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The requested URL /mbrowse/msdb/life/story/is/As+far+as+I+am+able+to+judge,+after+long+attending+to+the subject,+the+conditions+of+life+appear+to+act+in+two+ways,- directly+on+the+whole+organization+or+on+certain+parts+alone, and+indirectly+by+affecting+the+reproductive+system. With+respect+to+the+direct+action,+we+must+bear+in+mind+that in+every+case,+as+[[August_Weismann|Professor+Weismann]]+has+lately+insisted, and+as+I+have+incidentally+shown+in+my+work+on+Variation under+Domestication,+there+are+two+factors:+namely,+the nature+of+the+organism,+and+the+nature+of+the+conditions. [[[[[images/August+Weismann.jpg|August+Weismann]]]]] The+former+seems+to+be+much+the+more+important;+for+nearly similar+variations+sometimes+arise+under,+as+far+as+we+can judge,+dissimilar+conditions;+and,+on+the+other+hand, dissimilar+variations+arise+under+conditions+which+appear+to be+nearly+uniform. The+effects+on+the+offspring+are+either+definite+or+indefinite. They+may+be+considered+as+definite+when+all+or+nearly+all+the offspring+of+individuals+exposed+to certain+conditions+during+several+generations+are+modified+in the+same+manner. It+is+extremely+difficult+to+come+to+any conclusion+in+regard+to+the+extent+of+the+changes+which+have been+thus+definitely+induced. There+can,+however,+be+little+doubt+about+many+slight+changes,- such+as+size+from+the+amount+of+food,+colour+from+the+nature of+the+food,+thickness+of+the+skin+and+hair+from+climate,+&c. Each+of+the+endless+variations+which+we+see+in+the+plumage of+our+fowls+must+have+had+some+efficient+cause;+and+if+the same+cause+were+to+act+uniformly+during+a+long+series+of generations+on.+many+individuals,+all+probably+would+be modified+in+the+same+manner. [[[[[images/fowl.gif]]]]] Such+facts+as+the+complex+and+extraordinary+out-growths which+variably+follow+from+the+insertion+of+a+minute+drop+of poison+by+a+gall-producing+insect,+show+us+what+singular modifications+might+result+in+the+case+of+plants+from+a chemical+change+in+the+nature+of+the+sap.+Indefinite+variability is+a+much+more+common+result+of+changed+conditions+than definite+variability,+and+has+probably+played+a+more+important part+in+the+formation+of+our+domestic+races. [[[[[images/sap.jpg]]]]] We+see+indefinite+variability+in+the+endless+slight+peculiarities which+distinguish+the+individuals+of+the+same+species, and+which+cannot+be+accounted+for+by+inheritance+from either+parent+or+from+some+more+remote+ancestor. Even+strongly+marked+differences+occasionally+appear+in+the young+of+the+same+litter,+and+in+seedlings+from+the+same seed-capsule. [[[[[images/litter.gif]]]]]+[[[[[images/seedling.jpg]]]]]+[[[[[images/seeds.gif]]]]] At+long+intervals+of+time,+out+of+millions+of+individuals+reared+in the+same+country+and+fed+on+nearly+the+same+food,+deviations of+structure+so+strongly+pronounced+as+to+deserve+to+be+called monstrosities+arise; but+monstrosities+cannot+be+separated+by+any+distinct+line+from+slighter+variations. [[[[[images/monster.jpg]]]]]+ All+such+changes+of+structure,+whether+extremely+slight+or strongly+marked,+which+appear+amongst+many+individuals living+together,+may+be+considered+as+the+indefinite+effects+of the+conditions+of+life+on+each+individual+organism,+in+nearly the+same+manner+as+the+chill+affects+different+men+in+an indefinite+manner,+according+to+their+state+of+body+or constitution,+causing+coughs+or+colds,+rheumatism, or+inflammation+of+various+organs. [[[[[images/cough.jpg]]]]]+ With+respect+to+what+I+have+called+the+indirect+action+of changed+conditions,+namely,+through+the+reproductive+system of+being+affected,+we+may+infer+that+variability+is+thus+induced, partly+from+the+fact+of+this+system+being+extremely+sensitive to+any+change+in+the+conditions,+and+partly+from+the similarity,+as+Kreuter+and+others+have+remarked,+between+the variability+which+follows+from+the+crossing+of+distinct+species, and+that+which+may+be+observed+with+plants+and+animals when+reared+under+new+or+unnatural+conditions. Many+facts+clearly+show+how+eminently+susceptible+the reproductive+system+is+to+very+slight+changes+in+the+surrounding+conditions. Nothing+is+more+easy+than+to+tame+an+animal, and+few+things+more+difficult+than+to+get+it+to+breed+freely under+confinement,+even+when+the+male+and+female+unite. How+many+animals+there+are+which+will+not+breed,+though kept+in+an+almost+free+state+in+their+native+country! This+is+generally,+but+erroneously,+attributed+to+vitiated instincts. Many+cultivated+plants+display+the+utmost+vigour,+and+yet rarely+or+never+seed!+In+some+few+cases+it+has+been discovered+that+a+very+trifling+change,+such+as+a+little+more+or less+water+at+some+particular+period+of+growth,+will+determine whether+or+not+a+plant+will+produce+seeds. [[[[[images/wheatSeeds.jpg]]]]]+ I+cannot+here+give+the+details+which+I+have+collected+and elsewhere+published+on+this+curious+subject;+but+to+show+how singular+the+laws+are+which+determine+the+reproduction+of animals+under+confinement,+I+may+mention+that+carnivorous animals,+even+from+the+tropics,+breed+in+this+country+pretty freely+under+confinement,+with+the+exception+of+the plantigrades+or+bear+family,+which+seldom+produce+young; whereas+carnivorous+birds,+with+the+rarest+exceptions, hardly+ever+lay+fertile+eggs. [[[[[images/egg.jpg]]]]] Many+exotic+plants+have+pollen+utterly+worthless, in+the+same+condition+as+in+the+most+sterile+hybrids. When,+on+the+one+hand,+we+see+domesticated+animals+and plants,+though+often+weak+and+sickly,+breeding+freely+under confinement;+and+when,+on+the+other+hand,+we+see individuals,+though+taken+young+from+a+state+of+nature perfectly+tamed,+long-lived+and+healthy (of+which+I+could+give+numerous+instances), yet+having+their+reproductive+system+so+seriously+affected+by unperceived+causes+as+to+fail+to+act,+we+need+not+be surprised+at+this+system,+when+it+does+act+under confinement,+acting+irregularly,+and+producing+offspring somewhat+unlike+their+parents. [[[[[images/pollen.jpg]]]]] [[[OriginOfSpecies/msdbSearchText/title/Causes+of+Variability|Causes+of+Variability]]]/ was not found on this server.