es　very　different　from　those　which　surrounded　my　generation　in　the　seventies　of　the　last　century；　when　Virchow　and　his　great　contemporaries　laid　the　sure　and　deep　foundations　of　modern　pathology。　　Which　of　you　now　knows　the　〃Cellular　Pathology〃　as　we　did？　　To　many　of　you　it　is　a　closed　book；to　many　more　Virchow　may　be　thought　a　spent　force。　But　no；　he　has　only　taken　his　place　in　a　great　galaxy。　　We　do　not　forget　the　magnitude　of　his　labors；　but　a　new　generation　has　new　problemshis　message　was　not　for　youbut　that　medicine　today　runs　in　larger　moulds　and　turns　out　finer　castings　is　due　to　his　life　and　work。　It　is　one　of　the　values　of　lectures　on　the　history　of　medicine　to　keep　alive　the　good　influences　of　great　men　even　after　their　positive　teaching　is　antiquated。　　Let　no　man　be　so　foolish　as　to　think　that　he　has　exhausted　any　subject　for　his　generation。　Virchow　was　not　happy　when　he　saw　the　young　men　pour　into　the　old　bottle　of　cellular　pathology　the　new　wine　of　bacteriology。　Lister　could　never　understand　how　aseptic　surgery　arose　out　of　his　work。　Ehrlich　would　not　recognize　his　epoch…making　views　on　immunity　when　this　generation　has　finished　with　them。　　I　believe　it　was　Hegel　who　said　that　progress　is　a　series　of　negationsthe　denial　today　of　what　was　accepted　yesterday；　the　contradiction　by　each　generation　of　some　part　at　least　of　the　philosophy　of　the　last；　but　all　is　not　lost；　the　germ　plasm　remains；　a　nucleus　of　truth　to　be　fertilized　by　men　often　ignorant　even　of　the　body　from　which　it　has　come。　Knowledge　evolves；　but　in　such　a　way　that　its　possessors　are　never　in　sure　possession。　〃It　is　because　science　is　sure　of　nothing　that　it　is　always　advancing〃　（Duclaux）。
  History　is　the　biography　of　the　mind　of　man；　and　its　educational　value　is　in　direct　proportion　to　the　completeness　of　our　study　of　the　individuals　through　whom　this　mind　has　been　manifested。　I　have　tried　to　take　you　back　to　the　beginnings　of　science；　and　to　trace　its　gradual　development；　which　is　conditioned　by　three　laws。　In　the　first　place；　like　a　living　organism；　truth　grows；　and　its　gradual　evolution　may　be　traced　from　the　tiny　germ　to　the　mature　product。　Never　springing；　Minerva…like；　to　full　stature　at　once；　truth　may　suffer　all　the　hazards　incident　to　generation　and　gestation。　Much　of　history　is　a　record　of　the　mishaps　of　truths　which　have　struggled　to　the　birth；　only　to　die　or　else　to　wither　in　premature　decay。　Or　the　germ　may　be　dormant　for　centuries；　awaiting　the　fullness　of　time。
  Secondly；　all　scientific　truth　is　conditioned　by　the　state　of　knowledge　at　the　time　of　its　announcement。　　Thus；　at　the　beginning　of　the　seventeenth　century；　the　science　of　optics　and　mechanical　appliances　had　not　made　possible　（so　far　as　the　human　mind　was　concerned）　the　existence　of　blood　capillaries　and　blood　corpuscles。　Jenner　could　not　have　added　to　his　〃Inquiry〃　a　study　on　immunity；　Sir　William　Perkin　and　the　chemists　made　Koch　technique　possible；　Pasteur　gave　the　conditions　that　produced　Lister；　Davy　and　others　furnished　the　preliminaries　necessary　for　anaesthesia。　　Everywhere　we　find　this　filiation；　one　event　following　the　other　in　orderly　sequence〃Mind　begets　mind；〃　as　Harvey　（De　Generatione）　says；　〃opinion　is　the　source　of　opinion。　Democritus　with　his　atoms；　and　Eudoxus　with　his　chief　good　which　he　placed　in　pleasure；　impregnated　Epicurus；　the　four　elements　of　Empedocles；　Aristotle；　the　doctrines　of　the　ancient　Thebans；　Pythagoras　and　Plato；　geometry；　Euclid。〃＇2＇
  ＇2＇　Works　of　William　Harvey；　translated　by　Robert　Willis；　London；　1847；　p。　532。
  And；　thirdly；　to　scientific　truth　alone　may　the　homo　mensura　principle　be　applied；　since　of　all　mental　treasures　of　the　race　it　alone　compels　general　acquiescence。　　That　this　general　acquiescence；　this　aspect　of　certainty；　is　not　reached　per　saltum；　but　is　of　slow；　often　of　difficult　growth；marked　by　failures　and　frailties；　but　crowned　at　last　with　an　acceptance　accorded　to　no　other　product　of　mental　activity；is　illustrated　by　every　important　discovery　from　Copernicus　to　Darwin。
