are　larger　than　Watelai。
  The　whole　of　Aru　is　low；　but　by　no　means　so　flat　as　it　has　been　represented；　or　as　it　appears　from　the　sea。　Most　of　it　is　dry　rocky　ground；　with　a　somewhat　undulating　surface；　rising　here　and　there　into　abrupt　hillocks；　or　cut　into　steep　and　narrow　ravines。　Except　the　patches　of　swamp　which　are　found　at　the　mouths　of　most　of　the　small　rivers；　there　is　no　absolutely　level　ground；　although　the　greatest　elevation　is　probably　not　more　than　two　hundred　feet。　The　rock　which　everywhere　appears　in　the　ravines　and　brooks　is　a　coralline　limestone；　in　some　places　soft　and　pliable；　in　others　so　hard　and　crystalline　as　to　resemble　our　mountain　limestone。
  The　small　islands　which　surround　the　central　mass　are　very　numerous；　but　most　of　them　are　on　the　east　side；　where　they　form　a　fringe；　often　extending　ten　or　fifteen　miles　from　the　main　islands。　On　the　west　there　are　very　few；　Wamma　and　Palo　Pabi　being　the　chief；　with　Ougia；　and　Wassia　at　the　north…west　extremity。　On　the　east　side　the　sea　is　everywhere　shallow；　and　full　of　coral；　and　it　is　here　that　the　pearl…shells　are　found　which　form　one　of　the　chief　staples　of　Aru　trade。　All　the　islands　are　covered　with　a　dense　and　very　lofty　forest。
  The　physical　features　here　described　are　of　peculiar　interest；　and；　as　far　as　I　am　aware；　are　to　some　extent　unique；　for　I　have　been　unable　to　find　any　other　record　of　an　island　of　the　size　of　Aru　crossed　by　channels　which　exactly　resemble　true　rivers。　How　these　channels　originated　were　a　complete　puzzle　to　me；　till；　after　a　long　consideration　of　the　whole　of　the　natural　phenomena　presented　by　these　islands；　I　arrived　at　a　conclusion　which　I　will　now　endeavour　to　explain。　There　are　three　ways　in　which　we　may　conceive　islands　which　are　not　volcanic　to　have　been　formed；　or　to　have　been　reduced　to　their　present　condition；　by　elevation；　by　subsidence；　or　by　separation　from　a　continent　or　larger　island。　The　existence　of　coral　rock；　or　of　raised　beaches　far　inland；　indicates　recent　elevation；　lagoon　coral…islands；　and　such　as　have　barrier　or　encircling　reefs；　have　suffered　subsidence；　while　our　own　islands；　whose　productions　are　entirely　those　of　the　adjacent　continent；　have　been　separated　from　it。　Now　the　Aru　Islands　are　all　coral　rock；　and　the　adjacent　sea　is　shallow　and　full　of　coral；　it　is　therefore　evident　that　they　have　been　elevated　from　beneath　the　ocean　at　a　not　very　distant　epoch。　But　if　we　suppose　that　elevation　to　be　the　first　and　only　cause　of　their　present　condition；　we　shall　find　ourselves　quite　unable　to　explain　the　curious　river…channels　which　divide　them。　Fissures　during　upheaval　would　not　produce　the　regular　width；　the　regular　depth；　or　the　winding　curves　which　characterise　them；　and　the　action　of　tides　and　currents　during　their　elevation　might　form　straits　of　irregular　width　and　depth；　but　not　the　river…like　channels　which　actually　exist。　If；　again；　we　suppose　the　last　movement　to　have　been　one　of　subsidence；　reducing　the　size　of　the　islands；　these　channels　are　quite　as　inexplicable；　for　subsidence　would　necessarily　lead　to　the　flooding　of　all　low　tracts　on　the　banks　of　the　old　rivers；　and　thus　obliterate　their　courses；　whereas　these　remain　perfect；　and　of　nearly　uniform　width　from　end　to　end。
  Now　if　these　channels　have　ever　been　rivers　they　must　have　flowed　from　some　higher　regions；　and　this　must　have　been　to　the　east；　because　on　the　north　and　west　the　sea…bottom　sinks　down　at　a　short　distance　from　the　shore　to　an　unfathomable　depth；　whereas　on　the　east。　a　shallow　sea；　nowhere　exceeding　fifty　fathoms；　extends　quite　across　to　New　Guinea；　a　distance　of　about　a　hundred　and　fifty　miles。　An　elevation　of　only　three　hundred　feet　would　convert　the　whole　of　this　sea　into　moderately　high　land；　and　make　the　Aru　Islands　a　portion　of　New　Guinea；　and　the　rivers　which　have　their　mouths　at　Utanata　and　Wamuka；　might　then　have　flowed　on　across　Aru；　in　the　channels　which　are　now　occupied　by　salt　water。　Then　the　intervening　land　sunk　down；　we　must　suppose　the　land　that　now　constitutes　Aru　to　have　remained　nearly　stationary；　a　not　very　improbable　supposition；　when　we　consider　the　great　extent　of　the　shallow　sea；　and　the　very　small　amount　of　depression　the　land　need　have　undergone　to　produce　it。
