 Next , Diocletian was vulnerable in that he had no sons , just a daughter , Valeria , who could never succeed him . He was forced therefore to seek a co @-@ ruler from outside his family and that co @-@ ruler had to be someone he trusted . ( The historian William <unk> has argued that Diocletian , like heirless emperors before him , adopted Maximian as his filius Augusti ( " Augustan son " ) upon his appointment to the office . Some agree , but the historian Frank Kolb has stated that arguments for the adoption are based on <unk> of the <unk> evidence . Maximian did take Diocletian 's nomen ( family name ) Valerius , however . ) 
