becoming　short…legged　and　short…footed；　massive　animals；　the　proportions　of　which　suggest　aquatic　habits；　they　retained　four　digits　in　the　front　foot。　　The　animal　was　well　provided　with　weapons　in　the　large　canine　tusks；　but　was　without　horns。　　Some　members　of　this　group　extended　their　range　to　the　Old　World；　but　they　all　died　out　in　the　middle　Oligocene；　leaving　no　successors。

The　sub…family　of　the　true　rhinoceroses　cannot　yet　be　certainly　traced　farther　back　than　to　the　base　of　the　middle　Oligocene；　though　some　fragmentary　remains　found　in　the　lower　Oligocene　are　probably　also　referable　to　it。　　The　most　ancient　and　most　primitive　member　of　this　series　yet　discovered；　the　genus　Trigonias；　is　unmistakably　a　rhinoceros；　yet　much　less　massive；　having　more　the　proportions　of　a　tapir；　it　had　four　toes　in　the　front　foot；　three　in　the　hind；　and　had　a　full　complement　of　teeth；　except　for　the　lower　canines；　though　the　upper　canines　are　about　to　disappear；　and　the　peculiar　modification　of　the　incisors；　characteristic　of　the　true　rhinoceroses；　is　already　apparent；　the　skull　is　hornless。　　Representatives　of　this　sub…family　continue　through　the　Oligocene　and　Miocene　of　North　America；　becoming　rare　and　localised　in　the　Pliocene　and　then　disappearing　altogether。　　In　the　Old　World；　on　the　other　hand；　where　the　line　appeared　almost　as　early　as　it　did　in　America；　this　group　underwent　a　great　expansion　and　ramification；　giving　rise　not　only　to　the　Asiatic　and　African　forms；　but　also　to　several　extinct　series。

Turning　now　to　the　Artiodactyla；　we　find　still　another　group　of　mammals；　that　of　the　camels　and　llamas；　which　has　long　vanished　from　North　America；　yet　took　its　rise　and　ran　the　greater　part　of　its　course　in　that　continent。　From　the　lower　Eocene　onward　the　history　of　this　series　is　substantially　complete；　though　much　remains　to　be　learned　concerning　the　earlier　members　of　the　family。　　The　story　is　very　like　that　of　the　horses；　to　which　in　many　respects　it　runs　curiously　parallel。　　Beginning　with　very　small；　five…toed　animals；　we　observe　in　the　successive　genera　a　gradual　transformation　in　all　parts　of　the　skeleton；　an　elongation　of　the　neck；　limbs　and　feet；　a　reduction　of　the　digits　from　five　to　two；　and　eventually　the　coalescence　of　the　remaining　two　digits　into　a　〃cannon…bone。〃　　The　grinding　teeth；　by　equally　gradual　steps；　take　on　the　ruminant　pattern。　　In　the　upper　Miocene　the　line　divides　into　the　two　branches　of　the　camels　and　llamas；　the　former　migrating　to　Eurasia　and　the　latter　to　South　America；　though　representatives　of　both　lines　persisted　in　North　America　until　a　very　late　period。　　Interesting　side…branches　of　this　line　have　also　been　found；　one　of　which　ended　in　the　upper　Miocene　in　animals　which　had　almost　the　proportions　of　the　giraffes　and　must　have　resembled　them　in　appearance。

The　American　Tertiary　has　yielded　several　other　groups　of　ruminant…like　animals；　some　of　which　form　beautifully　complete　evolutionary　series；　but　space　forbids　more　than　this　passing　mention　of　them。

It　was　in　Europe　that　the　Artiodactyla　had　their　principal　development；　and　the　upper　Eocene；　Oligocene　and　Miocene　are　crowded　with　such　an　overwhelming　number　and　variety　of　forms　that　it　is　hardly　possible　to　marshal　them　in　orderly　array　and　determine　their　mutual　relationships。　　Yet　in　this　chaotic　exuberance　of　life；　certain　important　facts　stand　out　clearly；　among　these　none　is　of　greater　interest　and　importance　than　the　genealogy　of　the　true　Ruminants；　or　Pecora；　which　may　be　traced　from　the　upper　Eocene　onward。　　The　steps　of　modification　and　change　are　very　similar　to　those　through　which　the　camel　phylum　passed　in　North　America；　but　it　is　instructive　to　note　that；　despite　their　many　resemblances；　the　two　series　can　be　connected　only　in　their　far　distant　beginnings。　　The　pecoran　stock　became　vastly　more　expanded　and　diversified　than　did　the　camel　line　and　was　evidently　more　plastic　and　adaptable；　spreading　eventually　over　all　the　continents　except　Australia；　and　forming　to…day　one　of　the　dominant　types　of　mammals；　while　the　camels　are　on　the　decline　and　not　far　from　extinction。　　The　Pecora　successively　ramified　into　the　deer；　antelopes；　sheep；　goats　and　oxen；　and　did　not　reach　North　America　till　the　Miocene；　when　they　were　already　far　advanced　in　specialisation。　　To　this　invasion　of　the　Pecora；　or　true　ruminants；　it　seems　probable　that　the　decline　and　eventual　disappearance　of　the　camels　is　to　be　ascribed。

Recent　discoveries　in　Egypt　have　thrown　much　light　upon　a　problem　which　long　baffled　the　palaeontologist；　namely；　the　origin　of　the　elephants。　　（C。W。　Andrews；　〃On　the　Evolution　of　the　Proboscidea