Everywherethesamegeographicaldistributionasnow, withadifferenceintheparticulararea,asrespectsthenorthernportionofthecontinents,answeringtoawarmerclimatethenthanours, suchasallowedspeciesofhippopotamus, rhinoceros, andelephant, torangeeventotheregionsnowinhabitedbythereindeerandthemusk-ox, , , moderatesensibility, andthesimpleconditionsofanexistenceinamediumliketheocean,notsubjecttogreatvariationandincapableofsuddenchange, maywellaccountfortheircontinuance; while,ontheotherhand, themoreintense, howevergradual,climaticvicissitudesonland, whichhavedrivenalltropicalandsubtropicalformsoutofthehigherlatitudesandassignedtothemtheiractuallimits,wouldbealmostsuretoextinguishsuchhugeandunwieldyanimalsasmastodons, mammoths, andthelike, whosepowerofenduringalteredcircumstancesmusthavebeensmall.
, irrespectiveoftheirantecedents, leavesthisfactjustasmysteriousasiscreationitself; , , namely, , thoughnooneofthegrandtypesoftheanimalkingdomcanbetracedbackfartherthantherest, yetthelowerclasseslongprecededthehigher;thattherehasbeenonthewholeasteadyprogressionwithineachclassandorder; , however, , , allisasitshouldbeuponanhypothesisofderivation.
, curiouslyenough,themoststrikingclassofexceptions, ifsuchtheybe, ; forwhichtheformernamemaysuffice, asthedifferencebetweenthetwoisevanescent.