  But　the　fact　of　the　Aru　Islands　having　once　been　connected　with　New　Guinea　does　not　rest　on　this　evidence　alone。　There　is　such　a　striking　resemblance　between　the　productions　of　the　two　countries　as　only…　exists　between　portions　of　a　common　territory。　I　collected　one　hundred　species　of　land…birds　in　the　Aru　Islands；　and　about　eighty　of　them；　have　been　found　on　the　mainland　of　New　Guinea。　Among　these　are　the　great　wingless　cassowary；　two　species　of　heavy　brush　turkeys；　and　two　of　short　winged　thrushes；　which　could　certainly　not　have　passed　over　the　150　miles　of　open　sea　to　the　coast　of　New　Guinea。　This　barrier　is　equally　effectual　in　the　case　of　many　other　birds　which　live　only　in　the　depths　of　the　forest；　as　the　kinghunters　（Dacelo　gaudichaudi）；　the　fly…catching　wrens　（Todopsis）；　the　great　crown　pigeon　（Goura　coronata）；　and　the　small　wood　doves　（Ptilonopus　perlatus；　P。　aurantiifrons；　and　P。　coronulatus）。　Now；　to　show　the　real　effect　of　such　barrier；　let　us　take　the　island　of　Ceram；　which　is　exactly　the　same　distance　from　New　Guinea；　but　separated　from　it　by　a　deep　sea。　Cut　of　about　seventy　land…birds　inhabiting　Ceram；　only　fifteen　are　found　in　New　Guinea；　and　none　of　these　are　terrestrial　or　forest…haunting　species。　The　cassowary　is　distinct；　the　kingfishers；　parrots；　pigeons；　flycatchers；　honeysuckers；　thrushes；　and　cuckoos；　are　almost　always　quite　distinct　species。　More　than　this；　at　least　twenty　genera；　which　are　common　to　New　Guinea　and　Aru；　do　not　extend　into　Ceram；　indicating　with　a　force　which　every　naturalist　will　appreciate；　that　the　two　latter　countries　have　received　their　faunas　in　a　radically　different　manner。　Again；　a　true　kangaroo　is　found　in　Aru；　and　the　same　species　occurs　in　Mysol；　which　is　equally　Papuan　in　its　productions；　while　either　the　same；　or　one　closely　allied　to　it；　inhabits　New　Guinea；　but　no　such　animal　is　found　in　Ceram；　which　is　only　sixty　miles　from　Mysol。　Another　small　marsupial　animal　（Perameles　doreyanus）　is　common　to　Aru　and　New　Guinea。　The　insects　show　exactly　the　same　results。　The　butterflies　of　Aru　are　all　either　New　Guinea　species；　or　very　slightly　modified　forms；　whereas　those　of　Ceram　are　more　distinct　than　are　the　birds　of　the　two　countries。
  It　is　now　generally　admitted　that　we　may　safely　reason　on　such　facts　as　those；　which　supply　a　link　in　the　defective　geological　record。　The　upward　and　downward　movements　which　any　country　has　undergone；　and　the　succession　of　such　movements；　can　be　determined　with　much　accuracy；　but　geology　alone　can　tell　us　nothing　of　lands　which　have　entirely　disappeared　beneath　the　ocean。　Here　physical　geography　and　the　distribution　of　animals　and　plants　are　of　the　greatest　service。　By　ascertaining　the　depth　of　the　seas　separating　one　country　from　another；　we　can　form　some　judgment　of　the　changes　which　are　taking　place。　If　there　are　other　evidences　of　subsidence；　a　shallow　sea　　implies　a　former　connexion　of　the　adjacent　lands；　but　i￡　this　evidence　is　wanting；　or　if　there　is　reason　to　suspect　a　rising　of　the　land；　then　the　shallow　sea　may　be　the　result　of　that　rising；　and　may　indicate　that　the　two　countries　will　be　joined　at　some　future　time；　but　not　that　they　have　previously　been　so。　The　nature　of　the　animals　and　plants　inhabiting　these　countries　will；　however；　almost　always　enable　us　to　determine　this　question。　Mr。　Darwin　has　shown　us　how　we　may　determine　in　almost　every　case；　whether　an　island　has　ever　been　connected　with　a　continent　or　larger　land；　by　the　presence　or　absence　of　terrestrial　Mammalia　and　reptiles。　What　he　terms　〃oceanic　islands　〃possess　neither　of　these　groups　of　animals；　though　they　may　have　a　luxuriant　vegetation；　and　a　fair　number　of　birds；　insects；　and　landshells；　and　we　therefore　conclude　that　they　have　originated　in　mid…ocean；　and　have　never　been　connected　with　the　nearest　masses　of　land。　St。　Helena；　Madeira；　and　New　Zealand　are　examples　of　oceanic　islands。　They　possess　all　other　classes　of　life；　because　these　have　means　of　dispersion　over　wide　spaces　of　sea；　which　terrestrial　mammals　and　birds　have　not；　as　is　fully　explained　in　Sir　Charles　Lyell's　〃Principles　of　Geology；〃　and　Mr。　Darwin's　〃Origin　of　Species。〃　On　the　other　hand；　an　island　may　never　have　been　actually　connected　with　the　adjacent　continents　or　islands；　and　yet　may　possess　representatives　of　all　classes　of　animals；　because　many　terrestrial　mammals　and　some　reptiles　have　the　means　of　passing　over　short　distances　of　sea。　But　in　these　cases　the　number　of　species　that　have　thus　migrated　will　be　very　small；　and　there　will　be　great　deficiencies　even　in　birds　and　flying　insects；　which　we　should　imagine　could　easily　cross　over。　The　island　of　Timor　（as　I　have　already　shown　in　Chapter　XIII）　bears　this　relation　to　Australia；　for　while　it　contains　several　birds　and　insects　of　Australian　forms；　no　Australian　mammal　or　reptile　is　found　in　it；　and　a　great　number　of　the　most　abundant　and　characteristic　forms　of　Australian　birds　and　insects　are　entirely　absent。　Contrast　this　with　the　British　Islands；　in；　whi