LetusconsiderforamomentwhatmostofthetroubleandanxietywhichIhavereferredtoisabout，andhowmuchitisnecessarythatwebetroubled，oratleastcareful。Itwouldbesomeadvantagetoliveaprimitiveandfrontierlife，thoughinthemidstofanoutwardcivilization，ifonlytolearnwhatarethegrossnecessariesoflifeandwhatmethodshavebeentakentoobtainthem；oreventolookovertheoldday-booksofthemerchants，toseewhatitwasthatmenmostcommonlyboughtatthestores，whattheystored，thatis，whatarethegrossestgroceries。Fortheimprovementsofageshavehadbutlittleinfluenceontheessentiallawsofman’sexistence；asourskeletons，probably，arenottobedistinguishedfromthoseofourancestors。
Bythewords，necessaryoflife，Imeanwhatever，ofallthatmanobtainsbyhisownexertions，hasbeenfromthefirst，orfromlongusehasbecome，soimportanttohumanlifethatfew，ifany，whetherfromsavageness，orpoverty，orphilosophy，everattempttodowithoutit。Tomanycreaturesthereisinthissensebutonenecessaryoflife，Food。Tothebisonoftheprairieitisafewinchesofpalatablegrass，withwatertodrink；unlessheseekstheShelteroftheforestorthemountain’sshadow。NoneofthebrutecreationrequiresmorethanFoodandShelter。Thenecessariesoflifeformaninthisclimatemay，accuratelyenough，bedistributedundertheseveralheadsofFood，Shelter，Clothing，andFuel；fornottillwehavesecuredthesearewepreparedtoentertainthetrueproblemsoflifewithfreedomandaprospectofsuccess。Manhasinvented，notonlyhouses，butclothesandcookedfood；andpossiblyfromtheaccidentaldiscoveryofthewarmthoffire，andtheconsequentuseofit，atfirstaluxury，arosethepresentnecessitytositbyit。Weobservecatsanddogsacquiringthesamesecondnature。ByproperShelterandClothingwelegitimatelyretainourowninternalheat；butwithanexcessofthese，orofFuel，thatis，withanexternalheatgreaterthanourowninternal，maynotcookeryproperlybesaidtobegin？Darwin，thenaturalist，saysoftheinhabitantsofTierradelFuego，thatwhilehisownparty，whowerewellclothedandsittingclosetoafire，werefarfromtoowarm，thesenakedsavages，whowerefartheroff，wereobserved，tohisgreatsurprise，“tobestreamingwithperspirationatundergoingsucharoasting。“So，wearetold，theNewHollandergoesnakedwithimpunity，whiletheEuropeanshiversinhisclothes。Isitimpossibletocombinethehardinessofthesesavageswiththeintellectualnessofthecivilizedman？AccordingtoLiebig，man’sbodyisastove，andfoodthefuelwhichkeepsuptheinternalcombustioninthelungs。Incoldweatherweeatmore，inwarmless。
Theanimalheatistheresultofaslowcombustion，anddiseaseanddeathtakeplacewhenthisistoorapid；orforwantoffuel，orfromsomedefectinthedraught，thefiregoesout。Ofcoursethevitalheatisnottobeconfoundedwithfire；butsomuchforanalogy。Itappears，therefore，fromtheabovelist，thattheexpression，animallife，isnearlysynonymouswiththeexpression，animalheat；forwhileFoodmayberegardedastheFuelwhichkeepsupthefirewithinus——andFuelservesonlytopreparethatFoodortoincreasethewarmthofourbodiesbyadditionfromwithout——
ShelterandClothingalsoserveonlytoretaintheheatthusgeneratedandabsorbed。
Thegrandnecessity，then，forourbodies，istokeepwarm，tokeepthevitalheatinus。Whatpainsweaccordinglytake，notonlywithourFood，andClothing，andShelter，butwithourbeds，whichareournight-clothes，robbingthenestsandbreastsofbirdstopreparethisshelterwithinashelter，asthemolehasitsbedofgrassandleavesattheendofitsburrow！Thepoormaniswonttocomplainthatthisisacoldworld；andtocold，nolessphysicalthansocial，wereferdirectlyagreatpartofourails。Thesummer，insomeclimates，makespossibletomanasortofElysianlife。Fuel，excepttocookhisFood，isthenunnecessary；thesunishisfire，andmanyofthefruitsaresufficientlycookedbyitsrays；whileFoodgenerallyismorevarious，andmoreeasilyobtained，andClothingandShelterarewhollyorhalfunnecessary。