  The　difficulty　is　to　get　men　to　the　thinking　level　which　compels　the　application　of　scientific　truths。　　Protagoras；　that　〃mighty…wise　man；〃　as　Socrates　called　him；　who　was　responsible　for　the　aphorism　that　man　is　the　measure　of　all　things；　would　have　been　the　first　to　recognize　the　folly　of　this　standard　for　the　people　at　large。　　But　we　have　gradually　reached　a　stage　in　which　knowledge　is　translated　into　action；　made　helpful　for　suffering　humanity；　just　as　the　great　discoveries　in　physics　and　chemistry　have　been　made　useful　in　the　advance　of　civilization。　We　have　traced　medicine　through　a　series　of　upward　steps　a　primitive　stage；　in　which　it　emerged　from　magic　and　religion　into　an　empirical　art；　as　seen　among　the　Egyptians　and　Babylonians；　a　stage　in　which　the　natural　character　of　disease　was　recognized　and　the　importance　of　its　study　as　a　phenomenon　of　nature　was　announced；　a　stage　in　which　the　structure　and　functions　of　the　human　body　were　worked　out；　a　stage　in　which　the　clinical　and　anatomical　features　of　disease　were　determined；　a　stage　in　which　the　causes　of　disorders　were　profitably　studied；　and　a　final　stage；　into　which　we　have　just　entered；　the　application　of　the　knowledge　for　their　prevention。　Science　has　completely　changed　man's　attitude　towards　disease。
  Take　a　recent　concrete　illustration。　　A　couple　of　years　ago　in　Philadelphia　and　in　some　other　parts　of　the　United　States；　a　very　peculiar　disease　appeared；　characterized　by　a　rash　upon　the　skin　and　moderate　fever；　and　a　constitutional　disturbance　proportionate　to　the　extent　and　severity　of　the　eruption。　The　malady　first　broke　out　in　the　members　of　a　crew　of　a　private　yacht；　then　in　the　crews　of　other　boats；　and　among　persons　living　in　the　boarding…houses　along　the　docks。　It　was　the　cause　of　a　great　deal　of　suffering　and　disability。
  There　were　three　courses　open：　　to　accept　the　disease　as　a　visitation　of　God；　a　chastening　affliction　sent　from　above；　and　to　call　to　aid　the　spiritual　arm　of　the　church。　　Except　the　〃Peculiar　People〃　few　now　take　this　view　or　adopt　this　practice。　The　Christian　Scientist　would　probably　deny　the　existence　of　the　rash　and　of　the　fever；　refuse　to　recognize　the　itching　and　get　himself　into　harmony　with　the　Infinite。　　Thirdly；　the　method　of　experimental　medicine。
  First；　the　conditions　were　studied　under　which　the　individual　cases　occurred。　The　only　common　factor　seemed　to　be　certain　straw　mattresses　manufactured　by　four　different　firms；　all　of　which　obtained　the　straw　from　the　same　source。
  The　second　point　was　to　determine　the　relation　of　the　straw　to　the　rash。　One　of　the　investigators　exposed　a　bare　arm　and　shoulder　for　an　hour　between　two　mattresses。　　Three　people　voluntarily　slept　on　the　mattresses　for　one　night。　　Siftings　from　the　straw　were　applied　to　the　arm；　under　all　of　which　circumstances　the　rash　quickly　developed；　showing　conclusively　the　relation　of　the　straw　to　the　disease。
  Thirdly；　siftings　from　the　straw　and　mattresses　which　had　been　thoroughly　disinfected　failed　to　produce　the　rash。
  And　fourthly；　careful　inspection　of　the　siftings　of　the　straw　disclosed　living　parasites；　small　mites；　which　when　applied　to　the　skin　quickly　produced　the　characteristic　eruption。
  SANITATION
  WHEN　the　thoughtful　historian　gets　far　enough　away　from　the　nineteenth　century　to　see　it　as　a　whole；　no　single　feature　will　stand　out　with　greater　distinctness　than　the　fulfilment　of　the　prophecy　of　Descartes　that　we　could　be　freed　from　an　infinity　of　maladies　both　of　body　and　mind　if　we　had　sufficient　knowledge　of　their　causes　and　of　all　the　remedies　with　which　nature　has　provided　us。　Sanitation　takes　its　place　among　the　great　modern　revolutions　political；　social　and　intellectual。　　Great　Britain　deserves　the　credit　for　the　first　practical　recognition　of　the　maxim　salus　populi　suprema　lex。　In　the　middle　and　latter　part　of　the　century　a　remarkable　group　of　men；　Southwood　Smith；　Chadwick；　Budd；　Murchison；　Simon；　Acland；　Buchanan；　J。W。　Russell　and　Benjamin　Ward　Richardson；　put　practical　sanitation　on　a　scientific　basis。　　Even　before　the　full　demonstration　of　the　germ　theory；　they　had　grasped　the　conception　that　the　battle　had　to　be　fought　against　a　living　contagion　which　found　in　poverty；　filth　and　wretched　homes　the　conditions　for　its　existence。　　One　terrible　disease　was　practically　wiped　out　in　twenty…five　years　of　hard　work。　It　is　difficult　to　realize　that　within　the　memory　of　men　now　living；　typhus　fever　was　one　of　the　great　scourges　of　our　large　cities；　and　broke　out　in　terrible　epidemicsthe　most　fatal　of　all　to　the　medical　profession。　In　the　severe　epidemic　in　Ireland　in　the　forties　of　the　last　century；　one　fifth　of　all　the　doctors　in　the　island　died　of　typhus。　A　better　idea　of　the　new　crusade；　made　possible　by　new　knowledge；　is　to　be　had　from　a　consideration　of　certain　diseases　against　which　the　fight　is　in　active　progress。
  Nothing　illustrates　more　clearly　the　interdependence　of　the　sciences　than　the　reciprocal　impulse　given　to　new　researches　in　pathology　and　entomology　by　the　discovery　of　the　part　played　by　insects　in　the　transmission　of　disease。　　The　flea；　the　louse；　the　bedbug；　the　house　fly；　the　mosquito；　the　tick；　have　all　within　a　few　years　taken　thei