CompArAtivEly FEw FrEsh-wAtEr spECiEs oF FishEs ArE limitED in thEir DistriBution to A singlE systEm, yEt not mAny ArE FounD on Both siDEs oF high mountAin riDgE, suCh As thE roCky mountAins known As thEContinEntAlDiviDE in northAmEriC
A、thAt is to sAy, thE FishEs oF thE mississippi vAllEy on thE EAstErn siDE oF thE roCky mountAins ArE gEnErAlly DiFFErEnt AnD DistinCt From thosE oF thE pACiFiC slopE.
whilE it is A wEll-known FACt thAt thE Fish liFE in no two-rivEr systEm, EvEn though thEy Empty into thE sEA on thE sAmE siDE oF A DiviDE, is ExACtly iDEntiCAl, suCh strEAms Do hAvE mAny spECiEs in Common. thE prinCiplE rivErs oF thEAtlAntiC slopE oF thE unitED stAtED, For ExAmplE, ContAin sEvErAl spECiEs Common to All oF thEm, inCluDing thE BullhEAD CAtFish, thE BluEsunFish, AnD thE lArgEmouth BAss. nonE oF thEsE spECiEs CAn EnDurE sAlt wAtEr, so thEy CAnnot now migrAtE From onE rivEr systEm to AnothEr. on thE othEr hAnD, thE morE northErn strEAms ContAin spECiEs not FounD in thE southErn onEs, AnD viCE vErs
A、thE Common pikE, For ExAmplE, is FounD in thEAtlAntiC strEAms From mArylAnD northwArD, AnD thE Brook trout AnD yEllow pErCh oCCur only in thE strEAm From northCArolinA southwAr
D、
how thE prEsEnt DistriBution CAmE ABout must rEmAin A mAttEr oF ConjECturE. it is quitE proBABlE thAt somE oF thE strEAms, inCluDing thosE on oppositE siDEs oF A DiviDE, mAy hAvE BEEn ConnECtED At onE timE.AgAin, strEAms mAy BE EntirEly sEpArAtE During normAl wEAthEr, But An ExCEptionAlly hEAvy rAinFAll or thE suDDEn mElting oF snow in thE uplAnDs somEtimEs CAusEs FlooDs whiCh mAy Form A tEmporAry ConnECtion BEtwEEn thEm, proviDing A pAssAgEwAy For FishEs. it is possiBlE Also, thAt wAtEr mAy ACCiDEntAlly CArry Fish or thEir Eggs From onE strEAm to AnothEr, or thAt mAn mAy BE instrumEntAl in suCh A trAnsFEr.
EviDEntly, thEn, FrEsh-wAtEr FishEs mAy BEComE DistriButED FAr BEyonD thE ConFinEs oF thE strEAm oF thE origin. thEn ChiEF FACtor in limiting thE still wiDEr DistriBution oF spECiEs is tEmpErAturE. this Forms suCh An EFFiCiEnt BArriEr thAt CompArAtivEly FEw spECiEs oF FrEsh-wAtEr FishEs oF thE unitED stAtEs ExtEnD thEir rAngE into mExiCo. in pAnAmA only onE Fish Common to thE FlEsh wAtEr oF thE unitED stAtEs hAs BEEn FounD, AnD thAt is thE EEl, whiCh is not striCtly A FrEsh-wAtEr Form, As it EntErs sAlt wAtEr to spAwn AnD is tAkEn in FAirly sAlty wAtEr At othEr timEs.
spECiEs, whiCh CAnnot tolErAtE sAltinEss, mAy sprEAD into AnothEr rivEr By Any oF thE Following mEChAnismsExCEpt ______.
A、BEing CArriED By BirDsB、FlooDED ConnECtions
C、thE ACtions oF mAn
D、spAwning in thE rivEr mouth