▁" inter change " ▁among ▁the ▁sub system s ▁of ▁the ▁social ▁system ▁were ▁post ulated . ▁ ▁Par s ons ' ▁use ▁of ▁social ▁systems ▁analysis ▁based ▁on ▁the ▁AG IL ▁scheme ▁was ▁established ▁in ▁his ▁work ▁Econom y ▁and ▁Society ▁( with ▁N . ▁Sm el ser , ▁ 1 9 5 6 ) ▁and ▁has ▁pre va iled ▁in ▁all ▁his ▁work ▁ever ▁since . ▁However , ▁the ▁AG IL ▁system ▁existed ▁only ▁in ▁a ▁" r ud iment ary " ▁form ▁in ▁the ▁beginning ▁and ▁was ▁gradually ▁elabor ated ▁and ▁expanded ▁in ▁the ▁dec ades ▁which ▁followed . ▁A ▁brief ▁introduction ▁to ▁Par s ons ' ▁AG IL ▁scheme ▁can ▁be ▁found ▁in ▁Chapter ▁ 2 ▁of ▁The ▁American ▁University ▁( with ▁G . ▁Pl att , ▁ 1 9 7 3 ). ▁There ▁is , ▁however , ▁no ▁single ▁place ▁in ▁his ▁writing ▁in ▁which ▁the ▁total ▁AG IL ▁system ▁is ▁vis ually ▁displayed ▁or ▁explained : ▁the ▁complete ▁system ▁have ▁to ▁be ▁re construct ed ▁from ▁multiple ▁places ▁in ▁his ▁writing . ▁The ▁system ▁displayed ▁in ▁" The ▁American ▁University " ▁has ▁only ▁the ▁most ▁basic ▁elements ▁and ▁should ▁not ▁be ▁mistaken ▁for ▁the ▁whole ▁system . ▁ ▁Social ▁evolution ism ▁Par s ons ▁contributed ▁to ▁social ▁evolution ism ▁and ▁ne oe volution ism . ▁He ▁divided ▁evolution ▁into ▁four ▁sub - process es : ▁ ▁different iation , ▁which ▁creates ▁functional ▁sub system s ▁of ▁the ▁main ▁system , ▁as ▁discussed ▁above ; ▁ ▁adaptation ,