mindthe centralprincipleon which our concernfor other
beings is based. As I said in the first chapter, the only
legitimateboundarytoourconcernfortheinterestsofother
beingsisthepointatwhichitisnolongeraccuratetosaythat
theotherbeinghas interests.To haveinterests,in a strict,
nonmetaphoricalsense,abeingmustbecapableofsuffering
orexperiencingpleasure.Ifabeingsuffers,therecanbeno
moral justification for disregarding that suffering, or for
refusing to count itequally with the likesuffering of any
otherbeing.Buttheconverseofthisisalsotrue.Ifabeingis
notcapableofsuffering,orofenjoyment,thereisnothingto
take into account.
Sotheproblemofdrawingthelineistheproblemofdeciding
whenwearejustifiedinassumingthatabeingisincapableof
suffering. In my earlier discussion of the evidence that
nonhumananimalsarecapableofsuffering,Isuggestedtwo
indicatorsofthiscapacity:thebehaviorofthebeing,whether
itwrithes,utterscries,attemptstoescapefromthesourceof
pain,andsoon;andthesimilarityofthenervoussystemof
thebeingtoourown.Asweproceeddowntheevolutionary
scalewefindthatonboththesegroundsthestrengthofthe
evidenceforacapacity tofeelpaindiminishes.With birds
and mammals the evidence is overwhelming.
Reptilesandfishhavenervoussystemsthatdifferfromthose
ofmammalsinsomeimportantrespectsbutsharethebasic
structure of centrally organized nerve pathways. Fish and
reptilesshowmostofthepainbehaviorthatmammalsdo.In
most species there is evenvocalization, although itis not
audibletoourears.Fish,forinstance,makevibratorysounds,
anddifferent“calls”havebeendistinguishedbyresearchers,
including sounds indicating “alarm” and “aggravation.”^21
Fishalsoshowsignsofdistresswhentheyaretakenoutofthe