darwin and modern science-第69章

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e　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〃；　〃Phil。　Trans。　Roy。　Soc。〃　London；　Vol。　196；　1904；　page　99。）　　Early　representatives　of　this　order；　Mastodons；　had　appeared　almost　simultaneously　（in　the　geological　sense　of　that　word）　in　the　upper　Miocene　of　Europe　and　North　America；　but　in　neither　continent　was　any　more　ancient　type　known　which　could　plausibly　be　regarded　as　ancestral　to　them。　　Evidently；　these　problematical　animals　had　reached　the　northern　continents　by　migrating　from　some　other　region；　but　no　one　could　say　where　that　region　lay。　　The　Eocene　and　Oligocene　beds　of　the　Fayoum　show　us　that　the　region　sought　for　is　Africa；　and　that　the　elephants　form　just　such　a　series　of　gradual　modifications　as　we　have　found　among　other　hoofed　animals。　　The　later　steps　of　the　transformation；　by　which　the　mastodons　lost　their　lower　tusks；　and　their　relatively　small　and　simple　grinding　teeth　acquired　the　great　size　and　highly　complex　structure　of　the　true　elephants；　may　be　followed　in　the　uppermost　Miocene　and　Pliocene　fossils　of　India　and　southern　Europe。

Egypt　has　also　of　late　furnished　some　very　welcome　material　which　contributes　to　the　solution　of　another　unsolved　problem　which　had　quite　eluded　research；　the　origin　of　the　whales。　　The　toothed…whales　may　be　traced　back　in　several　more　or　less　parallel　lines　as　far　as　the　lower　Miocene；　but　their　predecessors　in　the　Oligocene　are　still　so　incompletely　known　that　safe　conclusions　can　hardly　be　drawn　from　them。　　In　the　middle　Eocene　of　Egypt；　however；　has　been　found　a　small；　whale…like　animal　（Protocetus）；　which　shows　what　the　ancestral　toothed…whale　was　like；　and　at　the　same　time　seems　to　connect　these　thoroughly　marine　mammals　with　land…　animals。　　Though　already　entirely　adapted　to　an　aquatic　mode　of　life；　the　teeth；　skull　and　backbone　of　Protocetus　display　so　many　differences　from　those　of　the　later　whales　and　so　many　approximations　to　those　of　primitive；　carnivorous　land…mammals；　as；　in　a　large　degree；　to　bridge　over　the　gap　between　the　two　groups。　　Thus　one　of　the　most　puzzling　of　palaeontological　questions　is　in　a　fair　way　to　receive　a　satisfactory　answer。　　The　origin　of　the　whalebone…whales　and　their　relations　to　the　toothed…whales　cannot　yet　be　determined；　since　the　necessary　fossils　have　not　been　discovered。

Among　the　carnivorous　mammals；　phylogenetic　series　are　not　so　clear　and　distinct　as　among　the　hoofed　animals；　chiefly　because　the　carnivores　are　individually　much　less　abundant；　and　well…preserved　skeletons　are　among　the　prizes　of　the　collector。　　Nevertheless；　much　has　already　been　learned　concerning　the　mutual　relations　of　the　carnivorous　families；　and　several　phylogenetic　series；　notably　that　of　the　dogs；　are　quite　complete。　　It　has　been　made　extremely　probable　that　the　primitive　dogs　of　the　Eocene　represent　the　central　stock；　from　which　nearly　or　quite　all　the　other　families　branched　off；　though　the　origin　and　descent　of　the　cats　have　not　yet　been　determined。

It　should　be　clearly　understood　that　the　foregoing　account　of　mammalian　descent　is　merely　a　selection　of　a　few　representative　cases　and