CHARLESDARWINTOALEXANDERBAIN。
Down，October9，1873。
MydearSir，Iamparticularlyobligedtoyouforhavingsendmeyouressay。Yourcriticismsareallwritteninaquitefairspirit，andindeednoonewhoknowsyouoryourworkswouldexpectanythingelse。WhatyousayaboutthevaguenessofwhatIhavecalledthedirectactionofthenervoussystem，isperfectlyjust。Ifeltitsoatthetime，andevenmoreoflate。I
confessthatIhaveneverbeenablefullytograspyourprincipleofspontaneity，aswellassomeotherofyourpoints，soastoapplythemtospecialcases。Butaswelookateverythingfromdifferentpointsofview，itisnotlikelythatweshouldagreeclosely。（ProfessorBainexpoundedhistheoryofSpontaneityintheessayherealludedto。Itwouldbeimpossibletodojusticetoitwithinthelimitsofafoot—note。Thefollowingquotationsmaygivesomenotionofit:——
\"BySpontaneityIunderstandthereadinesstopassintomovementintheabsenceofallstimulationwhatever;theessentialrequisitebeingthatthenerve—centresandmusclesshallbefreshandvigorous……Thegesticulationsandthecarolsofyoungandactiveanimalsaremereoverflowofnervousenergy;andalthoughtheyareveryapttoconcurwithpleasingemotion，theyhaveanindependentsource……Theyarenotproperlymovementsofexpression;theyexpressnothingatallexceptanabundantstockofphysicalpower。\"）