▁ 1 9 6 9 ) ▁was ▁an ▁American ▁zo ologist ▁and ▁prot ist ologist , ▁famous ▁for ▁giving ▁the ▁first , ▁strong ▁empir ical ▁proof ▁for ▁the ▁existence ▁of ▁a ▁sym bi otic ▁relationship ▁between ▁internal ▁micro organ isms ▁and ▁their ▁met az o an ▁host . ▁ ▁Cleveland ▁received ▁in ▁ 1 9 1 7 ▁his ▁B . S . ▁from ▁the ▁University ▁of ▁Mississippi , ▁where ▁he ▁spent ▁one ▁year ▁as ▁a ▁gradu ate ▁student ▁and ▁instruct or . ▁After ▁a ▁brief ▁period ▁of ▁military ▁service , ▁he ▁taught ▁for ▁two ▁years ▁at ▁Em ory ▁University ▁and ▁then ▁for ▁one ▁year ▁at ▁Kansas ▁State ▁College . ▁He ▁then ▁became ▁a ▁gradu ate ▁student ▁at ▁Joh ns ▁Hop kins ▁University , ▁where ▁he ▁in ▁ 1 9 2 3 ▁received ▁a ▁Ph . D . ▁and ▁from ▁ 1 9 2 3 ▁to ▁ 1 9 2 5 ▁held ▁a ▁National ▁Research ▁Council ▁Fellow ship ▁at ▁the ▁Joh ns ▁Hop kins ▁School ▁of ▁H yg iene ▁and ▁Public ▁Health . ▁From ▁ 1 9 2 5 ▁to ▁ 1 9 3 6 ▁Cleveland ▁worked ▁at ▁the ▁School ▁of ▁Trop ical ▁Medicine ▁of ▁Harvard ▁Medical ▁School . ▁From ▁ 1 9 3 6 ▁to ▁ 1 9 5 9 ▁he ▁worked ▁at ▁the ▁Bi ology ▁Department ▁of ▁Harvard ▁University , ▁where ▁he ▁became ▁a ▁full ▁professor ▁in ▁ 1 9 4 6 ▁and ▁retired ▁as ▁professor ▁emer it us ▁in ▁ 1 9 5 9 . ▁On ▁the ▁inv itation ▁of ▁R