▁gradually ▁decl ine ▁across ▁( say ) ▁ 2 5 ▁years , ▁it ▁is ▁still ▁not ▁proved ▁why ▁a ▁serious ▁economic ▁crisis ▁would ▁occur ▁precisely ▁at ▁the ▁end ▁of ▁that ▁period , ▁rather ▁than ▁( say ) ▁after ▁ 5 ▁years , ▁or ▁ 1 0 ▁years , ▁or ▁ 1 5 ▁years . ▁That ▁is , ▁by ▁demonstr ating ▁an ▁empir ical ▁profit ability ▁tr end , ▁the ▁main ▁causes ▁and ▁effects ▁of ▁the ▁tr end ▁are ▁not ▁yet ▁proved . ▁In ▁addition , ▁production ▁capital ▁is ▁a ▁smaller ▁and ▁smaller ▁fraction ▁of ▁the ▁total ▁mass ▁of ▁capital ▁accum ulated , ▁and ▁thus , ▁it ▁is ▁not ▁proved ▁how ▁the ▁reduced ▁profit ability ▁of ▁only ▁a ▁minor ▁part ▁of ▁the ▁total ▁capital ▁can , ▁by ▁itself , ▁throw ▁the ▁whole ▁of ▁capital ist ▁society ▁into ▁crisis . ▁ ▁What ▁can ▁be ▁definitely ▁proved , ▁is ▁that ▁sl umps ▁have ▁happened ▁fairly ▁regularly ▁in ▁the ▁history ▁of ▁industrial ▁capital ism ▁from ▁the ▁ 1 8 2 0 s ▁on ward , ▁some ▁being ▁more ▁severe ▁than ▁others . ▁In ▁the ▁real ▁economic ▁history ▁of ▁capital ism , ▁there ▁is ▁therefore ▁no ▁evidence ▁of ▁a ▁sp ont aneous ▁t endency ▁toward ▁economic ▁equilibrium : ▁capital ism ▁develop s ▁sp as mod ically , ▁through ▁bo oms ▁and ▁sl umps . ▁Every ▁crisis ▁is ▁supposed ▁to ▁be ▁the ▁last ▁one , ▁until ▁a ▁new ▁crisis ▁occurs . ▁That ▁was , ▁for ▁Marx , ▁a ▁good ▁reason ▁for ▁doing ▁away ▁with ▁the ▁capital ist ▁system ,