Complex sentence: When Mabillon suggested a new edition of the works of Hilary of Poitiers, it was Coustant whom the abbot general selected; there was before this time practically only one edition, namely the defective and uncritical one published by Erasmus (Basle, 1523).
Split into simpler sentences: When Mabillon suggested a new edition of the works of Hilary of Poitiers, it was Coustant whom the abbot general selected. Before this time, there was only one defective and uncritical edition of Poitiers' work, published by Erasmus (Basle, 1523).